Impact of sea ice on the structure of phytoplankton communities in the northern Antarctic Peninsula

The seasonal advance and retreat of sea ice around the northern Antarctic Peninsula can have a significant impact on phytoplankton, mainly due to alterations in the availability of ice-free areas, micronutrient inputs by meltwater and variations in water column structure. The aim of this work was to evaluate the effect of sea ice conditions on phytoplankton biomass and community composition in an area off the northern Antarctic Peninsula, a region undergoing important warming processes. In two consecutive summer cruises (2013 and 2014), seawater samples were analysed for nutrients and phytoplankton (through HPLC-CHEMTAX approach), and measurements were made for water column physical structure evaluation. Two contrasting conditions were studied: a strong environmental gradient around the sea ice edge, with a marked meltwater signal (summer 2013) and the same area with little indication of meltwater and no detectable sea ice conditions (summer 2014). In the first year, the phytoplankton communities were massively dominated by nanoflagellates such as cryptophytes, small dinoflagellates and Phaeocystis antarctica, but with differences between stations with less influence of meltwater (dominance of dinoflagellates type B, mainly Gymnodinium spp., mean chlorophyll a = 1.37 mg m−3) and stations closer to the sea ice edge (dominance of cryptophytes, mean chlorophyll a = 0.98 mg m−3). In the second year, cryptophytes were apparently replaced by diatoms type B (mainly Pseudonitzschia spp., 24% contribution, mean chlorophyll a = 0.93 mg m−3), although dinoflagellates were also important. Therefore, there was a clear distinction between the phytoplankton communities under sea ice influence, where mainly cryptophytes were associated with shallow mixed layers and high water column stability in 2013 and an important presence of diatoms in 2014, associated with deeper mixed layers, lower silicic acid concentrations and higher magnitudes of both salinity and temperature, under very little sea ice influence. Gymnodinioid dinoflagellates were an important component in both years, apparently occupying sites/conditions less favourable to cryptophytes. These results support previous suggestions that climate factors leading to shortening of the sea ice season in the region do have an important impact particularly in shaping the dominance of the main phytoplankton functional groups in the region

Large diatom bloom off the Antarctic Peninsula during cool conditions associated with the 2015/2016 El Niño

Diatoms play crucial functions in trophic structure and biogeochemical cycles. Due to poleward warming, there has been a substantial decrease in diatom biomass, especially in Antarctic regions that experience strong physical changes. Here we analyze the phytoplankton contents of water samples collected in the spring/summer of 2015/2016 off the North Antarctic Peninsula during the extreme El Niño event and compare them with corresponding satellite chlorophyll-a data. The results suggest a close link between large diatom blooms, upper ocean physical structures and sea ice cover, as a consequence of the El Niño effects. We observed massive concentrations (up to 40 mg m–3 of in situ chlorophyll-a) of diatoms coupled with substantially colder atmospheric and oceanic temperatures and high mean salinity values associated with a lower input of meltwater. We hypothesize that increased meltwater concentration due to continued atmospheric and oceanic warming trends will lead to diatom blooms becoming more episodic and spatially/temporally restricted

