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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.

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[1] In January 2008, a patch of high reflectance detected by ocean color satellite images was sampled during a cruise over the southern Argentinean continental shelf. High calcite concentrations (particulate inorganic carbon (PIC)) found at the patch were associated with dominance of the coccolithophorid Emiliania huxleyi. Relatively low chlorophyll concentrations (0.29 to 1.48 mg m −3 ) were found, but both particulate attenuation (0.27 to 1.15 m −1 ) and backscattering coefficients at 660 nm (0.003 to 0.042 m −1 ) were noticeably high. Particulate inorganic to organic carbon (POC) ratio (PIC:POC) was highly variable (0.02 to 1.1), but mostly high, showing a significant correlation with particulate backscattering coefficient at 660 nm (r = 0.83, p < 0.005). The spectral dependency of the backscattering coefficient followed Gordon et al. (2009). Both the time evolution analyses of normalized water leaving radiance at 551 nm (nLw551) and the high PIC:POC ratios suggested an advanced stage of the coccolithophorid bloom, therefore with high detached coccoliths:cell ratios. Moreover, this was supported by a strong correlation between PIC and both particulate backscattering (r = 0.81, p < 0.005) and particulate beam attenuation coefficient (r = 0.7, p < 0.05). Remote sensing reflectance data were strongly related to particle backscattering and backscattering ratio, but not to absorption. NASA operational algorithms overestimated chlorophyll by a factor of ∼2 and estimated PIC with a relatively high root-mean-square (RMS) error (RMS = 97.9 mg PIC L ) were achieved when we used the original PIC-specific backscattering coefficient (Balch et al., 2005).

We describe the phytoplankton community and biomass during a summer coccolithophorid bloom sampled over the Patagonian shelf (48.58S-50.58S). Those phytoplankton species can contribute to the flux of calcium carbonate out of surface waters. Results from both microscope and high-performance liquid chromatography (HPLC) analysis are shown to complement information on the phytoplankton community. From CHEMTAX analysis of HPLC data, the most important organisms and groups identified were the coccolithophorid Emiliania huxleyi, the haptophyte Phaeocystis antarctica, dinoflagellates, diatoms, cryptophytes, prasinophytes and cyanobacteria. Phytoplankton microscope counts were converted into phytoplankton group-specific biovolume estimates. Although some microscope-identified taxa could not be determined by CHEMTAX, e.g. the autotrophic ciliate Myrionecta rubra, cluster analyses from both techniques showed similar results for the main groups. Both Emiliania huxleyi cell concentration and biomass, and the pigment 19 ′ -hexanoyloxyfucoxanthin were the most important biological features during the sampling period. At surface, nitrate was moderately high (0.2 -4.2 mM) in coccolithophorid-dominated samples, whereas phosphate (,0.33 mM) and silicate (,1.35 mM) concentrations were low. Among the environmental factors low Si:N ratios were mainly associated with the dominance of E. huxleyi. Competition and probably differential grazing could also promote a coccolithophorid outgrowth over other photoautotrophs during the summer season in the Patagonian shelf.

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