Physical-biological drivers modulating phytoplankton seasonal succession along the Northern Antarctic Peninsula.

The Northern Antarctic Peninsula (NAP) shows shifts in phytoplankton distribution and composition along its warming marine ecosystems. However, despite recent efforts to mechanistically understand these changes, little focus has been given to the phytoplankton seasonal succession, remaining uncertainties regarding to distribution patterns of emerging taxa along the NAP. To fill this gap, we collected phytoplankton (pigment and microscopy analysis) and physico-chemical datasets during spring and summer (November, February and March) of 2013/2014 and 2014/2015 off the NAP. Satellite measurements (sea surface temperature, sea ice concentration and chlorophyll-a) were used to extend the temporal coverage of analysis associated with the in situ sampling. We improved the quantification and distribution pattern of emerging taxa, such as dinoflagellates and cryptophytes, and described a contrasting seasonal behavior and distinct fundamental niche between centric and pennate diatoms. Cryptophytes and pennate diatoms preferentially occupied relatively shallower mixing layers compared with centric diatoms and dinoflagellates, suggesting differences between these groups in distribution and environment occupation over the phytoplankton seasonal succession. Under colder conditions, negative sea surface temperature anomalies were associated with positive anomalies of sea ice concentration and duration. Therefore, based on sea ice-phytoplankton growth relationship, large phytoplankton biomass accumulation was expected during the spring/summer of 2013/2014 and 2014/2015 along the NAP. However, there was a decoupling between sea ice concentration/duration and phytoplankton biomass, characterizing two seasonal periods of low biomass accumulation (negative chlorophyll-a anomalies), associated with the top-down control in the region. These results provide an improved mechanistic understanding on physical-biological drivers modulating phytoplankton seasonal succession along the Antarctic coastal waters.

The impact of mesoscale eddies on the phytoplankton community in the South Atlantic Ocean: HPLC-CHEMTAX approach.

This study describes the pigment-based phytoplankton community within three South Atlantic anticyclonic eddies (at different ages) shed from the Agulhas Current retroflection crossing the South Atlantic Ocean. Seawater samples were collected over these mesoscale structures in June-July 2015 during the Following Ocean Rings in the South Atlantic (FORSA) cruise. Data on phytoplankton pigments, measured with high-performance liquid chromatography (HPLC), were processed using a chemical taxonomy (CHEMTAX) tool to determine and quantify phytoplankton taxonomic groups. In addition, physical (water column structure) and chemical (macronutrient) parameters were determined and related to the biological data. Our results showed that, in general, the community was composed mostly of small flagellates (haptophytes) and prokaryotes (Prochlorococcus) and that pelagophytes were prominent in the younger eddy. This ring, located in the eastern basin of the South Atlantic Ocean, represented a younger and stronger structure, with no evident deep chlorophyll maximum (DCM) depth and an evenly distributed biomass over a well-mixed upper layer, revealing a more diverse phytoplankton community. The weakened structures of the older western eddies presented a pronounced DCM depth below 100 m and similar phytoplankton community composition patterns marked by enhanced Prochlorococcus contributions but also the important occurrences of haptophytes at the DCM depth and Synechococcus and chlorophytes at the surface. The community distributions in all three structures were associated with the distribution of nutrients and acclimation to light conditions. This study contributes to a better understanding of the phytoplankton distribution and its association with the presence of mesoscale anticyclonic eddies in an undersampled and complex region of the South Atlantic Ocean.