Trophic Conditions and Stoichiometric Nutrient Balance in Subtropical Waters Influenced by Municipal Sewage Effluents in Mazatlán Bay (SE Gulf of California)

This work presents results of a study carried out between February 1995 and August 1996, on the trophic state of coastal waters influenced by a sewage outfall in Mazatlán Bay. Dissolved nutrients (ammonia, nitrate, nitrite, total inorganic nitrogen, reactive silicate and inorganic phosphate) and phytoplankton were used as parameters. The relative abundance by phytoplanktonic group changed in the different sites, in some sites (monthly sampled) bacillariophyte was the best-represented group with abundance ca. 70%. In additional sites around outfall and where red tides blooms occurred, this group was lower representation on the phytoplanktonic community. The application of two trophic indices based on nutrient concentrations corroborated along with the Shannon–Wiener diversity index indicates that Mazatlán Bay can be regarded as a predominantly eutrophic system. Additionally, the stoichiometric balance criterion in all selected sites confirmed that the mean ratio for the nutrients calculated does not reach the Redfield ratio Si:N:P of 16:16:1.