Hydrographic characteristics off the coast of Bahia Magdalena, BCS, Mexico during El Niño 2015.

Abstract

During 2015 an El Niño was recorded in the Eastern Pacific coast, which produced different environmental effects along the coasts of Mexico. The aim of this study was to describe the hydrological conditions,nutrients and chlorophyll variations along the coast of Bahia Magdalena, during 2015. Monthly samples were obtained during neap tides in 9 sites located within the lagoon margins. Temperature and salinity were measured in each sampling site using a CTD. Surface, midwater and bottom samples of seawater were obtained to measure nutrient concentrations (ammonium, nitrite, nitrate, phosphate and silicate) and chlorophyll a. The sea surface temperature data evidenced the presence of the El Niño, with values above the annual average (> 2.5 oC in July and August) with the domain of Subtropical Surface Water in the second semester. The water column was generally mixed, except in the warmer months, when some sites showed stratification. Nutrients mean values at surface,except silicate, tended to be lower than previously recorded, especially in the upwelling season. Chlorophyll data in the water column showed a wide range (0.05 to 12.94 mg m-3) with maximum values in midwater and bottom from April to June and minimum values in August and September. The warming effect produced by El Niño in neritic waters could have being mitigated by local processes, such as turbulent mixing, which may have carried nutrients to the mixed layer maintaining high biomass of phytoplankton and zooplankton. However, changes in Magdalena Bay were sufficient to modify the trophic web and produce an impact on the distribution and abundance of organisms in the area.