Lake Eymir Research Articles

Absence of typical diel vertical migration in Daphnia: varying role of water clarity, food, and dissolved oxygen in Lake Eymir, Turkey

Diel vertical migration (DVM) is a complex and dynamic behaviour against predation because the reaction of migrating organisms to light intensity plays a primary role, but is modified by other factors. In the relatively shallow but thermally stratified Lake Eymir, Daphnia pulex de Geers utilized vertical refugia afforded by the hypolimnion during both day and night. Differences in general vulnerability to fish predation determined the differences in their mean residence depths (MRDs) of different population categories such as most conspicuous and vulnerable individuals of adult with eggs inhabited the deepest depth, whereas juveniles stayed close the thermocline. In late spring, profoundly high amplitude of displacement within the hypolimnion, probably due to the hypolimnion being well-lit and relatively well-oxygenated for the fish and rather unsafe for the large-sized daphnids, was recorded. Therefore, the large-sized daphnids daytime refuge was close to the bottom whereas at night they moved upward to benefit from warmer water temperature along with food availability in the presence of fish predation but still remained below the thermocline. In summer, the insignificant amplitude of the hypolimnetic, which later became epilimnetic, displacements were probably due to the near-anoxic condition found below the thermocline. This might have deterred the fish, thus providing a safer refuge for daphnids in the below thermocline, which afterwards became the above thermocline. Low oxygen availability was regarded as the summer proximate factor. The abundant food and warmer water conditions found in the below/above thermocline also accounted for absence of DVM in summer. Consequently, this study suggests that DVM by Daphnia is an adaptation that is plastic to changing environmental conditions.

Catastrophic-like shifts in shallow Turkish lakes: a modeling approach

A generalized logistic model (GLM) was developed to determine occurrence of submerged macrophytes in shallow Lake Eymir, and the model was tested independently on the upstream shallow Lake Mogan using the data collected fortnightly from both lakes during 1997–2002. The independent variables included concentrations of chlorophyll-a (chl-a), suspended solids (SS) and total phosphorus (TP), Secchi disc transparency and z-scores of water levels. The dependent variable was the binary index of submerged plant occurrence. We used bootstrapping to determine the maximum number of epochs to train the model and to execute training when the corrected average cross entropy ( c-index) leveled off. The model predicted that SS concentration, z-scores of water levels and TP concentration were the most important variables for determining occurrence of submerged plants. Sensitivity analyses showed that the probability of submerged plant occurrence followed a strong hysterisis response to varying water levels and the concentrations of SS and TP, with the break points being ±50 cm, 12–17 mg l −1 and 200–300 μg l −1, respectively. This observed sensitivity was in accordance with the alternative stable states hypothesis of shallow lakes. For occurrence of submerged plants, chlorophyll-a concentration and Secchi disc transparency had low significance. This was in concert with both relevances of input variables and the field results. The model gave a good definition of the system since the c-index and corrected c-index on the training data were high (0.970 and 0.963, respectively). Testing the model on Lake Mogan produced a c-index of 0.815 with around 80% of the cases being correctly classified. This showed that the model had a high ability to generalize over a spatially independent test set; therefore, it had a great reliability as well. In addition, the predictive power of the model was indeed very high. Consequently, the model captured the relationships between the input and output variables successfully and consistently with alternative stable states hypothesis.

Restoration of the eutrophic Lake Eymir, Turkey, by biomanipulation after a major external nutrient control I

Nutrient loading in lakes is recognized as a serious threat to water quality. Over 25 years of raw sewage effluent discharge shifted Lake Eymir from a state dominated by submerged plants to a turbid water state. Successful effluent diversion undertaken in 1995 achieved 88% and 95% reductions in the areal loading of total phosphorus (TP) and dissolved inorganic nitrogen (DIN), respectively. Furthermore, the reduced load of TP was very close to the suggested threshold areal load (0.6 g m−2 yr−1) to attain recovery. Even though diversion also reduced the in-lake TP level by half, the poor water clarity and low submerged plant coverage (112 ± 43 cm and 2.5% coverage of the lake total surface area, respectively) persisted. Domination of the fish stock by planktivorous tench (Tinca tinca L.) and the benthivorous common carp (Cyprinus carpio L.) (66 ± 0.7 and 31 ± 1 kg CPUE, respectively) appeared to perpetuate the poor water condition. A substantial fish removal effort over 1 year achieved a 57% reduction in the fish stock which led to a 2.5-fold increase in Secchi disk transparency. This increase occurred largely because of a 4.5-fold decrease in the inorganic suspended solid concentration, and to some extent, a decrease in chlorophyll-a concentration. A strong top-down effect of fish on the large-sized grazers was evident as density and the body size of Daphnia pulexde Geer increased significantly after the fish removal. Even though the spring and annual euphotic depths occurred well above the maximum and mean depths of the lake, respectively, re-development of submerged plants was poor (6.2% coverage). A weak re-establishment of submerged plants might be attributed to an insufficiently viable seed bank, inappropriate chemical conditions of the sediment (severe oxygen deficiency), or to the high coot (Fulica atra L.) density. However, the top-down effect of fish appeared to be of great importance in determining water clarity, and in turn, conditions for submerged plant development in a warm temperate lake as recorded in the north temperate lakes. Furthermore, this study provides evidence for the importance of top-down control of fish, which, in turn, can be effectively utilised as a restoration strategy in warm-temperate lakes as well. More applications, along with long monitoring programs, are needed to develop a better understanding about requirements for biomanipulation success in this climate.

