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Broad-scale geographic variation in the organization of rocky intertidal communities in the 6 

Gulf of Maine 7 

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Elizabeth S. Bryson1, Geoffrey C. Trussell1*, Patrick J. Ewanchuk2 9 

1Marine Science Center and Department of Marine and Environmental Sciences, 430 Nahant 10 

Road, Nahant, MA 01908 11 

2Department of Biology, Providence College, Providence, RI 02912 12 

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*Corresponding author 16 

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Keywords: community organization, coastal oceanography, competition, disturbance, Gulf of 18 

Maine, herbivory, predation, recruitment, rocky intertidal 19 

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ABSTRACT 24 

A major challenge facing ecology is to better understand how large-scale processes 25 

modify local scale processes to shape the organization of ecological communities. Although the 26 

results of ecological experiments are repeatable on local scales, different results often emerge 27 

across broad scales, which can hinder the development of general predictions that apply across 28 

the geographical range of a community. Numerous studies in the southern Gulf of Maine have 29 

shaped our understanding of community organization and dynamics on New England rocky 30 

intertidal shores, where consumers strongly control recovery from disturbance on sheltered 31 

shores and high invertebrate recruitment and competition for space dictate recovery on wave-32 

exposed shores. It is unclear, however, whether the effects of consumers and recruitment 33 

variation on resulting community organization in this region apply more broadly to rocky 34 

intertidal habitats throughout the Gulf. 35 

We characterized variation in rocky intertidal community structure at 34 sites throughout 36 

the Gulf of Maine and experimentally examined the influence of consumers (present, absent) and 37 

wave energy (wave-exposed, sheltered) on community recovery from disturbance in the northern 38 

and southern Gulf. Our results reinforced previous work in the southern Gulf because consumers 39 

dictated the recovery of fucoid algae and mussels on sheltered shores, whereas high barnacle and 40 

mussel recruitment and competition for space shaped recovery on wave-exposed shores. 41 

However, on sheltered shores in the northern Gulf, neither consumers nor barnacle and mussel 42 

recruitment impacted recovery, which was dominated by fucoid algae. Moreover, recovery on 43 

wave-exposed shores in the northern Gulf was quite distinct from that observed in the southern 44 

Gulf: barnacle and mussel recruitment was negligible and fucoid algae dominated recovery 45 

including the long-term establishment of Ascophyllum nodosum, which is largely absent from 46 

 

 

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wave-exposed shores in the southern Gulf. Thus, distinct community types emerged in the 47 

northern and southern Gulf despite their sharing many of the same species. These patterns likely 48 

emerged because of regional differences in coastal oceanography that modulate the recruitment 49 

of barnacles and mussels. Hence, increased attention to regional factors should provide key 50 

insight into how rocky shore communities are organized in the Gulf of Maine and elsewhere. 51 

INTRODUCTION 52 

A central goal of ecology is to understand how large-scale processes modify local level 53 

processes to shape the distribution and abundance of species, and assembly, organization and 54 

dynamics of ecological communities (Wiens 1989, Levin 1992, McGill 2010, Hastings 2010). 55 

Because environmental gradients across larger scales can modify, for example, community 56 

assembly (Chase 2010, Hein and Gillooly 2011), the relative importance of bottom-up and top-57 

down processes (Menge 2000, Navarrete et al. 2005, Chase et al. 2010, Krenz et al. 2011), and 58 

the nature, intensity and scale of species interactions (Shurin and Allen 2001, Chase 2003, 59 

Sanford and Worth 2010, Menge et al. 2011), it is difficult to derive overarching assembly rules 60 

for community ecology. Understanding how differences in biotic and abiotic context mediate 61 

changes in species interactions and ultimately community assembly has been identified as a 62 

major gap in ecology (Agrawal et al. 2007, Weiher et al. 2011). 63 

Rocky intertidal communities in the southern Gulf of Maine have long served as a model 64 

system to understand how abiotic and biotic factors influence succession after disturbance and 65 

resulting community organization (Menge 1976, 1978a,b, Lubchenco, 1980, 1983, Petraitis 66 

1987, Petraitis and Dudgeon 1999, Dudgeon and Petraitis 2001, Bertness et al. 2002, 2004a,b). 67 

A defining feature of these and other rocky shores is the amount of wave action they experience 68 

(i.e., wave-exposed versus sheltered), which can influence larval and nutrient flux rates (Leonard 69 

 

 

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et al 1998, Jenkins and Hawkins 2003, Bertness et al. 2004a), the availability of space via 70 

disturbance (Paine and Levin 1981, Denny et al. 1985), the extent of air exposure as a result of 71 

wave splash (Harley and Helmuth 2003), and the abundance and efficacy of mobile consumers 72 

(Kitching et al. 1959, Menge 1976, Menge and Sutherland 1976, Etter 1989). Hence, patterns of 73 

succession and resulting community organization often differ substantially between wave-74 

exposed and sheltered shores. 75 

On wave-exposed shores in the southern Gulf of Maine, barnacle recruitment 76 

(Semibalanus balanoides) during a narrow window between late February and April (Barnes 77 

1957, Dudgeon and Petraitis 2001, Pineda et al. 2002, Kordas and Dudgeon 2009) often 78 

facilitates high mussel recruitment in the summer (Petraitis 1991, Bertness et al. 2004a). In the 79 

absence of consumer pressure, mussels eventually dominate these communities by overgrowing 80 

barnacles and outcompeting fucoid algae (Fucus vesiculosus, Ascophyllum nodosum) for space 81 

(Menge 1976). Of course, depending on the timing and scale that new bare space is made 82 

available by disturbance from waves and other factors, these habitats can also contain a mosaic 83 

of barnacles, mussels and Fucus at any given time (Menge 1976, 1978a,b, Bertness et al. 1999, 84 

2004a). In contrast, on sheltered shores in the southern Gulf the supply of barnacle and mussel 85 

larvae is reduced (Bertness et al. 2004a) and resulting low recruitment, coupled with more 86 

intense predation by dogwhelks (Nucella lapillus) and green crabs (Carcinus maenas), further 87 

reduces mussel and barnacle abundance (Lubchenco and Menge 1978). As a result, competition 88 

for space is relaxed allowing fucoid algae (initially Fucus vesiculosus followed by the slower 89 

growing Ascophyllum nodosum) to colonize, grow and form dense canopies that dominate the 90 

shore (Dudgeon and Petraitis 2001). Although snail (Littorina littorea) grazing on young 91 

recruits may slow fucoid algal recovery (Lubchenco 1983, Petraitis 1987), the eventually 92 

 

 

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dominant Ascophyllum canopy typically covers a sparse understory of mussels and barnacles at 93 

these sites (Bertness et al. 2004a), which contrasts with the dense mussel and barnacle 94 

communities typical of wave-exposed shores (Menge 1976, 1978a,b) 95 

The prevailing evidence indicates that these habitat-specific differences in succession and 96 

community organization in the southern Gulf typically depend on the high recruitment potential 97 

of dominant space occupying species and the impact of consumers on their abundance after 98 

settlement. However, recent work on Gulf of Maine shores has revealed geographically based 99 

differences in species interaction strength (Kordas and Dudgeon 2009, 2011) and community 100 

organization following disturbance (Petraitis and Dudgeon 1999, 2004; Dudgeon and Petraitis 101 

2001, Bertness et al. 2002, 2004a,b). Hence, there has been disagreement over whether 102 

consumers drive the dynamics and organization of these communities or whether spatial and 103 

temporal variation in recruitment levels of key species plays a more prominent role than 104 

previously thought. For example, experiments at multiple locations in the southern Gulf found 105 

that consumers prevented the recovery of fucoid algal canopies on sheltered shores (Bertness et 106 

al. 2002, 2004a,b). In contrast, work in central Maine’s Penobscot Bay found that variability in 107 

the timing and location of mussel, barnacle and fucoid algal recruitment and the size of patches 108 

created by disturbance could result in mussel dominated or algal dominated community states 109 

(Petraitis and Dudgeon 1999, Dudgeon and Petraitis 2001). These results also suggests that 110 

variation in barnacle and mussel recruitment in the Gulf of Maine may not solely depend on 111 

wave-exposure, but also local oceanographic processes that impact larval supply. Moreover, 112 

consumer pressure was quite different in these studies, with Bertness et al. (2004a) observing 113 

high predation rates within a single tidal cycle and Petraitis and Dudgeon (1999, 2004) observing 114 

predation rates that were not apparent for several weeks. 115 

 

 

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It is clear that there is the potential for substantial geographic variation in the factors 116 

affecting community succession and organization in the Gulf of Maine. This is not surprising 117 

because geographic variation in upwelling (Bustamante et al. 1995a, Menge et al. 1997, 2003, 118 

