Phycoerythrin-containing picoplankton in the Southern California Bight

Abstract

Abstract Flow cytometry was used to examine the distribution of phycoerythrin-rich picophytoplankton, referred to here as Synechococcus , off the Southern California coast during six California Cooperative Oceanic Fisheries Investigations (CalCOFI) cruises. Depth profiles revealed that Synechococcus was most abundant in the surface mixed layer, gradually disappearing with depth below the thermocline. Within the surface mixed layer, Synechococcus abundance was generally greater and more variable at stations shoreward of the California Current than at stations offshore of it. In waters associated with the California Current not impacted by upwelling, Synechococcus abundance increased with increasing bulk chlorophyll. In contrast, Synechococcus abundance declined with increasing bulk chlorophyll at stations that were impacted by upwelling. Synechococcus at stations impacted by upwelling also had more phycoerythrin per cell than at non-upwelling stations. Offshore of the California Current, Synechococcus cells in waters intruding from the Central North Pacific displayed higher side-scatter relative to forward scatter than did Synechococcus cells elsewhere in the region. Flow cytometrically distinct Synechococcus cell types were also detected below the thermocline at most of the stations where depth profiles were analyzed. These patterns in Synechococcus abundance and cellular characteristics might reflect physiological and/or genetic differences among Synechococcus associated with the various water masses that comprise the CalCOFI region. The data presented here provide a framework from which to launch more detailed and mechanistic studies examining the role of Synechococcus in the CalCOFI ecosystem.