Microbial food web contributions to satisfying mesozooplankton carbon requirements in the Southern Ocean along 170°W

Abstract Microzooplankton often consume large portions of Southern Ocean productivity and have been hypothesized to fill substantial carbon deficits for active metabolism and growth of mesozooplankton that cannot be explained by direct grazing on phytoplankton. Using measured grazing rates and empirically-calculated zooplankton carbon requirements (ZCR) as trophic constraints, that hypothesis was tested for copepod-dominated waters for four seasonal cruises of the Antarctic Environment and Southern Ocean Process Study Program along 170°W (53–72°S). Microzooplankton herbivory and bacterivory averaged 384 and 97 mg C m−2 d−1, respectively. For process cruise measurements, ZCR of 364 ± 45 mg C m−2 d−1 was satisfied with 55% from direct herbivory and 44% from microzooplankton. For the diatom bloom season, > 50% of ZCR was from microzooplankton. A late-summer grazing deficit revealed a previously unmeasured role for the pteropod Limacina in the former diatom bloom region. Lateral (convergent) advection was advanced to explain biomass and ZCR maxima and a local grazing budget deficit at the Polar Front. Overall, microzooplankton-mediated contributions to ZCR in this Southern Ocean region were similar to those of low-latitude open-ocean upwelling systems like the equatorial Pacific, suggesting that regional understanding of trophic structure and energy flows might benefit from better integration of microbial trophic interactions in field studies and models.