Persistent Coastal Eddy Drives Phytoplankton Hot Spot in Southern California

Abstract:

While most of the U.S. Pacific coast is dominated by strong spring upwelling and regular seasonal cycles of primary production driven by the California Current, the Southern California (SC) Bight, encompassing the southern California coast, has weak intermittent upwelling and low levels of primary production due to its irregular sheltered coastline. However, the Santa Barbara Channel (SBC), located within the northern SC Bight, is a hot spot for phytoplankton primary production, producing anomalously high levels of phytoplankton compared to the rest of the SC Bight. Located between the mainland coast and Northern Channel Islands, the circulation of the SBC is dominated by a mesoscale eddy that persists throughout the year. Using 3D ocean circulation (ROMS) and particle tracking models, satellite data, and field observations over 10 years (1998-2007), the eddy’s role in regulating the phytoplankton bloom development in the SBC was investigated. By implementing an eddy detection scheme on the ROMS flow fields, the eddy was found to persist 80% of the year in the SBC euphotic zone, the top 30m of water column, and varied in size up to 40 km in diameter. The ROMS temperature was used to identify the timing and size of upwelling events in the SBC by tracking changes in isotherm depth in the euphotic zone and surface wind stress. By developing a relationship between temperature and nitrate concentration from field observations, the nutrient availability from upwelling events was also estimated. Particle tracking driven by the ROMS was used to calculate the residence time of upwelled water within the eddy. By correlating the residence time of upwelled water within the eddy, nutrient availability, and satellite images of surface chlorophyll over 10 years, it was found that the eddy efficiently trapped upwelled water whenever an upwelling event occurred, even relatively weak event. After trapping, the eddy then sustained the nutrient-rich upwelled water in the euphotic zone often for weeks after the upwelling event was over, allowing phytoplankton blooms to initiate and grow in the SBC with a higher frequency than the surrounding region. Thus this study suggests that the eddy is the main driver in creating a phytoplankton hotspot in the SBC.