Biofilms on Rocky Shores

Biofilms are an important but often overlooked component of benthos and rocky shores worldwide. They are an important food resource and habitat for marine larvae. Microorganism communities within biofilms are regulated by biological and physico-chemical factors like grazing, nutrient availability and immersion stress. Variances of these factors can cause changes in the microorganism abundance. For example, previous work has shown that microbial abundance in the North East Atlantic is affected by general seasonal changes, along with increased grazing intensity during the summer period. While the majority of research on this type of biofilm has been conducted on exposed rocky shores, it has also been shown that there is a marked different between these biofilms and those found on sheltered rocky shores. The purpose of this study was to compare the microbial communities found on exposed and sheltered rocky shores during both summer and winter periods, and at different tidal levels.

Research for this study was conducted during 1993 and 1994 – a considerable time before this paper was written. Microbial communities were sampled using rock chips taken twice in each summer and twice in each winter from two exposed and two sheltered shores on the south coast of the Isle of Mann. They were taken from the lower, middle and upper shore levels to enable assessment of the effects of immersion.  Microbial biomass was quantified by extracting chlorophyll from the rock chips using methanol, while the abundance of organisms was determined using scanning electron microscopy (SEM). Previous work has shown that biofilms at these sites are dominated by diatoms and cyanobacteria, and so comparisons were restricted to these groups.

The results showed that the photosynthetic analysis of microbial biomass displayed a difference between different shores, shore levels and seasons. Wave exposure was also found to have an influence on this, particularly in winter samples. The abundance of diatoms varied between shores, with a much higher abundance found on exposed shores than sheltered ones. This was found irrespective of season or shore level, for which there was no significant effect. SEM showed a marked seasonal affect on cyanobacteria, which were virtually absent from sheltered shores and extremely scarce on exposed shores during the winter. Although greater abundance was found during the summer, the results were in fact not significant. Shore level had no significant difference on cyanobacterial abundance, apart from the summer of 1994, where a greater volume was found on the lower tidal line of exposed shores.

Overall, cyanobacteria, diatoms and photosynthetic microbial biomass were found to be more abundant on the exposed shores than sheltered ones, though the effects were considerably weaker for cyanobacteria than they were for either of the others. The causes of these patterns are not clear. In macroalgae opposite trends are usually found, with increased abundance on sheltered shores compared with exposed ones. One explanation could be increased water flow, which could enhance nutrient supply and the potential in-water photosynthetic activity. Another could be that molluscan grazers have a regulating effect on both macro and micro algae on rocky shores - a higher density of Limpets were found on the sheltered shore which could be why there were fewer microalga. The most important factor determining these results was considered to be wave action, due to its more damaging effects on macroalgae and grazers on exposed shores.

This paper demonstrates a point of interest as the results show the opposite trend to what is expected and to what is seen in macroalgae on rocky shores. It makes a good source for further reading and possible a future point for further study.

A review of Thompson R.C., Moschella P.S., Jenkins S.R., Norton T.A. and Hawkins S.J. (2005) Differences in Photosynthetic Marine Biofilms Between Sheltered and Moderately Exposed Rocky Shores, Marine Ecology Progress Series, 296, 53-63.