Trace-element Al composition of seston and plankton along the Portuguese coast

Surface water, seston. and material collected by 20 mum plankton nets mainly composed of phytoplankton were collected in February and July 2000. 1 mile from the mouth of major estuarine systems in Portugal and at two sites in the SW coast where freshwater inputs are negligible. Copper and Cd concentrations were higher in the NW coastal waters, evidencing the influence of river discharges on the water quality of this region. The levels of Cd reached extremely high values (19 nM) in the rainy season (February). The high values of seston (3.3-9.3 ma l(-1)), and Al content (1.7-3.0 mmol g(-1)) and low Fe/Al, Zn/Al, As/Al, Cu/Al and Cd/Al ratios confirm the presence of riverine-derived particles with a large lithogenic fraction. Under low-river flow conditions, Cu/Al and Cd/Al ratios were hi her (reaching 2.4 x 10(-3) and 27 x 10(-3), respectively), suggesting that the biogenic fraction of seston plays a major role in metal retention during this season. Comparison of metal distribution in seston and plankton-net material indicates that accumulation of As and Zn by phytoplankton along the coast rarely exceeds the levels existing in the lithogenic fraction. Otherwise, Cu and Cd accumulation appears as the dominant process in its distribution pattern along the coast. The distribution coefficients (Kd) of Cu and Cd were one order of magnitude higher in the SW coast, indicating a more effective transfer of these metals from water to the suspended phase, mainly to phytoplankton. (C) 2003 Editions scientifiques et medicales Elsevier SAS. All rights reserved.