Marta Nogueira

Spatial and seasonal variation of water quality in an impacted coastal lagoon (Óbidos Lagoon, Portugal).

The spatial distribution of silicate, ammonium, nitrate, nitrite, phosphate, chlorophyll a and dissolved oxygen in Óbidos lagoon was obtained by surveying five sites in eight campaigns, between October 2004 and October 2006. A confined inner branch of the lagoon showed higher availability of ammonium (1.2–81 μmol l−1), phosphate (1.9–17 μmol l−1), silicate (0.85–86 μmol l−1) and chlorophyll a (0.30–18 μg l−1) than other sites (0.47–25 μmol l−1, 0.10–3.9 μmol l−1, 0.47–25 μmol l−1, 0.25–11 μg l−1, respectively). According to several trophic classification tools, that branch is considered eutrophic to polytrophic, emphasising its deteriorated conditions, while the rest of the lagoon is of better quality. In autumn/winter nutrients were inversely correlated to salinity (r>0.93) reflecting the freshwater inputs enriched in nitrogen and phosphorous compounds to the inner branch. In warmer periods, dissolved oxygen concentrations dropped during the night, and sediments of the branch become an important source of ammonium and phosphate. The low DIN:P ratio (median=10) obtained in the branch, which suggests an excess of phosphate, that increased in warmer periods and changed the limiting nutrient in the entire lagoon. These results emphasize the spatial heterogeneity of water quality in Óbidos lagoon, its seasonal variability, and the importance of recognising these distributions before defining homogenous water body on the scope of Water Framework Directive.

Mercury in sediments and vegetation in a moderately contaminated salt marsh (Tagus Estuary, Portugal).

Depth variations of total mercury (Hg) and methylmercury (MeHg) concentrations were studied in cores from non-colonized sediments, sediments colonized by Halimione portulacoides, Sarcocorniafruticosa and Spartina maritima and belowground biomass, in a moderately contaminated salt marsh (Tagus Estuary, Portugal). Concentrations in belowground biomass exceeded up to 3 (Hg) and 15 (MeHg) times the levels in sediments, and up to 198 (Hg) and 308 (MeHg) times those found in aboveground parts. Methylmercury in colonized sediments reached 3% of the total Hg, 50 times above the maximum values found in non-colonized sediments. The absence of correlations between total Hg concentrations in sediments and the corresponding MeHg levels suggested that methylation was only dependent on the environmental and microbiological factors. The analysis of belowground biomass at high-depth resolution (2 cm) provided evidence that Hg and MeHg were actively absorbed from sediments, with higher enrichment factors at layers where higher microbial activity was probably occurring. The results obtained in this study indicated that the biotransformation of Hg to the toxic MeHg could increase the toxicity of plant-colonized sediments.

Mercury and methylmercury transport and fate in the water column of Tagus estuary (Portugal)

Six campaigns were performed in North Channel (CNOR), Barcas Channel (BC) and lower zones (EZ) of Tagus estuary to better understand methylmercury (MMHg) and mercury (Hg) transport and fate. Highest concentrations of particulate and dissolved MMHg were observed in CNOR in bottom waters and in the warmest months. The MMHg distribution coefficients between particulate and dissolved fractions were mainly influenced by particulate matter and dissolved organic carbon. The values were slightly higher in summer than in winter and in CNOR. Overall, results established that the tidal effect is a main driver on the transport and fate of Hg and MMHg from CNOR to outer areas, evidenced by the exportation of the Hg species from CNOR to the upstream station in high tide and to the downstream one in low tide. Therefore, CNOR may be considered a source of Hg and MMHg to the outer estuary.

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.