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

Abstract

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°S, 40°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). NO 3 :PO 4 depletion ratios were significantly higher and Si(OH) 4 :NO 3 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 (σ PSII ) 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 sizefraction (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. KEY WORDS: South Georgia · Phytoplankton blooms · Drifter tracks · Iron · Silicic acid · Diatoms Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 368: [75][76][77][78][79][80][81][82][83][84][85][86][87][88][89][90][91] 2008 enrichment is in the wake of the island of South Georgia, in the northern Scotia Sea (Fig. 1a), where large and intense phytoplankton blooms regularly occur and may last for 4 to 6 mo (Atkinson et al. 2001, Korb & Whitehouse 2004, Korb et al. 2004, Whitehouse et al. 2008. South Georgia's blooms are typically found extending from the island's southwestern shelf downstream into the generally cyclonic circulation of the Georgia Basin (Fig. 1b) and can cover an area of ca. 53 000 km 2 .Assuming an average integrated chl a concentration of 104 mg m -2 (using values from the present study), a carbon:chlorophyll ratio of between 75:1 and 150:1 (Larsson 2004), the standing sto...

Strong contribution of diatom resting spores to deep-sea carbon transfer in naturally iron-fertilized waters downstream of South Georgia

“…At P3, the mean summer mixed layer diatom assemblages (Korb et al, 2008) are dominated by F. kerguelensis (30%), followed by Pseudo-nitzschia (20%) and C. Hyalochaete vegetative stage (10%, Fig. 5).…”

“…To describe the change in diatom assemblage with depth, we compared the annual sediment trap record from P3 with mixed layer diatom assemblage from multiple summer surveys (February 2002, 2003and January 2005 at the P3 trap location (Korb et al, 2008) and surface sediment diatom assemblage from a core located very close to the trap (42°09.2′S-41°10.7′W, 3760 m, Allen et al, 2005).…”

“…The productive waters downstream of South Georgia support a very large biomass of mesozooplankton , macrozooplankton (Atkinson et al, , 2009, mesopelagic fish (Collins et al, 2012) and top predators (Boyd et al, 1994), with significant implications for the amount of POC export out of the mixed layer (Huntley et al, 1991). Upstream of South Georgia, the lack of iron limits phytoplankton growth and phytoplankton biomass is one order of magnitude lower than that is observed downstream of the island (Korb et al, 2008). Although the contribution of naked dinoflagellates to microplankton biomass is higher than in naturally-fertilized waters, the diatom species Fragilariospsis kerguelensis, Pseudo-nitzschia spp., small Thalassionema nitzschioides and Chaetoceros Hyalochaete species still dominate the microplankton assemblage .…”

“…The bloom is dominated by microplankton diatoms (Korb and Whitehouse, 2004) with a major contribution of Fragilariospsis kerguelensis, Chaetoceros Hyalochaete, Pseudo-nitzschia spp., Eucampia antarctica var. antarctica and Corethron (Korb et al, 2008. The productive waters downstream of South Georgia support a very large biomass of mesozooplankton , macrozooplankton (Atkinson et al, , 2009, mesopelagic fish (Collins et al, 2012) and top predators (Boyd et al, 1994), with significant implications for the amount of POC export out of the mixed layer (Huntley et al, 1991).…”

Planktic foraminifer and coccolith contribution to carbonate export fluxes over the central Kerguelen Plateau

“…Downstream of Subantarctic island plateaus, iron input from shelf sediments and glacial melt water can alleviate iron limitation and support large scale and long-lasting phytoplankton blooms (Blain et al, 2001Tarling et al, 2012). These blooms are dominated by diatoms Korb et al, 2008;Quéguiner, 2013) that respond to high macronutrient concentrations, marked turbulence, deep mixed layer depths and usually moderate light levels (Smetacek, 1985;Boyd, 2002;Strzepek et al, 2012). Diatom blooms result in a major contribution of biogenic silica to biomineral production of Southern Ocean waters, although biogenic production of calcium carbonate by calcifying planktonic organisms such as coccolithophores, foraminifers and pteropods can also occur.…”