Changes in Phytoplankton Abundance in Tidepools over the Period of Tidal Isolation

We examined changes in phytoplankton assemblages in tidepools during the period of tidal isolation, both at a time of low phytoplankton abundance (22 August 1991) and during the autumn bloom (9 October 1991) in the surrounding seawater. Four tidepools were sampled at each of three intertidal zones (mid, high and splash) on a rocky shore near Halifax, Nova Scotia, Canada. We sampled phytoplankton, micrograzers, and dissolved nutrients, and measured temperature, salinity and pH, immediately after the tide. receded and immediately before the pools were flooded on the returning tide. Although we detected changes in phytoplankton abundance over the period of tidal isolation of the pools, these changes varied among phytoplankton groups and within groups between sampling dates. Nanoflag-ellates increased significantly within the period of isolation in August, probably due to population growth. Centric diatoms, and consequently total phytoplankton, decreased significantly over the period of tidal isolation in October, probably due to grazing. Temperature was the most variable physical factor within the tidal cycle on both dates, and salinity was the least variable. Nitrate + nitrite and density of benthic micrograzers decreased significantly within the isolation period in August, but no significant differences were detected in October. Variability in the chemical and physical environment over the period of pool isolation do not adequately explain the few recorded changes in phytoplankton abundance. The abundance of most phytoplankton groups differed significantly between the two sampling dates.