2004), grazer impacts (Coleman et al. 2006), and the strength of positive and negative species 119 

interactions (Bertness and Leonard 1997, Leonard 2000) can influence the succession and 120 

organization of rocky shore communities. For example, large regional differences in community 121 

organization have been documented in the eastern Atlantic (Coleman et al. 2006, Jenkins et al. 122 

2008), which has many of the dominant species (Fucus, Ascophyllum, Semibalanus, Mytilus sp., 123 

Carcinus and Nucella) found on Gulf of Maine shores. In the eastern Atlantic, latitudinal 124 

differences in rocky shore community organization are likely driven by changes in the 125 

abundance and impact of two patellid limpet species, Patella depressa and P. vulgata (Coleman 126 

et al. 2006, Jenkins et al. 2008), which are absent from the western Atlantic and Gulf of Maine. 127 

In general, herbivore control of fucoid algae appears to be more intense in the eastern Atlantic 128 

than in the Gulf of Maine (Jenkins et al. 2008), whereas factors affecting barnacle and mussel 129 

abundance may be more important on rocky shores in the western Atlantic (i.e. the Gulf of 130 

Maine). 131 

Over the last 20 years, many studies have shown that broad scale spatial variation in 132 

rocky shore communities may reflect differences in oceanographic processes that impact larval 133 

supply (e.g. Menge et al. 1994, 2003, Broitman et al. 2001, Navarrete et al. 2005, Blanchette et 134 

al. 2008, Wieters et al. 2009). Because variation in barnacle and mussel recruitment can play a 135 

key role in the succession and organization of Gulf of Maine rocky shores, identifying 136 

geographic variation in coastal oceanography that influences larval supply may help reconcile 137 

the disparate influence of consumer control and recruitment variation on community succession 138 

 

 

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and organization throughout the Gulf of Maine. To examine whether distinct oceanographically 139 

driven biogeographic provinces occur within the Gulf of Maine, we examined the influence of 140 

physical stress (wave energy), recruitment, and consumers on community recovery after 141 

disturbance in the northern and southern Gulf. To broaden the geographic scope of these 142 

observations and experiments, we also examined between-site variation in community structure 143 

on 34 shores across the Gulf of Maine basin. Our results suggest that the succession and 144 

organization of rocky intertidal communities in the northern and southern Gulf are substantially 145 

different despite sharing virtually the same species assemblages. Moreover, these differences 146 

appear to be shaped by oceanographically driven recruitment variation that dictates subsequent 147 

species interactions. 148 

MATERIALS AND METHODS 149 

Community Structure Across the Gulf of Maine 150 

We characterized the structure of rocky intertidal communities with quadrat surveys at 34 151 

sites spanning the Gulf of Maine (Fig. 1, Appendix A). We recorded the percent cover of all 152 

visible, macroscopic, sessile species composing the canopy and understory in point-intercept 153 

quadrats (25 points per 0.25 m2 quadrat) haphazardly placed (N = 10) in the mid-intertidal at 154 

each site. This method provides reliable estimates of the abundance of common intertidal 155 

species, but may be less so in estimating the abundance of rare species. Quadrats were randomly 156 

tossed on horizontal emergent substratum within the fucoid algae zone. If a quadrat landed in a 157 

tidepool, or rested vertically against a ledge, then the quadrat was moved to the nearest emergent, 158 

horizontal surface. All algal or sessile invertebrate species located beneath an intercept were 159 

recorded to yield percent cover data that regularly exceeded 100% because of the presence of 160 

both the canopy and understory organisms. For any species that could not be conclusively 161 

 

 

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identified in the field (rarely the case), a representative sample was collected for identification in 162 

the laboratory. 163 

Sites were assigned to 3 geographic regions based on documented oceanographic 164 

circulation patterns (Pettigrew et al. 1998, 2005, see Fig. 1 for prevailing Gulf of Maine 165 

circulation patterns). The Southern Gulf region ranged from Cape Cod, Massachusetts to 166 

Penobscot Bay, Maine corresponding to the Western Maine Coastal Current (WMCC) that forms 167 

from the outflow of the Penobscot River and the Eastern Maine Coastal Current (EMCC; 168 

Churchill et al. 2005, Pettigrew et al. 2005, Manning et al. 2009). The Penobscot region ranged 169 

from Penobscot Bay to Great Wass Island, Maine, where a portion of the EMCC moves offshore 170 

to varying extents depending on seasonal and interannual variation. When offshore movement is 171 

high, a freshwater plume from the Penobscot River replaces the surface waters in this region 172 

(Pettigrew et al. 1998, 2005, Churchill et al. 2005, Hetland and Signell 2005, Pettigrew et al. 173 

2005). The Northern Gulf region ranged from Great Wass Island to Cobscook Bay, Maine, 174 

corresponding to the region where the Nova Scotia current and discharge from the St. John and 175 

St. Croix rivers meet near the mouth of the Bay of Fundy to form the EMCC and southwestward 176 

flow occurs (Hetland and Signell 2005, Pettigrew et al. 2005, Tilburg et al. 2012). The wave 177 

exposure of each site (sheltered vs. exposed) was characterized based on personal observations 178 

of coastal topography (e.g., headlands vs. bays) and wave action during calm and stormy periods, 179 

and/or dissolution rates of plaster clod cards (Appendix A). 180 

Variation in community structure was analyzed with multivariate analyses using the 181 

Vegan package (Oksanen et al. 2013) for R version 3.0.0 (R Core Development Team 2013). A 182 

Non-Metric Multidimensional Scaling (NMDS) plot was created to compare community 183 

composition between sites using the ‘metaMDS’ function with untransformed percent cover data 184 

 

 

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and a Bray-Curtis index. After the ordination plot was constructed from the metaMDS, the 185 

function ‘ordiellipse’ was used to plot 95% confidence ellipses for the mean ordination of each 186 

geographic region and wave exposure combination. We employed the ‘adonis’ function with a 187 

Bray-Curtis index with 999 permutations to conduct a multivariate ANOVA (PERMANOVA), 188 

with geographic region (Southern, Penobscot, Northern) and wave exposure (Exposed, 189 

Sheltered) as orthogonally crossed factors to test for differences in community structure. 190 

Subsequent Similarity Percentage (SIMPER) analyses using the ‘simper’ function determined the 191 

contributions of dominant species and bare rock to the similarity between regions and wave 192 

exposures. 193 

Recovery from Disturbance Experiment: Study Sites 194 

To examine patterns of recovery from disturbance, we chose two representative sites of 195 

each wave exposure (sheltered, exposed) in each of two regions (Northern Gulf [Lubec, Maine], 196 

Southern Gulf [Nahant, Massachusetts]). Due to the broad scale (distance between the two 197 

regions was > 400 km) and labor-intensive nature of this experiment (144 experimental plots) 198 

additional sites were not logistically feasible. Moreover, the results of the Gulf of Maine-wide 199 

community structure survey confirmed that the study sites used in the recovery experiment were 200 

representative of each geographic region. The exposed sites in the northern Gulf were on a 201 

rocky headland between Julia Cove and Hamilton Cove in Lubec, Maine and the sheltered sites 202 

were just north of Quoddy Head State Park in Lubec, Maine. In the southern Gulf, the two 203 

exposed sites were on East Point in Nahant, Massachusetts and the two sheltered sites were in an 204 

embayment northwest of East Point in Nahant, Massachusetts (see Appendix A for more 205 

information). One of the wave-exposed sites in the southern Gulf was used in Menge’s early 206 

studies (Menge 1976, 1978a,b) and the sheltered sites in the south were adjacent to those used by 207 

 

 

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Menge (1976, 1978a,b) and Lubchenco (1980, 1983) because their original sites are now subject 208 

to high levels of human activity. 209 

Consumer Density Surveys and Analysis 210 

We monitored mobile consumer density at experimental study sites by recording the 211 

number of mobile consumers in 10 randomly placed 0.25m2 quadrats in the mid-intertidal zone 212 

at each replicate site in July 2005. Density data for the dominant mobile consumers (predators: 213 

Nucella lapillus and Carcinus maenas and herbivores: Littorina littorea and Tectura testudinalis) 214 

were analyzed with a 3-factor, nested design with geographic region and wave exposure as fixed 215 

factors and site as a random factor nested within wave exposure and geographic region. Because 216 

the high frequency of zero counts in the quadrats resulted in heteroscedasticity that could not be 217 

corrected via transformation, we assessed the influence of geographic region and wave-exposure 218 

on mobile consumer densities with Generalized Linear Mixed Models (GLMMs) that had a 219 

negative binomial error distribution and a log link function (O’Hara and Kotze 2010, Linden and 220 

Mantyniemi 2011, Warton and Hui 2011) using the glmmADMB package (Fornier et al. 2011, 221 

Skaug et al. 2011) for R version 3.0.0 (R Core Development Team 2013). To determine the 222 

influence of geographic region and wave-exposure and their interaction on the density of each 223 

consumer, a model selection based approach to hypothesis testing using Akaike weights based 224 

upon corrected Akaike Information Criterion (AICc) was employed to determine the best-fit 225 

model (Burnham and Anderson 2002, 2004, Johnson and Omland 2004, Bolker et al. 2009). 226 

