|Title||The biological pump in the Costa Rica Dome: an open-ocean upwelling system with high new production and low export|
|Publication Type||Journal Article|
|Year of Publication||2016|
|Authors||Stukel M.R, Benitez-Nelson C.R, Decima M, Taylor A.G, Buchwald C., Landry MR|
|Journal||Journal of Plankton Research|
|Type of Article||Article|
|Keywords||biogeochemistry; carbon flux; eastern tropical pacific; equatorial; euphotic zone; northeast pacific; nutrients; pacific; particulate organic-carbon; Plankton; sargasso sea; sediment traps; small-volume; time-series; water column|
The Costa Rica Dome is a picophytoplankton-dominated, open-ocean upwelling system in the Eastern Tropical Pacific that overlies the ocean's largest oxygen minimum zone. To investigate the efficiency of the biological pump in this unique area, we used shallow (90-150 m) drifting sediment traps and Th-234: U-238 deficiency measurements to determine export fluxes of carbon, nitrogen and phosphorus in sinking particles. Simultaneous measurements of nitrate uptake and shallow water nitrification allowed us to assess the equilibrium balance of new and export production over a monthly timescale. While f-ratios (new: total production) were reasonably high (0.36+/-0.12, mean+/-standard deviation), export efficiencies were considerably lower. Sediment traps suggested e-ratios (export/C-14-primary production) at 90-100 m ranging from 0.053 to 0.067. ThE-ratios (Th-234 disequilibrium-derived export) ranged from 0.038 to 0.088. C: N and N: P stoichiometries of sinking material were both greater than canonical (Redfield) ratios or measured C: N of suspended particulates, and they increased with depth, suggesting that both nitrogen and phosphorus were preferentially remineralized from sinking particles. Our results are consistent with an ecosystem in which mesozooplankton play a major role in energy transfer to higher trophic levels but are relatively inefficient in mediating vertical carbon flux to depth, leading to an imbalance between new production and sinking flux.
Our results suggest that the CRD is a region with high nitrate uptake rates, but low vertical fluxes of carbon, nitrogen and phosphorus. Nitrate uptake in the region seems to be driven more by the activities of nano- and micro-phytoplankton than by the abundant picophytoplankton in the region, based on correlations between nitrate uptake and the production of each size class. This conclusion agrees with independent results from our cruise of Krause et al. (2016) who found that diatoms, despite low biomass, were disproportionately responsible for nitrate uptake and carbon export, and Landry et al. (2016b), who found that larger phytoplankton had a disproportionately important role in production compared with their biomass contribution.