Coupling between the thermohaline, chemical and biological fields during summer 2006 in the northeast continental shelf of the Gulf of Cádiz (SW Iberian Peninsula)

Abstract

SUMMARY:The coupling between the thermohaline, chemical and biological variables on the northeast continental shelf of the Gulf of Cádiz was determined during the Emigas I survey in summer 2006. Samples were collected to chemically characterize the different water types and to analyze the chlorophyll a distribution. Four different water masses were identified: North Atlantic Central Water (NACW), Surface Atlantic Water (SAW), considered a modification of NACW, and South and North Surface Waters. The highest nutrient levels were found in subsurface NACW, while surface waters were almost nutrientdepleted except in the Guadalquivir region. The isopycnal level of 26.3 kg m -3 marked the limit between nutrient-rich NACW and nutrient-poor surface waters. At the offshore stations, the subsurface cholorophyll a maximum was located at the depth of nitracline and associated with the 26.3 kg m -3 isopycnal level rather than with the pycnocline depth. At the inshore stations, chlorophyll a maxima were observed at the bottom, except for the surface maximum in the River Guadalquivir region.Keywords: nutrients, chlorophyll a, nitracline, River Guadalquivir, Gulf of Cádiz. RESUMEN: Acoplamiento de las variables termohalinas, químicas y biológicas durante el verano 2006 en la parte noreste de la plataforma continental del golfo de Cádiz (SO de la peninsula Iberica). -Durante el verano 2006 se llevó a cabo una campaña oceanográfica en la parte NE de la plataforma continental del Golfo de Cádiz para determinar el acoplamiento de las variables termohalinas, químicas y biológicas. En cada estación se tomaron muestras para la caracterización química de las diferentes masas de agua y el análisis de la distribución de clorofila-a. Se diferenciaron cuatro tipos distintos de masas de agua: North Atlantic Central Water (NACW); Surface Atlantic Water (SAW), que es una modificación de la primera y las aguas superficiales del Norte y del Sur. Las concentraciones más elevadas de nutrientes se encontraron asociadas con las aguas profundas (NACW); en cambio las aguas superficiales estaban prácticamente agotadas en nutrientes, con excepción de las estaciones próximas a la desembocadura del Río Guadalquivir. El límite entre NACW rica en nutrientes y las aguas con bajas concentraciones de nutrientes fue establecido en el nivel isopícnico de 26.3 kg m -3 . Para las estaciones exteriores, el máximo subsuperficial de clorofila-a no se encontró en la posición de la picnoclina sino en la de la nitraclina, asociado al nivel isopícnico de 26.3 kg m -3 . Para las estaciones interiores, el máximo de clorofila se localizó cerca del fondo, con excepción de la región del Río Guadalquivir, donde se observó un máximo superficial.

Author comments_Factors controlling pCO2 variability in the eastern Gulf of Cádiz (SW Iberian Península)

“…I notice that given the mean PO4 of 0.28 this mean NO3 is much less than what is expected from Redfield, is this typical for the area? AC: Low N/P relationships are typical for this study area (Anfuso et al, 2010). This information has been added in the text: Line 273-275: "The mean N/P ratio in surface waters for the whole study was 3.5 ± 2.0, similar to that estimated by Anfuso et al (2010) in the northeast continental shelf of the Gulf of Cádiz, which indicates a relative phosphate deficit with respect to the Redfield ratio (Redfield et al, 1963)".…”

<i>p</i>CO<sub>2</sub> variability in the surface waters of the eastern Gulf of Cádiz (SW Iberian Peninsula)

“…For this reason, the Gulf of Cádiz presents characteristics typical of this system: seasonal variability of a winds system favourable to the coastal upwelling (Fiúza et al, 1982), high biological productivity (Navarro and Ruiz, 2006), a system of fronts and zonal currents (García Lafuente and Ruiz, 2007) and a zone of water exchange between the coastal zone and open ocean (Sánchez et al, 2008). However, the fact that the coastline of the study area runs more in a W-E direction than the overall N-S direction, common to all the eastern boundary upwelling system phenomena, and the bilayer flow through the Strait of Gibraltar are two factors that complicate the simple eastern boundary upwelling system conceptual model (Arístegui et al, 2009;Peliz et al, 2009).…”

Biodiversity and distribution of macroinfauna assemblages on sandy beaches along the Gulf of Cadiz (SW Spain)

“…This region is characterized by a mean water-surface temperature ranging from 18°C to 22°C, a salinity range of 36.3‰ to 36.5‰, an average nitrate, phosphate and silicate concentration of about 0.33, 0.08 and 1.37 μM, respectively (Anfuso et al 2010), and a chlorophyll a concentration of around 10-40 mg m -2 (Prieto et al 1999). These features provide a suitable habitat for the development of several species, which makes this system a very diverse and productive area (Sobrino et al 1994).…”