Plankton dynamics in a cyclonic eddy in the Southern California Current System

Abstract

The California Current System is an eastern boundary upwelling system (EBUS) with high biological production along the coast. Oligotrophic offshore waters create cross‐shore gradients of biological and physical properties, which are affected by intense mesoscale eddy activity. The influence of eddies on ecosystem dynamics in EBUS is still in debate. To elucidate the mechanisms that influence the dynamics of ecosystems trapped in eddies, and the relative contribution of horizontal and vertical advection in determining local production, we analyze a particular cyclonic eddy using Lagrangian particle‐tracking analyses of numerical Eulerian. The eddy formed in a coastal upwelling system; coastal waters trapped in the eddy enabled it to leave the upwelling region with high concentrations of plankton and nutrients. The ecosystem was initially driven mainly by recycling of biological material. As the eddy moved offshore, production in its core was enhanced compared to eddy exterior waters through Ekman pumping of nitrate from below the euphotic zone; this Ekman pumping was particularly effective due to the shallow nitracline in the eddy compared to eddy exterior waters. Both eddy trapping and Ekman pumping helped to isolate and maintain the ecosystem productivity in the eddy core. This study shows the importance of cyclonic eddies for biological production in EBUS: they contribute both to the redistribution of the coastal upwelling ecosystem and are local regions of enhanced new production. Together, these processes impact cross‐shore gradients of important biological properties.

Biogeochemical properties of eddies in the California Current System

“…However, on average the relative proportions of different planktonic groups were the same in both types of eddies: 45% and 55% of small and large phytoplankton, respectively, and 20%, 35%, and 45% of small, large, and predator zooplankton, respectively. Similar proportions were diagnosed in the mature stage of a cyclonic eddy from a numerical simulation of the southern CCS [ Chenillat et al ., ].…”

“…Cyclonic eddies had a clear positive anomaly of both phytoplankton (+0.2 mmol N m −3 ) and zooplankton (+0.1 mmol N m −3 ) over the lifetime of the eddy, although these anomalies were sometimes negative near the coast (−0.1 mmol N m −3 ). These anomalously high coastal nitrate concentrations were carried with the cyclonic eddies, fueling the ecosystem as they propagated across shore [ Chenillat et al ., ]. Anticyclonic eddies, on the other hand, showed mixtures of positive and negative nitrate anomalies (from −2 to 2 mmol N m −3 ) and biological anomalies (from −0.1 to 0.05 mmol N m −3 ), both of which were weaker on average than those found in cyclonic eddies.…”

“…Chenillat et al . [] showed that the enhanced productivity and biomass in a cyclonic eddy was a consequence of the upward doming of the nitracline into the euphotic zone, combined with Ekman pumping of nitrate into the euphotic zone, i.e., a local upwelling driven by the surface wind stress curl. Although the Ekman pumping was not particularly high in the eddies relative to the surrounding waters, the shallow nitracline in the eddy core allowed an increased upward nitrate flux there.…”

“…This enhanced nitrate flux drove increased biomasses of both small and large phytoplankton and zooplankton in the waters between the base of the mixed layer and the bottom of the euphotic zone. The analyses presented here show that the dynamics revealed in the detailed examination of a single cyclonic eddy [ Chenillat et al ., ] can be generalized to all cyclonic eddies in the CCS. The average cyclonic eddy is formed within 200 km of the coast and propagates offshore toward the northwest.…”

“…This coupled model showed good agreement with observations from the California Cooperative Oceanic Fisheries Investigations (CalCOFI) program [ Chenillat et al ., , their Figure 3]; in particular, the Chl a maximum is dominated by large phytoplankton (diatom‐like, requiring silicon) close to shore, and by small phytoplankton (nondiatom‐like, not requiring silicon) offshore; the planktonic assemblage in a cyclonic eddy was dominated by small‐size classes in the surface mixed layer and by large‐size classes below the mixed layer.…”