Feast or famine for intertidal grazing molluscs: a mis-match between seasonal variations in grazing intensity and the abundance of microbial resources

Distinct seasonal variations in the abundance of photosynthetic microbiota and limpet grazing intensity were recorded at Port St Mary, Isle of Man bet ween January 1994 and June 1996. Microbial abundance was negatively correla ted with insolation stress, while grazing intensity was positively correlat ed with sea and air temperature. These patterns result in a mis-match betwe en the supply of and the demand for microbial resources with maximal grazin g intensity during the summer and autumn, but maximal microbial standing st ock during the winter and early spring. The importance of top-down control of microbial assemblages by grazing was demonstrated by experimental exclus ion of limpets during autumn 1993. This resulted in a four-fold increase in the abundance of cyanobacteria within 6 days, followed by a more gradual p roliferation of ephemeral algae during the next 4 weeks. The abundance of d iatoms remained relatively constant and was not influenced by the removal o f grazers at this time of year. The influence of microbial resource availab ility on the growth and mortality of limpets was examined using experimenta l enclosures of differing densities of either Patella vulgata or P. depress a. After 6 months, there were significant relationships between grazer dens ity and both mortality and growth with increased mortality and reduced grow th for P. vulgata at increased densities, and reduced growth for P. depress a at increased densities. Hence, the availability of microbial resources ma y also influence the biomass of grazers on rocky shorts from the bottom upw ards. A conceptual model is presented which describes seasonal and annual v ariations in microbial resources and grazing intensity and their potential consequences for other shore dwellers.