Description: Marine microbial plankton hold high structural and functional diversity, however, high-resolution data are lacking in a large part of the Global Ocean, such as in subpolar areas of the SW Atlantic. The Burdwood Bank (BB) is a submerged plateau (average depth 100 m) that constitutes the westernmost segment of the North Scotia Ridge (54°–55°S; 56°–62°W). The BB hosts rich benthic biodiversity in low chlorophyll waters of the southern Patagonian Shelf, Argentina, declared Namuncurá Marine Protected Area (NMPA) in 2013. So far, the pelagic microorganisms above the bank have not been described. During austral summer 2016, we assessed the microbial plankton (0.2–200 μm cell size) biomass and their taxonomical and functional diversity along a longitudinal transect (54.2–55.3°S, 58–68°W) from the Beagle Channel (BC) to the BB, characterized by contrasting hydrography. Results displayed a marked zonation in the composition and structure of the microbial communities. The biomass of phytoplankton 〉5 μm was 28 times higher in the BC, attributed mainly to large diatom blooms, than in oceanic waters above the BB, where the small coccolithophore Emiliania huxleyi and flagellates 〈10 μm dominated. In turn, the biomass of microheterotrophs above the BB doubled the biomass in the BC due to large ciliates. Notably, toxic phytoplankton species and their phycotoxins were detected, in particular high abundance of Dinophysis acuminata and pectenotoxins above the bank, highlighting their presence in open subpolar regions. Picophytoplankton (〈2 μm), including Synechococcus and picoeukaryotes, were remarkably important above the BB, both at surface and deep waters (up to 150 m). Their biomass surpassed by 5 times that of phytoplankton 〉 5 μm, emphasizing the importance of small-sized phytoplankton in low chlorophyll waters. The homogeneous water column and high retention above the bank seem to favor the development of abundant picophytoplankton and microzooplankton communities. Overall, our findings unfold the plankton configuration in the Southern Patagonian Shelf, ascribed as a sink for anthropogenic CO2, and highlight the diverse ecological traits that microorganisms develop to adjust their yield to changing conditions.

Description: Historical records (1980–2018) of potentially toxic phytoplankton and phycotoxins on the Argentine Continental Shelf (35°S-56.5°S) and adjacent ocean waters were systematically reviewed from scientific literature to assess their abundance and diversity over spatial and temporal scales. Records increased from 124 in the period 1980–1992 to 638 in 2006–2018, and the scanned area expanded from coastal to offshore waters including the shelf-break front. Alexandrium was the most reported genus (54%) during 1980–1992 and Pseudo-nitzschia (52%) during 1993–2005. By 2006–2018, a higher diversity was documented: Alexandrium (20%), Dinophysis (32%), Pseudo-nitzschia (31%), and the most recently described potentially toxic dinoflagellates of the family Amphidomataceae (8%). Likewise, a wider spectrum of phycotoxins was documented in the last decade, with lipophilic (LSTs) and paralytic shellfish toxins (PSTs) as the most recorded. Increased records are related to intensified monitoring, more detailed taxonomic analyses and more sensitive chemical techniques for marine biotoxin detection. This quantitative assessment brings light to the widespread occurrence of HABs along contrasting areas of the Patagonian Shelf and sets the basis for ecosystem risk evaluation. Moreover, comparison of toxic phytoplankton reported in the SW Atlantic with those in similar temperate seas in the North Atlantic and the Pacific Ocean, disclose ocean basin differences in strain toxicity of A. ostenfeldii, D. tripos and Azadinium species.

