Description: Monthly averaged satellite imagery of chlorophyll a in the south-western sector of the Atlantic Ocean shows intense and recurrent blooms around the Antarctic Peninsula, the Island of South Georgia and the South Sandwich Islands. Analysis of a 10 year long time series of SeaWiFS ocean color images allows studying seasonal trends and inter-annual variability. Although specific areas show a regular pattern of low or high productivity, certain years appear to be strikingly productive or well below the climatological average. Circulation patterns, continental shelves as well as Antarctic Circumpolar Front are shown to be the major factors controlling extension of highly productive patches

Description: Data coverage in the Southern Ocean is a limiting factor for a full understanding of physical and biological processes: still little is known about their seasonality as well as their regional distribution. In these conditions remote sensing becomes a key tool, as high resolution is possible both in space and time.Focus of this study is the Scotia Sea, included between the Drake Passage and the South Sandwich Islands, the North and South Scotia Ridge. Satellite based measurements of chlorophyll-a show an enhanced biological productivity in the vicinity of the island ecosystems, which is then redistributed by complex circulation patterns. As for other High Nutrient Low Chlorophyll regions, the iron deficiency hypothesis is a well accepted explanation for these confined and intense spring and summer phytoplankton blooms. Several studies suggest that melting of sea-ice, ocean circulation patterns, grazing and the presence of confined land (the so called island mass effect), can regulate their occurrence and extent, although the relative importance of these factors is difficult to ascertain. Here, we address interannual and spatial variability of phytoplankton blooms in the Scotia Sea region; a combination of different datasets is used to describe and study their dynamics. Aqua-MODIS measurements of surface chlorophyll-a concentration and sea surface temperature (SST) were obtained from the NASA Goddard Distributed Active Archive Center. Analysis of regional monthly averaged data allows to track the history of each bloom, as well as to locate its starting point and extension.Fronts and water masses can be contoured by following horizontal temperature gradients. Available Argo float CTD profiles will also contribute with values for the mixed layer depth. Results show a significant inter-annual variability of the bloom starting point, intensity and extent, especially around the South Sandwich and South Georgia islands. However, the complexity of interactions between the phytoplankton communities and the surrounding environment, responsible for bottom-up and top-down control (i.e. micro-nutrient enrichment and grazing) implies the need for more dedicated surveys and high resolution coupled physical-biogeochemical models; the latter would provide further information regarding nutrient fluxes and physical processes, especially during the undersampled winter season.

Description: Abstract Monthly averaged satellite imagery of chlorophyll a in the south-western sector of the Atlantic Ocean shows intense and recurrent blooms around the Antarctic Peninsula, the Island of South Georgia and the South Sandwich Islands. Analysis of a 10 year long time series of SeaWiFS ocean color images allows studying seasonal trends and inter-annual variability. Although specific areas show a regular pattern of low or high productivity, certain years appear to be strikingly productive or well below the climatological average. Circulation patterns, continental shelves as well as Antarctic Circumpolar Front are shown to be the major factors controlling extension of highly productive patches.

Description: The islands of the Southern Ocean, the largest of the High Nutrient Low Chlorophyll (HNLC) regions, have an important role in favouring phytoplankton growth and bloom development. Concentrations of iron, which has been demonstrated to be an important limiting micronutrient, play a major role in regulating the intensity and location of these blooms; however, sea-ice melting, ocean circulation patterns and the “island mass effect” may as well help explain such striking events for a HNLC region. This study will focus on the Southwest sector of the Atlantic Ocean, specifically on the Scotia Sea; this region [52°W – 30°W; 48°S – 56°S] is included between the North and South Scotia Ridge, and between the Drake Passage and the South Sandwich Islands. The main coastal ecosystems are those of the South Georgia, South Orkney and the South Sandwich islands; also the Antarctic Peninsula is included in the domain. The natural dynamics of phytoplankton blooms in the Scotia Sea have been investigated by analysing SeaWiFS, 9km resolution monthly composites of chlorophyll-a concentrations. The retrieved 12-year satellite time series (1997-2009) shows a clear seasonal trend in chlorophyll concentrations, which start increasing in August. Maximum concentrations (〉 3 mg/m^3) are detected between December and January after which bloom regions start shrinking in size, and chlorophyll-a concentrations return to the winter background concentrations. In this presentation we will also show how circulation patterns, bathymetry, continental shelves as well as the Antarctic Circumpolar Fronts are the major factors controlling the extension and location of these highly productive patches. The complexity of interactions between phytoplankton communities and the surrounding environment implies the need for more dedicated surveys and high resolution coupled physical-biogeochemical models; the latter would provide further information regarding nutrient fluxes and physical processes, especially during the under-sampled winter season, and in the deeper layers not captured by satellite images.

