Omnivory by copepods in the New Zealand Subtropical Frontal Zone

The importance of omnivorous feeding by epipelagic copepods for copepod nutrition, particle removal and generation of vertical export was investigated in Subantarctic (SA) and Subtropical (ST) water masses either side of the Subtropical Frontal Zone (SFZ) east of New Zealand, using biomass surveys and shipboard bottle incubations. Methodological biases in incubations are described. Ciliates were cleared from the food stock much more rapidly than phytoplankton by all copepod sizes, and heterotrophic nanoflagellates (HNAN) were shown to be a significant potential food source. Microheterotrophs (ciliates + HNAN) contributed at least 50% of the ration in the SA water mass, across copepod size classes. The summed auto- and heterotrophic rations were sufficient to exceed and nearly meet the metabolic and growth requirements for the small and medium-size copepods, respectively, but were well below the requirements for large copepods (including Neocalanus tonsus). Carbon flow calculations showed that this deficit for the larger copepods would be overcome if they were predators of the highly productive small copepods and their egg production. In spring, the copepod community consumed only similar to 3-8% of phytoplankton and HNAN daily production, whereas larger proportions of ciliate production were predated (similar to 21-38%). The estimated egestion (faecal production) rate by the spring SA copepod community was equal to half to one-third the sedimentation rate determined at 300 m, in spring SA sediment traps. Copepod omnivory, including predation on microheterotrophs and possibly cannibalism by large on small copepods, may increase vertical carbon flux because it increases connections in the food web to faecal production.