Recovery from Disturbance Experiment: Approach & Analysis 227 

This experiment was a 4-factor nested design with replicate sites (N = 2) as a random 228 

factor nested within each wave-exposure (sheltered, exposed) and geographic region (northern, 229 

southern). Hence in each region, there were two replicate wave-exposed and two replicate 230 

 

 

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sheltered sites. At each replicate site (N = 8 in total), there were 6 independent replicates of each 231 

caging treatment (cage, cage control, open), for a total of 144 experimental plots. 232 

Each replicate plot was a 1 m x 1 m clearing (each clearing was separated by at least 2 m) 233 

where all resident sessile algae and invertebrates were removed with paint scrapers and propane 234 

torches in mid October 2003. The successful removal of all organisms in each plot was 235 

confirmed in early March 2004 prior to the onset of barnacle recruitment. To maintain a 236 

consistent sampling, the corners of a single 20 cm x 20 cm plot located in the center of each 1 m 237 

x 1m clearing was marked with stainless steel lag screws installed into the substratum. In 238 

addition, the fucoid algal canopy surrounding each clearing was trimmed to prevent it from 239 

impacting the sampling area (e.g., whiplash, shading). All percent cover data were recorded in 240 

these 20 cm x 20 cm plots. 241 

Caging treatments (cage, cage control, or open) were applied to appropriate plots at each 242 

site in each location. Consumer exclusion cages (25 x 25 x 5 cm; mesh opening = 0.5 cm) 243 

constructed from stainless steel mesh were anchored into the rock to cover the 20 x 20 cm plot 244 

located in the center of each 1 m x 1m clearing. The edges of cages were pressed flush to the 245 

substratum and, if necessary, sealed with waterproof epoxy (Z-spar) to further ensure the 246 

exclusion of consumers. Cage controls were constructed and installed in a similar manner except 247 

that two sides of the cage were left open to allow access by consumers. Open plots were left 248 

uncovered to allow full access by consumers. 249 

We assessed recovery by photographing plots twice a year (spring and fall) over the 250 

following 2 years. Mussels in the northern Gulf may include M. edulis, M. trossulus and hybrids 251 

(Rawson et al. 2001), but field identification of these species is not feasible. Although the 252 

 

 

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general rarity of mussels at sites in the northern Gulf makes it unlikely that this distinction 253 

among species is important, all mussels are nevertheless referred to as Mytilus spp. 254 

The percent cover of sessile species was determined from resulting photographs by 255 

placing a layer of 36 random points over each photograph in Adobe Photoshop™ and recording 256 

species identity underneath each point. If no species was present, the point was scored as bare 257 

rock. If a point fell upon a mobile consumer, the sessile species to the immediate right of the 258 

mobile species was recorded. As recovery progressed and a fucoid algal canopy developed, it 259 

became necessary to collect percent cover data on understory species using a point-intercept 260 

quadrat (with 36 points) in the field. 261 

Percent cover data were analyzed with a 4-factor, nested ANOVA with replicate sites as a 262 

random factor nested within each wave exposure (sheltered, exposed) and geographic region 263 

(northern, southern) and caging treatment (cage, cage control, open) as a fixed factor. Although 264 

we present the full pattern of recovery in our figures, we conducted our analyses on the percent 265 

cover data collected during the last sampling date in Fall 2005 for the three dominant taxa (F. 266 

vesiculosus, S. balanoides, and Mytilus spp.) in our plots because they were the primary drivers 267 

of community recovery. Percent cover data were log10 (+1) transformed to meet the 268 

homogeneity of variance assumption (Warton and Hui 2010). These analyses were performed 269 

with JMP software for the Macintosh (version 10, SAS Institute, Cary, NC). 270 

To further address importance of geographic region, wave exposure and consumer 271 

exclusion to patterns of recovery, we determined the effect size (eta-squared, η2 = SSeffect/SStotal) 272 

for each factor in our ANOVA model. Because of the large effect (Cohen 1988) of geographic 273 

region (η2>0.22 for each species) and the significance of the region x wave exposure x caging 274 

interaction, we examined the effect sizes (η2) for wave exposure, caging and their interaction for 275 

 

 

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each geographic region. Separate 3-factor, nested ANOVAs with sites nested within wave 276 

exposure and crossed with caging were performed for each species in each region and the 277 

resulting sums of squares used to determine observed effect sizes. To examine relationships 278 

between dominant species on wave-exposed shores in the southern Gulf where recruitment, and 279 

likely competition for space, was highest, we used Pearson’s (r) correlations on percent cover 280 

data obtained in the spring and fall of 2005. 281 

Barnacle Recruitment 282 

Annual barnacle (S. balanoides) recruitment was measured at each experimental study 283 

site from 2004 to 2006 in 6 replicate ‘settlement’ plots (20 cm x 20 cm) that had been cleared of 284 

all sessile organisms in early spring of each year. Each plot was photographed in late spring of 285 

each year, after the completion of settlement. A grid of equally sized squares was then placed on 286 

each photo in Adobe Photoshop™ before counting the number of barnacles present in 10 287 

randomly chosen squares. Because barnacle settlement was quite high and uniform in southern 288 

plots, total recruitment was determined by scaling up the average number of barnacles in these 289 

subsamples to calculate barnacle density in each plot. Due to very low recruitment in the 290 

northern Gulf, this approach was not necessary and all barnacle recruits within the 20 x 20 cm 291 

plots were counted. Barnacle recruitment densities were analyzed with a four-factor nested 292 

ANOVA with site as a random factor nested within region and wave exposure, and crossed with 293 

year using JMP software for Macintosh (version 10, SAS Institute, Cary, NC). Although we 294 

used the same physical areas each year, we reasoned that a repeated measures design was not 295 

necessary because recruitment plots were re-cleared before each recruitment season and 296 

therefore provided an independent assessment of recruitment variation for each year. Prior to 297 

analysis, data were log transformed to meet the homogeneous variance assumption of ANOVA. 298 

 

 

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Long-term Ascophyllum recovery 299 

The long-term recovery of Ascophyllum in open plots at sheltered sites in both regions 300 

was monitored from 2004-2011 by annually photographing each plot in the spring and fall. 301 

Unfortunately, unanticipated changes in landowner restrictions in 2007 made such monitoring 302 

impossible for the wave-exposed sites in the northern Gulf. However, beginning in 2005 we 303 

were able to monitor long term Ascophyllum recovery in the open plots of another similar 304 

experiment that had one wave-exposed site in the northern Gulf that was unaffected by 305 

landowner restrictions and two wave-exposed sites in the southern Gulf.  Because long-term 306 

Ascophyllum recovery at each shore type was derived from two separate experiments with 307 

different time spans and levels of site replication, recovery data for the wave-exposed sites were 308 

analyzed with a one factor ANOVA that considered geographic region as a fixed effect. Because 309 

unequal variances between sites could not be correct via transformation or modeled through 310 

mixed model approaches, differences between these sites were assessed by adjusting the 311 

significance level to that of the Brown-Forsythe test for unequal variances. Long-term 312 

Ascophyllum recovery data at sheltered sites were analyzed with an ANOVA that considered 313 

geographic region as a fixed effect and site as a random effect nested within geographic region. 314 

Although figures present the full temporal pattern of recovery through time, we analyzed the 315 

endpoint data collected in Fall 2011. 316 

RESULTS 317 

Community Structure Across the Gulf of Maine 318 

Results of the metaMDS analysis indicated that sites in the northern and southern regions 319 

of the Gulf of Maine clustered based upon wave exposure (Fig 2). Moreover, wave-exposed and 320 

sheltered sites in the southern region clustered separately, whereas wave-exposed and sheltered 321 

 

 

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sites clustered together in the northern region. Community structure was strikingly similar for 322 

sites within the same wave exposure in the northern and southern regions. However, within the 323 

Penobscot region, site-to-site similarity was reduced, as indicated by the large 95% confidence 324 

ellipses (Fig. 2). One wave-exposed site (Milbridge, Maine) and one sheltered site (Grindstone 325 

Neck, Maine) in the Penobscot region that did not cluster with sites of similar wave exposure 326 

likely contributed to the reduced similarity in this region. 327 

Results of the PERMANOVA indicated that community structure differed based upon 328 

region and wave exposure, and that the effect of wave exposure differed between oceanographic 329 

regions (PERMANOVA, Region x Exposure, F2,28 =4.5058, P=0.0008, Appendix B). Wave-330 

exposure effects on community structure were far stronger in the southern Gulf than in the 331 

northern Gulf (Figs. 2, 3, 4). In the Penobscot region, sites grouped based upon wave exposure 332 

with most exposed sites and most sheltered sites clustering with their counterparts in the southern 333 

