Plankton metabolism in the Greenland Sea during the polar summer of 2007

Continuous daylight in the high-Arctic summer supports high plankton respiration rates compared to those supported in the dark

Plankton respiration rate is a major component of global CO₂ production and is forecasted to increase rapidly in the Arctic with warming. Yet, existing assessments in the Arctic evaluated plankton respiration in the dark. Evidence that plankton respiration may be stimulated in the light is particularly relevant for the high Arctic where plankton communities experience continuous daylight in spring and summer. Here we demonstrate that plankton community respiration evaluated under the continuous daylight conditions present in situ, tends to be higher than that evaluated in the dark. The ratio between community respiration measured in the light (R_light) and in the dark (R_dark) increased as the 2/3 power of R_light so that the R_light:R_dark ratio increased from an average value of 1.37 at the median R_light measured here (3.62 µmol O₂ L^-1 d^-1) to an average value of 17.56 at the highest R_light measured here (15.8 µmol O₂ L^-1 d^-1). The role of respiratory processes as a source of CO₂ in the Arctic has, therefore, been underestimated and is far more important than previously believed, particularly in the late spring, with 24 h photoperiods, when community respiration rates are highest.

The "degradative" and "biological" pumps controls on the atmospheric deposition and sequestration of hexachlorocyclohexanes and hexachlorobenzene in the North Atlantic and Arctic Oceans

The cycling of hexachlorobenzene (HCB) and hexachlorocyclohexanes (HCHs) has been studied in the North Atlantic and Arctic Ocean. Concentrations of HCHs and HCB were measured simultaneously in the atmosphere (gas and aerosol phases), seawater (dissolved and particulate phases), and phytoplankton. The atmospheric concentrations of HCHs decrease during transport over the Greenland Current with estimated e-folding times of 1.6 days, a trend not observed for HCB. This strong decrease in atmospheric concentrations of HCH is consistent with the estimated atmospheric depositional fluxes driven by the air-water disequilibrium. The removal of HCHs from the surface ocean by the degradative pump due to hydrolysis and microbial degradation and by the biological pump due to settling of particle-associated HCHs are estimated; the removal fluxes are within a factor of 2 of the atmospheric inputs for most sampling events, suggesting an important role of the degradative pump in the overall oceanic sink of HCHs. Conversely, the lack of degradation of HCB in surface waters and its relatively low hydrophobicity imply a lack of effective removal processes, consistent with the observed air and water concentrations close to equilibrium. This work is the first that estimates the relative importance of the biological and degradative pumps on the atmospheric deposition of the less persistent organic pollutants and points out the need for further research for quantifying the magnitude of degradative processes in the environment.