Spatial heterogeneity of phytoplankton assemblages in tidepools: Effects of abiotic and biotic factors

In any ecological system, the factors that regulate the abundance of species vary with spatial scale; therefore, the sources of spatial variability should be described. We examined different sources of variability in the spatial distribution of phytoplankton assemblages and biotic (e.g. planktonic and benthic micrograzers, mussels) and abiotic (e.g. nutrients, temperature, salinity, pH) factors that may regulate these assemblages in 4 tidepools at each of 3 intertidal zones (mid, high and splash) on a rocky shore in Nova Scotia, Canada, over a period of 15 mo. Stratum (defined as the depth within a pool) was a significant source of variability, particularly for pennate diatoms which were consistently more abundant near the bottom of pools. There was no indication of vertical zonation of the phytoplankton assemblages along the intertidal gradient, and differences among zones rarely explained more than 30% of the spatial variability in phytoplankton abundance. Also, among-zone variation was not apparent for the biotic and abiotic factors. We suggest that among-zone variability in these factors does not adequately explain vertical variability in phytoplankton assemblages. All groups of phytoplankton varied significantly among pools within intertidal zones on most sampling dates, and differences among pools explained up to 96% of the variability in phytoplankton abundance. Furthermore, there was significant variability among pools within zones for all biotic and abiotic characteristics of the pools on most sampling dates. We detected significant relationships between the density of benthic micrograzers and small mussels, and the concentration of nutrients in individual pools with the abundance of pennate diatoms, cryptomonads and chlorophytes. Among the abiotic characteristics of the tidepools, there was a significant relationship between flushing rate and temperature of individual pools, with the abundance of cryptomonads and chlorophytes. We suggest that the factors that regulate phytoplankton assemblages in tidepools probably operate more at the scale of the individual pool rather than the intertidal zone.