Water mass subduction and eddy effects on phytoplankton distributions in the Santa Barbara Channel, California

Observations using towed, undulating vehicles show that phytoplankton layers occur well below euphotic zone depths in the Santa Barbara Channel (SBC).  The deep layers are typically found where density surfaces slope steeply suggesting isopycnal mixing and advection cause downward transport of the phytoplankton. Significant correlation between salinity and chlorophyll along isopycnals supports this interpretation. The observations also indicate that eddy processes cause downward transport of phytoplankton. Data were obtained during 16 cruises conducted three times per year from 2001 to 2006 as part of the Santa Barbara Coastal Long Term Ecological Research project (SBC-LTER). Other sampling during the cruises reveals that wind-driven upwelling and cyclonic eddies largely control the spatial patterns of phytoplankton primary productivity. Cyclonic eddies influence productivity by uplifting isopycnal surfaces and by occasionally entraining phytoplankton and nutrients from water upwelled near Point Conception. Eddy-enhanced productivity changes were superimposed on variable levels of channel-wide productivity caused by wind-driven upwelling. The combined effects of upwelling and cyclonic circulation enhance phytoplankton biomass and productivity in the SBC compared to elsewhere in the Southern California Bight.