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Grazer effects on algal assemblages andmussel recruitment in two differentmid‐intertidal communities in the CookStrait, New ZealandNicole E. Phillips a & Elanor Hutchison b ca Victoria University Coastal Ecology Laboratory, School ofBiological Sciences , Victoria University of Wellington , P.O. Box600, Wellington, New Zealand E-mail:b Victoria University Coastal Ecology Laboratory, School ofBiological Sciences , Victoria University of Wellington , P.O. Box600, Wellington, New Zealandc Department of Zoology , University of Otago , P.O. Box 56,Dunedin, New ZealandPublished online: 19 Feb 2010.
To cite this article: Nicole E. Phillips & Elanor Hutchison (2008) Grazer effects on algalassemblages and mussel recruitment in two different mid‐intertidal communities in the CookStrait, New Zealand, New Zealand Journal of Marine and Freshwater Research, 42:3, 297-306, DOI:10.1080/00288330809509957
To link to this article: http://dx.doi.org/10.1080/00288330809509957
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New Zealand Journal of Marine and Freshwater Research, 2008, Vol. 42: 297-3060028-8330/08/4203-0297 © The Royal Society of New Zealand 2008
297
Grazer effects on algal assemblages and mussel recruitment in twodifferent mid-intertidal communities in the Cook Strait, New Zealand
NICOLE E. PHILLIPSELANOR HUTCHISON*
Victoria University Coastal Ecology LaboratorySchool of Biological SciencesP.O. Box 600Victoria University of WellingtonWellington, New Zealandemail: [email protected]
*Present address: Department of Zoology, P.O. Box56, University of Otago, Dunedin, New Zealand.
Abstract Molluscan grazers were experimentallyexcluded for 24 months (December 2003 - December2005) from the mid-intertidal zone of the rockyshore at two sites at each of two locations inNew Zealand where intertidal communities differdramatically: Wellington Harbour (dominated bysessile invertebrates), and the Cook Strait (= south)coast (mostly bare rock). Excluding grazers resultedin immediate increases in foliose algae, and gradualincreases in filamentous algae. After 2 years, themean cover of both groups was similar in exclusionplots in both locations (22-24%). Crustose algaeand microalgae also increased in grazer exclusionplots, but mussel recruitment did not. There were nodifferences in response to grazer exclusion betweenthe two locations in the final algal assemblage ordensity of new mussel recruits, but the species ofmussel were different: Mytilus galloprovincialisrecruited in Wellington Harbour, and Xenostrobuspulex on the south coast. Thus, molluscan grazersin this system have a strong effect on the mid-zonealgal assemblage of the intertidal, and this effect wasgenerally similar across these two markedly differentintertidal communities.
M08004; Online publication date 2 September 2008Received 15 January 2008; accepted 18 June 2008
Keywords limpets; rocky intertidal; macroalgae;community structure; temperate reefs
INTRODUCTION
The dramatic impact that macro-invertebrategrazers can have on rocky intertidal assemblageshas been well documented from numerous studiesacross a variety of temperate systems (see reviewsby Lubchenco & Gaines 1981; Hawkins & Hartnoll1983; Underwood 2000). In mid to high intertidalzones, grazers often directly influence the abundanceand distribution of algae such that the removal orexclusion of grazers can result in dramatic increasesin algal cover and vertical distribution (Underwood1980; Lubchenco 1983; Cubit 1984; Jernakoff1985; Moreno & Jaramillo 1985). Grazers canalso have profound effects on sessile invertebrates,both directly and indirectly. Recruitment of manybarnacle species is directly, negatively affected bygrazers that ingest new recruits or "bulldoze" themoff the substrate (Dayton 1971; Petraitis 1983;Miller & Carefoot 1989). However, grazers mayalso influence recruitment of other sessile speciesindirectly via recruit responses to grazer effectson algae or microfilms, and these effects maybe either positive or negative (e.g., Underwoodet al. 1983; Jernakoff 1985; Menge et al. 1986;Petraitis 1990; Anderson 1995). Succession ofintertidal assemblages is also often dependent onboth direct effects of grazers (such as the removalof early colonising algal species and preferencesof different grazers for particular types or speciesof algae), as well as indirect effects via alteringspecies interactions (Lubchenco 1978; Lubchenco& Menge 1978; Benedetti-Cecchi & Cinelli 1993;Anderson & Underwood 1997).
