Metal and nutrient dynamics in a eutrophic coastal lagoon (Obidos, Portugal): the importance of observations at different time scales.

Water and sediment quality was monitored at four sites of Obidos coastal lagoon (Portugal) in February, May, July and October 2006, covering different hydrological conditions. Concentrations of nutrients and metals increased in autumn/winter, particularly in an inner branch with symptoms of eutrophication that receives a small tributary contaminated by agro-industrial activities. Moreover, concentrations of PO(4)(3-), Si(OH)(4) and Mn (diffusive gradients of thin films (DGT)-measured) varied inversely with salinity. Additionally, that branch was monitored over 26-h in July 2006 to assess variations of water quality parameters, nutrients and metals on short timescale. During the night, O(2) in water reached a minimum of 40% saturation followed by a pronounced increase of DGT-measured metals and nutrients in water column: Fe and Mn (ten times); Cr, Co, PO(4)(3-) and Si(OH)(4) (six times). Enhancements were also registered for metal/Al ratios in suspended particulate matter: Mn, Cr and Cd (four to six times); Fe, Ni and Co (1.5 times). The metal distribution coefficients calculated along the 26-h survey showed a maximum at daylight suggesting a preferential association of metals with suspended particles. Data recorded under different hydrological conditions and over the 26-h survey allowed to address the influence of external and internal sources on water quality. The results of this study highlight the importance of day/night cycles on the availability of nutrients and metals in eutrophic environments.