Chlorophyll a and chemical signatures during an upwelling event off the South Portuguese coast (SW Iberia)

The present paper describes the chemical characteristics and biological implications during an upwelling event that took place in the beginning of October 2006 off the South Portuguese coast. The study is based on a total of 90 CTD casts regularly distributed in 10 meridional 35 km long sections, along with wind and current ADCP velocity measurements. Stations were sampled from the surface to a maximum depth of 900 dbar. Five sections covering the entire region, including the main areas of influence of rivers outflow, were selected for characterization of dissolved oxygen, chlorophyll a and nutrients (nitrate, phosphate and silicate). Previous and during the cruise time, northwesterly winds, with an upwelling favourable component, were predominant and upwelling extended far eastward into the Gulf of Cadiz, which is uncommon for this time of the year, in the end of the upwelling season. In the coastal stations, cold low salinity waters with high chlorophyll a (maximum 3.1 μg l−1) and low nutrient concentrations were observed, particularly in the retention area close to the western limit of the Gulf of Cadiz. No influence from rivers discharges was observed, since the survey was carried out during a dry period in autumn. Offshore, warmer and saltier waters recirculating from the Gulf of Cadiz, typically oligotrophic (low concentrations of chlorophyll a and nutrients), were observed. Deeper waters along the continental slope, at levels close to 400 dbar, reveal the influence of the Mediterranean outflow, characterised by an increase of temperature and salinity along with a decrease of nutrients. The present study reveals that although the inhomogeneities observed along the South Portuguese coast, the water characteristics are in conformity with those of the Gulf of Cadiz and similar to those observed in the NW Iberia and NW Africa, showing a continuity of the upper layers of northern branch of the Canary Upwelling System.