Top down and bottom-up regulation of phytoplankton assemblages in tidepools

We examined the relative importance of bottom-up (nutrient availability) and top-down (grazing) factors in regulating phytoplankton assemblages in tidepools on a rocky shore near Halifax, Nova Scotia, Canada. We manipulated the concentration of nutrients and density of micrograzers in pools in the high intertidal and splash zones in 3 repeated, 1 to 2 wk experiments in November 1992 and June and August 1993. For each experiment, we set up 4 orthogonal treatments in enclosures in each of 3 or 4 pools: (1) micrograzers removed and nutrients enriched, (2) micrograzers removed and nutrients at natural levels, (3) micrograzers at natural densities and nutrients enriched, and (4) both micrograzers and nutrients at natural levels. For each treatment, we measured the change in abundance over 1 wk intervals of 5 taxonomic groups of phytoplankton: centric diatoms, pennate diatoms, cryptomonads, prasinophytes, and chlorophytes. We examined the effects of nutrient concentration, grazer density, and pool on the phytoplankton assemblage using multivariate analysis of variance. There were significant effects of grazer density in June and August, and of nutrient concentration in August, which varied among phytoplankton groups and tidepools. In 1 pool in June, reduction in grazer density had a negative effect on pennate diatoms, cryptomonads and chlorophytes in the first week of the experiment, but a positive effect on pennate diatoms in the second week. In another pool in the second week, grazer reduction had a positive effect on prasinophytes but a negative effect on chlorophytes. In 1 pool in August, nutrient enrichment had a positive effect on prasinophytes in the first week of the experiment, and grazer reduction had a negative effect on cryptomonads and chlorophytes in the second week. In another pool in the first week, nutrient enrichment had a negative effect on chlorophytes. Based on the frequency of significant effects, we concluded that grazing is more important than the nutrient regime in regulating phytoplankton assemblages in tidepools. The large variability among tidepools in the response of phytoplankton to our manipulations supports our previous suggestion that regulation of these assemblages occurs at the scale of the individual pool rather than the intertidal zone.