New Zealand has an extensive coastline and adiverse fauna of macro-invertebrate grazers (Creese1988). Asinmany other regions of the world, intertidalgrazer assemblages are often dominated by molluscssuch as limpets, which in New Zealand are diverseand often abundant in the rocky intertidal (Morton &
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Miller 1968; Menge et al. 1999; Dunmore & Schiel2003). Nevertheless, the role that grazers play instructuring intertidal communities in New Zealandis still relatively unexplored (but see Rafaelli 1979;Menge et al. 1999; Dunmore & Schiel 2003).
a t the southern tip of the North island, thereis a dramatic change in intertidal communitystructure over the relatively short distance (i.e.,<3km) between Wellington Harbour and theWellington south coast that faces cook Strait(hereafter referred to as south coast). The rockyintertidal shores of Wellington Harbour house atypical intertidal community (i.e., after Stephenson& Stephenson 1949) where the mid-zone faunais generally dominated by mussels (in particularthe blue mussel, Mytilus galloprovincialis) andthe high-zone fauna by barnacles (largely thechthamalid Chaemaesipho columna). The rockyshores of the south coast are a stark contrast withno mussel beds, only sparse and patchy coverage ofbarnacles in the high zone, and a high proportion ofbare rock substrate in both zones (Morton & Miller1968; Gardner 2000; Helson & Gardner 2004).unlike the sessile invertebrates, the assemblage ofgrazing molluscs appears to be relatively similaracross these two locations with generally the samespecies of limpets, chitons and snails found acrossboth locations (e.g., the topshell Melagraphiaaethiops, winkles Austrolittorina spp., the chitonSypharochiton pelliserpentis, the cat's eye snailTurbo smaragdus, limpets Siphonaria australis,Cellana denticulata, C. radians, and C. ornata;Morton & Miller 1968).
it was the purpose of this study to examine theeffect of molluscan grazers in the mid-intertidalzone of these two locations (Wellington Harbourand south coast) that have such different intertidalcommunities. We tested the hypothesis that theresponse of the algal community to molluscangrazer exclusion (in functional groups and finalcover of dominant species) is different across thesetwo locations. For mussel recruitment, we wereparticularly interested in testing the followinghypotheses: (1) that excluding molluscan grazersincreases the recruitment of mussels; and (2) thatthe response of mussel recruitment to molluscangrazer exclusion varies across locations. if musselsrecruit to algae (e.g., Bayne 1964; Petersen 1984),then the exclusion of grazers may enhance musselrecruitment as suggested by Petraitis (1990), but thiseffect may be different in the harbour where musselbeds are well established, compared with the southcoast where they are not.
MATERIALS AND METHODS
Experimental designFour rocky shore sites were selected, two on the southcoast, island Bay (41°20.9 S, 174°46.05 e), and justwest of the Victoria university coastal ecologylaboratory ( V u c e l 41°20.9 S, 174°45.6 e) andtwo in Wellington Harbour, Shelly Bay (41°17.9 S,174°49.05 e) and Worser Bay (41°18.5 S, 174°49.9 e). The south coast sites were separated byapproximately 0.5 km, the harbour sites wereseparated by 5 km. The rocky shores of the southcoast are generally more wave exposed comparedwith the more protected harbour. We wanted tominimise confounding effects of wave exposure, andthese south coast sites were selected because theyare at least partially sheltered by broken offshorerocks and islets. The topography of the coast isheterogeneous, with a high degree of vertical relief,so it was not possible to control for variability in slopeor aspect with respect to sun exposure. Thereforewe used a blocked design. a t each site, two blockswere established in the mid-intertidal zone, withinapproximately a 10 m stretch of coast. Blocks wereseparated by a minimum of 20 m. The rock type wassedimentary greywacke across all sites.
