Magnitude and maintenance of the phytoplankton bloom at South Georgia: a naturally iron-replete environment

We investigated phytoplankton blooms around the island of South Georgia, in the South Atlantic sector of the Southern Ocean, during 3 austral summer cruises. Blooms developed largely to the northwest, downstream of the island, in the Georgia Basin. Drifter buoys approached the island from the southwest and diverged in the region of ca. 55 degrees S, 40 degrees W, to pass either to the west or east of the island and into the main bloom area of the Georgia Basin. The divergence zone indicated a likely site of upwelling of nutrient-rich deepwater, whilst the eastward flowing drifters indicated nutrient enrichment through shelf sediment interactions along the Southern and eastern shelf. South Georgia's summer phytoplankton blooms were characteristic of those found in Fe-replete environments. Water column standing stocks of chl a and primary production rates were 3 times higher at stations situated within the main bloom (in-stations) compared to those outside of the bloom (out-stations). NO3:PO4 depletion ratios were significantly higher and Si(OH)(4):NO3 depletion ratios lower at in-stations compared to out-stations and were in the range expected under Fe-replete conditions. High photochemical quantum efficiency (F-v/F-m,) and low functional absorption cross-section (alpha(psm)) values, measured during our January 2005 cruise, further supported the view that in-stations were Fe-rich. However, on all cruises, both in- and out-stations were strongly dominated by the largest chl a size-fraction (microphytoplankton), and diatoms accounted for >63%, of the total cell count. Reduced availability of Fe at out-stations may have prevented very large species of diatoms from blooming there, but did allow a modest accumulation of smaller diatoms. Simultaneous limitation of Fe with silicic acid or temperature may also account for the species composition and reduced productivity observed at some out-stations. Conversely, a steady supply of Fe and macronutrients, together with shallow mixed layers and slightly elevated temperatures, could account for the blooms of giant diatoms observed at in-stations.