Abstract:
The Tagus estuary (Portugal) supports diverse uses and activities and its
ecological value is well recognized. This study aimed to characterize the water quality in
the Tagus estuary during one year, as part of the Tagus estuary observatory. Three field
campaigns were performed in 2018 covering distinct seasons (Spring – May; Summer –
September; Autumn – November). Seven sampling stations were chosen, covering the
whole estuary and its water bodies (WB). At each station, physical, chemical and
biological data were collected during one semidiurnal tidal cycle (~12 hours).
Temperature, salinity, pH and dissolved oxygen were measured in-situ. Water samples
were collected to determine in laboratory the concentration of nutrients, chlorophyll-a and
total suspended solids. Results showed marked spatial gradients, typically with larger
concentrations of chlorophyll-a, nitrate and silicate upstream. Silicate and nitrate presented
a conservative behaviour, contrarily to ammonium and phosphate. Seasonally, the highest
nutrients and suspended solids concentrations were found in Autumn, after a period of
rainfall, pointing out to the relevance of land runoff for material supply into the estuary.
These conditions were favourable for phytoplankton development, once chlorophyll-a was
maximum during this campaign (24 μg/l in the upper estuary – WB4). Following Caetano
et al. (2016) the Tagus estuary was classified relative to the nutrients concentrations. Both
the Tagus WB1 and WB2 presented “Good” status. The Tagus WB3 and WB4 presented
“Medium” status due to, respectively, ammonium, and nitrate and phosphate
concentrations. The data acquired will be combined with historical data and numerical
modelling, to contribute to a better understanding of the biogeochemical buffering capacity
of the Tagus estuary and its susceptibility to climate change and anthropogenic pressures