The interaction of physical and biological factors drives phytoplankton spatial distribution in the northern California Current

Transitions in phytoplankton community composition are typically attributed to ecological succession even in physically dynamic upwelling systems like the California Current Ecosystem (CCE). An expected succession from a high-chlorophyll (similar to 10 mu g L-1) diatom-dominated assemblage to a low-chlorophyll (< 1.0 mu g L-1) non-diatom dominated assemblage was observed during a 2013 summer upwelling event in the CCE. Using an interdisciplinary field-based space-for-time approach leveraging both biogeochemical rate measurements and metatranscriptomics, we suggest that this successional pattern was driven primarily by physical processes. An annually recurring mesoscale eddy-like feature transported significant quantities of high-phytoplankton-biomass coastal water offshore. Chlorophyll was diluted during transport, but diatom contributions to phytoplankton biomass and activity (49-62% observed) did not decline to the extent predicted by dilution (18-24% predicted). Under the space-for-time assumption, these trends infer diatom biomass and activity and were stimulated during transport. This is hypothesized to result from decreased contact rates with mortality agents (e.g., viruses) and release from nutrient limitation (confirmed by rate data nearshore), as predicted by the Disturbance-Recovery hypothesis of phytoplankton bloom formation. Thus, the end point taxonomic composition and activity of the phytoplankton assemblage being transported by the eddy-like feature were driven by physical processes (mixing) affecting physiological (release from nutrient limitation, increased growth) and ecological (reduced mortality) factors that favored the persistence of the nearshore diatoms during transit. The observed connection between high-diatom-biomass coastal waters and non-diatom-dominated offshore waters supports the proposed mechanisms for this recurring eddy-like feature moving seed populations of coastal phytoplankton offshore and thereby sustaining their activity.