The fate of production in the central Arctic Ocean – top–down regulation by zooplankton expatriates?

Abstract

We estimated primary and bacterial production, mineral nutrients, suspended chlorophyll a (Chl), particulate organic carbon (POC) and nitrogen (PON), abundance of planktonic organisms, mesozooplankton fecal pellet production, and the vertical flux of organic particles of the central Arctic Ocean (Amundsen basin, 89–88° N) during a 3 week quasi-Lagrangian ice drift experiment at the peak of the productive season (August 2001). A visual estimate of ≈15% ice-free surface, plus numerous melt ponds on ice sheets, supported a planktonic particulate primary production of 50–150 mg C m −2 d −1 (mean 93 mg C m −2 d −1, n = 7), mostly confined to the upper 10 m of the nutrient replete water column. The surface mixed layer was separated from the rest of the water column by a strong halocline at 20 m depth. Phototrophic biomass was low, generally 0.03–0.3 mg Chl m −3 in the upper 20 m and <0.02 mg Chl m −3 below, dominated by various flagellates, dinoflagellates and diatoms. Bacterial abundance (typically 3.7–5.3 × 10 5, mean 4.1 × 10 5 cells ml −1 in the upper 20 m and 1.3–3.7 × 10 5, mean 1.9 × 10 5 cells ml −1 below) and Chl concentrations were closely correlated ( r = 0.75). Mineral nutrients (3 μmol NO 3 l −1, 0.45 μmol PO 4 l −1, 4–5 μmol SiO 4 l −1) were probably not limiting the primary production in the upper layer. Suspended POC concentration was ∼30–105 (mean 53) mg C m −3 and PON ∼5.4–14.9 (mean 8.2) mg N m −3 with no clear vertical trend. The vertical flux of POC in the upper 30–100 m water column was ∼37–92 (mean 55) mg C m −2 d −1 without clear decrease with depth, and was quite similar at the six investigated stations. The mesozooplankton biomass (≈2 g DW m −2, mostly in the upper 50 m water column) was dominated by adult females of the large calanoid copepods Calanus hyperboreus and Calanus glacialis (≈1.6 g DW m −2). The grazing of these copepods (estimated via fecal pellet production rates) was ≈15 mg C m −2 d −1, being on the order of 3% and 20% of the expected food-saturated ingestion rates of C. hyperboreus and C. glacialis, respectively. The stage structure of these copepods, dominated by adult females, and their unsatisfied grazing capacity during peak productive period suggest allochthonous origin of these species from productive shelf areas, supported by their long life span and the prevailing surface currents in the Arctic Ocean. We propose that the grazing capacity of the expatriated mesozooplankton population would match the potential seasonal increase of primary production in the future decreased ice perspective, diminishing the likelihood of algal blooms.