Changes in copepod egg production rates in relation to frontal gradients and microplankton concentrations in the California Current Ecosystem

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Fronts, i.e., ocean regions of high horizontal spatial gradients, often separate coastal, high productivity waters from oligotrophic, offshore zones. However, frontal features themselves may also be areas of elevated nutrient fluxes and phytoplankton and microzooplankton concentrations. Under such frontal conditions, suspension-feeding copepods would be expected to have elevated ingestion rates. We hypothesized that this increased food intake could result in an increase in copepod reproductive success through enhanced rates of egg production and recruitment. During California Current Long Term Ecological Research (CCE-LTER) process cruises, three species of copepods (Calanus pacificus, Metridia pacifica, and Eucalanus californicus) were collected at sites across a gradual gradient and two frontal regions. The copepods were incubated at sea in simulated in situ conditions to ascertain egg production rates and egg hatching success. Several indices of food availability were also measured for each collection site. The rates and concentrations were analyzed to test the hypothesis that regions of sharp frontal gradients enhance copepod reproductive success through increased food availability. Egg production rates varied between species and across the gradients but did not appear to be consistently elevated within frontal regions.