Microzooplankton grazing in the coastal Gulf of Alaska: Variations in top-down control of phytoplankton

Microzooplankton grazing rates on three phytoplankton size fractions (< 5, 5-20, and > 20 mu m) were measured during spring and summer 2001 in the northern coastal Gulf of Alaska (CGOA). To a first approximation, microzooplankton consumed all production by phytoplankton < 20 mu m in size and nearly half the production by phytoplankton > 20 mu m, mainly diatoms. Microzooplankton (ciliate plus heterotrophic dinoflagellate) biomass ranged from 9.6 mu g C L-1 to 82.2 mu g C L-1. The highest levels were associated with diatom blooms and equaled those previously reported for highly productive coastal upwelling regions. Regulation of microzooplankton grazing differed according to size class. Grazing on phytoplankton < 5 mu m in size averaged 0.48 d(-1) and was closely correlated with phytoplankton growth rates in the same size class. In contrast, grazing on phytoplankton > 20 mu m averaged 0.17 d(-1) and was unrelated to phytoplankton growth rate in this size class. Variations in grazing pressure on these largest phytoplankton arose mainly through variations in the biomass of the larger (> 40 mu m) ciliates and dinoflagellates. This biomass, in turn, became more closely correlated with > 20 mu m chlorophyll as the season progressed, indicating removal of top- down control on these ciliates and dinoflagellates as Neocalanus spp. copepods left the upper water column. Because microzooplankton directly consume much of the phytoplankton production in the CGOA, processes that regulate this trophic linkage have major implications for food web structure and secondary production in this coastal ecosystem.