Hydrography, oxygen saturation, suspended particulate matter, and chlorophyll- a fluorescence in an oceanic region under freshwater influence

Abstract

During the northern hemisphere summer of 1999, we analyzed the hydrography, oxygen saturation, suspended particulate matter, and chlorophyll- a fluorescence of the Southern Gulf of Mexico, an oceanic region under the influence of freshwater from the Grijalva–Usumacinta Rivers system. This rivers system induces thermal and haline fronts within the Southern Gulf, with surface temperature and salinity increasing offshore. A westward shelf circulation is inferred from the tilting of isopycnes and cold water on the bottom of the shelf break. Oxygen saturation shows low values in the region under freshwater influence. The composition of suspended particulate matter shows high variability, with the inorganic fraction dominant over the organic fraction. At the edge of the continental slope, the organic fraction is very high within the euphotic layer. The vertical distribution of chlorophyll- a fluorescence indicates different trophic conditions within the water column at three studied regions: the eastern shallow region, the middle shelf, and the continental slope. We found mesotrophic or eutrophic waters at the chlorophyll- a maximum level and oligotrophic waters at the background level. The behavior of the analyzed variables enables us to identify three ecological regions: (1) the inner shelf, where chlorophyll- a fluorescence increases near the seafloor, related to the thermocline; in this region the euphotic layer comprises the entire water column; (2) the middle shelf, with a sharp maximum of chlorophyll- a at mid-depths, strongly associated with the thermocline; and (3) the outer region above the continental slope that shows a deep-level chlorophyll- a maximum associated with low-light conditions. We propose that the vertical structure of chlorophyll- a fluorescence in the study area is dependant on thermal and light structures, and in some cases on the thermal and haline fronts associated with the river plumes, as well as on the regional circulation pattern.