MEPS 401:101-111 (2010)  -  DOI: https://doi.org/10.3354/meps08481

Past and present grazing boosts the photo-autotrophic biomass of biofilms

M. W. Skov1,5,*,**, M. Volkelt-Igoe1,**, S. J. Hawkins2,5, B. Jesus3, R. C. Thompson4, C. P. Doncaster1

1School of Biological Sciences, University of Southampton, Boldrewood Campus, Southampton SO16 7PX, UK
2Marine Biological Association of the UK, The Laboratory, Citadel Hill, Plymouth, PL1 2PB, Devon, UK
3Instituto de Oceanografia, Faculdade de Ciências da Universidade de Lisboa, Campo Grande, 1749-016, Lisbon, Portugal
4Marine Biology & Ecology Research Centre, Marine Institute, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK
5Present address: School of Ocean Sciences, Bangor University, Menai Bridge, Anglesey, LL59 5AB, UK
**Email: **These 2 authors contributed equally to this work

ABSTRACT: Little is known about the long-term consequences of grazing effects on microphytes. This study tested density-dependent responses to grazer removal on the biomass (chlorophyll a, chl a) and composition of natural high rocky-shore biofilms over a 7 mo period. Gastropod snails Melarhaphe neritoides graze entirely within circular halos generated in biofilms surrounding their refuges. The experiment crossed 3 levels of original snail density per halo with 3 levels of grazing intensity (generated by 100, 50 and 0% snail removal). Areas inside halos from which all snails had been removed sustained significantly higher chl a than never-grazed control areas outside the halos. This effect of grazing history was still present after 7 mo, suggesting that past grazing had an enduring positive influence on biofilm biomass. Against expectation, chl a-biomass was not increased by removing snails, regardless of original grazer density. Half- and fully-grazed halos peaked to a higher chl a than ungrazed halos in spring. Grazing did not affect the presence of major biofilm taxonomic groups, although it did alter their relative contributions. Never-grazed areas were covered by a thick biofilm-detritus complex and had proportionally more filamentous cyanobacteria than grazed areas, which sustained abundant clusters of coccoid cyanobacteria and lichen within micro-pits inaccessible to snail radulae. The present study shows that effects of grazing history are not exclusive to macrophytic systems. Grazers boosted the concentration of micro-autotrophs relative to non-chl a biofilm constituents, probably by removing an unproductive biofilm canopy and facilitating light and nutrient penetration for new growth.


KEY WORDS: Standing stock · Epilithic biofilm · Micro-algae · Grazing · Refuge · Rocky shore · Detritus · Littorina


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Cite this article as: Skov MW, Volkelt-Igoe M, Hawkins SJ, Jesus B, Thompson RC, Doncaster CP (2010) Past and present grazing boosts the photo-autotrophic biomass of biofilms. Mar Ecol Prog Ser 401:101-111. https://doi.org/10.3354/meps08481

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