Seasonal evolution of cholorophyll-<i>a</i> and cyanobacteria (<i>Prochlorococcus</i> and <i>Synechococcus</i>) on the northeast continental shelf of the Gulf of Cádiz: relation to thermohaline and nutrients fields

Abstract

SUMMARY:The seasonal evolution of nutrients, chlorophyll-a (chl-a) and cyanobacteria (Synechococcus and Prochlorococcus) on the northeast continental shelf of the Gulf of Cádiz was observed during four cruises in summer and autumn 2006 and winter and spring 2007. Samples were collected to determine the distribution of chl-a and cyanobacteria abundance and to analyse their coupling with thermohaline and chemical properties in the gulf region. Surface nutrient distributions showed clear seasonal variability. Maximum levels were recorded for the entire study region during winter due to winter mixing and the strong influence of the Guadalquivir River and Bay of Cádiz outflow. Maximum chl-a levels were reached during spring for the entire region and for the entire water column. Minimum values of chl-a were observed during summer, under prevailing oligotrophic conditions, and winter when in spite of relatively high nutrient concentrations, phytoplankton growth was probably light-limited. The distribution of both cyanobacteria populations also varied seasonally in association with the oceanographic conditions. Prochlorococcus maximum cell abundance was observed at the sea surface during the four cruises, except in the northern region during summer. However, Synechococcus showed a maximum concentration at the sea surface in autumn and winter and at the subsurface chlorophyll maximum in autumn and spring.Keywords: chlorophyll-a, cyanobacteria, seasonal evolution, Guadalquivir River, Gulf of Cádiz.RESUMEN: Evolución Estacional dE clorofila-a, y cianobactEria (Prochlorococcus y synechococcus) En la partE noroccidEntal dE la plataforma continEntal dEl golfo dE cádiz: rElación con las propiEdadEs tErmohalinas y dE nutriEntEs. -Durante cuatro campañas, verano y otoño 2006 e invierno y primavera 2007, se examinó la evolución estacional de nutrientes, clorofila-a (chl-a) y cianobacterias en la parte noroccidental de la plataforma continental del Golfo de Cádiz. Se tomaron muestras para el analisis de la distribución de chl-a y cianobacterias y para analizar su acoplamiento con las propiedades termohalinas y químicas en la región del golfo. La distribución superficial de nutrientes presentaba una clara estacionalidad, de hecho las máximas concentraciones se registraron en toda el área de estudio durante el invierno debido a la mezcla invernal y a la fuerte influencia del los aportes del rio Guadalquivir y la Bahía de Cádiz. Los máximos niveles de chl-a se alcanzaron en primavera en toda la región y en toda la columna de agua. Mínimos valores de chl-a se observaron en verano, cuando prevalecían condiciones oligotróficas y en invierno cuando, a pesar de una alta concentración de nutrientes, el crecimiento del fitoplancton estaba limitado por la luz. La distribución de ambas poblaciones de cianobacterias varió tambien estacionalmente asociada a las condiciones oceanográficas. La máxima abundancia de células de Prochlorococcus se observó en aguas superficiales durante las cuatro campañas, a excepción de la región norte en verano. Po...

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