Mick Whitehouse

Seasonal and annual change in seawater temperature, salinity, nutrient and chlorophyll a distributions around South Georgia, South Atlantic

Data collected between 1926 and 1990, during the Discovery Investigations and fourteen subsequent cruises, have allowed the description of spatial and temporal variability of temperature, salinity, phosphate, silicate, nitrate and chlorophyll a in the surface waters around the subantarctic island of South Georgia. Measurements made in Antarctic Circumpolar Current water were compared with others made in Weddell Sea water, and profiles from shelf, shelf-slope and oceanic sites were considered separately. In summer, Weddell Sea surface water was significantly colder than that of the Antarctic Circumpolar Current (1.66 and 2.59°C, respectively), but no changes of temperature corresponded with bathymetry. There were no systematic differences between the salinity measurements made in Weddell Sea surface water and those in Antarctic Circumpolar Current water; however, oceanic waters were always more saline than those over the shelf-slope and shelf (33.91, 33.89 and 33.86, respectively). Silicate levels correlated well with seawater temperature, and Weddell Sea surface water concentrations were substantially higher than those of the Antarctic Circumpolar Current (51 and 25 mmol m−3, respectively, in winter; 29 and 13 mmol m−3 in summer). No such differences were found for phosphate or nitrate, and no systematic differences in any of the nutrient levels were attributable to bathymetry. Although summer chlorophyll a levels appeared to be highest in Antarctic Circumpolar Current water over the shelf and shelf-slope (2.7–3.0 mg m−3), no significant differences were attributable to water-mass or bathymetry. A clear seasonal pattern was evident, with the warmest seawater conditions, minimum nutrient concentrations and highest chlorophyll a levels found between December and March. Phosphate and nitrate were never exhausted: the lowest recorded phosphate concentrations were around 0.6 mmol m−3 and for nitrate 11 mmol m−3. However, low concentrations of silicate (∼ 1.0 mmol m−3) were evident during some summer surveys from the 1920s through to the present day. Average nutrient deficits calculated either between winter and summer mixed-layer concentrations, or between summer mixed-layer and Tmin values, produced similar estimates of carbon fixation for both phosphate and silicate, while nitrate appeared to underestimate carbon production. Phosphate and silicate deficits were considered to be satisfactory predictors of carbon production, which was about 30–40 g C m−2 year−1 in a mixed-layer depth of 50 m. Considerable interannual variability was found, with winter-like conditions prevailing until January on some occasions, and apparent year-to-year variability in the timing and magnitude of nutrient utilisation (especially silicate) by phytoplankton. A relationship was found between sea surface conditions around South Georgia in summer and the preceding winters fast-ice duration at the South Orkney Islands, which implied that some of this variability was attributable to large-scale change over the Scotia Sea as a whole, as opposed to local influences.

An anticyclonic circulation above the Northwest Georgia Rise, Southern Ocean

Data from a variety of sources reveal a warm-core anticyclonic circulation above the Northwest Georgia Rise (NWGR), an similar to2000-m high bathymetric feature north of South Georgia. The sense of the circulation is opposite to the general cyclonic flow in the Georgia Basin. The circulation shows the characteristics of a stratified Taylor column: dimensional analysis shows that the local bathymetry and hydrography are conducive to the formation of such. ERS2 altimeter data show that the column, whilst not fully permanent, is nonetheless a recurring feature. High concentrations of chlorophyll-a are observed at the centre of the circulation, indicating that the modulation of the physical environment has significant consequences for the local biogeochemical system via enhanced primary production. Enhanced chlorophyll-a extends in a long plume from the NWGR along pathways indicated by drifters; this passive redistribution may have consequences for the larger (basin-) scale ecosystem.

Variations in phytoplanktonic nitrogen assimilation around South Georgia and in the Bransfield Strait (Southern Ocean)

Abstract Nitrogen assimilation was measured in two austral summers in the Scotia Sea around the island of South Georgia as well as the Bransfield Strait. Nitrate and ammonium assimilation was measured using 15 N techniques and the population was divided into two size classes, less than and greater than 20 μ m. Water column integrated nitrogen assimilation rates varied between 2.43 and 26.50 mmol N m −2 day −1 , the distribution being highly heterogeneous. The highest assimilation rate was found at a station near South Georgia, where the chlorophyll standing stock was elevated. A high assimilation rate was observed at a station in the Bransfield Strait and was associated with a localised, shallow mixed-layer feature. The less than 20 μ m size fraction contributed to a variable but frequently significant proportion of the total assimilation (14–78%). f ratios were generally low, signifying a high dependence of the population on ammonium as a nitrogen source. Small phytoplankton exhibited a statistically significant greater preference for ammonium than large species, and the total community f ratio was influenced strongly by the proportion of the less than 20 μ m fraction and ammonium concentration. It is suggested that nutrients may play a more important role in the ecology of the phytoplankton in the Southern Ocean than is usually supposed.

