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

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<ul><li><p>1</p><p> 1 2 3 4 5</p><p>Broad-scale geographic variation in the organization of rocky intertidal communities in the 6Gulf of Maine 7</p><p> 8Elizabeth S. Bryson1, Geoffrey C. Trussell1*, Patrick J. Ewanchuk2 9</p><p>1Marine Science Center and Department of Marine and Environmental Sciences, 430 Nahant 10Road, Nahant, MA 01908 11</p><p>2Department of Biology, Providence College, Providence, RI 02912 12 13 14 15</p><p>*Corresponding author 16 17Keywords: community organization, coastal oceanography, competition, disturbance, Gulf of 18Maine, herbivory, predation, recruitment, rocky intertidal 19 20 21 22 23</p></li><li><p>2</p><p>ABSTRACT 24A major challenge facing ecology is to better understand how large-scale processes 25</p><p>modify local scale processes to shape the organization of ecological communities. Although the 26results of ecological experiments are repeatable on local scales, different results often emerge 27across broad scales, which can hinder the development of general predictions that apply across 28the geographical range of a community. Numerous studies in the southern Gulf of Maine have 29shaped our understanding of community organization and dynamics on New England rocky 30intertidal shores, where consumers strongly control recovery from disturbance on sheltered 31shores and high invertebrate recruitment and competition for space dictate recovery on wave-32exposed shores. It is unclear, however, whether the effects of consumers and recruitment 33variation on resulting community organization in this region apply more broadly to rocky 34intertidal habitats throughout the Gulf. 35</p><p>We characterized variation in rocky intertidal community structure at 34 sites throughout 36the Gulf of Maine and experimentally examined the influence of consumers (present, absent) and 37wave energy (wave-exposed, sheltered) on community recovery from disturbance in the northern 38and southern Gulf. Our results reinforced previous work in the southern Gulf because consumers 39dictated the recovery of fucoid algae and mussels on sheltered shores, whereas high barnacle and 40mussel recruitment and competition for space shaped recovery on wave-exposed shores. 41However, on sheltered shores in the northern Gulf, neither consumers nor barnacle and mussel 42recruitment impacted recovery, which was dominated by fucoid algae. Moreover, recovery on 43wave-exposed shores in the northern Gulf was quite distinct from that observed in the southern 44Gulf: barnacle and mussel recruitment was negligible and fucoid algae dominated recovery 45including the long-term establishment of Ascophyllum nodosum, which is largely absent from 46</p></li><li><p>3</p><p>wave-exposed shores in the southern Gulf. Thus, distinct community types emerged in the 47northern and southern Gulf despite their sharing many of the same species. These patterns likely 48emerged because of regional differences in coastal oceanography that modulate the recruitment 49of barnacles and mussels. Hence, increased attention to regional factors should provide key 50insight into how rocky shore communities are organized in the Gulf of Maine and elsewhere. 51</p><p>INTRODUCTION 52A central goal of ecology is to understand how large-scale processes modify local level 53</p><p>processes to shape the distribution and abundance of species, and assembly, organization and 54dynamics of ecological communities (Wiens 1989, Levin 1992, McGill 2010, Hastings 2010). 55Because environmental gradients across larger scales can modify, for example, community 56assembly (Chase 2010, Hein and Gillooly 2011), the relative importance of bottom-up and top-57down processes (Menge 2000, Navarrete et al. 2005, Chase et al. 2010, Krenz et al. 2011), and 58the nature, intensity and scale of species interactions (Shurin and Allen 2001, Chase 2003, 59Sanford and Worth 2010, Menge et al. 2011), it is difficult to derive overarching assembly rules 60for community ecology. Understanding how differences in biotic and abiotic context mediate 61changes in species interactions and ultimately community assembly has been identified as a 62major gap in ecology (Agrawal et al. 2007, Weiher et al. 2011). 63</p><p>Rocky intertidal communities in the southern Gulf of Maine have long served as a model 64system to understand how abiotic and biotic factors influence succession after disturbance and 65resulting community organization (Menge 1976, 1978a,b, Lubchenco, 1980, 1983, Petraitis 661987, Petraitis and Dudgeon 1999, Dudgeon and Petraitis 2001, Bertness et al. 2002, 2004a,b). 