Factors shaping microbial community structure and dissolved organic matter composition during austral spring in the San Jorge Gulf

Abstract

Marine biological productivity depends on phytoplankton and heterotrophic bacterial interactions, influencing the energy and carbon transfer to higher trophic levels including fish, birds and marine mammals. The aim of this project was to study major factors governing the microbial food web and dissolved organic matter characterization in the San Jorge Gulf, the largest basin of the Patagonian Shelf. San José Gulf campaign was carried out during austral spring 2017. Surface water samples were collected for biological (chlorophyll a, microbial abundances) and chemical analyses (dissolved inorganic nutrients, colored dissolved organic matter (CDOM), humic substances). Moreover, the Simpson stability parameter was calculated to estimate water mixing condition based on CTD data. A clear district of the stratified waters observed mostly in the central part of the Gulf from the well-mixed coastal areas was observed. Stratified waters were characterized by higher small phytoplankton (pico- and nanophytoplankton), heterotrophic bacteria, nanoflagellates abundances and the relative contribution of protein-like (C1) compound. Lower temperature and higher wind speed observed in southern coastal area extending over the South Bank and southern edge of the Gulf contributed to well mixed water conditions likely supporting larger phytoplankton cells growth. Additionally, higher biological index (BIX), the relative contribution of humic-like (C4) compound and humic substance concentrations were recorded. Northern coastal waters were characterized by higher salinity, the relative contribution of humic-like (C2, C3) compounds and humification index (HIX).