No abstract

No abstract

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.

No abstract

Diatoms are considered the main base of the Southern Ocean food web as they are responsible for more than 85% of its annual primary production and play a crucial role in the Antarctic trophic structure and in the biogeochemical cycles. Within this context, an intense diatom bloom reaching > 45 mg m−3 of chlorophyll a was registered in the Northern Antarctic Peninsula (NAP) during a late summer study in February 2016. Given that nutrient concentrations and grazing activities were not identified here as limiting factors on the bloom development, the aim of this study was to evaluate the effect of water column structure (stability and upper mixed layer depth) on the phytoplankton biomass and composition in the NAP. The diatom bloom, mainly composed by the large centric Odontella weissflogii (mostly > 70 μm in length), was associated with a local ocean carbon dioxide uptake that reached values greater than −60 mmol m−2 d−1. We hypothesize that the presence of a vertically large water column stability barrier, just below the pycnocline, was the main driver allowing for the development of the intense diatom bloom, particularly in the Gerlache Strait. Contrarily, a shift from diatoms to dinoflagellates (mainly Gymnodiniales < 20 μm) was observed associated with conditions of a highly stable thin layer. The results suggest that a large fraction of this intense diatom bloom is in fast sinking process, associated with low grazing pressure, showing a crucial role of diatoms for the efficiency of the biological carbon pump in this region.

In recent years, the global distribution of phytoplankton functional types (PFT) and phytoplankton size classes (PSC) has been determined by remote sensing. Many of these methods rely on interpretation of phytoplankton size or type from pigment data, but independent validation has been difficult due to lack of appropriate in situ data on cell size. This work uses in situ data (photosynthetic pigments concentration and cell abundances) from the north-east Atlantic, along a trophic gradient, sampled from 2005 to 2010, as well as Atlantic Meridional Transect (AMT) data for the same region, to test a previously developed conceptual model, which calculates the fractional contributions of pico-, nano-and micro-plankton to total phytoplankton chlorophyll biomass (Brewin et al., 2010). The application of the model proved to be successful, as shown by low mean absolute error between data and model fit. However, regional values obtained for the model parameters had some effect on the relative distribution of size classes as a function of chlorophyll-a, compared with the results according to the original model. The regional parameterisation yielded a dominance of micro-plankton contribution for chlorophyll-a concentrations greater than 0.5 mg m −3 , rather than from 1.3 mg m −3 in the original model. Intracellular chlorophyll-a (Chla) per cell, for each size class, was computed from the cell enumeration results (microscope counts and flow cytometry) and the chlorophyll-a concentration for that size class given by the model. The median intracellular chlorophyll-a values computed were 0.004, 0.224 and 26.78 pg Chla cell −1 for pico-, nano-, and micro-plankton respectively. This is generally consistent with the literature, thereby providing an indirect validation of the method based on pigments to assign size classes. Using a satellite-derived composite image of chlorophyll-a for the study area, a map of cell abundance was generated based on the computed intracellular chlorophyll-a for each size-class, thus extending the remote-sensing method for mapping size classes of phytoplankton from chlorophyll-a concentration to mapping cell numbers in each class. The map reveals the ubiquitous presence of pico-plankton, and shows that all size classes are more abundant in more productive areas.

The effectiveness of two HPLC methods (a monomeric C 18 column with a high ion strength solvent gradient and a monomeric C 8 column with a pyridine-containing mobile phase) in the separation, identification, and quantification of pigments are compared, using phytoplankton, microphytobenthos, and algal cultures. Although the two studied methods showed good analytical resolution, the C 18 method presents a higher discrimination for several carotenoid pigments. On the other hand, the C 8 method allows the separation of chlorophylls c 1 , c 2 , and Mg-2, 4-divinyl phaeoporphyrin a 5 monomethyl ester as well as the pair chlorophyll a/divinyl Chl a. The C 18 method had a shorter elution program and a lower solvent flow rate, making it cheaper and faster than the C 8 method. Furthermore, the C 18 method showed significantly lower detection and quantification limits, a particularly important advantage for pigments present in trace amounts in both phytoplankton and microphytobenthic samples. Differences in the sensitivity of the two studied methods were due mainly to differences in flow rate, with significantly higher peak areas for slower flow rates. No method is ideal for all pigments and sample types and the choice of method should be made in accordance with the objective of each study, taking into consideration the advantages and disadvantages identified for each method.

No abstract