Dynamics of the Southern Ocean Circulation Off West Antarctica

This thesis presents an investigation of the dynamics of the ocean circulation off West Antarctica, a region characterised by intense ice shelf loss. The Ross Gyre, located in the western side of West Antarctica, is studied through remote sensing techniques that retrieve information of the sea level from sea ice leads. The seasonality of the gyre is driven by the winds through two distinct modes. The first mode, associated with circumpolar winds, reduces sea level at the end of summer through northward Ekman transport. Regional sea level rises in winter due to ice cover, which reduces the wind stress on the ocean. The second mode, associated with the Amundsen Sea Low (ASL), controls the transport and area of the Ross Gyre and the coastal throughflow with a semi-­‐ annual period, with peaks in May and November. A deeper ASL allows the gyre’s expansion and intensification, and enhances the westward throughflow. On the eastern side of West Antarctica, warm Circumpolar Deep Water (CDW) inflows to the Amundsen Sea are studied with a regional model and in situ data. The key inflow-­‐controlling mechanisms are identified by decomposing CDW temperature variability into two components associated with (1) changes in the depth of isopycnals (heave; HVE), and (2) changes in the temperature of isopycnals (water mass property changes; WMP). In the western Amundsen Sea, the deeper thermocline and shallower shelf break hinder CDW access onto the shelf, and CDW inflow is regulated by the uplift of isopycnals at the shelf break – which is itself controlled by wind-­‐driven variations in the speed of an undercurrent flowing eastward along the continental slope. In contrast, the shallower thermocline and deeper shelf break in the eastern Amundsen Sea permit CDW to persistently access the continental shelf, and CDW temperature in the area responds to wind-­‐driven modulation of the water mass’ on-­‐ shelf volume by changes in the rate of inflow across the shelf break and in Ekman pumping-­‐ induced vertical displacement of isopycnals within the shelf. A multi-­‐year mooring located in the Dotson Trough (western Amundsen Sea) supports our model-­‐based conclusions. This mooring shows a transition to cooler conditions between 2010 and 2015, associated with WMP induced by weakening winds at the shelf break. The ASL was identified as the main driver of the local winds. A deeper ASL favours strengthening of the easterlies, weakening the undercurrent and reducing access of CDW to the continental shelf. In turn, a shallow ASL favours strengthening of the westerlies, intensifying the undercurrent and increasing access of CDW to the continental shelf. Thus, the Amundsen and Ross Seas are connected by a coastal current accelerated during periods of deeper ASL, which favours the export to the Ross Sea of meltwater from the Amundsen Sea coast. A future change in the ASL strength could impact both regions, with consequences for the regional heat budget and melting of the ice shelves downstream

New insights on the dominance of cryptophytes in Antarctic coastal waters: A case study in Gerlache Strait

Changes in phytoplankton composition from large diatoms to small cryptophytes and their implications to the food web have been previously associated with rapid warming of surface waters in the western Antarctic Peninsula (WAP). However, ecological and physiological attributes that favor dominance of these flagellates in the region have not been fully explored. The overall aim of this work was to characterize the phytoplankton pigments and assemblages in relation to environmental conditions during three successive summer cruises (2013, 2014 and 2015) in the Gerlache Strait − a coastal area in the northern WAP. Data on phytoplankton (through HPLC/CHEMTAX pigment analysis) and associated physical (water column structure) and chemical (macronutrients) parameters were determined. Cryptophytes were conspicuously found in shallow mixed layers, under stratified conditions, as the main contributors to total phytoplankton biomass. Their greatest contributions were associated with warmer surface waters at the northwestern sector of the strait. Other phytoplankton groups (Phaeocystis antarctica in 2013 and small diatoms in both 2014 and 2015) were also important components. Photoprotective carotenoids (mainly alloxanthin), with an important role in preventing photodamage caused by excess light, were closely linked with the dominance of cryptophytes at surface layers. The results of this study suggest that the prevalence of cryptophytes in WAP coastal waters can be, to a great extent, due to a particular ability of those small flagellates to successfully grow in highly illuminated conditions in shallow upper mixed layers and strong water column stratification

Baroclinic ocean response to climate forcing regulates decadal variability of ice‐shelf melting in the Amundsen Sea

Warm ocean waters drive rapid ice-shelf melting in the Amundsen Sea. The ocean heat transport toward the ice shelves is associated with the Amundsen Undercurrent, a near-bottom current that flows eastward along the shelf break and transports warm waters onto the continental shelf via troughs. Here we use a regional ice-ocean model to show that, on decadal time scales, the undercurrent's variability is baroclinic (depth-dependent). Decadal ocean surface cooling in the tropical Pacific results in cyclonic wind anomalies over the Amundsen Sea. These wind anomalies drive a westward perturbation of the shelf-break surface flow and an eastward anomaly (strengthening) of the undercurrent, leading to increased ice-shelf melting. This contrasts with shorter time scales, for which surface current and undercurrent covary, a barotropic (depth-independent) behavior previously assumed to apply at all time scales. This suggests that interior ocean processes mediate the decadal ice-shelf response in the Amundsen Sea to climate forcing