Seasonal fluctuations of nutrients in a hypersaline Mediterranean lagoon

The Korissia lagoon is located in the southwest of Corfu Island, Greece. It is a shallow coastal ecosystem of great aesthetic and ecological importance, included in the Natura 2000 network of protected areas. The lagoon is connected to the sea by an artificial channel and its depth varies between 0.5 and 1.2 m. During summer it is hypersaline (salinity exceeds 50 psu) and surface temperature is 23.6–26.1°C, while DO values are 75–100%. On the other hand, in winter salinity falls to about 19 psu, surface temperature is 13.5–16.2°C and DO varies between 91 and 115%. Salt water intrusion occurs only to wells adjacent to the lagoon. Ammonia, nitrites, nitrates, phosphates and total organic carbon have been measured in six surface water samples from the lagoon as well as in 10 well water samples collected from wells adjacent to the lagoon in two sampling periods, September 2005 and March 2006. Ammonia is the main dissolved inorganic N (DIN) form in the lagoon (79%), while nitrates predominate in the wells (99%), with values indicating fertilizers contamination. In general, DIN forms in the lagoon tend to increase from late summer to late winter and especially ammonia whose values are strongly related to sedimental N. On the other hand, nitrates values are affected by water exchange between the lagoon and the Ionian Sea. The presence of increased amounts of phosphates in the wells constitutes one more proof of extensive use of fertilizers in the region. On the contrary, P is the limiting factor in the lagoon on both sampling periods. According to eutrophication criteria, the lagoon is eutrophic concerning N and oligotrophic concerning P. Generally, the region’s well water is proper for human consumption, as far as the chemical parameters presented in this study are concerned, based on Council Directive 98/83/EC. Some wells are excepted because of increased salinity, nitrates and ammonia. Increased TOC values were measured in the lagoon where it ranged between 4.12–5.65 mg/l and in the wells between 0.13–2.93 mg/l. The results of our study indicate that Korissia lagoon is an interesting and probably fragile system that needs further study and a scientific management plan in order to avoid environmental degradation.