estimating marine reserve effects through quantification

IntroductionMacro-algal phase shifts have been commonly observed on Caribbean coral reefs1-3 (Fig.1) No-take marine reserves have been implementedas fisheries management and conservation tools on the premise that natural food-webs will be restored; herbivorous fish populations willrecover, restoring sufficient algal grazing, thus enhancing reef resiliency to such shifts 4, 5. Whilst estimation of herbivory and macro-algalpercentage cover provides important insight into reef health and the ecological impact of marine reserves, this understanding of reserveeffects on reef community dynamics may be enhanced through assessment of relative macroalgal biomass, since competitive algalinteractions and the extent to which cover is controlled by herbivory, will depend largely on algal size and morphology 2, 4, 7. This study aimsto investigate the effects on macro-algal biomass of a well-established, actively enforced marine reserve in the Grand Cayman (CaymanIslands), an island with negligible land run-off or agricultural impact but a significant artisanal and recreational fishery.

Objectives• Quantification of relative biomass of macro-algae inside reserve versus unprotected reef;

• Identification of broad algal community structure around Grand Cayman.

Methods

Estimating Marine Reserve Effects through Quantification of Macro-Algal Biomass on a North-West Caribbean Coral Reef

Croy McCoy1,2, Laura Richardson1,2, John Russell Turner1

1 School of Ocean Sciences, University of Bangor, Westbury Mount, Menai Bridge, Anglesey, Wales, LL59 5AB2 Department of Environment, 580 North Sound Road, Grand Cayman KY1-1106, Cayman Islands

Results

Discussion

References1 Hughes, T. P. (1994). Catastrophes, Phase Shifts, and Large-Scale Degradation of a Caribbean Coral Reef. Science, 265, 1547-1551.2 Bellwood, D. R.et al. (2004). Confronting the coral reef crisis. Nature, 429, 827-833. 3 Gardner, T. A.,et al. (2003). Long-Term Region-Wide Declines in Caribbean Corals. Science, 301, 958-960.4 Hughes, T. P.,et al. (2006). No-take areas, herbivory and coral reef resilience. TRENDS in Ecology and Evolution, 22 (1), 1-3. 5 Mumby, P. J.et al. (2006). Fishing, trophic cascades, and the process of grazing on coral reefs. Science, 311, 98–101. 6 DeGeorges, A. (1990). Land-Based Pollution and Its Impact on Coral Reefs and Related Ecosystems, the Caribbean Experience Implications for East African Coastal Tourism, U.S.

Agency for International Development: Washinton, DC, USA.7 Renken, H.,et al. (2010). Effects of physical environmental conditions on the patch dynamics of Dictyota pulchella and Lobophora variegata on Caribbean coral reefs. Marine

Ecology Progress Series, 403, 63-74.8 Harborne, A. R. et al. (2008). Tropical coastal habitats as surrogates of fish community structure, grazing, and fisheries value. Ecological Applications, 18 (7), 1689-1701.9 Glynn, P. W. (1976). Some physical and biological determinants of coral community structure in the eastern Pacific. Ecological Monographs, 46, 431-436.10 Vergés, A. et al. (2011). Spatial patterns in herbivory on a coral reef are influenced by structural complexity but not by algal traits. PLoS ONE, 6 (2): e17115.

doi:10.1371/journal.pone.001711511 Hoey, A. S. and Bellwood, D. R. (2009). Limited Functional Redundancy in a High Diversity System: Single Species Dominates Key Ecological Process on Coral Reefs. Ecosystems,

12, 1316–1328.

AcknowledgementsMany thanks to Gene Parsons, Paul Chin, Janice Blumenthal, Matthew Cottam, Jeremy Olynik, Phillipe Bush, James Gibb, andNeal Haddaway. for advice and assistance with fieldwork

• Study area: Grand Cayman (Cayman Islands) located in the north-west Caribbean. All samples were taken fromshallow terrace reef sites at a depth of 8-10 m.

• Data collection: Turf and macro algae was extracted from 5 x 0.25m2 randomly positioned quadrats at 4 MPAprotected sites and 8 unprotected sites on Grand Cayman’s shallow reef terrace (Nov.-Dec. 2010). Samples weresorted to genus level and dry weighed (g) for measure of relative biomass.

