Plankton community structure south and west of South Georgia (Southern Ocean): Links with production and physical forcing

During late December 2004 and early January 2005 the plankton community to the south and west of South Georgia was investigated. Satellite imagery had shown the surface expression of a bloom over the southern shelf I month prior to the cruise, although by the time of sampling a well-defined sub-surface chl-a maximum was evident at 26 of the 57 stations located mainly at the western end of the southern shelf (and the bloom was declining). Nonetheless, integrated chl-a was still greater over the shelf than elsewhere (18-362 mg m(-2)). Macronutrient distributions essentially mirrored the distribution of chl-a biomass, with depletion greatest in the on-shelf waters at the western end of South Georgia, where the most intense surface bloom had occurred during the preceding November. Nearest neighbour clustering of microplankton and mesozooplankton data revealed the presence of two major station groups within each analysis with broadly congruent distributions. Within the microplankton analysis a southern and western shelf grouping of 18 stations was dominated by Corethron spp., Eucampia antarctica and Thalassiothrix spp. This group corresponded spatially to a shelf zooplankton grouping (12 of the 18 stations in both groups in common) in which mesozooplankton abundance was greatest. Here small copepods such as Oithona spp. and the neritic clausocalaniid Drepanopus forcipatus dominated, along with the thecate pteropod Limacina helicina, appendicularians and calanoid copepod naupliar stages. Acoustic doppler current profiler (ADCP) measurements indicated that water flow over the shelf was low and variable (< 15 cm s(-1)). In contrast the largest station groups in both ordinations were distributed along the southern shelf-break and further off-shelf in water flowing rapidly (up to 55 cm s(-1)) to the southeast. Nitzschia spp., Pseudonitzschia spp., and Fragilariopsis kerguelensis were abundant here, and the zooplankton, in addition to Oithona spp., was characterized by Metridia spp., Ctenocalanus spp., Oncaea spp., and the polychaete Pelagobia longicirrata. A third group of 13 stations disclosed by the mesoplankton ordination was confined to the north and west and generally comprised outer shelf stations in deeper waters. Here zooplankton abundance was less than in the adjacent major station groupings, although Calanus simillimus was considerably more abundant than in other groups. Relationships of both micro- and zooplankton ordinations with environmental variables were modest (Spearman rank correlation, rho w = 0.49-0.59), albeit complex, with interactions likely to have occurred over different timescales. High levels of ammonium over the shelf, probably resulting from microbial breakdown and zooplankton excretion, contributed most to explaining both ordinations, along with the Si(OH)(4):NO3 deficit ratio, a measure of past nutrient use. Model output from Ocean Circulation and Climate Advanced Modelling (OCCAM) supported ADCP-derived flow measurements. Specifically, release of particles along a transect to the southwest suggested there was an extended residence time (in excess of 3 months) over the southern shelf and a slow but significant northwards transport into the Georgia Basin. The spatial extent of the shelf and the current speed and direction implied that in situ production was locally important and had the potential to contribute significantly to downstream ecosystems. (C) 2007 Elsevier Ltd. All rights reserved.