Timing of nutrient depletion, diatom dominance and a lower-boundary estimate of export production for Irminger Basin, North Atlantic

During the North Atlantic spring bloom, a seasonal phytoplankton community succession takes place from diatoms to non-siliceous phytoplankton. Diatoms rely on silica to form their frustules and are out-competed by other species when silica becomes depleted. Diatoms are also expected to contribute significantly to export production in the North Atlantic. We suggest that a lower boundary to export production can be estimated as the component of total production that occurs between the start of the spring bloom and the time when silica becomes depleted.This method has been tested in the Irminger Basin, located between Greenland and Iceland, in the North Atlantic. A technique to estimate silica concentration from satellite-derived sea surface temperature and chlorophyll a concentration has been developed and used to determine silica concentration at high spatial and temporal resolution. This facilitates an estimation of the timing of silica depletion and thus the timing of the transition from a phytoplankton community dominated by diatoms to dominance by non-siliceous species. The timing of the initiation of the bloom, defined as a pronounced and sustained increase in biomass, is estimated from a Sea-viewing Wide Field-of-View Sensor (SeaWiFS)-derived chlorophyll a concentration. A lower-boundary estimate of export production is made and, additionally, estimates of the contribution to export production by diatoms and non-diatoms are made by considering silica-to-nitrate drawdown ratios. We estimate export production in this region to be ~60 gC m–2 yr–1, of which diatoms account for ~65%

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

The majority of the Southern Ocean is a high-nutrient 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 iron stress 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