Factors affecting the current status of a eutrophic shallow lake (Lake Uluabat, Turkey): Relationships between water physical and chemical variables

Abstract

Lake Uluabat (Apolyont), a protected wetland area, the shorelines of which host urban and industrial sites, is influenced by a high human impact. The lake has eutrophic hardwater characteristics. Temperature, pH value, total dissolved substance, electrical conductivity, dissolved oxygen, biochemical oxygen demand, total hardness, carbonate and bicarbonate alkalinity, nitrate, soluble reactive phosphate, sulphate, soluble reactive silica, secchi disc depth, water depth, water volume, and chlorophyll a values were measured. Physical and chemical variables were determined from samples taken monthly at 10 stations from July 1998 to June 1999 to assess the present status of the lake. Significant correlations between physical and chemical variables were observed, and these relationships reflected the interactions among internal nutrient loadings. Two-way ANOVA test results also showed spatial and temporal variations. Internal nutrient loadings have been stimulated by a combination of different factors. Principal-component analyses showed that pH and alkalinity were the major factors that influenced the internal nutrient loadings in the lake. Results also indicated that nitrate, sulphate, and soluble reactive silica concentrations significantly induced the soluble reactive phosphorus loading. Direct or indirect interactions among primary nutrients were important in the nutrient loading in Lake Uluabat. Mustafakemalpaşa stream and its two branches Orhaneli and Emet, which drain into the lake, receive sewage water from households and industry. For this reason, soluble reactive phosphate, sulphate, nitrate, nitrite, ammonia, boron, and suspended solids were determined in samples collected bi-monthly from the Ayazköy station located on the Mustafakemalpaşa stream during the same study period to determine the effects of the main streams on the lake. Results indicated that the pollution load of Lake Uluabat is highly affected by Mustafakemalpaşa stream inflow. In addition, the present nutrient status, compared with previous years’ data (1986–1990), collected only during summer months, showed that the physical and chemical characteristics of the lake have changed according to former years. The former and recent years’ data also showed that the fluctuation of the water depth also influenced the chemical composition of the lake water. According to qualitative and quantitative observations, the effects of human impact and current status of the lake were determined. The physical and chemical characteristics of the lake water have changed according to human activities and nutrient loadings. Although external nutrient loadings are important because of the inflow of pollutants, internal nutrient loadings are the major factors that determined the trophic status of the lake. In the light of all findings, the lake balance is going to be disturbed, and some measures must be taken to prevent these circumstances.