Abstract
Vistonis Lagoon is located in Northern Greece in the Region of Thrace. It is protected by the Ramsar Treaty and is considered of first priority in the Network “Natura 2000” of the European Union, because of the great variety of rare birds that leave in this lagoon. In the 80s, the sources of nutrients to Vistonis Lagoon were domestic wastewater from the City of Xanthi, industrial wastes and agricultural runoff pollution. In a research conducted in 1985, the lagoon was rated as “eutrophic”. Today, the main type of pollution in the lagoon is non-point source pollution from its drainage area, since point sources of pollution have been eliminated. The change of cultivated species, which has occurred over the last 15 years within the agricultural part of the drainage area of Vistonis, has had also as a result the reductions in nitrogen and phosphorus loadings from fertilizers by 16.7% and 16%, respectively. Main objective of this doctoral dissertation was the study of the eutrophication sta ...
Vistonis Lagoon is located in Northern Greece in the Region of Thrace. It is protected by the Ramsar Treaty and is considered of first priority in the Network “Natura 2000” of the European Union, because of the great variety of rare birds that leave in this lagoon. In the 80s, the sources of nutrients to Vistonis Lagoon were domestic wastewater from the City of Xanthi, industrial wastes and agricultural runoff pollution. In a research conducted in 1985, the lagoon was rated as “eutrophic”. Today, the main type of pollution in the lagoon is non-point source pollution from its drainage area, since point sources of pollution have been eliminated. The change of cultivated species, which has occurred over the last 15 years within the agricultural part of the drainage area of Vistonis, has had also as a result the reductions in nitrogen and phosphorus loadings from fertilizers by 16.7% and 16%, respectively. Main objective of this doctoral dissertation was the study of the eutrophication status and water quality of Vistonis aquatic ecosystem. To achieve this objective, the following tasks were undertaken: A monitoring program was designed within the main torrents emptying into Vistonis (i.e., Kosyntos, Kompsatos and Travos), which provided flow and sediment and nutrient loading data. In addition, data were collected on the annual cycles of nutrients and phytoplankton within the lagoon. There were in total 57 discharge measurements and water quality samples collected in 9 stations within the torrents. Approximately 2000 chemical analyses were made for the determination of nutrient concentrations (e.g., total phosphorus, orthophosphorus, nitrates, and ammonia nitrogen). Within the lagoon there were 15 sampling deployments at 7 stations, 3 sampling depths, leading to a total of 375 samples collected. There were approximately 2250 chemical analyses for the determination of nutrient and chlorophyll_a concentrations. In addition, mathematical models for the drainage area and the lagoon were developed, calibrated, verified and used to study management alternatives, aiming at lagoon water quality improvement. From the study of the time and spatial variation of the nitrates and ortho-phosphates within the torrents, it was concluded that the highest concentrations and most variability occurred within the lower plain area of the watershed where the agricultural activities take place. Intense seasonal variation was observed in the physicochemical and biological parameters measured within the lagoon. There was salinity variation, with the highest values observed during the summer months. This is due to the tidal influence which is greater in the summer, when the torrent discharge is minimal or zero. The greater concentrations of total phosphorus and ortho-phosphates in the lagoon occurred in August. The data showed that this was due to phosphorus release from the bottom sediments. On the contrary, during the wintertime, phosphorus loading was mainly due to the torrents, with Kompsatos carrying nearly the 2/3 of the total loading. The seasonal variation of the total inorganic nitrogen in the lagoon, follows the seasonal pattern of nitrogen concentrations in the torrents. Again, the data showed that ammonia nitrogen is released from the bottom sediments mainly in the summer months. There was also a great variation in chlorophyll a concentration, with the highest chlorophyll_a concentrations in August. Using stepwise regression, there were mathematical expressions developed, relating chlorophyll_a concentrations with various physicochemical parameters. In addition, there were empirical models developed relating chlorophyll_a to total phosphorus and water transparency. Finally, well-accepted eutrophication indices were used to characterize the eutrophication status of the lagoon, which was characterized as hypereutrophy. The data collected in the torrents (discharge, and concentrations of sediments, nitrogen and phosphorus) were also used in the calibration and verification of a mathematical model developed for the drainage area using SWAT. The validation results were very satisfactory. In addition, data collected within Vistonis lagoon and available tidal elevations were used in the calibration and verification of a mathematical model developed for this lagoon based on WASP5 (sub-models DYNHYD5, TOXI5 and EUTRO5). The validation results for this model also were very satisfactory. The validated model SWAT was then used for testing alternatives regarding management of the drainage area, e.g., land use and crop changes within the agricultural area. Results showed that there is significant reduction in nitrogen and phosphorus loadings to the lagoon, when the main crop is cotton. The validated model WASP5 was used to test alternatives regarding the management of the lagoon. Results showed that the greatest reduction of chlorophyll_a concentration is obtained after stabilizing and/or treating the lagoon’s bottom sediments.
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