Gulf. However, the overall similarity of sites within each wave exposure grouping was lower in 334 

the Penobscot region, as evidenced by the larger ellipses (Fig. 2), than in other regions. The 335 

similarity between wave-exposed and sheltered sites in the northern Gulf was likely driven by 336 

the dominance of Ascophyllum at both wave exposures in this region, whereas this alga was only 337 

dominant at sheltered sites in the southern Gulf and the Penobscot region (Figs. 3, 4, Table 1, 338 

also see Appendix C). In contrast, Fucus dominated wave-exposed sites in the southern Gulf and 339 

the Penobscot region. Hence, Ascophyllum and Fucus contributed to the lack of similarity 340 

between wave-exposed and sheltered sites in the southern Gulf and the Penobscot region but 341 

played a minor role in the lack of similarity among wave exposures in the northern Gulf. 342 

Differences in the abundance of these fucoids also drove the dissimilarity between the wave-343 

 

 

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exposed sites among the three regions, but lower barnacle and higher bare space percent cover 344 

distinguished northern sheltered sites from those in the other regions. 345 

Consumer Densities 346 

Quadrat surveys indicated that predators and herbivores with strong effects on dominant 347 

intertidal species were most abundant at sheltered sites in the southern Gulf. Green crabs (C. 348 

maenas) were observed only at southern sheltered sites (Fig. 5a). The best fit model included 349 

both region and wave exposure as fixed effects (GLMM, wi = 0.7451, Appendix D) and models 350 

that lacked either of these effects were poor predictors of crab density (GLMM, wi = 0.0093, 351 

Appendix D). Predatory (N. lapillus) and herbivorous (T. testudinalis) gastropods were most 352 

abundant at southern wave-exposed sites and northern sheltered sites, respectively (Figs. 5b,c). 353 

The greatest density of N. lapillus occurred at southern, wave-exposed sites (~150 snails m-2, 354 

Fig. 3b) and models without either wave exposure or region received minimal support (GLMM, 355 

wi = 0.0002 and < 0.0001 for wave exposure and region, respectively, Appendix D). T. 356 

testudinalis density surpassed 10 limpets m-2 at the northern sheltered sites (Fig. 5c), and the best 357 

model included both region and wave-exposure as fixed effects (GLMM, wi = 0.6390 Appendix 358 

D). Finally, the herbivorous snail, L. littorea, reached densities of 25 snails m-2 at the southern 359 

sheltered sites, and was comparatively absent from all other sites (Fig. 5d). The best fit model 360 

includes both fixed effects and the interaction (GLMM, wi = 0.9978, Appendix D) and all other 361 

models had minimal weight, indicating that although higher densities occurred at sheltered sites 362 

in both regions, the magnitude of this effect was greater in the southern than in the northern Gulf. 363 

Community Recovery at Wave-Exposed Sites 364 

At northern wave-exposed sites, Fucus dominated community recovery and mussels and 365 

barnacles were relatively unimportant (Fig. 6, Appendix E). In fact, we observed no mussel 366 

 

 

17

recruits in our plots at these sites and although barnacles recruited to our plots (Fig. 7a, Appendix 367 

F), their abundance was relatively low and they never occupied more than 20% of the available 368 

space. Surprisingly, the presence and absence of consumers had little impact on recovery (linear 369 

contrast, P = 0.2045). In fact, Fucus recovery was strongest in uncaged, open plots reaching 370 

100% cover after 2 years, whereas recovery was slower in caged and cage control plots, 371 

presumably because of cage abrasion. When cages and cage controls were removed in Fall 372 

2005 for photographic sampling, we frequently observed damaged fucoid fronds in our sampling 373 

plots (E.S. Bryson, personal observation). 374 

Recovery patterns at southern wave-exposed sites were dramatically different (Fig. 6, 375 

Appendix E). At these sites, Fucus recruitment was quite low and recovery was dominated by 376 

the recruitment and establishment dynamics of barnacle and mussels. In spring 2004, new 377 

barnacle recruits dominated all plots regardless of consumer exclusion treatment but by the fall 378 

mussels had replaced barnacles as the primary occupier of space. During the winter of 2004-379 

2005, mussel density declined in open plots, but remained high in cage and cage control plots. It 380 

is unlikely that consumer exclusion drove this pattern because consumers are inactive during 381 

winter. Instead, winter disturbance likely caused mussel decline in open plots whereas the 382 

physical structure provided by cages and cage controls prevented storm-induced mussel 383 

dislodgement (Menge 1976). The following spring (2005) barnacle recruitment into bare patches 384 

remained high (Fig. 5a), but was considerably lower in cage and cage control plots where mussel 385 

abundance was high (Figs. 6a,c). By fall of 2005, mussel abundance had declined in all plots, 386 

possibly because of heat waves during the summer. However, a mix of Fucus, barnacles, and 387 

mussels were observed only in open plots where mussel cover had declined over winter and was 388 

low in spring 2005 (Fig. 6). Pearson’s correlations indicate that mussel cover in the spring of 389 

 

 

18

2005 explained over 50% of the variation in barnacle and Fucus cover in the fall of 2005 390 

(Pearson’s r = -0.5262, -0.6734, and p < 0.0001, 0.0004 respectively, Table 2). Higher spring 391 

mussel cover corresponded to lower barnacle and Fucus cover in the fall. However, mussel 392 

cover in the fall of 2005 appeared independent of the barnacle, Fucus or mussel cover the 393 

previous spring (Pearson’s r = 0.1391, -0.1399, 0.0575 respectively, p > 0.4157 for all, Table 2). 394 

Moreover, the presence and absence of consumers appeared to be unimportant to recovery on 395 

these shores. 396 

Community Recovery at Sheltered Sites 397 

Recovery on northern sheltered sites was remarkably similar to that found on their wave-398 

exposed counterparts (Appendix E). Consumer pressure was unimportant to the recovery of 399 

Fucus, which dominated all plots (linear contrast, P = 0.4071; Fig. 8). Mussel recruitment and 400 

establishment did not occur and barnacle recruitment was low so that barnacles never occupied 401 

more than 5% of the available space, regardless of consumer exclusion. 402 

In contrast, recovery on southern sheltered sites was strongly influenced by the presence 403 

of consumers (Fig. 8, Appendix E). Initial barnacle recruitment in spring 2004 was high at these 404 

sites (Fig. 7b), especially in cage and cage controls perhaps because these structures either 405 

reduced thermal stress or water velocity, thereby enhancing larval settlement and survivorship. 406 

By fall 2004, the effects of consumer exclusion began to emerge. Barnacle abundance declined 407 

in open plots and cage controls and both mussels and Fucus began to dominate plots where 408 

consumers were excluded. By spring 2005, the positive effects of consumer exclusion on Fucus 409 

and mussels intensified. At the end of the experiment, consumer exclusion (caged plots) led to 410 

communities dominated by Fucus (~60% canopy cover; linear contrast, caged vs. open, P = 411 

0.0026) with an understory of mussels (~20% understory cover; linear contrast, caged vs. open, P 412 

 

 

19

= 0.0018), barnacles (~35% understory cover; linear contrast, caged vs. open, P = 0.0003) and 413 

bare space (~45%). By contrast, in open plots, Fucus still formed a canopy in the presence of 414 

consumers despite its relatively lower abundance (~25-30%), and barnacles (~35%) and bare 415 

space (~65%) dominated the understory while mussels were generally absent presumably 416 

because of predation by green crabs and Nucella. 417 

Long-term Ascophyllum Recovery 418 

Recovery rates of Ascophyllum on wave-exposed shores differed between northern and 419 

southern sites (ANOVA, P < 0.0004, Fig. 9, Appendix G). As expected, Ascophyllum remained 420 

absent at southern wave-exposed sites whereas recovery at northern wave-exposed sites became 421 

evident after 3 years, reaching ~15% cover after 7 years. On sheltered shores, Ascophyllum 422 

recovery occurred at both northern and southern sites but was more substantial (ANOVA, P = 423 

0.0224) at the northern sites reaching 40-50% cover after 8 years. 424 

DISCUSSION 425 

Our survey of 34 established rocky intertidal communities throughout the Gulf of Maine 426 

revealed substantial differences in the distribution and abundance of key shared species that 427 

likely influence how these communities are organized. In the northern Gulf, sites had a dense 428 

canopy of Ascophyllum regardless of wave exposure and barnacles and mussels were either rare 429 

or altogether absent. In the southern Gulf, extant community structure was consistent with 430 

previous work (Menge 1976, Menge 1978a,b, Lubchenco 1980, Bertness et al. 2004a) showing 431 

that barnacles, mussels and Fucus dominate wave-exposed shores and Ascophyllum dominates 432 

sheltered shores. Communities in the Penobscot region of central Maine were generally aligned 433 

with those on sheltered and wave-exposed shores in the southern Gulf. However, one wave-434 

exposed site (Milbridge, Maine) with abundant Ascophyllum was more consistent with 435 

 

 

