Spatial and temporal variation in mesozooplankton grazing across the California Current Ecosystem

We assess mesozooplankton grazing across a spectrum of ocean conditions as a component of the California Current Ecosystem Long Term Ecological Research (CCE-LTER) program. Our objectives are to (1) test the hypothesis of elevated grazing in the vicinity of sub-mesoscale frontal features, and (2) to utilize spatial variations in mesozooplankton-related grazing mortality in a space-for-time exchange, in order to project future changes in grazing processes. We conduct experiments in a Lagrangian design, while following discrete water parcels and entrained zooplankton using satellite-tracked drifters. Mesozooplankton are sampled in the upper 210 m of the water column, anaesthetized, size-fractionated into five size classes (0.2, 0.5, 1, 2, 5 mm), and ingestion of autotrophic prey measured by gut fluorescence. We will present results from at least two contrasting cruises, one of which (Aug. 2012) sampled a region of sharp frontal gradients across an eddy dipole, the other (Aug. 2014) which reflected anomalously warm upper ocean conditions, low wind forcing, and minimal frontal gradients. Our results illustrate spatial differences in the size distribution of mesozooplankton grazing and suggest selective impacts on parts of the phytoplankton assemblage.