Water quality and impacts of pollution sources for Eymir and Mogan Lakes (Turkey)

Mogan and Eymir Lakes are two shallow lakes, interconnected hydrologically in the close vicinity of Ankara, Turkey. A total of 245 km 2 of the total 971.4 km 2 watershed is under environmental protection status as “Gölbaşı Specially Protected Area”. Potential impacts from extensive agriculture, recreation, incomplete infrastructure and other human activities, such as residential settlements, are discussed with reference to previous and more recent pollution monitoring. Six monitoring stations enabling follow-up of previous work were selected in this study. These were on the creeks feeding the lake systems. Generally, summer months showed heavier pollution loads, with Eymir Lake concentrating the pollutants due to flow from Mogan Lake. When compared with the 1995 study; COD, total-P, Kjeldahl-N in the six stations were close or slightly decreased in the present study. Suspended solids significantly decreased; possibly due to erosion control measures and decreased domestic wastewater. The improvement in the pollution state of the lakes is attributed to the construction of a sewage system going around Mogan Lake and collecting wastewater discharges and restrictions to urban settlement development around the lakes brought by the 1/25000 land use plan controlling further impact from residential developments within the protected area boundaries. The study, while addressing water quality and interactions due to human activities in shallow lakes, also discusses problems associated with human impacts in protected areas with the aim of presenting a complicated case study.

Hydrogeochemical Evaluation of Heavy Metal Loadings to Waters and Sediments by Leachate: A Case Study from the Golbasi Waste Disposal Site, Ankara, Turkey

At sites of groundwater contamination, predictions of contaminant behavior and evaluation of remedial strategies depend on identification and characterization of the geochemical processes affecting contaminant migration. Heavy-metal loadings to waters and sediments by leachate from the Golbasi waste disposal site in Ankara, Turkey, have been evaluated quantitatively using hydrogeochemical modeling. The groundwater of the waste disposal area, characterized by high concentrations of Ca, K, Cl, Cd, Pb, Zn, Cu, B, Ni, SO4, and NO3, contaminates the waters and sediments in the down-gradient area, Eymir Lake and a swamp along the flow path. An advective mass transport duration is ∼15 years for unretarded contaminants to move from the waste disposal well area to the southern shoreline of Eymir Lake. Mixing calculations suggest that the down-gradient groundwater is formed by mixing of 40 to 72% upgradient groundwater and 28 to 60% waste-disposal-area groundwater, as well as Eymir Lake surface-water ion concentrations formed by mixing of down-gradient groundwater (3%-25%) and swamp-water ion concentrations (75-97%) along the flow path. An evaluation of the changes in concentration of trace ion-related precipitation/dissolution and exchange reactions between water and sediments for the formation of both Eymir Lake surface-water composition and the down-gradient groundwater composition indicate considerable trace-ion content of the clays (exchangers) and related reactions in the system. These results suggest that the amounts of contaminants removed from or added both to the down-gradient groundwater and to surface waters through mixing, dilution, and evaporation processes are rather small. The amounts of ions in the waters at the present stage of the contamination process are predominantly governed by exchange and dissolution/precipitation reactions.

Case studies on the use of neural networks in eutrophication modeling

Artificial neural networks are becoming more and more common to be used in development of prediction models for complex systems as the theory behind them develops and the processing power of computers increase. A three layer Levenberg–Marquardt feedforward learning algorithm was used to model the eutrophication process in three water bodies of Turkey (Keban Dam Reservoir, Mogan and Eymir Lakes). Despite the very complex and peculiar nature of Keban Dam, a relatively good correlation (correlation coefficient between 0.60 and 0.75) was observed between the measured and calculated values. For Mogan and Eymir, which are much smaller and more homogenous lakes compared to Keban Dam Reservoir, correlation values as high as 0.95 were achieved between the measured and calculated values. Neural network models were able to model non-linear behavior in eutrophication process reasonably well and could successfully estimate some extreme values from validation and test data sets which were not used in training the neural network.

Hydrogeochemical modeling of waters in Mogan and Eymir Lakes special environmental protection area, Ankara, Turkey

On the basis of water‐rock interactions, hydrogeochemical characteristics of spring, stream, wetland, and lake waters in the Mogan and Eymir Lakes special environmental protection area of Turkey were modeled using a reaction‐path simulation method. Geochemical characteristics of surface waters, mineralogical sources of ion concentrations in the springs, mixing, and evaporation/dilution relationships between the compositions of input flows and the lakes and between the compositions of inflows and wetlands were quantitatively determined. Major ion distributions in spring water reflect mineralogical characteristics of the lithologies that have been traversed by related springs before reaching the surface. Mixing and evaporation/dilution calculations show that approximately 41 to 83% of the ion concentrations in the Mogan Lake is provided by groundwater and the remaining is supplied by streams. In addition to the mixing, cation exchange and carbon dioxide (CO2) outgasing reactions are also required to obtain the observed lake composition. Chemistry of the wetland waters provided by Mogan Lake by way of groundwater and the regulator canal is primarily controlled by CO2 ingasing and exchange reactions. Small amounts of dilution by other sources are also required. The majority of ion concentrations in Eymir Lake water, on the other hand, is supplied by the wetland area. Ion concentration contributions of groundwater from other areas to the Eymir Lake are calculated to be approximately 8 to 59% of the total input. Silica precipitation, exchange, and evaporation are the effective processes in the production of Eymir Lake water concentrations. In general, the ranges of model results are consistent with those of hydrological data.