Selenium speciation in subantarctic and subtropical waters east of New Zealand: trends and temporal variations

Surveys examining selenium speciation in two South Pacific water masses were conducted during austral autumn, winter, spring and summer to investigate the in situ effect of phytoplankton growth on the speciation and recycling of oceanic selenium. The water masses studied, subantarctic surface water (SASW) and subtropical surface water (STW), possess distinct biological signatures. SASW is a high nitrate low chlorophyll (HNLC) water mass, while in STW algal growth exhibits a classical cycle of spring and autumn blooms. The ‘nutrient type’ vertical profiles obtained for selenite and selenate in the study area, and the observed selenium content of phytoplankton (4.4±1.7 μg Se g dw−1), indicate that selenium is biologically utilized by phytoplankton in SASW and STW. Surface selenium concentration in the two water masses (Se(tot)=0.62–0.95 nM; 0–50 m) was similar to previously reported sub-tropical surface ocean concentrations. For the majority of the year, inorganic forms of selenium (selenite and selenate) were dominant over organic forms (typically, Se(org) ∼0.10 nM). In seasons with different primary production rates selenium speciation trends in the HNLC SASW remained relatively constant. In contrast, at the end of the algal growth season in STW, following spring phytoplankton blooms, decreased levels of bio-available inorganic selenium coincided with a decrease in concentration of the nutrient phosphate. Thus, the reduction in inorganic selenium concentration was presumably due to the uptake of inorganic selenium by phytoplankton. A corresponding increase in organic selenium concentration observed in STW is likely due to the regeneration of biogenic particles (derived from algal cells) that contain selenium. This study, representing the first report of significant short-term variations in oceanic selenium speciation related to phytoplankton processes, indicates that seasonality can be important in the interpretation of trends in oceanic selenium speciation.