title: Pentadal variability (2000-2004) of thermohaline properties and chlorophyll a distribution in the NW and N Iberian shelf during the winter-spring transition
Creators: name: Nogueira, Enrique
name: Bode, Antonio
name: Varela, Manuel
name: Morán, Xosé Anxelu G.
Publisher: name: ASC 2004 - P - Theme session
Year: 2024
Subjects: Technologies and data
Ecosystem observation, processes and dynamics

description: No abstracts are to be cited without prior reference to the author.Temperature, salinity and chlorophyll a profiles obtained in the NW and N Iberian shelf during the winter-spring transition (March-April) between the years 2000 and 2004 were analysed in order to characterise oceanographic variability at regional (&gt;100 km) and intermediate (10-100 km) scales. Hydrographic features and phytoplankton distribution showed large regional-scale differences between the Galician shelf and the Cantabrian Sea when the Iberian poleward current (IC) still persisted in the NW region, as in 2000 and 2002. Under these circumstances, salinity and temperature displayed a clear along-shelf gradient. The position of the front between the relatively saline and warm waters (&gt;35.70, &gt;13.8 °C) distinctive of the IC and the relatively less saline and colder waters (&lt;35.55, &lt;13.0 °C) associated with the general anticyclonic circulation (AC) characteristic of the southern Bay of Biscay, varied from year-toyear (e.g. up to 8°W in 2000, and 6°W in 2002). When large precipitation, and thus runoff, occurred all over the region, as in 2001 and 2003, salinity and temperature presented a marked across-shelf pattern, and the thermohaline front is located over the shelf-break. In contrast, quite homogeneous conditions were observed in 2004, with only traces of IC water over the westernmost part of the shelf and lower-than-average precipitation. Mesoscale variability was largely associated with runoff and upwelling-downwelling processes. Haline fronts associated to freshwater discharges from rivers and estuaries occurred at mid-shelf or coastal locations (e.g. in 2000 between 6-7° W). Coastal upwelling was observed mainly in the W part of the shelf (e.g. at 42.5° N, off the Rías Baixas, in 2004) and in the vicinity of capes (e.g. at 7.6° and 6° W, Capes Ortegal-Estaca de Bares and Peñas respectively, in 2003). Most of the variability of the distribution of phytoplankton biomass (chlorophyll a) occurred at the meso-scale, not fully resolved by our sampling strategy. However, co-variation with the hydrographic field could be discerned, more clearly in the vertical than in the horizontal planes. Low chlorophyll a concentrations (i.e. &lt;0.5 mg·m-3) were found under water column mixing conditions (e.g. NW part of the shelf in 2002), while increased concentrations (i.e. &gt;1 mg·m-3) were associated to the development of the seasonal thermocline (e.g. East Cantabrian Sea in 2002) or the formation of haloclines (e.g. vicinity of the Rías Baixas in 2003). In the horizontal, relatively high concentrations were found in frontal areas or where convergent processes, such as anticyclonic eddies (e.g. at 6°W in 2002) or coastal downwelling (e.g. off the Rías Baixas in 2001), occurred. Coastal upwelling (e.g. at 5°W in 2003) and plumes associated to runoff (e.g. at 41°N in 2002), acted as dispersive agents, thus lowering chlorophyll a concentration.