The Southern Antarctic Circumpolar Current Front: physical and biological coupling at South Georgia

The coupling of physics and biology was examined along a 160 km long transect running out from the north coast of South Georgia Island and crossing the Southern Antarctic Circumpolar Current Front (SACCF) during late December 2000. Surface and near surface potential TS properties indicated the presence of three water types: a near-shore group of stations characterised by water which became progressively warmer and fresher closer to South Georgia, an offshore grouping in which sea surface temperatures and those at the winter water level were relatively warm (1.81C and 0.51C, respectively), and a third in which surface and winter water temperatures were cooler and reflected the presence of the SACCF. The transect bisected the SACCF twice, revealing that it was flowing in opposite directions, north-westward closest to South Georgia and south-eastwards at its furthest point from the island. The innermost limb was a narrow intense feature located just off the shelf break in 2000–3500 m of water and in which rapid surface baroclinic velocities (up to 35 cm s � 1 ) were encountered. Offshore in the outermost limb, shown subsequently to be a mesoscale eddy that had meandered south from the retroflected limb of the SACCF, flow was broader and slower with peak velocities around 20 cm s � 1 . Chlorophyll a biomass was generally low (o 1m g m � 3 ) over much of the transect but increased dramatically in the region of the innermost limb of the SACCF, where a deepening of the surface mixed layer was coincident with a subsurface chlorophyll maximum (7.4 mg m � 3 ) and elevated concentrations down to 100 m. The bloom was coincident with depleted nutrient concentrations, particularly silicate, nitrate and phosphate, and although ammonium concentrations were locally depleted the bloom lay within an elevated band (up to 1.5 mmol m � 3 ) associated with the frontaljet. Increased zoopl ankton abundance, higher copepod body carbon mass and egg production rates all showed a strong spatial integrity with the front. The population structure of the copepods Calanoides acutus and Rhincalanus gigas at stations within the front suggested that rather than simply resulting from entrainment and concentration within the jet, increased copepod abundance was the result of development in situ. Estimates of bloom duration, based on silicate and carbon budget calculations, set the likely duration between 82 and 122 d, a figure supported by the development schedule of the two copepod species. Given this timescale, model outputs from FRAM and OCCAM indicated that particles that occurred on the north

Estimates of Southern Ocean primary production : constraints from predator carbon demand and nutrient drawdown

In view of the wide range of estimates for the total primary production for the Southern Ocean south of the Subantarctic Front—current estimates range from 1.2 to 3.5 Gtonne C year−1—we have examined two indirect methods for assessing primary production. First, we have estimated the primary production needed to sustain the carbon requirements of the endotherm top predators in the ecosystem. Estimation of the carbon requirements for crabeater seals of about 7 Mtonne C year−1 is extrapolated to a value for all endotherm predators of 15–30 Mtonne C year−1. Current data indicate that 70–80% of the diet of this suite of predators is zooplankton (predominantly the euphausiid krill), making for highly efficient transfer from primary production to top predators. Our best estimate of Southern Ocean primary production by this method is of the order of 1.7 Gtonne C year−1, or an averaged areal primary production of about 30–40 g C m−2 year−1. Our second approach is to estimate primary production from the drawdown of inorganic nutrients, based on the limited suite of studies from which an annual nutrient deficit can be calculated. Again, this indicates annual primary production of the order of 1.5 Gtonne. Although both methods have inherent uncertainties, taken together they provide a relatively robust constraint on annual primary production. For both methods to underestimate primary production by the 1–1.5 Gtonne C implied by the higher current estimates, carbon export from the Southern Ocean pelagic ecosystem would need to be much higher than is normally found in other oceans.

Variations in the distribution of chlorophyll a and inorganic nutrients around South Georgia, South Atlantic

Data collected on four large-scale surveys around the subantarctic island of South Georgia provide information on the variability in the distribution of chlorophyll and inorganic nutrients during the austral summer and winter. During three summer surveys, surface water cholorophyll and nutrient concentrations were highly patchy over scales ranging from a few to hundreds of kilometres. The highest measurement of chlorophyll a was 8 mg m−3 and a wide range of nutrient concentrations were found; 5–32 mmol m−3 NO3−N, 1.1–2.2 mmol m−3 PO4−P and 8–60 mmol m−3 Si(OH)4−Si. In winter, chlorophyll and nutrient levels were far more uniform, with chlorophyll concentrations lower and nutrient concentrations generally higher than in summer. The spatial variability in nutrient concentrations was due to a variety of factors acting over a range of scales, however biological processes appeared most important in creating the mesoscale patchiness around the island. Although phytoplankton abundance and nutrient concentrations were not directly correlated, the scales of variability were clearly similar.