The abundance and distribution of total autotrophic picophytoplankton (PFP), temperature, salinity, PAR, and chlorophyll a were determined in two presumably contrasting environments: (1) two coastal areas (close to the mouths of three rivers), and (2) one oceanic area (Campeche Canyon), of the southern Gulf of Mexico, during the "dry season" (June-July, 2004). The picoprokaryotes Prochlorococcus and Synechococcus were identified by TEM, whereas Synechococcus and picoeukaryotes populations were also recognized by flow cytometry. The highest PFP abundance (1.67×105 cells ml-1) was found in shallow waters (~10 m depth) around the Grijalva-Usumacinta river mouth, followed by that found at a station close to the Coatzacoalcos River (1.19×105 cells ml-1); PFP abundances in the Campeche Canyon were usually lower (maximum 1.53×104 cells ml-1). Greater variability in PFP abundances was found in coastal stations than in oceanic waters, and weak relationships appeared between the patterns of chlorophyll a and PFP abundance. Peaks of PFP were detected in both coastal and more oceanic areas, but in the Campeche Canyon they were located deeper (60 m), relatively closer to the deep maximum of chlorophyll (located at about 75 m). Results suggest that PFP populations include a substantial photosynthetic component in both coastal and oceanic waters of the southern Gulf of Mexico.
Abundance; Distribution; Picophytoplankton; Prochlorococcus; southern Gulf of Mexico; Synechococcus