20

communities typical of northern wave-exposed sites. In addition, a sheltered site in this region 436 

(Grindstone Neck, Maine) was dominated by Fucus and barnacles rather than Ascophyllum and 437 

was therefore more similar to wave-exposed sites in the southern region. Recovery patterns after 438 

disturbance reinforced the notion that communities in the northern and southern Gulf are 439 

organized differently. 440 

Consumer Driven Dynamics in the Southern Gulf of Maine 441 

Recovery in the southern Gulf indicates that the strong influence of recruitment, 442 

competition for space, and consumer control on community organization depends on wave 443 

exposure, which is consistent with previous research (Menge 1976, 1978a,b, Lubchenco and 444 

Menge 1978, Bertness 2004a). On wave-exposed shores, barnacles recruited densely in the 445 

spring of 2004, but were replaced by mussels in the fall. This transition may reflect barnacle 446 

facilitation of mussel recruitment, which has been observed by others within the Gulf of Maine 447 

(Menge 1976, Petraitis 1987). The subsequent impact of physical factors (e.g., thermal stress 448 

and storm-induced dislodgement) likely drove cyclical patterns in mussel cover, which when 449 

high, shaped subsequent recovery patterns by overgrowing barnacles and limiting the space 450 

available for Fucus establishment. For example, barnacle recruitment in the year (2005) 451 

following mussel establishment (Fig. 6) was much lower despite remarkably high and consistent 452 

barnacle supply over the 3 years it was measured (Fig. 7a) and the abundance of Fucus remained 453 

relatively low in cages and cage controls throughout the 2 year experiment. Only in open plots, 454 

where mussel populations declined over winter because of dislodgement by storms (Menge 455 

1976, Paine and Levin 1981, Denny et al. 1985, Hunt and Scheibling 2001, Carrington 2002), 456 

did Fucus and barnacles establish larger populations. Thus, although open plots at wave-457 

exposed sites included a mix of all three primary space occupying species, rather than mussel 458 

 

 

21

dominance as observed by Menge (1976), the increase in both Fucus and barnacles after the 459 

spring of 2005 appeared to largely hinge on mussel dynamics. These cyclical patterns of mussel 460 

dominance are likely driven by variation in disturbance rather than by interactions with barnacles 461 

and Fucus (Table 2). We suggest that this high recruitment-high turnover system creates 462 

unstable biotic secondary substratum that may contribute to the lower recovery of Fucus and to 463 

the absence of Ascophyllum from wave-exposed shores in this region. 464 

Previous work has shown that Nucella can strongly control the abundance of barnacles 465 

and mussels on wave-exposed shores in this region (Bertness et al. 2004a), but our experiment 466 

provided no evidence of consumer control at these sites. Although high wave energies can limit 467 

the effectiveness of Nucella (Burrows and Hughes 1989, Etter 1989, 1996), we were surprised to 468 

find no evidence of consumer control because all aspects of our experimental cages, cage 469 

controls and open plots were identical to those used by Bertness et al. (2004a) in southern Maine. 470 

These disparate patterns of consumer effects on wave-exposed shores may reflect regional 471 

differences within the southern Gulf of Maine similar to those observed by Kordas and Dudgeon 472 

(2011) who found weaker consumer effects on Ascophyllum in Massachusetts than in southern 473 

Maine. Thus, Nucella-barnacle and Nucella- mussel interactions may be stronger in southern 474 

Maine than in Massachusetts where caging artifacts overrode any consumer effects. 475 

Nevertheless, the resulting impacts of cages on mussels influenced subsequent recovery. For 476 

example, mussel abundance after the first settlement season in fall 2004 was high, occupying 477 

~60% of the available space in all treatments, but had declined only in open plots by the 478 

following spring. Because cage controls maintained the same high mussel abundance as full 479 

cages, we suggest that the physical structure provided by both cages and cage controls had a 480 

positive effect on mussels by reducing the risk of dislodgement during winter storms. Indeed, 481 

 

 

22

observations at our southern wave-exposed sites in the winter of 2004-2005 after storms revealed 482 

substantial unoccupied space in our open plots, indicating that physical stress, rather than species 483 

interactions, caused this decline. By fall 2005, cages and cage controls were full of empty 484 

mussels, sand and shell hash, which negatively impacted mussel survival as well as the few 485 

Fucus individuals present. Thus, the disparate patterns of consumer effects on mussels at wave-486 

exposed sites between this study (i.e., reduced consumer pressure) and past work (Bertness et al. 487 

2004a) may reflect either greater thermal stress, greater wave exposure, or both, at our sites in 488 

Massachusetts. 489 

On sheltered sites in the southern Gulf, the presence of consumers strongly influenced the 490 

recovery of Fucus and mussels (Fig. 8). After 1 year of recovery, a Fucus canopy with a mix of 491 

mussels and barnacles in its understory developed when consumers were excluded. When 492 

consumers were present, however, bare space dominated plots with moderate cover of Fucus and 493 

barnacles after 2 years. Although adult fucoids are relatively resistant to snail grazing, L. littorea 494 

was abundant at these sites (Fig. 5d) and it can slow fucoid recovery by consuming germlings 495 

and young recruits before they attain a size refuge and begin to produce chemical and structural 496 

defenses (Lubchenco and Gaines 1981, Lubchenco 1983, Barker and Chapman 1990, Rhode et 497 

al. 2004). Those Fucus that had escaped snail grazing, either via substratum or size-related 498 

refugia (Lubchenco 1983), eventually led to this alga being more prevalent even in open plots 499 

(~25% cover) after 2 years of recovery. 500 

Consumer exclusion clearly enhanced mussel recovery presumably by preventing green 501 

crabs, which were also abundant at these sites (Fig. 5a), from accessing them. Even after 2 502 

years, mussels were not present in open plots, suggesting that the effects of consumer control by 503 

green crabs on this species is stronger than that imposed by L. littorea on Fucus. This control of 504 

 

 

23

mussel abundance, coupled with Fucus escapes from grazing, will presumably lead to even 505 

greater Fucus abundance over the longer term (Menge 1976, 1978a, Lubchenco 1980, 1982, 506 

1983, Bertness et al. 2004a). In contrast, we saw no strong evidence of consumer control on 507 

barnacle abundance, perhaps because their primary predator, Nucella, was not abundant at these 508 

sites (Fig. 5b). 509 

Recruitment Limitation in the Northern Gulf of Maine 510 

A striking feature of communities in the northern Gulf was that mussels and barnacles 511 

were unimportant to community recovery on both wave-exposed and sheltered shores. Unlike 512 

their southern counterparts, mussel recruitment and establishment was nonexistent and barnacle 513 

recruitment and abundance was quite low at all sites in the northern Gulf (Figs. 6, 7, 8); these 514 

recruitment patterns have continued in the years following this study (Bryson and Trussell, 515 

unpublished). 516 

In the northern Gulf, currents move southward along the Scotian shelf before flowing 517 

northward to the mouth of the Bay of Fundy where intense tide-driven mixing occurs and the 518 

southwestward movement of the Eastern Maine Coastal Current (EMCC) begins (Xue et al. 519 

2000, Pettigrew et al. 2005). The proximity of the EMCC to the eastern Maine coastline can 520 

vary seasonally and annually (Hetland and Signell 2005, Pettigrew et al. 2005) but analyses of 521 

drifter data suggest high connectivity between Cutler, Maine (in the northern region) and sites in 522 

western Nova Scotia (Manning et al. 2009). However, results of an experimental particle release 523 

on the western coast of Nova Scotia suggest that planktonic larvae do not accumulate in eastern 524 

Maine (Xue et al. 2008) because they either become entrained within the Bay of Fundy, 525 

aggregate near Penobscot Bay, or are driven offshore. Moreover, it is unlikely that planktonic 526 

larvae are transported northward from southern populations due to the strong southwestward 527 

 

 

24

prevailing flow of the EMCC, which can exceed 0.15 m s-1 (Pettigrew et al. 2005, Manning et al. 528 

2009, Tilburg et al. 2012). Finally, freshwater input from the St. Croix River into 529 

Passamaquoddy Bay along the mouth of the Bay of Fundy (Pettigrew et al. 2005) may restrict 530 

larval transport from either within the Bay of Fundy or from the Scotian shelf to this region. 531 

Thus, the limited barnacle and mussel recruitment in the northern Gulf is likely derived from 532 

either the sparse local populations or larvae from more northern populations that deviate from 533 

prevailing flow patterns. This low recruitment may relax competition for space and may 534 

promote algal dominance in these communities. 535 

Although we suggest that local and meso-scale oceanographic conditions are responsible 536 

for geographic variation in recruitment in the Gulf, more detailed studies of larval transport 537 

along the Gulf of Maine coast will be essential to better address the role that oceanography plays 538 

in shaping geographic variation in recruitment. Recent studies have improved the clarity and 539 

detail of summer circulation patterns in the Gulf of Maine (Pettigrew et al. 1998, 2005, Churchill 540 

et al. 2005, Hetland and Signell 2005, Manning et al. 2009), but winter circulation patterns are 541 

less understood (but see Xue et al. 2000), which is unfortunate because barnacle larvae in the 542 