Description: Amidst high-nutrient low-chlorophyll waters of the south-western Atlantic sector of the Southern Ocean, an intense phytoplankton bloom is observed annually north of South Georgia (37°W, 55°S). South Georgia blooms have a vital role in the ecosystem surrounding the island, and have been linked to one of the strongest seasonal atmospheric-carbon uptake in the open Southern Ocean. Which environmental conditions drive such remarkable productivity are still under debate, and were investigated in the current study using a multidisciplinary approach. Satellite-derived observations of surface chlorophyll a concentrations and circulation patterns were used to study the annual and inter-annual variability of phytoplankton blooms in the region. Our analysis reveals a time series of very regular blooms, controlled in space by circulation and regulated in time by surface silicate concentrations, temperature and light. The role of the fundamental, yet limiting, micronutrient iron was investigated with the coupled hydrodynamic-biogeochemical model ROMS_AGRIF-PISCES. Model results, validated against available observations, suggest a continuous supply of dissolved iron from the island's shallow shelves that is redistributed in the region by local circulation. Conversely, aeolian sources of iron have a negligible role in the main bloom area, but appear to be more important outside the influence of the South Georgia island mass effect.

Description: In high-nutrient low-chlorophyll waters of the south-western Atlantic sector of the Southern Ocean, an intense phytoplankton bloom is observed annually north of South Georgia (37°W, 55°S). South Georgia blooms have a vital role in the ecosystem surrounding the island, and have been linked to one of the strongest seasonal atmospheric-carbon uptake in the open Southern Ocean. Which environmental conditions drive such remarkable productivity are still under debate, and were investigated in the current study using a multidisciplinary approach. Satellite-derived observations of surface chlorophyll a concentrations and circulation patterns were used to study the annual and inter-annual variability of phytoplankton blooms in the region. Our analysis reveals a time series of very regular blooms, controlled in space by circulation and regulated in time by surface silicate concentrations, temperature and light. The role of the fundamental, yet limiting, micronutrient iron was investigated with the coupled hydrodynamic-biogeochemical model ROMS_AGRIF-PISCES. Model results, validated against available observations, suggest a continuous supply of dissolved iron from the island's shallow shelves that is redistributed in the region by local circulation. Conversely, aeolian sources of iron have a negligible role in the main bloom area, but appear to be more important outside the influence of the South Georgia island mass effect.

Description: In high-nutrient low-chlorophyll waters of the western Atlantic sector of the Southern Ocean, an intense phytoplankton bloom is observed annually north of South Georgia. Multiple sources, including shallow sediments and atmospheric dust deposition, are thought to introduce iron to the region. However, the relative importance of each source is still unclear, owing in part to the scarcity of dissolved iron (dFe) measurements in the South Georgia region. In this study, we combine results from a recently published dFe data set around South Georgia with a coupled regional hydrodynamic and biogeochemical model to further investigate iron supply around the island. The biogeochemical component of the model includes an iron cycle, where sediments and dust deposition are the sources of iron to the ocean. The model captures the characteristic flow patterns around South Georgia, hence simulating a large phytoplankton bloom to the north (i.e. downstream) of the island. Modelled dFe concentrations agree well with observations (mean difference and root mean square errors of �0.02nM and �0.81 nM) and form a large plume to the north of the island that extends eastwards for more than 800 km. In agreement with observations, highest dFe concentrations are located along the coast and decrease with distance from the island. Sensitivity tests indicate that most of the iron measured in the main bloom area originates from the coast and very shallow shelf-sediments (depths 〈 20 m). Dust deposition exerts almost no effect on surface chlorophyll a concentrations. Other sources of iron such as run-off and glacial melt are not represented explicitly in the model, however we discuss their role in the local iron budget.

