A comparison of satellite and cruise chlorophyll-a measurements in the Scotia Sea, Antarctica

We compared both SeaWiFS (Sea-viewing Wide Field-of-View Sensor) and MODIS (Moderate-resolution Imaging Spectroradiometer) chlorophyll-a (chl-a) measurements with simultaneous ship based data obtained during a 2003 British Antarctic Survey (BAS) research cruise. This cruise provided in situ data from a large area of the Scotia Sea containing areas of extreme contrasts in terms of chl-a concentration. We present the results of correlation analysis between the in situ ship based chl-a measurements and the satellite chl-a products (SeaWiFS, and from MODIS the semi-analytic, SeaWiFS analog OC3M, HPLC empirical algorithms). The results confirm the good correlation between SeaWiFS and in situ chl-a measurements. The results indicate Terra MODIS chl-a measurements show reduced correlation to in situ values when compared to SeaWiFS. In addition, we compared chl-a averages from the various algorithms, over wider geographical regions of greater ecological relevance than point measurements. Over an area of 3° × 3°, SeaWiFS estimates could be as much as 2 times higher than estimates from MODIS

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

Summer microplankton community structure across the Scotia Sea: implications for biological carbon export

During the austral summer of 2008, we carried out a high resolution survey of the microplankton communities along a south to north transect covering a range of environments across the Scotia Sea, Southern Ocean; high and low productivity, sea-ice to open water conditions, and over a number of oceanographic fronts and bathymetric features. Cluster analysis revealed five distinct communities that were geographically constrained by physical features of bathymetry and fronts. From south to north the communities were: (1) the South Orkney group, a mixed community of naked dinoflagellates and heavily silicified diatoms, (2) southern Scotia Sea, a mixed community of cyptophytes and naked dinoflagellates, (3) central Scotia Sea, dominated by naked dinoflagellates, (4) southwest of the island of South Georgia, lightly silicified diatoms and naked dinoflagellates (5) northwest of South Georgia, dominated by diatoms. Data from a previous summer cruise (2003) to the Scotia Sea followed a similar pattern of community distribution. MODIS images, Chlorophyll a and macronutrient deficits revealed dense phytoplankton blooms occurred around the island of South Georgia, were absent near the ice edge and in the central Scotia Sea and were moderate in the southern Scotia Sea. Using these environmental factors, together with community composition, we propose that south of the Southern Antarctic Circumpolar Current Front, biogenic silica is preferentially exported and north of the front, in the vicinity of South Georgia, carbon is exported to depth

Formation, transport and decay of an intense phytoplankton bloom within the High-Nutrient Low-Chlorophyll belt of the Southern Ocean

The blooms associated with South Georgia coincide with the largest predicted carbon sink in the Southern Ocean. A major injection of iron and silica is required to sustain them over a long growth season that exceeds 4 months. The mechanisms for this are still poorly known but a cruise in January 2005 provided an opportunity to sample waters both upstream and downstream of the North Scotia Ridge to examine the processes involved. SeaWiFS imagery gave a misleading impression of bloom progression due to pronounced but highly regionalised subsurface chlorophyll-a (chl-a) maxima. Our combined measurements of oceanography, macronutrients, chl-a and floral composition showed that a single bloom that had seeded over the western shelf of South Georgia two months before the cruise developed into two distinct entities. Water retention and turbulent mixing south of the island led to severe nutrient depletion, low chl-a concentrations in surface waters (and in SeaWiFS images) yet pronounced subsurface maxima. By contrast the bloom over the northern shelf persisted in a shallow mixed layer, was florally different and appeared to be advected downstream into oceanic waters to the north. We found evidence for upwelling of nutrients on the upstream shelf edge of the island where the flow diverges to pass around each side of its shelf. This would provide one mechanism for the injection of iron and silica to sustain these massive South Georgia blooms and seed waters downstream for an extended growing season

Role of krill versus bottom-up factors in controlling phytoplankton biomass in the northern Antarctic waters of South Georgia

The extent to which Antarctic phytoplankton stocks are controlled by 'bottom-up' and/or 'top-down' factors is highly variable. Here we consider data collected at South Georgia during 3 summer surveys that recorded substantial hydrographic variability. A suite of bottom-up and top-down controlling factors were measured simultaneously at the mesoscale. Sea surface temperature varied by >2 degrees C, macronutrients ranged from near-winter concentrations to near-depleted, while mean densities of a major grazer, krill Euphausia superba, varied between near-zero and >400 g wet mass m(-2). A general linear model was used to identify the main factors implicated in the observed differences in phytoplankton biomass. Despite east-to-west and on- to off-shelf temperature gradients, temperature per se was not implicated in phytoplankton variability. Also, while there was an abundance of NO3-N in surface waters, NH4-N was the key nutrient throughout. A domed relationship between phytoplankton and krill peaked between 2 and 4 mg chlorophyll a m(-3) and 6 and 30 g krill m(-2). The positive side of this dome was represented by the west off-shelf region downstream of South Georgia. Here, an ample supply of micro- and macronutrients promoted high primary production, and low densities of krill presumably had little grazing effect, This positive relationship between krill and phytoplankton biomasses was interpreted as krill accumulating in areas of good feeding conditions. The negative side of the dome was typified by the east off-shelf region, where macronutrients remained high, primary production rates were low, and krill densities were very high. The grazing rates calculated here suggested that krill affect their food stocks severely, and the negative krill-phytoplankton relationship in this region may reflect locally high krill densities driving down their food supply

Rapid warming of the ocean around South Georgia, Southern Ocean, during the 20th century: Forcings, characteristics and implications for lower trophic levels

The Southern Ocean is known to have warmed considerably during the second half of the 20th century but there are few locations with data before the 1950s. In addition, assessments of change in this region are hampered by the strong seasonal bias in sampling, with the vast majority of data collected during the austral summer. However, oceanographic measurements near South Georgia span most of the last century, and we here consider almost year-round data from this location over an 81-year period (1925–2006). We observe significant warming between the early and late 20th century, with differential warming between summer and winter months and an indication that late 20th century summer temperatures peaked 6 days earlier. To quantify the longterm warming trend in this highly variable data, a mixed model utilising a Residual Maximum Likelihood (REML) method was used. Over the 81-year period, a mean increase of 0.9 1C in January and 2.3 1C in August was evident in the top 100m of the water column. Warming diminished below 100m and approached 0 at 200m. Thus the longterm warming around South Georgia is substantial—more so than documented previously for the circumpolar warming of the Southern Ocean. We examine potential causal effects of this trend, including local atmospheric and cryospheric change, the influence of upstreamwaters and the role of coupled modes of climate variability such as El Nin˜o/Southern Oscillation and the Southern Annular Mode (SAM). It is likely that all of these play a part in the observed temperature increase. However, the role of the SAM is strongly indicated, via its likely role in the circumpolar warming trend in the Southern Ocean, and also by the atypical response of the South Georgia region to changes in heat fluxes associated with the SAM. Furthermore, the combination of a regional decline in ice extent and strong upstream warming likely explains a significant part of the strong seasonal variation apparent in the warming trend. In addition, we consider the implications that long-term warming has for South Georgia’s lower trophic levels. For Euphausia superba, at their northern limit, we find a significant negative relationship between summer South Georgia water temperatures and mean summer density of E. superba across the southwest Atlantic sector of the Southern Ocean. Simple abundance and growth rate relationships with our long-term temperature data appear to show declining habitat suitability for E. superba. In contrast, the warming trend is likely to favour other macro- and mesozooplankton species that occupy the more northerly parts of the Antarctic Circumpolar Current, and it is likely to promote phytoplankton growth

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