Laboratory studies over the past decade have shown that adult females of planktonic copepods often have reduced reproductive success when fed high concentrations of diatoms. However field evidence for a “diatom effect” has been equivocal. The goal of this dissertation is to unambiguously determine whether or not the diatom effect can be measured in situ, and whether it is ecologically significant to the target copepod populations. This work was part of a larger field study that addressed these questions in Dabob Bay, Washington, from February – April and in July of 2002 and 2003. Here I present data on hydrographic conditions during the study, abundance and distribution of the target copepod species Calanus pacificus and Pseudocalanus newmani, reproductive success of C. pacificus, and results of a modeling approach to evaluate the ecological significance of the observations. Highest abundances of C. pacificus females were found below the surface layer (25-50 m), whereas P. newmani female maximum abundance and maximum chlorophyll concentrations were both found above 10 m. Reduced survival of C. pacificus nauplii occurred during blooms of diatoms of the genus Thalassiosira in February of both years, but no negative effects on the egg production rate, clutch size, or egg hatching success were observed, and egg production rate and clutch size were significantly positively related to both chlorophyll a and microplankton carbon concentrations. Although the episode of reduced naupliar survival was ephemeral, model results suggest C. pacificus recruitment was depressed in comparison to estimates of recruitment generated using averaged naupliar viability. There was little difference in recruitment between estimates using averaged naupliar viability and 100% naupliar survival. In Dabob Bay other sources of mortality are likely more important to the population dynamics of this species. Nevertheless, this study shows that naupliar viability must be accounted for in models of copepod recruitment dynamics.

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