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