Description: Historical records (1980–2018) of potentially toxic phytoplankton and phycotoxins on the Argentine Continental Shelf (35°S-56.5°S) and adjacent ocean waters were systematically reviewed from scientific literature to assess their abundance and diversity over spatial and temporal scales. Records increased from 124 in the period 1980–1992 to 638 in 2006–2018, and the scanned area expanded from coastal to offshore waters including the shelf-break front. Alexandrium was the most reported genus (54%) during 1980–1992 and Pseudo-nitzschia (52%) during 1993–2005. By 2006–2018, a higher diversity was documented: Alexandrium (20%), Dinophysis (32%), Pseudo-nitzschia (31%), and the most recently described potentially toxic dinoflagellates of the family Amphidomataceae (8%). Likewise, a wider spectrum of phycotoxins was documented in the last decade, with lipophilic (LSTs) and paralytic shellfish toxins (PSTs) as the most recorded. Increased records are related to intensified monitoring, more detailed taxonomic analyses and more sensitive chemical techniques for marine biotoxin detection. This quantitative assessment brings light to the widespread occurrence of HABs along contrasting areas of the Patagonian Shelf and sets the basis for ecosystem risk evaluation. Moreover, comparison of toxic phytoplankton reported in the SW Atlantic with those in similar temperate seas in the North Atlantic and the Pacific Ocean, disclose ocean basin differences in strain toxicity of A. ostenfeldii, D. tripos and Azadinium species.

Description: The species composition and structure (e.g. abundance and biomass) of protistan plankton (cell size 〉 5 µm) and in situ chorophyll a were assessed in a shallow (〈50 m depth) inner shelf area of the Argentine Shelf called El Rincón (38º-41°S). Surface water samples (5 m depth) for plankton quantification) were taken with Niskin bottles during four oceanographic cruises (two in early austral spring and two in late austral summer- early fall), onboard the vessel B. Houssay accounting for a total of 36 sampling stations. These samples were analyzed under optical microscopy following the inverted microscope technique with sedimentation chambers. Cells enumeration and identification was made up to species, genus or family level, which were afterward categorized in taxonomical groups: diatoms, dinoflagellates, coccolithophores and nanoflagellates. The studied area supports important fishes of commercial interest, therefore plankton biodiversity records are neccesary to understand possible shifts at the population and community levels that might have cascading effects on marine ecosystems' productivity.

Description: The species composition and structure (e.g. abundance and biomass) of protistan plankton (cell size 〉 5 µm) and in situ chorophyll a were assessed in a shallow (〈50 m depth) inner shelf area of the Argentine Shelf called El Rincón (38º-41°S). Surface water samples (5 m depth) for plankton quantification were taken with Niskin bottles during four oceanographic cruises (two in early austral spring and two in late austral summer- early fall), onboard the vessel B. Houssay accounting for a total of 36 sampling stations. These samples were analyzed under optical microscopy following the inverted microscope technique with sedimentation chambers. Cells enumeration and identification was made up to species, genus or family level, which were afterward categorized in taxonomical groups: diatoms, dinoflagellates, coccolithophores and nanoflagellates. The studied area supports important fishes of commercial interest, therefore plankton biodiversity records are neccesary to understand possible shifts at the population and community levels that might have cascading effects on marine ecosystems' productivity.

Description: The species composition and structure (e.g. abundance and biomass) of protistan plankton (cell size 〉 5 µm) and in situ chorophyll a were assessed in a shallow (〈50 m depth) inner shelf area of the Argentine Shelf called El Rincón (38º-41°S). Surface water samples (5 m depth) for plankton quantification were taken with Niskin bottles during four oceanographic cruises (two in early austral spring and two in late austral summer- early fall), onboard the vessel B. Houssay accounting for a total of 36 sampling stations. These samples were analyzed under optical microscopy following the inverted microscope technique with sedimentation chambers. Cells enumeration and identification was made up to species, genus or family level, which were afterward categorized in taxonomical groups: diatoms, dinoflagellates, coccolithophores and nanoflagellates. Carbon content was calculated following the method of Menden-Deuer et al. (2000) in which biovolume was estimated assigning a geometrical shape to each species (Hillebrand et al., 1999). The biomass is the result of multiplying the carbon content of a species by its abundance in the sample. The studied area supports important fishes of commercial interest, therefore plankton biodiversity records are neccesary to understand possible shifts at the population and community levels that might have cascading effects on marine ecosystems' productivity.