67A 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</p></li><li><p>4</p><p>et al 1998, Jenkins and Hawkins 2003, Bertness et al. 2004a), the availability of space via 70disturbance (Paine and Levin 1981, Denny et al. 1985), the extent of air exposure as a result of 71wave 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 73succession and resulting community organization often differ substantially between wave-74exposed and sheltered shores. 75</p><p>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 771957, Dudgeon and Petraitis 2001, Pineda et al. 2002, Kordas and Dudgeon 2009) often 78facilitates high mussel recruitment in the summer (Petraitis 1991, Bertness et al. 2004a). In the 79absence of consumer pressure, mussels eventually dominate these communities by overgrowing 80barnacles 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 82available by disturbance from waves and other factors, these habitats can also contain a mosaic 83of barnacles, mussels and Fucus at any given time (Menge 1976, 1978a,b, Bertness et al. 1999, 842004a). In contrast, on sheltered shores in the southern Gulf the supply of barnacle and mussel 85larvae is reduced (Bertness et al. 2004a) and resulting low recruitment, coupled with more 86intense predation by dogwhelks (Nucella lapillus) and green crabs (Carcinus maenas), further 87reduces mussel and barnacle abundance (Lubchenco and Menge 1978). As a result, competition 88for space is relaxed allowing fucoid algae (initially Fucus vesiculosus followed by the slower 89growing Ascophyllum nodosum) to colonize, grow and form dense canopies that dominate the 90shore (Dudgeon and Petraitis 2001). Although snail (Littorina littorea) grazing on young 91recruits may slow fucoid algal recovery (Lubchenco 1983, Petraitis 1987), the eventually 92</p></li><li><p>5</p><p>dominant Ascophyllum canopy typically covers a sparse understory of mussels and barnacles at 93these sites (Bertness et al. 2004a), which contrasts with the dense mussel and barnacle 94communities typical of wave-exposed shores (Menge 1976, 1978a,b) 95</p><p>The prevailing evidence indicates that these habitat-specific differences in succession and 96community organization in the southern Gulf typically depend on the high recruitment potential 97of dominant space occupying species and the impact of consumers on their abundance after 98settlement. However, recent work on Gulf of Maine shores has revealed geographically based 99differences in species interaction strength (Kordas and Dudgeon 2009, 2011) and community 100organization following disturbance (Petraitis and Dudgeon 1999, 2004; Dudgeon and Petraitis 1012001, Bertness et al. 2002, 2004a,b). Hence, there has been disagreement over whether 102consumers drive the dynamics and organization of these communities or whether spatial and 103temporal variation in recruitment levels of key species plays a more prominent role than 104previously thought. For example, experiments at multiple locations in the southern Gulf found 105that consumers prevented the recovery of fucoid algal canopies on sheltered shores (Bertness et 106al. 2002, 2004a,b). In contrast, work in central Maines Penobscot Bay found that variability in 107the timing and location of mussel, barnacle and fucoid algal recruitment and the size of patches 108created 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 110variation in barnacle and mussel recruitment in the Gulf of Maine may not solely depend on 111wave-exposure, but also local oceanographic processes that impact larval supply. Moreover, 112consumer pressure was quite different in these studies, with Bertness et al. (2004a) observing 113high predation rates within a single tidal cycle and Petraitis and Dudgeon (1999, 2004) observing 114predation rates that were not apparent for several weeks. 115</p></li><li><p>6</p><p>It is clear that there is the potential for substantial geographic variation in the factors 116affecting community succession and organization in the Gulf of Maine. This is not surprising 117because geographic variation in upwelling (Bustamante et al. 1995a, Menge et al. 1997, 2003, 1182004), grazer impacts (Coleman et al. 2006), and the strength of positive and negative species 119interactions (Bertness and Leonard 1997, Leonard 2000) can influence the succession and 120organization of rocky shore communities. For example, large regional differences in community 121organization have been documented in the eastern Atlantic (Coleman et al. 2006, Jenkins et al. 1222008), which has many of the dominant species (Fucus, Ascophyllum, Semibalanus, Mytilus sp., 123Carcinus and Nucella) found on Gulf of Maine shores. In the eastern Atlantic, latitudinal 124differences in rocky shore community organization are likely driven by changes in the 125abundance and impact of two patellid limpet species, Patella depressa and P. vulgata (Coleman 126et al. 2006, Jenkins et al. 2008), which are absent from the western Atlantic and Gulf of Maine. 