Abstract
Sixteen monthly cruises were carried out at a station 6.5 km to the NW of the mouth of Magdalena Bay (Mexico) to study the temporal variability of phytoplankton biomass and primary production rates, and the response of phytoplankton to the prevailing hydrographic characteristics. During each cruise, temperature and salinity were measured in the water column, and discrete samples were collected to measure inorganic nutrient concentrations (nitrate, phosphate, and silicate) and chlorophyll a content (as a proxy for phytoplankton biomass); in addition, in situ incubations were performed to estimate primary production (PP) rates using the 14C technique. During spring and early summer (March–July), the water column at the study site was cold (14.4 ºC at 80 m depth and 17.7 ºC at the surface) and well mixed, with high nutrient concentrations (nitrate = 6.39 μM, phosphate = 0.91 μM, and silicate = 11.87 μM) at the surface, typical of intense upwelling systems. On the contrary, from August to February the water column was stratified (23.5 ± 3.2 ºC above the thermocline and 17.0 ± 1.7 ºC at 80 m depth) and had low nutrient concentrations (nitrate = 3.90 μM, phosphate = 0.47 μM, and silicate = 10.30 μM) at the surface. Phytoplankton abundance, chlorophyll a concentration, and PP rates in the euphotic zone were higher during the upwelling season, with maximum cell density of 1.7 × 106 cells·L–1 (nanoplankton fraction), integrated chlorophyll a content of 231 mg·m–2, and integrated PP rates of 553 mg C·m–2·h–1). Abundance of cells <20 μm, as determined by flow cytometry, revealed the importance of the nanoplankton and picoplankton fractions in this region. This study showed the seasonal variability of phytoplankton communities (biomass and productivity) in waters off Magdalena Bay and the coupling of phytoplankton to environmental variability. Seasonal variability was similar to that observed for other typical coastal upwelling regions, that is, higher phytoplankton abundances and PP rates in spring and summer and lower values in autumn and winter.
 Key words: biomass, primary productivity, hydrography, coastal zone, Magdalena Bay.
Highlights
Temperature and salinity were measured in the water column, and discrete samples were collected to measure inorganic nutrient concentrations and chlorophyll a content; in addition, in situ incubations were performed to estimate primary production (PP) rates using the 14C technique
This study showed the seasonal variability of phytoplankton communities in waters off Magdalena Bay and the coupling of phytoplankton to environmental variability
En cada muestreo se midieron la temperatura y salinidad en la columna de agua y se determinaron las concentraciones de nutrientes inorgánicos y el contenido de clorofila a; además, se hicieron incubaciones in situ para estimar la producción primaria (PP) con el método de 14C
Summary
Coastal upwelling areas are the most productive zones in the marine environment, especially the ones in regions with eastern boundary currents (Hill et al 1998). High phytoplankton biomass (chlorophyll a ~150 mg·m–2) and primary production (~3.5 g C·m–2·d–1) values are observed during upwelling events. In the Peruvian-Chilean coastal system, one of the most productive coastal systems in the world (4 to 20 g C·m–2·d–1, Daneri et al 2000), upwelling of deep waters with high partial pressure of carbon dioxide, low temperatures, low oxygen concentrations, and high nutrient concentrations (e.g., Strub et al 1998) results in high phytoplankton productivity within a narrow coastal band This high productivity is mainly due to the contribution of large cells (>20 μm) such as diatoms, which is prompted by an increase in nitrate concentrations (Chavez and Messié 2009, Varela et al 2010). Samples collected in 200-mL flasks between April and June 2013 were fixed with Lugol acetate and analyzed to determine cell abundance and the main phytoplankton species during intense upwelling periods (Zaytsev et al 2003). The integration method used for the measured variables was the sum of the areas of simple polygons
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