Composition and abundance of phytoplankton in two areas of Gulf of California, one near (ND) and one far (FD) from shrimp farms discharge, were studied in 3 seasons: late fall (farms finishing operations); spring (farms not operating); and summer (farms operating). In ND, 61 diatoms, 33 dinoflagellates, 4 cyanobacteria, and 2 silicoflagellates were identified; in FD, 72 diatoms, 38 dinoflagellates, 5 cyanobacteria, and 4 silicoflagellates were found. Thirty-three species were recorded only in ND (20 diatoms, 11 dinoflagellates, 1 silicoflagellate), whereas 39 species appeared exclusively in the FD (28 diatoms, 9 dinoflagellates, 1 cyanobacteria, 1 silicoflagellate). Thirty-seven species were common for both areas (23 diatoms, 10 dinoflagellates, 3 cyanobacteria and 1 silicoflagellate). In ND, 9 species potentially toxic (3 diatoms, 5 dinoflagellates, 1 cyanobacteria) were identified. From FD, 3 species potentially toxic (2 diatoms and 1 cyanobacteria) were found. Total abundance of phytoplankton was more than double in ND than in FD. The species richness and diversity, were greater in FD. Higher phytoplankton abundance was observed when farms were operating or finishing operations. The composition and abundance of phytoplankton is a good indicator of shrimp effluents impact, diminishing the species richness and diversity, but augmenting the abundance.
