Microzooplankton bacterivory and herbivory in the NE subarctic Pacific

Despite a multi-decade time series of observations in the high nutrient, low chlorophyll regions of the NE subarctic Pacific, remarkably little is known about heterotrophic microbial processes, especially the role of microbes in the cycling of heterotrophic carbon through the lower food web. In this study, we quantified the seasonal patterns of ingestion of phytoplankton (i.e.>0.7 μm chlorophyll a), bacteria and Synechococcus by microzooplankton in the surface layer at Ocean Station Papa. During spring and summer microzooplankton ingestion rates (∼7–13 μg C l−1 d−1) were significantly greater than in winter (∼2–3 μg C l−1 d−1). On an annual basis, bacteria comprised a significantly greater fraction of the total ingested ration (x̄=58%) than chlorophyll a (x̄=26%) or Synechococcus (x̄=16%). Temperature appeared to be an important predictor of bacterial abundance and growth, and bacterivory by microzooplankton. The Q10 for bacterial growth and bacterivory of 2.3 to 2.5 was about two-fold higher than for total ingestion and percent herbivory. Significant predictive relationships (adj-r2=0.75–0.99) were found between ingestion of bacteria, Synechococcus and total ingestion, and ambient prey stocks and temperature. These predictive models were tested with data we collected from other coastal and oceanic sites in the NE subarctic Pacific, and were found to have highly significant predictive capabilities. This study shows that microzooplankton in the NE subarctic Pacific obtain most of their nutrition from heterotrophic and autotrophic picoplankton and that herbivory is not their major trophic mode. Our results do not support the paradigm that microzooplankton effectively constrain phytoplankton stocks and production in this region.