Description: Despite the high macro-nutrient content of waters transported by the Antarctic Circumpolar Current, most of the Southern Ocean waters fall in the High Nutrient Low Chlorophyll category. Exceptions are found downstream from islands or along coastal shelves, which are believed to be sources of the bio-limiting nutrient iron. Nevertheless, further top-down and bottom-up controls may play an important role in affecting the recurrence and distribution of phytoplankton blooms. Analyzing a 13-year long time series of satellite ocean color imagery for the Atlantic sector of the Southern Ocean, we have detected two well confined regions, in the southern Drake Passage and downstream from South Georgia, where inter-annual variability is surprisingly low. We thus combined satellite based sea surface height data and diverse langrangian measurements of circulation (Argo floats and surface drifters) to investigate existing connections between local hydrography and the observed chl-a patterns. Our results highlight how the occurrence and distribution of the observed pigment biomass is strongly controlled by surface circulation, which not only is responsible for the dispersal of chlorophyll-a patches but also that of micro-nutrients, possibly including iron. Our observations lead us to the conclusion that the topographic steering of the currents may also determine the stability of the observed patterns. Furthermore, we observe how both in the Southern Drake Passage, and downstream from South Georgia, higher chlorophyll-a concentrations are measured where a meander is formed. Combining our observations with those derived from other natural iron enrichment studies in the Southern Ocean (i.e. in the Crozet and Kerguelen Islands), we argue that downstream from a nutrient source the presence of a meander with greater elocities, yet embracing calmer waters, may act as a preconditioner to the observed higher chlorophyll-a concentrations.

Description: Primary production in the Southern Ocean (SO) is believed to be mostly iron limited; despite the high macronutrient content of waters transported by the Antarctic Circumpolar Current, the SO is considered a High Nutrient Low Chlorophyll region. However, ocean color imagery shows a variable and patchy environment, where sharp chlorophyll concentration (chl-a) gradients separate highly productive regions, which are found mostly downstream from islands and along coastal shelves, from the less productive ones, where pigment biomass remains low. In the western sector of the Atlantic SO, an intense and long-lasting phytoplankton bloom is found northwest of the Island of South Georgia, while very low chl-a are persistently measured southwest of the Shackleton Transverse Ridge (STR), in the southern Drake Passage. In both cases, local circulation, which is steered by bottom topography, plays a major role in controlling biogeochemistry and thus the distribution and intensity of chl-a. By combining surface drifter trajectories with satellite based measurements of sea surface height and ocean color, we relate local flow regimes to the observed pigment biomass patterns. Basing our analysis on 13 years of SeaWiFS imagery, the intense and long-lasting phytoplankton bloom developing northwest of South Georgia appears to be recurrent in time with little inter-annual variability; furthermore,our results show how the bloom is clearly confined to the area enclosed by the cyclonic circulation flowing along the periphery of the South Georgia Basin (SGB). Here, current velocities appear to gradually decrease towards the center of the basin possibly favoring the accumulation of shelf-derived iron, and thus the growth of phytoplankton cells. Southwest of the STR, the available ocean color time-series highlights an area with very low productivity values, which can be detected yearly and with little inter-annual variability. This region is adjacent to the more productive one found to the northeast of the ridge, above the Ona Basin. The former appears to be related to the intense Shackleton Jet flowing along the ridge, while the latter to the calmer cyclonic circulation located above the Ona Basin; just above the STR lies the area where maximum chl-a gradients can be measured. Absolute dynamic topography values retrieved for the two regions together with surface drifter trajectories, suggest a clear spatial and temporal correspondence between local circulation patterns and those of surface chl-a; furthermore, the AVISO time-series confirms the low inter-annual variability of the two previously described flow patterns. We argue how similarly above the STR and the SGB, the presence of the cyclonic circulation acts as a precondition to the observed higher chl-a. In both cases, nutrient (i.e. iron) rich waters may be entrained in the cyclone, and separated from those lying outside its borders. Similar observations have been made in the Crozet and the Kerguelen regions.