Contrasting spring and summer phytoplankton dynamics in the nearshore Southern California Bight

Abstract

We analyzed a 9‐yr record of ocean color data from the Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) together with a 4‐yr record of satellite‐derived sea surface temperature (SST) measurements to investigate potential mechanisms influencing nearshore phytoplankton variability in the Southern California Bight. We also compared in situ physicochemical conditions at four shoreline stations during a year with an extensive nearshore dinoflagellate red tide (2005) to those of the following year when there was no extended red tide (2006). SeaWiFS data indicated that in spring, chlorophyll a (Chl a) and nitrate (NO3−) were inversely correlated to SSTs, consistent with an upwelling‐dominated system, but in summer Chl a and NO3− were unrelated to SST. Summer dissolved inorganic nitrogen concentrations were not well correlated between alongshore sampling stations, but showed a significant effect of tide stage (low vs. high). Mean summer nearshore Chl a was positively, linearly related to rainfall in the previous winter. The relationship between summer Chl a and winter precipitation could be due either to direct freshwater inputs during the winter rainy season or to delayed input via groundwater discharge. A first‐order analysis of nutrient delivery from both surface runoff and groundwater discharge showed that they could sustain summer blooms of up to 7540 and 1700 km2, respectively—areas on the order of the study site. However, precipitation‐induced physical changes to the nearshore water column cannot be conclusively ruled out as a potential causative mechanism.