Seasonal cycle of CO2 from the sea ice edge to island blooms in the Scotia Sea, Southern Ocean

The Scotia Sea region contains some of the most productive waters of the Southern Ocean. It is also a dynamic region through the interaction of deep water masses with the atmosphere. We present a first seasonally-resolved time series of the fugacity of CO2 (fCO2) from spring 2006, summer 2008, autumn 2009 and winter (potential temperature minimum) along a 1000 km transect from the pack ice to the Polar Front to quantify the effects of biology and temperature on oceanic fCO2. Substantial spring and summer decreases in sea surface fCO2 occurred in phytoplankton blooms that developed in the naturally iron fertilised waters downstream (north) of South Georgia island (54-55S, 36-38W) and following sea ice melt (in the seasonal ice zone). The largest seasonal fCO2 amplitude (fCO2) of 159 uatm was found in the South Georgia bloom. In this region, biological carbon uptake dominated the seasonal signal, reducing the winter maxima in oceanic fCO2 by 257 uatm by the summer. In the Weddell-Scotia Confluence, the southern fringe of the Scotia Sea, the shift from wintertime CO2-rich conditions in ice covered waters to CO2 undersaturation in the spring blooms during and upon sea ice melt created strong seasonality in oceanic fCO2. Temperature effects on oceanic fCO2 ranged from fCO2sst of 55 uatm in the seasonal ice zone to almost double that downstream of South Georgia (98 uatm). The seasonal cycle of surface water fCO2 in the high-nutrient low-chlorophyll region of the central Scotia Sea had the weakest biological control and lowest seasonality. Basin-wide biological processes dominated the seasonal control on oceanic fCO2 (fCO2bio of 159 μatm), partially compensated (43%) by moderate temperature control (fCO2sst of 68 μatm). The patchwork of productivity across the Scotia Sea creates regions of seasonally strong biological uptake of CO2 in the Southern Ocean

Iron Biogeochemistry in the High Latitude North Atlantic Ocean

Iron (Fe) is an essential micronutrient for marine microbial organisms, and low supply controls productivity in large parts of the world’s ocean. The high latitude North Atlantic is seasonally Fe limited, but Fe distributions and source strengths are poorly constrained. Surface ocean dissolved Fe (DFe) concentrations were low in the study region (<0.1 nM) in summer 2010, with significant perturbations during spring 2010 in the Iceland Basin as a result of an eruption of the Eyjafjallajökull volcano (up to 2.5 nM DFe near Iceland) with biogeochemical consequences. Deep water concentrations in the vicinity of the Reykjanes Ridge system were influenced by pronounced sediment resuspension, with indications for additional inputs by hydrothermal vents, with subsequent lateral transport of Fe and manganese plumes of up to 250–300 km. Particulate Fe formed the dominant pool, as evidenced by 4–17 fold higher total dissolvable Fe compared with DFe concentrations, and a dynamic exchange between the fractions appeared to buffer deep water DFe. Here we show that Fe supply associated with deep winter mixing (up to 103 nmol m−2 d−1) was at least ca. 4–10 times higher than atmospheric deposition, diffusive fluxes at the base of the summer mixed layer, and horizontal surface ocean fluxes

Physical and biogeochemical controls on the variability in surface pH and calcium carbonate saturation states in the Atlantic sectors of the Arctic and Southern Oceans

Polar oceans are particularly vulnerable to ocean acidification due to their low temperatures and reduced buffering capacity, and are expected to experience extensive low pH conditions and reduced carbonate mineral saturations states (Ω) in the near future. However, the impact of anthropogenic CO2 on pH and Ω will vary regionally between and across the Arctic and Southern Oceans. Here we investigate the carbonate chemistry in the Atlantic sector of two polar oceans, the Nordic Seas and Barents Sea in the Arctic Ocean, and the Scotia and Weddell Seas in the Southern Ocean, to determine the physical and biogeochemical processes that control surface pH and Ω. High-resolution observations showed large gradients in surface pH (0.10–0.30) and aragonite saturation state (Ωar) (0.2–1.0) over small spatial scales, and these were particularly strong in sea-ice covered areas (up to 0.45 in pH and 2.0 in Ωar). In the Arctic, sea-ice melt facilitated bloom initiation in light-limited and iron replete (dFe>0.2 nM) regions, such as the Fram Strait, resulting in high pH (8.45) and Ωar (3.0) along the sea-ice edge. In contrast, accumulation of dissolved inorganic carbon derived from organic carbon mineralisation under the ice resulted in low pH (8.05) and Ωar (1.1) in areas where thick ice persisted. In the Southern Ocean, sea-ice retreat resulted in bloom formation only where terrestrial inputs supplied sufficient iron (dFe>0.2 nM), such as in the vicinity of the South Sandwich Islands where enhanced pH (8.3) and Ωar (2.3) were primarily due to biological production. In contrast, in the adjacent Weddell Sea, weak biological uptake of CO2 due to low iron concentrations (dFe<0.2 nM) resulted in low pH (8.1) and Ωar (1.6). The large spatial variability in both polar oceans highlights the need for spatially resolved surface data of carbonate chemistry variables but also nutrients (including iron) in order to accurately elucidate the large gradients experienced by marine organisms and to understand their response to increased CO2 in the future

