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

Environ Monit Assess (2009) 153:281292 DOI 10.1007/s10661-008-0355-x

Spatial and seasonal variation of water qualityin an impacted coastal lagoon (bidos Lagoon, Portugal)

Patrcia Pereira & Hilda de Pablo & Carlos Vale &

Vanda Franco & Marta Nogueira

Received: 6 November 2007 /Accepted: 14 April 2008 / Published online: 30 May 2008 # Springer Science + Business Media B.V. 2008

Abstract The spatial distribution of silicate, ammonium, nitrate, nitrite, phosphate, chlorophyll a and dissolved oxygen in bidos 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 (1.281 mol l1), phosphate (1.9 17 mol l1), silicate (0.8586 mol l1) and chlorophyll a (0.3018 g l1) than other sites(0.4725 mol l1, 0.103.9 mol l1, 0.4725 mol l1, 0.2511 g l1, 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 bidos lagoon, its seasonal variability, and the importance of recognising these distributions before defining homogenous water body on the scope of Water Framework Directive.

Keywords Coastal lagoon . Eutrophication . Nutrient dynamics . Sediment . Water Framework Directive

Introduction

Small rivers running towards the coast do not always reach the sea. Discharge to the ocean is often prevented by active beach-ridges, forcing water and suspended particulate matter to accumulate in small coastal lagoons. The ecology of the lagoons is determined to a large extent by freshwater inputs and the mixing and circulation processes with the adjacent sea (Postma 1981; Ittkoot et al. 2000). In lagoons with a permanent connection to the sea and strong tidal amplitudes, the circulation is highly influenced by semi-diurnal and fortnight tidal cycles,...