Impacts of Mesoscale Eddies on Nutrient Supply to the Upper Ocean
What are the mechanisms by which nutrients are supplied to the surface layers of the open ocean? These processes constitute a fundamental constraint on the productivity of marine ecosystems, and set an upper limit on the so-called “biological pump” in the ocean’s carbon cycle. Geochemical tracers indicate rates of net community production in the oligotrophic gyres that far surpass that which can be sustained by physically-driven nutrient inputs from wintertime convective mixing and turbulent diffusion. What process(es) supply the rest of the nutrients? Although the answer may be related to biological processes such as nitrogen fixation, other physical mechanisms may be at work.
The notion that mesoscale flows (sometimes referred to as the internal weather of the sea) could result in localized episodic sources of nutrients to the upper ocean dates back to the mid-1980s. In my thesis research, I began to investigate these processes in the context of the JGOFS North Atlantic Bloom Experiment (McGillicuddy et al., 1995a,b). However, it was not until a few years later that I identified a mechanism by which the dynamics of mesoscale eddies could bring about a net upward transport of nutrients when averaged over a suitably large area of the ocean (McGillicuddy and Robinson, 1997a; McGillicuddy et al., 1998a). The eddy upwelling mechanism can be conceptualized by considering a density surface with mean depth coincident with the base of the euphotic zone. This surface is perturbed vertically by the formation, evolution, and destruction of mesoscale features. Shoaling density surfaces lift nutrients into the euphotic zone and are rapidly utilized by the biota. Deepening density surfaces push nutrient-depleted water out of the well-illuminated surface layers, and therefore do not produce a net ecosystem response. The asymmetry imposed by the light field thus rectifies vertical displacements of both directions into a net upward transport of nutrients, which is presumably balanced by a commensurate export flux of either sinking particulates or dissolved organic material. I have been involved in three independent estimates of the space-time integrated flux associated with this mechanism: (1) high-resolution regional numerical simulations (McGillicuddy and Robinson, 1997a), (2) a statistical model driven by both satellite and in situ observations (Siegel et al., 1999), and (3) a basin-scale eddy-resolving model (McGillicuddy et al., in press). All three studies suggest that eddy-induced transport is sufficient to balance the nutrient demand implied by geochemical estimates of net community production.
Analysis of observations capable of resolving mesoscale fluctuations reveals that they are replete with this phenomenology. High-resolution time series from the Bermuda Testbed Mooring program (Tommy Dickey, lead PI) documented a nutrient pulse and increase in chlorophyll and particulate material associated with the passage of an eddy (McNeil et al., 1999). Mesoscale biogeochemical surveys carried out as part of the Bermuda Atlantic Time-series Study (BATS) validation activities demonstrated the impact of mid-ocean eddies on nutrient and biomass distributions (McGillicuddy et al., 1999 ). These observations showed that eddy-induced upward displacement of density surfaces can inject nutrients into the euphotic zone, causing accumulation of phytoplankton biomass in the overlying waters. Similar patterns emerge from an analysis of satellite-derived sea-surface temperature and pigment fields (McGillicuddy et al., 2001a). These data reveal that higher (lower) pigment biomass occurs in mesoscale features consisting of cold (warm) temperature anomalies. In addition, both the autocorrelation scales as well as the magnitude of the covariation are consistent with the process of eddy-induced upwelling. Finally, retrospective analysis of time-series data from BATS using a data assimilative regional hindcast model (McGillicuddy and Kosnyrev, 2001 ) indicates connections between eddy activity, changes in phytoplankton species composition, and export from the euphotic zone (Sweeney et al., 2003 ). Related links
Modeling Mesoscale Biogeochemical Processes in the Topex/Poseidon Diamond Surrounding the U.S. JGOFS Bermuda Atlantic Time-series Study (BATS) The Role of Eddies in Basin-scale Biogeochemical Budgets of the North Atlantic Eddy Dynamics, Mixing, Export, and Species Composition (EDDIES) Transatlantic VPR cruise - August - September 2003 |
