Abstract
Mesoscale eddies in the south west Pacific region are prominent ocean features that represent distinctive environments for phytoplankton. Here we examine the seasonal plankton dynamics associated with averaged cyclonic and anticyclonic eddies (CE and ACE, respectively) off eastern Australia. We do this through building seasonal climatologies of mixed layer depth and surface chlorophyll-a for both CE and ACE by combining remotely sensed sea surface height (TOPEX/Poseidon, Envisat, Jason-1, and OSTM/Jason-2), remotely sensed ocean colour (GlobColour) and in situ profiles of temperature, salinity and pressure from Argo floats. Using the CE and ACE seasonal climatologies, we assimilate the surface chlorophyll-a data into both a single (WOMBAT), and multi-phytoplankton class (EMS) biogeochemical model to investigate the level of complexity required to simulate the phytoplankton chlorophyll-a. For the two eddy types, the data assimilation showed both biogeochemical models only needed one set of parameters to represent phytoplankton but needed different parameters for zooplankton. To assess the simulated phytoplankton behavior we compared EMS model simulations with a ship-based experiment that involved incubating a winter phytoplankton community sampled from below the mixed layer under ambient and two higher light intensities with and without nutrient enrichment. By the end of the 5-day field experiment, large diatom abundance was four times greater in all treatments compared to the initial community, with a corresponding decline in pico-cyanobacteria. The experimental results were consistent with the simulated behavior in CE and ACE, where the seasonal deepening of the mixed layer during winter produced a rapid increase in large phytoplankton. Our model simulations suggest that CE off East Australia are not only characterized by a higher chlorophyll-a concentration compared to ACE, but also by a higher concentration of large phytoplankton (i.e. diatoms) due to the shallower CE mixed layer. The model simulations also suggest the zooplankton community is different in the two eddy types and this behavior needs further investigation.
Highlights
Mesoscale eddies play crucial roles in ocean circulation and dynamics, stimulating phytoplankton growth and enhancing the global primary production by ∼20% (Falkowski et al, 1991; McWilliams, 2008)
The mixed layer depth (MLD) of anticyclonic eddies (ACE) is generally deeper than cyclonic eddies (CE) (Dufois et al, 2014) and this changes the supply of nutrients to the euphotic zone when the eddy undergoes seasonal deepening of the MLD with additional impacts to light levels (Dufois et al, 2014; McGillicuddy, 2015)
We firstly explore the level of phytoplankton complexity required to estimate the phytoplankton chlorophyll-a (Chl-a) for eddies in the East Australian system, using a single (WOMBAT) and a multi-phytoplankton class model (EMS)
Summary
Mesoscale eddies play crucial roles in ocean circulation and dynamics, stimulating phytoplankton growth and enhancing the global primary production by ∼20% (Falkowski et al, 1991; McWilliams, 2008). The shoaling of the nutricline helps supply nutrient-rich waters to the euphotic zone when mixed layer depth (MLD) undergoes seasonal deepening (Dufois et al, 2014; McGillicuddy, 2015) and, thereby stimulating phytoplankton growth (Jenkins, 1988; Falkowski et al, 1991; McGillicuddy and Robinson, 1997). Anticyclonic eddies (ACE) are associated with high sea level anomalies, depression of the isopycnals and deepening of the nutricline (McGillicuddy, 2015). One example is the formation of eddies off Western Australian where the Leeuwin Current generates ACE initialized with high Chl-a derived from the coastal water (Moore et al, 2007)
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