Phytoplankton blooms are strongly impacted by microzooplankton grazing in coastal North Pacific waters

Phytoplankton growth and microzooplankton grazing were measured in two prod uctive coastal regions of the North Pacific: northern Puget Sound and the c oastal Gulf of Alaska. Rates of phytoplankton growth (range: 0.09-2.69 day( -1)) and microzooplankton grazing (range: 0.00-2.10 day(-1)) varied seasona lly, with lowest values in late fall and winter, and highest values in spri ng and summer. Chlorophyll concentrations also varied widely (0.19-13.65 mu g l(-1)). Large(>8 mum) phytoplankton cells consistently dominated phytopla nkton communities under bloom conditions, contributing on average 65% of to tal chlorophyll biomass when chlorophyll exceeded 2 mug l(-1) Microzooplank ton grazing was an important loss process affecting phytoplankton, with gra zing rates equivalent to nearly two-thirds (64%) of growth rates on average . Both small and large phytoplankton cells were consumed, with the ratio of grazing to growth (g:mu) for the two size classes averaging 0.80 and 0.42, respectively. Perhaps surprisingly, the coupling between microzooplankton grazing and phytoplankton growth was tighter during phytoplankton blooms th an during low biomass periods, with g:mu averaging 0.78 during blooms and 0 .49 at other times. This tight coupling may be a result of the high potenti al growth and ingestion rates of protist grazers, some of which feed on blo om-forming diatoms and other large phytoplankton. Large ciliates and Gyrodi nium-like dinoflagellates contributed substantially to microzooplankton bio mass at diatom bloom stations in the Gulf of Alaska, and microzooplankton b iomass overall was strongly correlated with >8 mum chlorophyll concentratio ns. Because grazing tended to be proportionally greater when phytoplankton biomass was high, the absolute amount of chlorophyll consumed by microzoopl ankton was often substantial. In nearly two-thirds of the experiments (14/2 3), more chlorophyll was ingested by microzooplankton than was available fo r all other biological and physical loss processes combined. Microzooplankt on were important intermediaries in the transfer of primary production to h igher trophic levels in these coastal marine food webs.