Within each block, eight 0.0625 m2 plots (0.25 m ×0.25 m) were created. Plots were randomly assigned toone of four treatments: (1) molluscan grazer exclusion(hereafter referred to as "exclusion" plots; (2) paintcontrol; (3) epoxy control; and (4) unmanipulated. allbut the unmanipulated plots were manually clearedof organisms and subsequently scrubbed with a wirebrush. We cleared treatment plots initially to removepotential effects of the initial difference in sessileinvertebrate cover between the locations.
For exclusion plots, first a thin boundary ofmarine epoxy (Z-spar brand, Splash Zone 788, Kop-coat inc., united States) was applied to bare rocksubstrate, with care taken that the barrier was aslow as possible, and smoothed flat and flush withthe natural rock contours to avoid causing water topool in the plots. a s the epoxy was curing we appliedcopper-based antifouling paint to it (Micron c S cantifouling Paint, interlux Paint co., united States).copper-based paint has been used extensively insimilar studies to exclude molluscan grazers (e.g.,cubit 1984; Farrell 1988; Paine 1992; Menge etal. 1999; Dunmore & Schiel 2003). The paintcontrol plot was established the same way, exceptthat paint was only applied to 50% of the epoxyboundary discontinuously, with alternating paintedand unpainted areas. This treatment allowed for the
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Phillips & Hutchison—Grazer effects on intertidal assemblages 299
movement of molluscs into the plots, but controlledfor potential effects the paint might have other thanto exclude molluscan grazers. The epoxy controlplots were also cleared and scrubbed similar to theexclusion and paint control plots, but only the fourcorners were marked with small buttons of epoxy.This treatment controlled for potential effects of theepoxy barrier itself. These treatments were similar tothose used by Menge et al. (1999). unmanipulatedplots were marked at the corners with epoxy buttonsand otherwise untouched.
We photographed all plots immediately beforeand after their establishment in December 2003, andsubsequently on a low tide every 3 months throughDecember 2005. Plots were examined monthly, andpaint reapplied as needed. occasionally molluscangrazers were found in the exclusion plots and wereremoved at this time.
We calculated the percentage cover of sessileorganisms for each quadrat using the random pointcontact method (Dethier et al. 1993). We overlaid50 randomly generated dots onto the image of eachquadrat, and identified the species underneath. Forease of plotting and analysis, algae were classifiedinto functional groups after Steneck & Dethier(1994). a l l mobile invertebrates were identifiedand counted in each image.
In December 2005, after the final photographswere taken, plots were examined thoroughly,particularly for small juvenile mussels that mightbe hidden under or within the algal canopy that haddeveloped in the exclusion plots. We removed thealgal canopy from the exclusion plots to quantify thefinal percentage cover of encrusting algae that mayhave been otherwise obscured by the canopy.
Data analysisTreatment effects on algalcover and mussel recruitment
Treatment and spatial effects were particularlyof interest, and so were the only ones tested. Toidentify treatment effects, we used a nested a N o Vato examine the total number of grazing molluscsand total percentage cover of algae in the plotson two dates: after 1 year (December 2004) and2 years (December 2005). location (two levels:Wellington Harbour and south coast) and treatment(four levels: exclusion, paint control, epoxy control,and unmanipulated) were fixed factors. Sites wereconsidered a random factor nested within locationsand blocks were a random factor, nested within sitesand locations. We also included the treatment × block(site, location) interaction. Post-hoc Tukey tests were
used to further explore significant differences (Zar1984). Proportional data often require transformationto fulfil assumptions of ANOVA, therefore algalpercentage cover data were arcsin square-roottransformed before analysis as recommended by Zar(1984). log10- (x + 1) transformation was the best atimproving homoscedascity for the molluscan data.all transformed data were examined for equality ofvariance using Cochran's test (Underwood 1997).
