Μελέτη της υδροδυναμικής και φυσικοχημικής κατάστασης των παράκτιων ανοξικών λεκανών

Abstract

During the last five decades, the oxygen content in the open ocean appears to have decreased in most areas. At the same time, low oxygen areas, also known as “dead zones”, have spread in the coastal oceans. These changes in oxygen are an increasingly important topic due to large impacts on the ecosystems, living resources, and biogeochemical cycles. Over the recent years, the research interest has been focused on the relationship of the organic and nutrient load increase in coastal zone, with the proliferation of hypoxic or anoxic environments and the deterioration of oxygen conditions in environments where anoxia/hypoxia prevalence is the result of natural processes. However, human intervention in coastal areas is not limited to eutrophication, but can alter morphological and/or hydrological characteristics of coastal environments. Such changes are of great importance in enclosed/semi-enclosed basins since they regulate hydrodynamic circulation, governing the balance of physical and biogeochemical processes. The aim of this dissertation was the study the physicochemical and hydrodynamic characteristics of enclosed/semi-closed coastal basins, the investigation of natural processes and anthropogenic interventions contribution in the prevalence of anoxic conditions, in their water column, and the approach of different management practices, in order to control the intensity and extent of anoxic conditions, as well as their consequences. To achieve this objective, Aitoliko lagoon was used as a case study in this thesis. It is about a semi-closed, natural anoxic coastal basin, in which nutrient load increase, played a supplementary role in anoxic conditions aggravation, during the last decades. Aitoliko lagoon is a representative example, of a coastal basin which degradation resulted from the alteration of its hydrological and morphological characteristics. The intensive human activities in the area culminated in the 1990's and influenced the intensity and extent of anoxic conditions in the basin. In the present study a systematic monitoring of the physico-chemical, the oceanographic and the qualitative characteristics of Aitoliko lagoon was conducted, for two years on a monthly basis. The three dimensional model that was used to study hydrodynamic circulation in Aitoliko lagoon was calibrated and verified using field data. For the first time this thesis refers to the oxygenation of Aitoliko lagoon hypolimnion during the winter months. The presence of dissolved oxygen in lagoon’s bottom water was attributed to the high salinity water influx from the adjacent Messolonghi lagoon, as a result of the sill’s dredging, held in an effort to facilitate water discharges between the two lagoons, improving the anoxic conditions prevailing in the Aitoliko basin. Ecosystem restoration, describes the necessary actions to bring the environment back to its previous physical condition. Thereafter, the oxygenation of the deep layers in a natural anoxic basin, there should not be an objective of any management practice. The aim should be the reduction of the anoxic layer extension in the area occupied prior to environmental degradation. The case of semi-closed Aitoliko lagoon is a very good example proving that management plans should be based on reliable tools. The three-dimensional hydrodynamic model, used in this thesis, described the mechanism of dissolved oxygen advection at the basin’s bottom layers and highlighted the role of the hydrology and sill’s morphology into the lagoon’s hydrodynamic circulation and the extent of water column anoxia. Such a mathematical model can be used as a management tool for restoration, in basins where human activities have altered morphological and hydrological characteristics, aggravating anoxic conditions. Aitoliko lagoon qualitative characteristics assessment, highlighted the importance of the "internal load" of an anoxic basin (ammonia, sulfides and phosphates which are accumulated in the anoxic layer), since it can potentially alter basin’s water quality and threaten aquatic organisms. Aiming to the management of anoxic basins’ "internal load" and based on practices widely used for eutrophic ecosystems restoration, in this thesis the ability of zeolite, was examined, a natural material, to control ammonia, phosphates and sulfides availability in aqueous solutions. It was concluded that there is a natural material capable to remove nitrogen, phosphorus and sulfur forms from aqueous solutions. This ability gives to zeolite the perspective, of its future use in anoxic basins "internal load" control. As zeolite’s ammonia and phosphate removal capacity was known, in this study its efficiency to remove sulfides from aqueous solutions, was examined. It turned out that zeolite is characterized by particularly high removal capacity depending on sulfides initial concentration.