Effects of the intense summer desiccation and the autumn filling on the water chemistry in some Mediterranean ponds

The objective of this research was to compare the effects of evaporation and total desiccation, as well as the influence of the rate of relling, on the concentrations of major ions and nutrients (nitrogen and phosphorus) in Mediterranean ponds. Water level uctuations in these ecosystems can have a decisive role in community structuring and functioning and may affect their conservation values. The results obtained enable an assessment of the consequences of water quality changes resulting from the increased evaporation, drastic fluctuations in water levels and higher incidence of droughts in summer. Desiccation was observed to cause an increase in the bicarbonate and chloride concentrations, whilst sulphate content was frequently found to have decreased. With respect to nutrients, a drastic reduction in water volume increased orthophosphate levels, with a consequent drop in the N-nitrate: SRP ratio. Pond refilling following total desiccation resulted in higher total anion content, especially among the waters with higher mineral content, the extent of which was related to the rate of refilling. When refill followed desiccation, sulphate concentrations increased the most, whilst alkalinity decreased in most ponds. An increase in chloride concentration was only observed when refilling occurred slowly. Orthophosphate and nitrate concentrations increased at the start of the refilling period, although the most pronounced increase was in nitrate which, together with potassium, was probably washed into the ponds through soil leaching following autumn rainfall.