Trophic state, nutrient dynamics and net metabolism of a subtropical coastal lagoon (Gulf of California) receiving wastewater

Abstract

The trophic state, nutrient dynamics and net metabolism of a subtropical coastal lagoon located on the east coast of the Gulf of California were evaluated. The Santa María lagoon is a receiving system for urban, shrimp farming, and agricultural wastewater. In the lagoon and adjacent sea, water samples were taken in summer and winter to gather information about temperature, salinity, dissolved oxygen, dissolved inorganic nutrients, and chlorophyll a. The trophic state was evaluated by the TRIX index, while nutrient fluxes and net ecosystem metabolism were estimated by the LOICZ biogeochemical model. The lagoon exhibited a mesotrophic state both in summer and winter. Estimations of the flushing time were 22 days in summer and 10 days in winter. The nutrient fluxes showed that the lagoon was a sink in summer (ΔNID = –0.84 mmol/m2/and y ΔFID = –0.12 mmol/m2/day) and a source in winter (ΔNID = 0.04 mmol/m2/day and ΔFID = 0.15 mmol/m2/day). In summer, the net ecosystem metabolism (NEM) was autotrophic (13 mmol C/m2/day) and the nitrogen fixation process predominated (+ 1.2 mmol/m2/ day). In winter, the NEM was heterotrophic (–16.36 mmol/m2/day) and denitrification predominated (–2.3 mmol/m2/day). It is concluded that the Santa María lagoon functions by assimilating nutrients with a predominance of autotrophic metabolism and nitrogen fixation in summer, as well as exporting nutrients with a predominance of heterotrophic metabolism and denitrification in winter. The lagoon maintains a mesotrophic state in both seasons, showing the capacity to assimilate, transform, and export nutrient loads without manifesting adverse eutrophication processes.