Oceanic phytoplankton structure off western Patagonia during the austral summer: Implications for harmful algal blooms

Resumen: Changes in phytoplankton composition and abundance are controlled by multiple factors, including physical forcing and nutrient stoichiometry. This study seeks to assess the interplay between the environmental and biogeochemical conditions in shaping the phytoplankton community structure in open ocean waters off western Patagonia. For this purpose, we used biological, hydrographic, and chemical data measured aboard the R/V Mirai during the austral summer of 2017, combined with remote sensing and reanalysis products. Although no first-order predictive relationships were found between the environmental and biological data, the results showed a latitudinal gradient of the phytoplankton structure, favoring maximum abundances of diatoms (∼10×10⁴ cells L^-1) in the northern area (∼43-48°S) characterized by high stratification and freshwater content. The high diatom abundances of Thalassiosira spp., Chaetoceros spp., Pseudo-nitzschia cf. australis and Pseudo-nitzschia cf. pseudodelicatissima taxa in this area results in a nutrient content decreases (nitrate < 9 μmol kg⁻¹, phosphate < 0.9 μmol kg⁻¹, silicic acid < 1.5 μmol kg⁻¹), and low N:Si (<8) and N:P (<10) ratios. Nonetheless, the total dinoflagellates abundance was equal to or higher than those of diatoms (∼10.2×10⁴ cells L^–1) throughout most of the sampling region. Specifically, maximum abundances (∼6.3×10⁴ cells L^–1) of Karenia spp. were found in the north under the freshwater influence from the Gulf of Penas, where the highest ammonium:nitrate ratio (∼4 to 11.5) was recorded. In contrast, high dinoflagellate abundances of Azadinium spp. were found in the south (∼48-55°S), where an increase of nutrient content (nitrate ∼12μmol kg^–1, phosphate ∼1 μmol kg^–1, ammonium ∼0.4 μmol kg^–1), and high N:Si and N:P ratios (∼35 and ∼12) were observed. This study highlights a major presence of potentially toxic phytoplankton species in the oceanic zone off northern Patagonia under conditions of strong latitudinal and cross-shore gradients in nutrient content and salinity, related to the interplay between oceanic water masses and freshwater input.