To examine mussel recruitment after 2 years ofmolluscan grazer exclusion, a nested a N o V a wasconducted on the total number of mussel juveniles inplots on the final sample date in December 2005. Thefactors were: location, treatment (both fixed factors),sites nested with location, blocks nested with site andlocation (both random factors), and the treatment byblock (nested within site and location) interaction.For this analysis only the exclusion and two types ofcontrol plots were used as some unmanipulated plotsin the harbour had adult mussels in them which mayconfound the results if mussel larvae use presenceof adults as a settlement cue (Pawlik 1992). Thesedata were log10- (x + 1) transformed before analysis.a l l statistical analyses were performed using thesoftware package JMP (v7).
Effects of molluscan grazer exclusionon algal community over 2 years
only using the exclusion plots, we used nestedaNoVas on three of the major algal functionalgroups that occurred in the plots (filamentousalgae, foliose algae, and microalgae) to examinedifferences among locations (fixed factor) with sitesnested within location, and blocks nested withinsites and locations (both random factors) on threedates: 3 months after the experiment started (i.e.,the first sampling date), after 1 year, and after 2years. Because we used photographs to quantifyalgal cover, we were unlikely to obtain an accurateestimate of crustose algal cover, as it may havebeen growing obscured beneath the algal canopies.Therefore, on the final date (December 2005) wequantified the percentage cover of crustose algaeafter the removal of the algal canopy.
We then used a nested a N o V a to examine thefinal species composition of the exclusion plots foreach of the dominant algal species or groups (non-calcareous crustose algae, microalgae, Ulva spp.,and Scytothamnus australis) across locations (fixedfactor), with sites nested within location, and blocksnested within sites and locations (random factors).all percentage cover data were arcsine square-roottransformed before analysis.
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Mar04
Jun04
Sep04
Dec04
Mar05
Jun05
Sep05
Dec05
Sample dates
Mar Jun Sep Dec Mar Jun Sep Dec04 04 04 04 05 05 05 05
Fig. 1 Treatment effects on mean abundance of molluscan grazers (± 1 Se, n = 4) in experimental plots across sites.A, island Bay; B, Victoria university coastal ecology laboratory; C, Shelly Bay; and D, Worser Bay. Data werepooled across plots at each site.
RESULTS
exclusion plots were generally but not completelysuccessful in excluding grazing molluscs (Fig. 1).after 1 year and 2 years, the abundance of molluscangrazers was significantly lower in exclusion plotscompared with all other plot types (Table 1, Tukeytest, P < 0.05). The number of molluscan grazersdid not vary across sites or location, but did vary byblocks after two years (Table 1). Molluscan grazersthat were occasionally found in low numbers inexclusion plots included the topshell M. aethiops,and winkle Austrolittorina spp., on two occasions thechiton S. pelliserpentis and on single occasions thecat's eye snail T. smaragdus, and limpets S. australis,C. denticulata, and C. ornata. The most abundantmolluscan grazers in control and unmanipulatedplots were C. denticulata, followed by M. aethiops,Austrolittorina sp., S. pelliserpentis and two otherCellana species, C. ornata and C. radians.
Total percentage cover of algae was much greaterin exclusion plots than any other plot type overthe entire 2-year course of the experiment (Fig.2). After both 1 and 2 years, the only significanteffect on algal cover was the treatment effect, where
algal cover in exclusion plots was greater than inany other plot type (Table 2, Tukey test, P < 0.05),and no significant differences in algal cover acrosslocations, sites, or blocks.
