Maria José Brogueira

Physical, chemical and sedimentological aspects of the Mediterranean outflow off Iberia

A multidisciplinary study of the Mediterranean outflow in the region west of the Strait of Gibraltar and off the southern and southwestern coasts of the Iberian Peninsula was developed in the frame of the Canary Islands Azores Gibraltar Observations Project. Two high-resolution CTD surveys, which included water sampling for chemical (nutrients and dissolved oxygen) and sedimentological analyses, took place in September 1997 (summer cruise) and January 1998 (winter cruise) in the study region. The correspondence between the high-salinity Mediterranean Water (MW) layer and the low-nutrient content and relatively high abundance of particles was a general result. Further details of the thermohaline analysis and of the geostrophic computations, especially for the layer of MW, are compared with the results obtained for the chemical properties and the sedimentological characteristics.

Intermediate water masses off south-southwest Portugal: Chemical tracers

Biogeochemical tracers, as dissolved oxygen and nutrients, were measured during several surveys carried out between 1999 and 2001, off south-southwestern Portugal. The dense vertical and horizontal sampling allows the extensions of intermediate water masses present in the region to be resolved. A subsurface minimum salinity water layer, exhibiting values of apparent oxygen utilization (AOU) in the range 40-60 μmol/kg and intruding into the study area mainly from the south-southwest, is proposed to be a remnant of the Subarctic Intermediate Water (SAIW). A deeper salinity minimum displaying high concentrations of nutrients and AOU (∼100 μmol/kg) and identified as a branch of the Antarctic Intermediate Water (AAIW), is observed to derive from the western African coast and to penetrate the area predominantly from the south-southeastern side. The salty and warm Mediterranean Water (MW) present in the same density range as the fresher intermediate waters, reveals differentiated chemical properties at the various depths depending on the surrounding waters. Analysis of water masses gives an indication that the collapse of the fresher waters into a narrow range of salinity values (35.58-35.64) at intermediate levels is favored by the strong presence of the MW outflow, and that the admixture of the fresher waters with the MW outflow very likely induces the formation, splitting and spreading of several MW cores extending westwards along the Portuguese coast.

Aspects of Phytoplankton Communities Response to Climate Changes

Climate changes concerning shifts in seasonal dominant winds and increases in atmospheric CO2 are underway. Thus, changes are occurring in coastal upwelling regimes as well as increases in CO2 absorption by the ocean/acidification impacting phytoplankton communities in terms of abundance and diversity. In order to illustrate these apparent changes we present results obtained from coastal waters adjacent to the Tagus estuary under different hydrological conditions. Upwelling events prevailed in winter 1994, were absent in winter 2001 and were present in summer 2002. Chemical and biological properties are examined: in March 1994, a strong bloom of phytoplankton developed (chlorophyll a up to 40 mg m–3) which was attributed to the combined effect of intense freshwater runoff and upwelling, leading to the establishment of a strong frontal boundary and a supply of a considerable amount of nutrients. In March 2001, under an extremely intense river discharge and absence of upwelling, only values up to 1.5 mg m–3 of chlorophyll a were measured. On the other hand, in June 2002, when Tagus river inflow was reduced and nutrient levels were quite low, chlorophyll a levels attained 5 mg m–3 despite the occurrence of upwelling.