Exchange Dynamics Through the Mouth of a Coastal Lagoon
Georgios K. Sylaios, Vassilios A. Tsihrintzis, Christos Akratos, Athanassios C. Tsikliras, and Kiriaki Haralambidou
The tidal circulation and the temporal variability of water quality parameters (salinity, nutrients and chlorophyll-a) at the mouth of a coastal lagoon in Northern Greece are described. Vassova is a small and shallow coastal lagoon located at the western bank of Nestos River, in the region of East Macedonia. The lagoon is a closed system (i.e., there is no freshwater input except directly from rainfall and through limited seepage from adjoining agricultural lands). The lagoon has a single entrance canal, which connects its main basin to the open North Aegean Sea (Kavala bay), thus considered as the only water renewal mechanism for this system. Bottom and side friction acting at the entrance channel modifies and distorts the form of the incoming tidal wave. This study is based on an intensive sampling program of physical and chemical parameters obtained at the mouth of Vassova lagoon during 4 separate tidal cycles, aiming at understanding tidal circulation and estimating the instantaneous and residual fluxes of water, salt and nutrients. Field data of hydrographic and water quality parameters were collected under various tidal conditions (neap, intermediate and spring tidal cycles) and under winter and summer circulation, to account for the fortnight and seasonal effects in the lagoon. The analysis showed that the Eulerian residual transport and the Stokes drift are under certain conditions positive in direction, thus pushing water, salt and nutrients inward Vassova lagoon. The water volume entering the lagoon during flood was nearly double that leaving during ebb. Freshwater into the lagoon was found dependent only on precipitation. A near zero return flow factor was calculated showing the important flushing of Vassova lagoon due to the presence of coastal alongshore currents in Kavala Gulf. Lagoon’s residence time was found of the order of 4 to 5 days, dependent mostly on the tidal status prevailing in the area.