Surface chlorophyll concentration as a mesoplankton biomass assessment tool in the Southern Ocean region

AIM: Mesoplankton of the Southern Ocean is the key trophic link between phytoplankton and other consumers and an important part of the biological carbon pump, a critical component of climate regulation. Although general patterns of mesoplankton distribution and life cycles are known, we still cannot estimate mesoplankton standing stocks at specified ocean‐basin scales. LOCATION: Southern Ocean, 34.4−56.9°S, 0–14.4°E. TIME PERIOD: December 2009. MAJOR TAXA STUDIED: All mesoplankton 0.2–20.0 mm. METHODS: We assessed biomass of dominant taxa and total wet mesoplankton biomass (B, mg/m³) using surface chlorophyll a concentration (Chl) as a proxy of phytoplankton biomass and surface temperature (ST), a basic hydrological parameter. We sampled three strata within the layer 0–300 m at 43 stations with a Judey net (0.1 m², 180 µm). RESULTS: We identified 163 taxa and calculated their biomass, biomass of major trophic groups, and total mesoplankton biomass. Monthly Chl and ST derived from satellite imaging were averaged at various temporal and spatial scales. B in the upper layer and in the integrated layer 0–300 m was significantly linked to antecedent Chl signals; the most significant regressions were found between B and monthly Chl and ST averaged two months prior to the survey and 10° upstream of the Antarctic Circumpolar Current. Larger mesoplankton fractions showed longer response periods to Chl signal than smaller fractions. MAIN CONCLUSIONS: Our results suggest the integral mesoplankton stock is 2.47–3.69 Gt wet weight wₜₒₜ (ww), or 0.12–0.18 Gt C, in the whole Southern Ocean. Mesoplankton was dominated by omnivores (69%–74%) followed by carnivores (25%) and herbivores (20%), which suggests the leading role of omnivory in the Southern Ocean pelagic during the austral summer. We anticipate that these results will help to provide a deeper insight into the Southern Ocean ecosystems and will benefit models predicting changes in the carbon–climate Earth system.