Characterization of chemical parameters in surface sediments from a semi-enclosed frontal system: The case of the San José Gulf, Patagonia Argentina

Abstract

The San José Gulf (SJG) is a highly productive basin where a seasonal frontal system takes place, supporting a small-scale fishery on bivalves. In terms of benthonic resources, sediments and the overlying water column are strongly linked and both are affected by the prevalent bottom hydrodynamics. In this work, several chemical variables were assessed to evaluate sediment variability during the frontal system formation and were linked to grain size sediments and bottom water circulation. Four sectors were identified within SJG. Sector 1 comprises two reduced areas characterized by the highest contents of the studied chemical parameters (total organic matter, phytopigments, and total ammonium) due to low hydrodynamic conditions, which facilitate the sedimentation of fine grains and other particles. Sector 2 is characterized by moderate values of chemical parameters, encompassing the surrounding area of Sector 1. Sectors 3 and 4 represent the largest area of SJG, which show low concentrations of chemical variables associated with coarse-grain sediments and with the prevalence of high-energy hydrodynamic conditions. The higher amount of total organic matter in the whole SJG in spring than summer emphasizes the role of seasonal changes in temperate regions, with the formation of a thermocline during spring-summer. This is in concordance with the observed falling of redox potential in summer, suggesting consumption of the available oxygen by the activity of aerobic microorganisms and the transformation from oxidized to reduced sediments. Phytopigments and TOM were good descriptors of the environmental quality, being in the same order than those reported for marine coastal sediments in meso-eutrophic regions.