After 2 years, there were no significant differencesin the number of mussel recruits for any effectsexamined (aNoVa, all factors and interactions,P > 0.05). on average, after 2 years, there was atotal of four juvenile mussels per plot. althoughthe magnitude of mussel recruitment was similarin both locations, the species were different. a l lmussel juveniles from the harbour were identified bymorphology as the blue mussel M. galloprovincialis,and all mussels from the south coast were identifiedas the little black mussel Xenostrobus pulex.
in plots that excluded molluscan grazers, algalfunctional group composition changed over the 2years (Fig. 3, Table 3). Three months after the startof the experiment, foliose algal cover was similarlyhigh at island Bay and Worser Bay, but not presentat Shelly Bay or V u c e l (Tukey test, P < 0.05).after 1 and 2 years, there were no differences infoliose algal cover between blocks, sites or locations(Table 3), although V u c e l tended to have lowestcover of this functional group on most sampling
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Phillips & Hutchison—Grazer effects on intertidal assemblages 301
O
ro
ra
Mar Jun Sep Dec Mar Jun Sep Dec
04 04 04 04 05 05 05 05
100 -,
80 -
60 -
40 -
20 -
0
— o — Paint Control
•^ù— Epoxy control
Unmanipulated
Mar Jun Sep Dec Mar Jun Sep Dec04 04 04 04 05 05 05 05
Sample dates
Fig. 2 Treatment effects on mean total percentage cover of algae (± 1 Se, n = 4) in experimental plots across sites.A, island Bay; B, Victoria university coastal ecology laboratory; C, Shelly Bay; and D, Worser Bay. Data werepooled across plots at each site.
Table 1 Results of nested aNoVas examining treatment effects (molluscan grazer exclusion, paint control, epoxycontrol and unmanipulated) on molluscan grazer abundance after 1 and 2 years. Data were log10- (x + 1) transformedbefore analysis. F ratios reported are generated from JMP output using denominator MS values synthesised for modelswith random effects. (ns, not significant.)
Source of variation
locationTreatmentTreatment DLocationSite (location)Treatment × Site (location)Block (site, location)Treatment ×Block (site, location)error
d.f.
13326412
32
afterMS
0.54031.05860.05130.27510.05820.14120.0865
0.1087
1 yearF
1.963718.18810.88092.43640.67281.63250.7961
P
ns< 0.005
nsnsnsnsns
MS
0.58611.34570.05150.34240.02490.14810.0393
0.1334
after 2 yearsF
1.719154.0221
2.06832.56020.63463.77260.2942
P
ns< 0.005
nsnsns
< 0.005ns
dates (Fig. 3). Microalgal cover was beginning toincrease after 3 months, and although there weredifferences among blocks, there were no differencesbetween sites or locations (Table 3). after 1 year,there was significantly higher microalgal cover at thesouth coast compared with Wellington Harbour, but
after 2 years this location effect was not significant(Table 3). Filamentous algae gradually increasedat three of the sites over the first year molluscangrazers were excluded, first appearing 9 months afterthe experiment began and generally continuing toslowly increase in cover over the second year (Fig.
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90 -,80 -70-60 -50 -40 -30 •
20 -10 -0
Mar Jun Sep Dec Mar Jun Sep Dec2004 2004 2004 2004 2005 2005 2005 2005
Sample dates
Fig. 3 Change in mean percentage cover of each algalfunctional group over time in molluscan grazer exclusionplots at each site (± 1 Se, n = 4). A, Filamentous algae;B, foliose algae; C, microalgae. Solid lines/filled symbolsare sites from Wellington Harbour: squares, Shelly Bay;triangles Worser Bay. Dashed lines/open symbols aresites from the south coast: circles, island Bay; diamonds,Victoria university coastal ecology laboratory.
3). Filamentous algae were not present at ShellyBay, which led to a significant site effect after 1year (Tukey test, P < 0.05, Fig. 3); however, aftertwo years, there were only significant differencesbetween blocks (Table 3).
