Differences in photosynthetic marine biofilms between sheltered and moderately exposed rocky shores

Thompson, R.C., Moschella, P.S., Jenkins, S.R., Norton, T.A. and Hawkins, Stephen J. (2005) Differences in photosynthetic marine biofilms between sheltered and moderately exposed rocky shores. Marine Ecology Progress Series, 296, 53-63. (doi:10.3354/meps).

Abstract

Photosynthetic microbial biofilms are an important functional component of rocky intertidal
habitats worldwide. The abundance of organisms within these films varies both seasonally and
according to vertical emersion gradients. However, the effects of horizontal gradients of wave exposure,
which have a key influence on macrobiotic assemblages, have received little attention. Here,
we examine the relative importance of exposure to wave action, tidal elevation and season on intertidal
microbial assemblages. The abundance of micro-organisms was quantified using counts of
major functional groups and total photosynthetic biomass, assessed using extracted chlorophyll.
These data were compared between 2 moderately exposed and 2 sheltered shores on replicated sampling
dates during summer and winter at 3 tidal levels. Diatoms were approximately 6 times more
abundant on moderately exposed shores than on sheltered shores. This pattern was consistent
between sites and seasons. The percentage cover of cyanobacteria and total photosynthetic biomass
was also typically greater on exposed shores. Seasonal variations were also apparent with a trend of
greater photosynthetic microbial biomass during the winter than during the summer. Photosynthetic
biomass and, to a lesser extent, the abundance of cyanobacteria were greater on the lower shore than
on the upper shore. The possible causes of the differences in microbial assemblages between sheltered
and exposed shores are discussed. We suggest that indirect effects of habitat amelioration from
macroalgal canopy and grazer density, which also covary along the wave exposure gradient, may be
more important than the direct effects of wave action itself.

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