• Data analysis: A linear mixed effects model with quasipoisson error distribution (R v.2.12.2) was used to test fordifferences in total biomass (ln transformed since model residuals were not Normal) betweenprotected/unprotected sites and between sites of different aspect (north, south, west), allowing for nesting ofreplicates (quadrats) between sites. An alpha threshold of 0.017 was applied to facilitate post-hoc testing withmultiple groups. Multivariate analysis (PRIMER v.6) comparing reserve effect on algal community data (Ln (x+1)transformed) included cluster analysis using the Bray-Curtis index of similarity, non-metric multidimensional scalingto visualise groupings, and an a priori ANOSIM to test for differences (according to protection and aspect groupings).SIMPER analysis was applied to ascertain average similarity within and dissimilarity between groups and identifythose species driving any differences.

Fig. 2 Map of Grand Cayman, showing sampling sites, reserveboundaries and location of Cayman Islands within widerCaribbean region (red star).

• A total dry weight of 544.45 g was collected, with mean biomass at sites ranging from3.47-6.26 g in the west (MPA), 3.80-5.90 g north (non-MPA), and 6.82-26.05 g south(non-MPA). Lobophora variegata, Halimeda spp., Dictyota spp., filamentous turfalgae and other turfing algae dominated total biomass at most sites (Fig. 4).

• Difference in total algal biomass between reserve sites and unprotected sites couldnot be tested due to high variance among unprotected sites (north≠south; LME, F1,

37=19.239, P<0.001). There was no significant difference in algal communitystructure between protected/unprotected reefs (ANOSIM, R=-0.061, P=0.604).

Univariate and multivariate statistical analyses showed differences between sitesbased on aspect differentials.

Dictyota spp., Halimeda spp., Lobophora variegata and various turfing algae were themost common genera found, typically also dominant on coral reefs in the widerCaribbean region 6, 7.

Differences in biomass, species distribution and community structure around GrandCayman may be indicative of species specific algal patch-dynamics and of naturalvariation between reefs with different exposure or currents 7. Such physical conditionscan serve to determine habitat structure and complexity, fish community structure,nutrient levels and available spores for algal recruitment, all of which can inhibit ordrive algal growth and proliferation 8, 9, 10. Intra-habitat variation between northern andsouthern reefs may be responsible for concealing a clear reserve effect as a result ofpotential increased herbivory in the west where the island’s main reserve is located.

Inherent characteristics of the herbivore community (such as mouth size and feedingmodes and preferences) and macro-algal characteristics (i.e. maximum size,morphology and life history) will also likely influence grazing pressure, particularly asalgae reaches a ‘size refuge’ whereby keystone herbivores no longer graze upon them 7,

11.

Fig. 4 Single quadrat sample comprised of three of the most dominant algal genera identified during this study. From left to right, Lobophora variegata, Halimeda spp. and Dictyota spp. (images: L. Richardson).

Fig. 1 Smooth trunkfish on macro-algal dominatedreef , Grand Cayman (image: L. Richardson).

• Algal biomass was significantly greaterin the south than in the north (LME, F1,

37=19.239, P<0.001) and west (LME, F1,

37=3.854, P<0.001). No significantdifference was found between north andwest (LME, F1, 37=0.522, P=0.475; Fig. 5).

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• Species composition also varied between island aspects with southern sitesdisplaying a significantly different algal community structure (ANOSIM, R=0.685,P=0.003; Fig. 6 and 7) driven by comparatively minimal biomass of Lobophoravariegata, consistently large proportion of Halimeda spp. and greater biomass ofturfing algae than sites in the north and west (SIMPER).

Fig. 5 Mean algal biomass (dry weight g) ± 2SE inreserve (MPA: grey) and un-protected reefs. Matchedletters above bars, indicate significant differencesbetween aspects (P≤0.017).

Fig. 3 Macro-algal extraction using suction and scrapingdevices. Stretched net mesh size 6 mm (images: L. Richardson).

Sampling sitesNo-take MPAs

Fig. 6 Multi-dimensional scaling plot of macro-algalcommunity structure at each site, clustered by averagesite similarity. (Blue line used to aid visualinterpretation).

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Fig. 7 Mean biomass of algal genera in the north, southand west (MPA).