Final species composition of the algae in exclusionplots in the different locations were dominated byfour taxonomic groups: Ulva spp. (foliose green),S. australis (filamentous brown), non-calcareouscrust, and microalgae (Fig. 4). The final percentagecover of Ulva spp. and S. australis did not varysignificantly across locations (for Ulva spp.: F1,2= 8.45, P > 0.05; for S. australis: F1,2 = 1.39, P >0.05), or sites nested within locations (for Ulva spp.:F2,4= 0.25,P >0.05; for S. australis: F2,4 = 1.28,P >0.05). Ulva spp. cover did not vary by blocks nestedwithin sites and locations (F4,8 = 0.62, P > 0.05),but S. australis cover did (F4,8 = 4.24, P < 0.05).The final percentage cover of each of these twodominant algal species was similar to each other,with 22-24% cover of each in exclusion plots at theend of the 2 years (Fig. 4). The final percentage coverof non-calcareous crust (F12=0.028, P > 0.05) andmicroalgae (F12=0.89, P >0.05) also did not differby location, and only crust varied significantly acrosssites (for crust: F2,4=36.24, P < 0.05; for microalgae:F2,4 = 0.90, P > 0.05), owing to the much highercover of crustose algae at Worser Bay than the othersites. Final cover of neither crust (F4,8 = 0.18, P >0.05), nor microalgae (F4,8 = 1.79, P > 0.05) variedby blocks. Total cover of canopy forming algae aftermolluscan grazers had been excluded for 2 years wassimilarly high at three of the four sites (>50%) butlower at Shelly Bay (<20%).
Table 2 Results of nested aNoVas examining treatment effects (molluscan grazer exclusion, paint control, epoxycontrol and unmanipulated) on total algal cover after 1 and 2 years. Data were arcine square-root transformed beforeanalysis. F ratios reported are generated from JMP output using denominator MS values synthesised for models withrandom effects. (ns, not significant.)
Source of variation
locationTreatmentTreatment × locationSite (location)Treatment × Site (location)Block (site, location)Treatment ×Block (site, location)error
d.f.
13326412
32
after
MS
0.35082.90230.06040.11140.01990.08240.0933
0.0486
1 yearF
3.1486145.1859
3.022012.2120.21440.88351.9185
P
ns< 0.005
nsnsnsnsns
MS
0.59292.79090.03040.24950.05240.03690.0502
0.0402
after 2 yearsF
2.376653.30560.58056.38721.04240.73531.2485
P
ns< 0.005
nsnsnsnsns
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Phillips & Hutchison—Grazer effects on intertidal assemblages 303
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DISCUSSION
The molluscan grazer exclusion plots in thisexperiment were largely successful, particularlyat excluding limpets. The most abundant grazeracross all of the control and unmanipulatedplots combined was the limpet C. denticulata,and limpets were rarely found in the exclusionplots. The species that were most often found inexclusion plots were the relatively small topshellM. aethiops and the tiny winkles Austrolittorinaspp. which likely washed into the plots by waveaction. The use of copper-based paint to excludemolluscan grazers, although used in a largenumber of studies, has also been criticised whenappropriate controls are not used (e.g., Johnson1992; Benedetti-cecchi & cinelli 1997). in thisexperiment, however, there were no differencesamong the paint and epoxy control plots, so theresults appear robust and attributable to molluscangrazer exclusion rather than artifacts of theexclusion method used.
Despite the vast differences in the mid-zonesessile invertebrate assemblage between these twolocations, there was no evidence of differences inthe grazing molluscan assemblage between thesouth coast and Wellington Harbour. Further, after2 years, the final algal assemblage in response tograzer exclusion was also similar in the dominantspecies or taxonomic groups: Ulva spp., S.australis, non-calcareous crust, and microalgae.although the abundance of crust was particularlyhigh at one site, and S. australis varied at the
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304 New Zealand Journal of Marine and Freshwater Research, 2008, Vol. 42
smaller scale among blocks, the response of algaeto the removal of grazers was generally similar atthe larger scale between locations.
