Abstract
We examined the biogeochemical impact of pairs of mesoscale cyclones and
anticyclones in spatial proximity (<200 km apart) in the North
Pacific Subtropical Gyre. While previous studies have demonstrated that
upwelling associated with the intensification of cyclonic eddies
supplies nutrients to the euphotic zone, we find that cyclonic eddies in
their mature stage sustain plankton growth by increasing the diapycnal
flux of nutrients to the lower portion of the euphotic zone. This
increased supply results from enhanced vertical gradients in inorganic
nutrients due to erosion of the nutricline that accompanied plankton
growth during eddy intensification. From a biological standpoint,
increased nutrient flux was linked with expansion of eukaryotic
phytoplankton biomass and intensification of the deep chlorophyll
maximum layer. This perturbation in the plankton community was
associated with increased fluxes of biominerals (opal and calcium
carbonate) and isotopically enriched nitrogen in particles exported in
the cyclone. The time-integrated effects of thermocline uplifts and
depressions were predictable deficits and surpluses of inorganic
nutrients and dissolved oxygen in the lower euphotic zone. However, the
stoichiometry of changes in oxygen and inorganic nutrients differed from
that predicted for production and consumption of phytoplankton biomass,
consistent with additional biological processes that decouple changes in
oxygen and nutrient concentrations. The dynamics revealed by this study
may be a common feature of oligotrophic ecosystems, where mesoscale
biogeochemical perturbations are buffered by the deep chlorophyll
maximum layer, which limits the ecological impact of eddies in the
well-lit, near-surface ocean.