Seasonality of phytoplankton growth limitation by iron and manganese in
subantarctic waters
Abstract
Phytoplankton indirectly influence the climate, through their role in
the ocean biological carbon pump. Hence factors limiting phytoplankton
growth directly impact the strength of the biological carbon pump and
consequently climate. In the Southern Ocean, the subantarctic zone
represents an important carbon sink, yet variables limiting
phytoplankton growth are not fully constrained. Co-limitation by iron
(Fe) and manganese (Mn) has recently been observed in the coastal and
offshore Southern Ocean, but very few studies have focused on the
subantarctic zone. In addition, no study has investigated the seasonal
variability of Mn (co-)limitation of phytoplankton growth in the
Southern Ocean. Using three shipboard bioassay experiments, we evaluated
the seasonality of Fe and Mn co-limitation of subantarctic phytoplankton
growth, south of Tasmania. We observed a strong seasonal variation in
phytoplankton Fe limitation, and that the response of phytoplankton to
Mn was subtle and thus readily masked by the responses to Fe. Combined
addition of Fe and Mn enhanced carbon uptake of nanoeukaryotes in spring
and microeukaryotes in summer while the addition of Mn alone stimulated
the growth of picocyanobacteria in autumn. These results suggest the
importance of Mn may vary seasonally and its control on phytoplankton
growth may be associated with specific taxa.