exclusion of grazers initially resulted in increasesin microalgae and foliose algae (primarily Ulvaspp.), similar to other studies (e.g., Southward &Southward 1978; underwood 1980; Hawkins 1981;Farrell 1988). The timing varied by site (althoughnot location): Ulva spp. were abundant at WorserBay and Island Bay by the first sampling period, 3months after the experiment was established, butdid not increase at the other sites until after 9-12months, and did not attain high cover at V u c e l . inother studies, Ulva spp. and other ephemeral algaecolonise very rapidly after grazer exclusion (e.g.,underwood 1980), but a recent study by Jenkins etal. (2005) showed that over a year, the developmentof ephemeral green algae in limpet exclusion plotswas variable on a geographic scale: at the isle ofMan (united Kingdom) colonisation was rapid (i.e.,within 2 months), whereas in SW england it occurred6-8 months later. in general, recent studies whichexplicitly examine grazer effects over a variety ofspatial scales have found heterogeneous responsesdepending on the geographical location of the study(e.g., Benedetti-cecchi 2001; Boaventura et al.2002; Jenkins et al. 2005; coleman et al. 2006), andindicate that an initial strong response by ephemeralalgae to grazer removal is not universal.
in this study, foliose algae only appeared to bedeclining over time at Worser Bay. These resultsare similar to other studies which have found thatephemeral species are often able to persist overyears when grazers are excluded or disturbancereduced (lubchenco 1978; Sousa 1979; underwood1980). By comparison, filamentous algae, mostlythe brown alga S. australis, was not very abundantat any of the sites until about a year after grazershad been excluded, and appeared to be graduallyincreasing in abundance at all but one site. Theseresults are in contrast to those of Rafaelli (1979)who reported that S. australis did not respond to theremoval of grazers from experiments he conductedin New Zealand. in some Northern Hemispherelocations, when grazers are removed, ephemeralsare gradually replaced by perennial brown fucoids(e.g., lubchenco 1978; Hawkins 1981; Hartnoll &Hawkins 1985). it is not clear whether S. australisin this study would have continued to increase overtime, however, S. australis is reportedly an annualspecies that in southern New Zealand appears insummer but disappears in winter (Naylor 1956).We noted that this species persisted through winter
suggesting that further study is needed to ascertainthe life cycle of this alga, and the scale at which itsapparent gradual dominance in response to grazerexclusion likely occurs, or whether it is a localphenomenon.
although algae flourished in the absence ofgrazers, mussel recruitment did not increase, thusthe hypothesis of Petraitis (1990) that removal ofgrazers can positively affect mussel recruitmentis not supported in this system. it was particularlyunexpected that the magnitude of recruitment onthe south coast where adult mussels are largelyabsent was similar to that of Wellington Harbour,a location that supports a well developed musselcommunity. The difference in species across thelocations suggests that both mussel species (M.galloprovincialis and X. pulex) are equally likelyto recruit to algae, as has been found for otherspecies (e.g., Bayne 1964; Petersen 1984), as tobare rock (here mussel juveniles were largely foundin crevices).
The lack of M. galloprovincialis recruitment to thesouth coast suggests either differential larval supplyor early juvenile survival in these two regions. Theseresults are consistent with other studies of recruitmentin the region. Helson & Gardner (2004) found ordersof magnitude lower numbers of planktonic mussellarvae and settlement on the south coast comparedwith Wellington Harbour, suggesting differencesin larval supply, but species were not identified intheir study. another recruitment study (N.e. Phillipsunpubl. data) found low but continuous monthlyrecruitment of X. pulex to the south coast over a 2-year period, but almost none of M. galloprovincialis.Both of these studies suggest there may be differentiallarval supply of the two mussel species to these tworegions. However, it has also been suggested thatlow planktonic food supply in cook Strait limitsmussel survival on the south coast, preventing theestablishment of populations (Gardner 2000; Helsonet al. 2007). Food limitation has not been tested forearly juveniles or for X. pulex explicitly, but species-specific differences in early post-settlement mortalityin the two regions, owing to planktonic food supplyor other factors, may also contribute to the patternsfound here.
ACKNOWLEDGMENTS
We thank S. Geange, E. Liggins, and J. Long for fieldassistance, Victoria university coastal ecology laboratoryfor logistical support and Victoria university of Wellingtonfor funding.
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