Mesozooplankton and particulate matter responses to a deep-water frontal system in the southern California Current System

We analyzed the abundance of mesozooplankton and suspended particulate matter across the deep-water “A-Front” in the southern sector of the California Current System. We characterized the A-Front with two novel devices, a free-fall Moving Vessel Profiler (MVP) and an Underwater Vision Profiler 5 (UVP5), together with quantitative bongo samples analyzed by ZooScan. The MVP permitted real-time visualization of vertical density structure, chlorophyll a fluorescence and particle size structure (from a laser optical particle counter) across the front to a depth of 200 m with the research vessel moving at 6 m s. The UVP5 quantified in situ vertical distributions from digital images of planktonic organisms and particles in profiles to 300 m. Both the MVP and UVP5 indicated that organic aggregates increased several-fold at the A-Front. The A-Front was a region of elevated abundance of mainly particle-grazing mesozooplankton, including calanoid copepods, Oithona spp., appendicularians and euphausiids, as well as a site of elevated ratio of nauplii copepod. In contrast, poecilostomatoid copepods, ostracods, chaetognaths and radiolaria, most of which are more carnivorous or omnivorous, were all elevated in abundance to the south of the front. We provide evidence that submesoscale fronts can be regions of locally elevated plankton abundance and production, as well sites of faunal transitions.