Variability of the trophic state and phytoplankton biomass of Bahia Concepcion, Gulf of California (1997-1999)

Abstract

To determine the effect of the phytoplankton biomass and the trophic status in Bahía Concepción in the centralwest part of the Gulf of California an hydro-biologic study was conducted from 1997 to 1999. Temperature, dissolved oxygen, inorganic nutrients (nitrate, nitrite, phosphate and silicate) and phytoplankton biomass (chlorophyll a) concentrations were measured. Hydrographic conditions shown a mixing period (October-April), two transition periods (April-May and September-October) and a stratification period (June-September). Warmest temperatures were observed during 1998 and coldest in 1997. Dissolved oxygen values shown an hypoxic layer (<2.7±0.78 ml l-1), during the stratified period below 20 m depth. During the transition period two peaks of phytoplankton biomass were observed, on spring and summer, which matched with those reported for coastal lagoons at similar latitudes, however the highest biomasses were observed in the stratification period, particularly below 15 m depth. The scale of nutrients showed an eutrophy for phosphate and silicate all the year. The trophic index revealed an oligotrophy state for phosphate, nitrate and nitrite had a dominant status with the exception for the transition periods, when the mesotrophic conditions were presents. These results suggest that phytoplankton biomass is associated with the natural inputs of nutrients and that trophic conditions (mesotrophy or eutrophy) were determined for the hydrographic of the area.