127In general, herbivore control of fucoid algae appears to be more intense in the eastern Atlantic 128than in the Gulf of Maine (Jenkins et al. 2008), whereas factors affecting barnacle and mussel 129abundance may be more important on rocky shores in the western Atlantic (i.e. the Gulf of 130Maine). 131</p><p>Over the last 20 years, many studies have shown that broad scale spatial variation in 132rocky shore communities may reflect differences in oceanographic processes that impact larval 133supply (e.g. Menge et al. 1994, 2003, Broitman et al. 2001, Navarrete et al. 2005, Blanchette et 134al. 2008, Wieters et al. 2009). Because variation in barnacle and mussel recruitment can play a 135key role in the succession and organization of Gulf of Maine rocky shores, identifying 136geographic variation in coastal oceanography that influences larval supply may help reconcile 137the disparate influence of consumer control and recruitment variation on community succession 138</p></li><li><p>7</p><p>and organization throughout the Gulf of Maine. To examine whether distinct oceanographically 139driven biogeographic provinces occur within the Gulf of Maine, we examined the influence of 140physical stress (wave energy), recruitment, and consumers on community recovery after 141disturbance in the northern and southern Gulf. To broaden the geographic scope of these 142observations and experiments, we also examined between-site variation in community structure 143on 34 shores across the Gulf of Maine basin. Our results suggest that the succession and 144organization of rocky intertidal communities in the northern and southern Gulf are substantially 145different despite sharing virtually the same species assemblages. Moreover, these differences 146appear to be shaped by oceanographically driven recruitment variation that dictates subsequent 147species interactions. 148</p><p>MATERIALS AND METHODS 149Community Structure Across the Gulf of Maine 150 We characterized the structure of rocky intertidal communities with quadrat surveys at 34 151sites spanning the Gulf of Maine (Fig. 1, Appendix A). We recorded the percent cover of all 152visible, macroscopic, sessile species composing the canopy and understory in point-intercept 153quadrats (25 points per 0.25 m2 quadrat) haphazardly placed (N = 10) in the mid-intertidal at 154each site. This method provides reliable estimates of the abundance of common intertidal 155species, but may be less so in estimating the abundance of rare species. Quadrats were randomly 156tossed on horizontal emergent substratum within the fucoid algae zone. If a quadrat landed in a 157tidepool, or rested vertically against a ledge, then the quadrat was moved to the nearest emergent, 158horizontal surface. All algal or sessile invertebrate species located beneath an intercept were 159recorded to yield percent cover data that regularly exceeded 100% because of the presence of 160both the canopy and understory organisms. For any species that could not be conclusively 161</p></li><li><p>8</p><p>identified in the field (rarely the case), a representative sample was collected for identification in 162the laboratory. 163</p><p>Sites were assigned to 3 geographic regions based on documented oceanographic 164circulation patterns (Pettigrew et al. 1998, 2005, see Fig. 1 for prevailing Gulf of Maine 165circulation patterns). The Southern Gulf region ranged from Cape Cod, Massachusetts to 166Penobscot Bay, Maine corresponding to the Western Maine Coastal Current (WMCC) that forms 167from the outflow of the Penobscot River and the Eastern Maine Coastal Current (EMCC; 168Churchill et al. 2005, Pettigrew et al. 2005, Manning et al. 2009). The Penobscot region ranged 169from Penobscot Bay to Great Wass Island, Maine, where a portion of the EMCC moves offshore 170to varying extents depending on seasonal and interannual variation. When offshore movement is 171high, 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. 1732005). The Northern Gulf region ranged from Great Wass Island to Cobscook Bay, Maine, 174corresponding to the region where the Nova Scotia current and discharge from the St. John and 175St. Croix rivers meet near the mouth of the Bay of Fundy to form the EMCC and southwestward 176flow occurs (Hetland and Signell 2005, Pettigrew et al. 2005, Tilburg et al. 2012). The wave 177exposure of each site (sheltered vs. exposed) was characterized based on personal observations 178of coastal topography (e.g., headlands vs. bays) and wave action during calm and stormy periods, 179and/or dissolution rates of plaster clod cards (Appendix A). 180 Variation in community structure was analyzed with multivariate analyses using the 181Vegan package (Oksanen et al. 2013) for R version 3.0.0 (R Core Development Team 2013). A 182Non-Metric Multidimensional Scaling (NMDS) plot was cre...</p></li></ul>

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