Water quality of Vistonis Lagoon, Northern Greece: seasonal variation and impact of bottom sediments

The aim of this study is to present the seasonal variation of nutrients in the water column and the bottom sediments of Vistonis Lagoon, a hypereutrophic Mediterranean coastal lagoon located in Northern Greece, and to estimate the impact of bottom sediments on the water quality of this lagoon. Nutrient concentrations in the water column and in bottom sediments were determined throughout seven seasonal sampling cruises from May 2003 to October 2004. Physicochemical parameters, such as transparency, temperature, salinity, pH and dissolved oxygen, were measured in situ in the water column using suitable equipment. Nutrient concentrations in bottom water were found generally higher than those in surface water. Nitrogen, mainly as nitrates, and phosphorus are released into the water column from the bottom sediments, especially during the summer period. Anoxia in the bottom water, as well as resuspension of the sediments are the main factors affecting nutrient internal loading in this lagoon. An approximate calculation showed that total phosphorus release was about 80 mg m–2 d–1 for the period March– August 2004. Vistonis Lagoon restoration will be possible only through the minimization or elimination of both external and internal nutrient loadings.

Transparency in Vistonis Lagoon increased from the freshwater influence zone to the part of the lagoon near the tidal channels. Dissolved oxygen concentration in the bottom water was low, especially in March and August 2004, while supersaturation values were present in the surface water in spring and autumn periods, when photosynthesis was more intense. There was a significant spatial and temporal variability of nutrient concentrations in the water column of Vistonis Lagoon. Nutrient concentrations in bottom sediments are high and act as a significant pool that reloads the water column under both sediment resuspension and anoxia events. Particularly, phosphorus release from the bottom sediments, occurring mainly in the summer period, appears to increase the water column concentrations more efficiently than external loadings. An approximate estimation of total phosphorus release rate for March to August 2004 period showed that this rate is comparable to values estimated at other lakes. An integrated management plan should be implemented in this lagoon to reduce or eliminate nutrient external and internal loadings, and restore its water quality.