Iron biogeochemistry in (sub-) Polar waters

Iron represents an important control on primary production in high nutrient low chlorophyll(HNLC) regimes and has received considerably attention during the last two decades. Thiswork has focussed on the biogeochemistry of iron in two oceanic environments; the highlatitude North Atlantic and the Scotia Sea in the Southern Ocean. The mechanisms of ironsupply and the biological response of resident phytoplankton communities to iron wereaddressed in both study areas. Two cruises to the high latitude North Atlantic Ocean (&gt;55 °N)during late July-early September 2007 indicated that nitrate concentrations of 2 to 5 ?Mpersisted in the surface waters. The concentration of dissolved iron (dFe) in the surface waterswas very low, with an average of 0.093 (&lt;0.010-0.218, n=43) nM, and in situ chlorophyllconcentrations were &lt; 0.5 mg m-3. In vitro iron addition experiments demonstrated that theaddition of iron increased photosynthetic efficiencies (Fv/Fm) and resulted in enhancedchlorophyll in treatments amended with iron when compared to controls. A number ofphytoplankton taxa, including the coccolithophore Emiliania huxleyi, were observed toincrease their net growth rates following iron addition. These results provide strong evidencethat iron limitation within the post spring bloom phytoplankton community contributes to theobserved residual macronutrient pool during summer. Low atmospheric iron supply and suboptimalFe:N ratios in winter overturned deep water are suggested as proximal causes for thisseasonal High Nutrient Low Chlorophyll (HNLC) condition, which represents an inefficiencyof the biological (soft tissue) carbon pump. Large areas of the Southern Ocean arecharacterised as HNLC. Satellite chlorophyll data indicate that phytoplankton blooms occurin vicinity to Southern Ocean Island systems. The bloom associated with South Georgia hasthe largest spatial extent and duration (16-20 weeks). Detailed measurements were made onaustral spring and summer cruises to the Scotia Sea during November – early December 2006and January – February 2008. This work presents the first comprehensive study of seasonalvariations in phytoplankton biomass and iron availability in the Scotia Sea. The drawdown ofnitrate between the two seasons in the South Georgia bloom was 16 ?M indicative ofsubstantial new production. Surface water concentrations of dissolved iron (dFe) were slightlyhigher during summer than spring (0.31 nM compared to 0.20 nM, with P&gt;0.05). We suggestthat the South Georgia bloom is sustained by a continuous benthic supply of iron from theSouth Georgia shelf. In addition, enhanced dFe (0.34 nM) was observed in a cryptophytedominated bloom in the southern Scotia Sea in the vicinity of South Orkney Islands. Thedifference in the community composition between the two natural occurring blooms highlightthat Southern Ocean island systems have individual characteristics and should be viewedindependently

Sedimentary and atmospheric sources of iron around South Georgia, Southern Ocean: a modelling perspective

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 ~0.02 nM 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 &lt; 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

The South Georgia island mass effect: observations from satellite imagery and biogeochemical modeling

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

Áhrif áreiðanleika áreita á raðhrif í sjónkynjun

As the visual environment offers us an overload of visual information embedded in an abundance of noise, we are able to use previous visual experience to facilitate processing and perception of current scenes. Rather than attending to all items in our surroundings, the perceptual system filters out and prioritizes items that are more important to us. In order to make more accurate predictions about our visual environment, the attractive bias of previously attended targets pulls perception of subsequently attended targets towards their specific features, while distractors seem to have an opposing, repulsive effect on perception. While these biases generally reduce uncertainty and stabilize our perception of the world, they can also distort it from the real visual environment. While these biases are considered automatic processes, little is known as to how they affect the metaperception of one’s own performance – confidence. This bachelor thesis contains two similar, but separate experiments investigating whether these perceptual biases are modulated by stimulus uncertainty. Both experiments contained an odd-one-out search task of an oddly oriented line within 35 distractor lines. In Experiment 1 uncertainty was produced by higher standard deviations in the distribution of distractor orientations, increasing difficulty in target detection. Uncertainty in Experiment 2 was manipulated by changing the contrast of all items, and therefore increasing overall item discrimination. Attractive bias – also known as serial dependence – was apparent in all conditions of both experiments, while the repulsive bias of distractors was rather weak. Uncertainty had little to no effect on the biases, but more so on observers’ confidence judgments of their own task performance

Seasonal and spatial dynamics of iron availability in the Scotia Sea

The Southern Ocean is the world's largest high nutrient low chlorophyll (HNLC) region. However, satellite images highlight several areas associated with island chains and shallow topographic features which display high phytoplankton biomass. Here we present the first study of seasonal variations in phytoplankton biomass and iron availability in the Scotia Sea over both austral spring and summer seasons. Based on dissolved iron (dFe) and Chlorophyll a (Chl a) concentrations, the study area is be divided into three regions: North of South Georgia, south of South Georgia and the vicinity of South Orkney Islands. The Scotia Sea to the south of South Georgia exhibited low dFe concentrations (&lt; 0.027–0.05 nM) in surface waters during both the spring and summer seasons. Nevertheless, nitrate concentrations were considerably lower in spring compared to summer (difference ~ 8 ?M). Summer Chl a concentrations were ~ 1.4 mg m? 3 and in situ phytoplankton populations displayed evidence of iron stress, suggesting the development of seasonal iron limitation. Surface water dFe concentrations in the South Georgia bloom waters (north of the islands) were elevated and slightly lower during spring than summer (0.20 nM compared to 0.31 nM, P &gt; 0.05). Nitrate concentrations were 16 ?M lower in summer compared to spring, whilst Chl a standing stocks remained high. Enhanced dFe (~ 0.25 nM) and Chl a concentrations were furthermore observed in the vicinity of the South Orkney Islands, located in the southern Scotia Sea. Iron addition experiments showed that in situ phytoplankton were iron replete spring and summer north of South Georgia and in the vicinity of South Orkney Islands during summer. We thus suggest that increased iron supply in high productivity areas including the area north of South Georgia and the South Orkney Islands, was sustained by a continuous benthic supply from their shelf systems, with a potential additional input from seasonally retreating sea ice in the South Orkney system