The effects of episodic wind-event mixing on vertical chlorophyll structure in the southern California Current Ecosystem

Subsurface chlorophyll maxima are ubiquitous features in the ocean that are thought to be foraging habitat for many zooplankters including larval fishes. Disruption of these plankton structures by episodic wind events was hypothesized to increase rates of larval fish mortality due to degraded feeding conditions. Previous research, however, found no association between wind events and larval fish mortality across five different species, motivating an examination of the connections between wind events and the incidence of plankton layers. Here, vertical chlorophyll profiles from 2004 to 2010 within the southern California Current Ecosystem were used to identify vertical chlorophyll structures (VCS) indicative of high densities of phytoplankton cells. We also identified wind events from reanalysis model output and tested their association with VCS that occurred within five days of the wind event. We asked three questions to examine the importance of episodic wind events to VCS: (i) was the occurrence of VCS less frequent immediately following wind events? (ii) Were the maximal chlorophyll concentrations associated with VCS during windy periods lower than the maxima when there were no wind events? And lastly, (iii) were there increases in mixed-layer nitrate concentrations during periods that were proceeded by wind events? We found that VCS was less frequent with the occurrence of wind events, and the magnitude of the subsurface chlorophyll maxima decreased when associated with wind events. Additionally, there was no effect of wind events on mixed-layer nitrate outside the expected climatology. The results were consistent with expectations that mixing induced by atmospheric forcing can disrupt concentrations of phytoplankton potentially serving as forage for larval fishes.