Gulf are typically released during January and February. 543 

The relative absence of consumers also may contribute to fucoid algal dominance on 544 

northern Gulf shores (Figs. 6, 8), and we saw no evidence of consumer control on either Fucus 545 

vesiculosus or Ascophyllum nodosum. For example, grazing by the herbivorous snail L. littorea 546 

can slow fucoid algal recovery (Lubchenco and Gaines 1981, Lubchenco 1983, Barker and 547 

Chapman 1990, Trussell et al. 2002, 2003), but this grazer was conspicuously absent from our 548 

study sites and many others in the region perhaps due to either low recruitment or intense 549 

harvesting (Bryson & Trussell, personal observations). Moreover, limpets (T. testudinalis) were 550 

 

 

25

present at northern sites, particularly on sheltered shores, but previous work (Steneck 1982, 551 

Steneck and Watling 1982, Watson and Norton 1987) and our results indicate that they do not 552 

influence fucoid algal recovery. Our results suggest that both the relative absence of consumers 553 

and low barnacle and mussel recruitment in the northern Gulf may contribute to fucoid algal 554 

dominance in this region. This pattern is consistent with experimental work in the eastern 555 

Atlantic where mussel recruitment is often patchy (Jenkins et al. 2008). In the eastern Atlantic, 556 

fucoid algae recover quickly and dominate wave-exposed shores when either patellid limpets are 557 

experimentally excluded (Hawkins 1981, Hawkins et al. 1992, Jenkins et al. 1999, 2005, 558 

Coleman et al. 2006) or outside of their bioegeographic range (Cervin et al. 2004, Ingolfsson and 559 

Hawkins 2008). Moreover, although competitive interactions between mussels and fucoid algae 560 

have received little attention in Europe, there is evidence that suppression of mussels by crab 561 

predators promotes the establishment of fucoid canopies (Janke 1990). 562 

We should also note that increased nutrient availability in the northern Gulf of Maine 563 

might contribute to the rapid recovery of fucoid algae in this region. High levels of inorganic 564 

nutrients (nitrate, nitrite) occur within the EMCC (Pettigrew et al. 1998, Townsend et al. 2001) 565 

due to upwelling caused by cyclonic circulation over Jordan basin and intense tidal flushing 566 

(Townsend et al. 1987). Thus, the combined effects of weak top-down control and bottom-up 567 

effects on nutrient availability and the recruitment of invertebrate competitors may ultimately 568 

drive algal dominance in the northern Gulf. 569 

Long-term Ascophyllum Recovery 570 

Long-term monitoring of open plots revealed that Ascophyllum recovery is consistent 571 

with previous work in the southern Gulf: Ascophyllum did not become established on wave-572 

exposed shores (Fig. 9). In contrast, Ascophyllum began to recover on sheltered shores in the 573 

 

 

26

southern Gulf, even in the presence of consumer pressure, achieving ~3-8% on sheltered shores 574 

after 8 years. 575 

Perhaps more interesting was the dominance of Ascophyllum in our community surveys 576 

(Fig. 3a,b, Table 1) and its observed recovery (Fig. 9) at all of our northern wave-exposed sites. 577 

In contrast to mussel and barnacle dominance on southern wave-exposed shores, dense 578 

Ascophyllum canopies dominated these shores in the northern Gulf. Although it has long been 579 

held that Ascophyllum is unable to achieve high abundance on wave-exposed shores in the Gulf 580 

of Maine due to wave stress (Vadas et al 1990,) or competition from high densities of mussels 581 

and barnacles (Dudgeon and Petraitis 2001), this species can become established on wave-582 

exposed shores in the northeastern Atlantic (Jenkins et al. 2008). Ascophyllum recovery on 583 

sheltered shores was more rapid in the north than the south, presumably because of the scarcity 584 

of grazers like L. littorea, which can affect Ascophyllum recovery (Trussell et al. 2002). 585 

Continued monitoring of these plots indicates that Ascophyllum may eventually replace Fucus as 586 

the dominant canopy-forming alga on sheltered shores, which contradicts previously 587 

hypothesized Fucus dominance once its becomes established (McCook and Chapman 1993, 588 

1997, Dudgeon and Petraitis 2001). Ascophyllum recovery and dominance on wave exposed and 589 

sheltered shores may occur because Ascophyllum is a more northern, boreal species than Fucus 590 

(Vadas et al. 2004, Jenkins et al. 2008, Svensson et al. 2009), there is less competition from 591 

mussels and barnacles (McCook and Chapman 1993, 1997, Dudgeon and Petraitis 2001), and the 592 

EMCC may increase nutrient availability (Townsend et al. 1987, 2001, Pettigrew et al. 1998). 593 

Regional Community Dynamics in the Gulf of Maine? 594 

Views of how rocky intertidal communities are organized have long focused on the 595 

dominant role of top-down, or consumer control (Paine 1966, Menge 1976, 1978a, b, Lubchenco 596 

 

 

27

1983, Bertness et al. 2002, 2004a,b, Jenkins et al. 2005, 2008, Coleman et al. 2006, Davies et al. 597 

2007). More recently, however, growing evidence suggests that coastal oceanography and 598 

resulting bottom-up forcing can strongly influence these communities (Bustamante et al. 599 

1995a,b, Menge et al. 1997, 2003, Menge 2000, Menge and Branch 2001, Leslie et al. 2005, 600 

Navarrete et al. 2005, Barth et al. 2007, Blanchette & Gaines 2007, Blanchette et al. 2008, 601 

Broitman et al. 2008, Witman et al. 2010, Benedetti-Cecchi and Trussell 2013, Menge and 602 

Menge 2013). For example, shifts from persistent upwelling in central and northern California to 603 

intermittent upwelling along the coast of Oregon are associated with marked increases in 604 

barnacle recruitment (Connolly and Roughgarden 1998, Connolly et al. 2001, Broitman et al. 605 

2008, Krenz et al. 2011, Menge et al. 2011) and similar relationships between upwelling, 606 

recruitment patterns and onshore community structure have been documented in New Zealand 607 

(Menge et al. 1999), Chile (Lagos et al. 2005, Navarrete et al. 2005) and the Galapagos Islands 608 

(Witman et al. 2010) 609 

Compared to rocky shore communities on many western coastlines (e.g., US Pacific 610 

coast, Chilean coast), coastal upwelling is relatively unimportant in the Gulf of Maine. Although 611 

the influence of oceanographic processes on onshore community organization have been less 612 

intensively studied in the Gulf of Maine, coastal downwelling may contribute to the patterns we 613 

observed. The rapid southward movement of surface waters of the EMCC in the northern Gulf 614 

and the slower, southward flow of the WMCC in the southern Gulf may create conditions of 615 

strong and weak coastal downwelling in the north and south, respectively. Recent studies 616 

indicate that strong downwelling can reduce ecological subsidies, larval recruitment, and growth 617 

as well as weaken species interactions in New Zealand (Menge et al. 2002, 2003, Menge and 618 

Menge 2013), and we observed similar patterns in the northern Gulf. However, the importance 619 

 

 

28

of downwelling in this system remains unclear because the prevailing flow of the EMCC 620 

opposes the prevailing northwestward wind direction in the GOM. Thus, more research is 621 

necessary to address the strength and potential impacts of downwelling in the Gulf of Maine. 622 

Although the precise mechanism remains unclear, it appears that regional variation in 623 

coastal oceanography, which is characterized by distinct cyclonic circulation and along-shore 624 

currents (Brooks 1985, Pettigrew et al. 1998, 2005, Xue et al. 2008), strongly drives barnacle and 625 

mussel recruitment that is consistently low and high in the northern and southern Gulf, 626 

respectively. Coastal oceanography, in addition to intense human harvesting in the north, may 627 

also drive the north-south increase in the abundance of L. littorea, which, like barnacles and 628 

mussels, has a planktonic larval phase. Thus, we suggest that the deterministic recruitment 629 

variation between the northern and southern Gulf is a primary driver of disparate community 630 

organization in these two regions, particularly on wave-exposed shores. In the north, consumers 631 

were rare and thus unimportant to recovery, and low barnacle and mussel recruitment may allow 632 

fucoid algae, particularly Ascophyllum, to dominate both sheltered and wave-exposed shores. In 633 

the south, high barnacle and mussel recruitment and ensuing disturbance and competitive 634 

dynamics appeared to be most important on wave-exposed shores. Moreover, we observed 635 

strong consumer control only at southern sheltered sites, which suggests that consumer control 636 

may not be a pervasive feature of Gulf of Maine rocky shores. 637 

Our study cannot directly address the possibility of alternative stable community states 638 

driven by recruitment variation because our clearings were below the size threshold (12m2) 639 

thought necessary (Dudgeon and Petraitis 2001, Petraitis and Dudgeon 2004) for the 640 

development of alternative community states. However, indirect evidence suggests that the 641 

