Dynamics of a temperate fluvial estuary in early winter

In order to evaluate how current speed, salinity and water temperature stratification
behaves under the influence of a freshwater source in a tidal domain, a 25 hour survey
campaign was performed in the upper sector of the Vouga River estuary, the major
source of freshwater of Ria de Aveiro, a coastal lagoon located in the Northwest of the
Iberian Peninsula.
The sampling campaign started at 10:30 AM of January 15th 2004 and ended next day,
covering two tidal cycles, during neap tide condition (tidal range of about 1.7 m) and low
river runoff (58 m3 s-1). Observations of the thermohaline properties and currents were
undertaken at an anchor station located about 9 km upstream of the mouth of the lagoon.
Local meteorological variables were also measured.
Vertically lunar hourly profiles of salinity, water temperature and current speed were
obtained and the results indicate that the estuary is forced by semi-diurnal tides. The
profiles of current intensity indicated an asymmetry between flood (v>0) and ebb (v<0)
being the ebb currents higher than the flood ones, with maximum values of about -1.1
and 0.7 m s-1, respectively. The major ebbing velocity was enhanced by the freshwater
runoff. The lunar hourly variations of salinity and water temperature reveal that during the
ebb, the vertical stratification decreases, increasing during the flood period. The salinity
ranges between 0 and 32 psu and the water temperature between 10.9 and 13.7 ºC.
Near the surface, the water temperature is influenced by the air temperature, decreasing
along the day. At mid-water and near the bed the salinity and water temperature
increases during the flood periods, decreasing during the ebb. From the time mean
salinity and current profiles for the 25 hours period were obtained the stratification and
circulation parameters of the classical Stratification-Circulation Diagram (Hansen and
Rattray, 1966), revealing that the estuary can be considered partially mixed-highly
stratified.