Water quality trends in Polyphytos reservoir, Aliakmon River, Greece.

A water quality monitoring program was undertaken from June 2004 to May 2005, on a monthly basis, in Polyphytos Reservoir of Aliakmon River. Depth, water temperature, dissolved oxygen, conductivity, pH and transparency (Secchi disk) were measured in situ, while collected water samples were analyzed in the laboratory for the determination of BOD, COD, total phosphorus (TP), ortho-phosphate (OP), chlorophyll-a (Chl-a), ammonium, nitrite and nitrate, total Kappajeldahl nitrogen (TKN) and heavy metals (Fe, Mn, Cu, Cr, Pb, Ni and Cd). Measured concentrations were compared to those from two previous studies conducted in July 1987 to June 1988 and January 1991 to February 1993. The following conclusions are drawn: the effect of the watershed on the lake environment, mostly through Aliakmon River, is significant, and it accelerates the eutrophication of the lake. The anoxic zones, which were defined in the lake, reinforce this conclusion. Nitrate, nitrite and ammonia were measured at lower concentrations compared to previous studies, while total phosphorus and chlorophyll-a were found at increased concentrations. The current trophic state of Polyphytos reservoir is eutrophic, based on the OECD method and Carlson's Trophic State Indices (TSI). The concentration of BOD and COD ranged at low levels. Furthermore, the mean concentrations of metals Fe, Mn, Cu, Cr, Pb, and Cd were below the potable water standards set by WHO and EU. During most part of the study period the ratio N/P for Polyphytos reservoir was higher than 7.2:1, and therefore, phosphorus is the limiting nutrient for algal growth.