Penobscot Region, the location of studies by Petraitis and Dudgeon, may have the conditions 642 

 

 

29

necessary for the emergence of alternative (barnacle and mussel versus Ascophyllum) dominated 643 

community states. Unlike the southern Gulf, consumer pressure is relaxed in this region and 644 

unlike the northern Gulf there is the potential for high barnacle and mussel recruitment that 645 

varies in space and time (Dudgeon and Petraitis 2001). For example, this region has many small 646 

bays that may retain barnacle and mussel larvae either because of differences in flushing rates 647 

(e.g., Gaines and Bertness 1992) or because of onshore-offshore movement of the thermal 648 

boundary established by the EMCC that may strongly dictate larval transport (Hayhurst and 649 

Rawson 2009, Owen and Rawson 2013). Finally, our community surveys revealed high affinity 650 

between sites in the Penobscot Region and those in the southern Gulf, but two Penobscot sites 651 

diverged from this pattern. One sheltered site in the Penobscot Region had high Fucus and 652 

barnacle abundance and one wave-exposed site had high Ascophyllum abundance. Although 653 

more work is clearly needed, these patterns are certainly consistent with the outcome expected 654 

from substantial recruitment variation and relaxed consumer pressure rather than that produced 655 

by deterministic consumer control. 656 

In summary, our study reveals that despite largely sharing the same suite of species, the 657 

factors dictating rocky shore community organization differ substantially across the Gulf of 658 

Maine. Long-standing views of community organization that were primarily shaped by classic 659 

work in the south do not apply in the northern Gulf and may be unreliable in the Penobscot 660 

region. Although these conclusions emerged because of the broad scale nature of this study, it is 661 

clear that attention to both local and regional factors is also needed to more fully understand 662 

pattern and process. 663 

664 

665 

 

 

30

ACKOWLEDGMENTS 666 

We thank G. Bernatchez, M. Doellman, T. Dwyer, and C. Matassa for their assistance in 667 

the field, C. Matassa for her assistance in photograph analysis, and P. Petraitis, M. Bertness, M. 668 

Bracken, S. Vollmer and three anonymous reviewers for their valuable input of this manuscript. 669 

We are also grateful to S. Navarrete for his considerable and constructive advice and T. Gouhier 670 

for helping generate the map for the Gulf of Maine. This work was submitted in partial 671 

fulfillment of the requirements for the PhD in Ecology, Evolution and Marine Biology from 672 

Northeastern University and was supported by grants from the US National Science Foundation 673 

to G.C.T. (OCE-0240265, OCE-0727628) and Sigma Xi to E.S.B. This is contribution no. 303 674 

from the Marine Science Center and the Department of Marine and Environmental Sciences. 675 

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SUPPLEMENTAL MATERIAL 995 

Appendix A: Study sites, listed from south to north, used to assess natural community structure. 996 

Sites are categorized based upon oceanographic region (Southern, Penobscot or Northern) and 997 

wave energy (Sheltered, Wave-exposed). 998 

999 

Appendix B: Results of a 2-factor multivariate ANOVA assessing variance in community 1000 

structure with geographic region and wave exposure as fixed factors using a Bray-Curtis index 1001 

and 999 permutations. 1002 

 

 

45

Appendix C: Average percent similarity between geographic region or both high and low wave 1003 

energy regimes based upon SIMPER analyses of Bray Curtis similarities, and the contributions 1004 

of dominant space-occupiers (Ascophyllum nodosum, Fucus vesiculosus, Mytilus spp., 1005 

Semibalanus balanoides) and Bare Rock to the dissimilarity between sites. 1006 

1007 

Appendix D: Akaike weights of full and reduced Generalized Linear Mixed Models with 1008 

Negative Binomial Error Distribution to examine differences in consumer density due to region 1009 

and wave exposure. 1010 

1011 

Appendix E: Results of a 4-factor nested ANOVA on the percent cover of sessile species and 1012 

effect sizes (η2). 1013 

1014 

Appendix F: Results of a 4-factor nested ANOVA on Semibalanus balanoides recruitment 1015 

density. 1016 

1017 

Appendix G: Analyses of Ascophyllum nodosum percent cover in the Fall of 2011. 1018 

1019 

1020 

1021 

1022 

1023 

1024 

 

 

46

Table 1: Average percent similarity between wave-exposed and sheltered sites in each 1025 

geographic region based upon SIMPER analyses of Bray Curtis similarities, and the 1026 

contributions of dominant space-occupiers (Ascophyllum nodosum, Fucus vesiculosus, Mytilus 1027 

spp., Semibalanus balanoides) and Bare Space to the dissimilarity between sites. 1028 

1029 

Region Species Exposed

Abundance

Sheltered

Abundance

Contribution to

Dissimilarity (%)

Southern

A. nodosum

4.18 86.20 34.98

F. vesiculosus 71.56 12.60 24.87

Mytilus sp. 16.58 1.27 6.36

S. balanoides 44.74 44.53 5.11

Bare Rock 17.70 27.93 6.68

Penobscot

A. nodosum

30.00 73.00 28.49

F. vesiculosus 66.65 24.87 27.41

Mytilus sp. 4.40 0.00 2.44

S. balanoides 37.90 34.40 10.85

Bare Rock 21.10 22.00 4.64

Northern

A. nodosum

95.90 98.60 4.49

F. vesiculosus 2.80 1.20 3.63

 

 

47

Mytilus sp. 0.00 0.00 0.00

S. balanoides 0.20 1.90 2.32

Bare Rock 32.80 45.30 24.88

1030 

1031 

1032 

1033 

1034 

1035 

1036 

1037 

1038 

1039 

1040 

1041 

1042 

1043 

1044 

1045 

1046 

1047 

1048 

 

 

48

Table 2. Matrix of Pearson’s r correlations between the percent cover of dominant space 1049 

occupying species (Semibalanus balanoides, Mytilus sp. and Fucus vesiculosus) in the spring and 1050 

fall of 2005. N=36 for all correlations and asterisks indicate corresponding p-values <0.01. 1051 

1052 

S. balanoides cover Spring 2005

Fucus cover Spring 2005

Mytilus sp. cover Spring 2005

S. balanoides cover Fall 2005

0.7861* -0.1083 -0.5262*

Fucus cover Fall 2005

0.1256 0.4915* -0.6734*

Mytilus sp. cover Fall 2005

0.1391 -0.1399 0.0575

1053 

1054 

1055 

1056 

1057 

1058 

1059 

1060 

1061 

1062 

1063 

1064 

1065 

 

 

49

FIGURE LEGENDS 1066 

Figure 1. Map of the Gulf of Maine with prevailing oceanographic currents. The Eastern Maine 1067 

Coastal Current (EMCC, blue) flows southwesterly along the coast from the mouth of the Bay of 1068 

Fundy until the Penobscot Bay region, at which point water either moves offshore or continues 1069 

southwesterly to form the Western Maine Coastal Current (WMCC, red). Also shown are the 1070 

Nova Scotia Current (black) and cyclonic circulation around Jordan Basin (yellow). Points along 1071 

the coastline indicate sites where community structure surveys were conducted. Due to the 1072 

proximity of some sites, some points may overlap. Blue triangles indicate sites in the Northern 1073 

region, black circles indicate sites in the Penobscot region, and red squares indicate sites in the 1074 

Southern region. Solid points indicate wave-exposed sites and open points indicate sheltered 1075 

sites; the symbols for some site overlap. 1076 

1077 

Figure 2. Two-dimensional Non-metric Multidimensional Scaling (NMDS) plot constructed 1078 

using the ‘metaMDS’ function with a Bray-Curtis distance index of untransformed community 1079 

structure data (Stress = 0.07876). Each point represents a single site and ellipses represent 95% 1080 

confidence intervals of the mean ordination for each region and exposure combination. Blue 1081 

triangles indicate northern sites, black circles indicate Penobscot sites and red squares indicate 1082 

southern sites. Solid points and lines indicate wave-exposed sites and open points and dashed 1083 

lines indicate sheltered sites. 1084 

1085 

Figure 3. Average percent cover of dominant space-occupying species and bare rock in 1086 

established communities on wave-exposed shores: (A) Fucus, (B) Ascophyllum, (C) 1087 

 

 

50

Semibalanus, (D) Mytilus sp. and (E) Bare Rock. On the x-axis, sites are ordered by latitude 1088 

from southernmost (left) to northernmost (right). 1089 

1090 

Figure 4. Average percent cover of dominant space-occupying species and bare rock in 1091 

established communities on sheltered shores: (A) Fucus, (B) Ascophyllum, (C) Semibalanus, (D) 1092 

Mytilus sp. and (E) Bare Rock. On the x-axis, sites are ordered by latitude from southernmost 1093 

