Physiological state of phytoplankton communities in the Southwest Atlantic sector of the Southern Ocean, as measured by fast repetition rate fluorometry

Abstract

The majority of the Southern Ocean is a highnutrient low- chlorophyll (HNLC) ecosystem. Localized increases in chlorophyll concentration measured in the wake of bathymetric features near South Georgia demonstrate variations in the factors governing the HNLC condition. We explore the possibility that the contrast between these areas of high-chlorophyll and surrounding HNLC areas is associated with variations in phytoplankton photophysiology. Total dissolvable iron concentrations, phytoplankton photophysiology and community structure were investigated in late April 2003 on a transect along the North Scotia Ridge (53–54S) between the Falkland Islands and South Georgia (58–33W). Total dissolvable iron concentrations suggested a benthic source of iron near South Georgia. Bulk community measurements of dark-adapted photochemical quantum efficiency (Fv/Fm) exhibited a sharp increase to the east of 46W coincident with a decrease in the functional absorption cross-section (rPSII). Phytoplankton populations east of 46W thus displayed no physiological symptoms of iron or nitrate stress. Contrasting low Fv/Fm west of 46W could not be explained by variations in the macronutrients nitrate and silicic acid and may be the result of taxon specific variability in photophysiology or iron stress. We hypothesize that increased Fv/Fm resulted from local relief from ironstress near South Georgia, east of Aurora Bank, an area previously speculated to be a ‘‘pulse point’’ source of iron. Our measurements provide one of the first direct physiological confirmations that iron stress is alleviated in phytoplankton populations near South Georgia.