(left) to northernmost (right). 1094 

1095 

Figure 5. Average density of mobile consumers on wave-exposed (black bars) and sheltered 1096 

(stippled bars) shores in the southern and northern Gulf of Maine. (A) Carcinus maenas and (B) 1097 

Nucella lapillus are important consumers of barnacles (Semibalanus balanoides) and mussels 1098 

(Mytilus spp.) and the herbivorous gastropods (C) Tectura testudinalis and (D) Littorina littorea 1099 

consume algae. Error bars indicate 1 standard error of the mean and n = 20 at each location and 1100 

wave exposure. Note: y-axis scale varies for each consumer. 1101 

1102 

Figure 6. Mean (±SE) percent cover of the dominant space-occupying organisms (A) 1103 

Semibalanus balanoides, (B) Fucus vesiculosus, and (C) Mytilus spp. over 2 years on wave-1104 

exposed shores in the southern (red lines) and northern (blue lines) Gulf of Maine. Effects of the 1105 

presence (open plots, circles) and absence (cage, squares) of consumers and cage controls 1106 

(triangles) are also shown. Although recovery through time is shown, only end point data were 1107 

analyzed. 1108 

1109 

 

 

51

Figure 7. Mean (± SE) recruitment density (no. m-2) of Semibalanus balanoides into 1110 

experimental recruitment clearings in the spring of 2004-2006 on (A) wave-exposed and (B) 1111 

sheltered shores in the southern (red bars) and northern (blue bars) Gulf of Maine. 1112 

1113 

Figure 8. Mean (±SE) percent cover of the dominant space-occupying organisms (A) 1114 

Semibalanus balanoides, (B) Fucus vesiculosus, and (C) Mytilus spp. over 2 years on sheltered 1115 

shores in the southern (red lines) and northern (blue lines) Gulf of Maine. Effects of the 1116 

presence (open plots, circles) and absence (cage, squares) of consumers and cage controls 1117 

(triangles) are also shown. Although recovery through time is shown, only end point data were 1118 

analyzed. 1119 

1120 

Figure 9. Mean (± SE) percent cover of Ascophyllum nodosum in open plots on (A) wave-1121 

exposed and (B) sheltered shores in the southern (red lines) and northern (blue lines) Gulf of 1122 

Maine. We monitored the recovery of open plots from the original manipulative recovery 1123 

experiment on sheltered shores for 8 years beginning in 2004. Because of unanticipated changes 1124 

in site accessibility, we were only able to monitor recovery at one wave-exposed site in the 1125 

northern Gulf and two wave-exposed sites in the southern Gulf for 7 years beginning in early 1126 

2005. Note that for the southern wave-exposed sites there was no recovery so data points for 1127 

each site overlap. Although recovery through time is shown, only end point data were analyzed 1128 

and data points and error bars reflect standard errors of the mean. 1129 

1130 

 

 

52

Figure 1. 1132  

1133  

1134  

1135  

1136  

1137  

1138  

1139  

1140  

1141  

1142  

1143  

1144  

1145  

1146  

1147  

1148  

1149  

1150  

1151  

1152  

1153  

1154  

−71 −70 −69 −68 −67 −66 −65 −64

4041

4243

4445

4647

Longitude (°W)

Latit

ude

(°N

)

Maine

MA

NH

Nova Scotia

New Brunswick

Bay of Fundy

−1.0 −0.5 0.0 0.5 1.0

−0.5

0.0

0.5

NMDS1

NM

DS2

North ExposedNorth ShelteredPenob. ExposedPenob. ShelteredSouth ExposedSouth Sheltered

 

 

54

Figure 3. 1168  

A B 1169  

1170  

   C D 1171  

 1172  

E 1173  

 1176  

1177  

 1178  

 1179  

 1180  

0

20

40

60

80

100

Fucu

s Pe

rcen

t Co

ver

0

20

40

60

80

100

Asco

phyl

lum

Per

cent

Cov

er

0

20

40

60

80

100

Sem

ibal

anus

Per

cent

Cov

er

0

20

40

60

80

100

Myt

ilus

sp. P

erce

nt C

over

0

20

40

60

80

100

Turr

ett

Cunn

er L

edge

East

Poi

ntPi

geon

Cov

eCa

pe N

eddi

ckCa

pe E

lizab

eth

Gian

t St

eps

Pem

equi

d Pt

.Ch

ambe

rlain

Pt.

Cham

berla

in E

ast

Bass

Har

bor

Otte

r Pt

.Sc

hood

ic P

t.M

cLel

lan

Park

Wes

tern

Hea

dHa

milt

on C

ove

Quod

dy H

ead

Quod

dy L

ight

Bare

Roc

k Per

cent

Cov

er

 

 

55

Figure 4. 1181  

A B 1182  

1183  

C D 1184  

1185  

E 1186  

1187  

1188  

1189  

1190  

1191  

1192  

0

20

40

60

80

100

Fucu

s Pe

rcen

t Co

ver

0

20

40

60

80

100

Asco

phyl

lum

Per

cent

Cov

er

0

20

40

60

80

100

Sem

ibal

anus

Per

cent

Cov

er

0

20

40

60

80

100

Myt

ilus

sp. P

erce

nt C

over

0

20

40

60

80

100

Cano

e Be

ach

Fort

y St

eps

Past

For

tyLo

bste

r Co

ve

Cham

berla

in B

ayPo

rt C

lyde

New

port

Cov

e

Bar

Harb

orGr

inde

ston

e Ne

ck

Fraz

er P

oint

So.

Addi

son

Roqu

e Bl

uffs

Stea

mbo

at W

harf

Carr

ying

Cov

e

Nort

h Qu

oddy

Coas

t Gu

ard

Bare

Roc

k Per

cent

Cov

er

 

 

56

Figure 5. 1193  

A B 1194  

1195  

           1196  

1197  

1198  

1199  

1200  

1201  

C D 1202  

1203  

1204  

1205  

1206  

1207  

1208  

1209  

1210  

1211  

1212  

1213  

1214  

0

5

10

15

20

Tect

ura

test

udin

alis

dens

ity (

no. m

-2)

South North0

10

20

30

40

50

LItt

orin

a lit

tore

a den

sity

(no.

m-2

)

South North

0

1

2

3

4

5ExposedSheltered

Carc

inus

mae

nas d

ensit

y (n

o. m

-2)

South North0

50

100

150

200

Nuce

lla la

pillu

s den

sity

(no.

m-2

)

South North

 

 

57

Figure 6. 1215  

A 1216  

         1217   1218  

1219  

1220  

1221  

1222  

B 1223  

1224  

1225  

1226  

1227  

1228  

1229  

C 1230  

1231  

1232  

1233  

1234  

1235  

1236  

0

20

40

60

80

100

S F S F

Barn

acle

Per

cent

Cov

er

2004 2005

O

C O C CC

CC

0

20

40

60

80

100

S F S F

Fucu

s Pe

rcen

t Co

ver

2004 2005

O

O

C

C

CC

CC

0

20

40

60

80

100

S F S F

Mus

sel P

erce

nt C

over

2004 2005

O

O C CC

CCC

 

 

58

Figure 7. 1237  

A   1238  

1239  

1240  

1241  

1242  

1243  

1244  

B 1245  

1246  

1247  

1248  

1249  

1250  

1251  

1252  

1253  

1254  

1255  

1256  

1257  

1258  

10

102

103

104

105

106

2004 2005 2006

Barn

acle

Rec

ruitm

ent

Dens

ity (

no. m

-2) Wave Exposed

10

102

103

104

105

106

2004 2005 2006

Barn

acle

Rec

ruitm

ent

Dens

ity (

no. m

-2) Sheltered

 

 

59

Figure 8. 1259  

A 1260  

1261  

1262  

1263  

1264  

1265  

1266  

B 1267  

1268  

1269  

1270  

1271  

1272  

1273  

C 1274  

1275  

1276  

1277  

1278  

1279  

1280  

0

20

40

60

80

100

S F S F

Barn

acle

Per

cent

Cov

er

2004 2005

O

C CC

C CC O

0

20

40

60

80

100

S F S F

Fucu

s Pe

rcen

t Co

ver

2004 2005

O

CCO

CC

CC

0

20

40

60

80

100

S F S F

Mus

sel P

erce

nt C

over

2004 2005

C

CC OC CC O

 

 

60

Figure 9. 1281  

A 1282  

         1283   1284  

1285  

1286  

1287  

1288  

1289  

1290  

B 1291  

1292  

1293  

1294  

1295  

1296  

1297  

1298  

1299  

1300  

1301  

1302  

0

10

20

30

40

50

60As

coph

yllu

m P

erce

nt C

over

05 06 07 08 09 10 11

Wave Exposed

0

10

20

30

40

50

60

Asco

phyl

lum

Per

cent

Cov

er

Sheltered

04 05 06 07 08 09 10 11