Water quality assessment in a river-sea transition zone. Recent results from distinct aquatic environments of the danube delta biosphere reserve area, Romania

This study aims to compare the water quality in distinct aquatic systems, situated in different representative areas of the Danube Delta Biosphere Reserve, Romania - a unique natural area of national and international importance. These areas are largely populated by a vast diversity of aquatic organisms, including rare species of plants, animals, aquatic birds and fish, many of them being endangered species. Any imbalances in these ecosystems, owing to natural and human-induced changes in the water quality may affect aquatic life. In this sense, water samples were collected from the surface layer of a freshwater environment (i.e., Babina, Rădăcinoasele and Ciorticuț lakes), as well as from mixed environments (i.e., Musura and Sahalin Bays) during August 2018. The following water quality indicators were considered: temperature, pH, dissolved oxygen, nitrite-nitrogen, nitrate-nitrogen, ammonium-nitrogen, orthophosphates, chlorophyll a, total organic carbon, silica, electrical conductivity, total dissolved solids, sulphates, turbidity, total suspended solids, transparency and oxido-reduction potential. These parameters were analysed and discussed in relation to national and international environmental standards. Maps of spatial distribution of some quality parameters were obtained, showing variation as a result of local specific environmental circumstances, which change in time and space. These results suggest that the environmental indicators measured in drought conditions and low water levels showed generally good to moderate water quality status, in spite of many local triggers associated with natural and anthropogenic causes that can alter or impair the quality of the water. After this investigation performed during August 2018, it can be appreciated that the analysed water samples maintain appropriate levels that are suitable for the ecological status of the investigated aquatic systems, and, as expected, the aquatic habitats are ecologically balanced environments. The paper recommends the routine monitoring and periodical testing of the water quality to create and develop a database to reflect the natural variation and human related implications for water quality within these areas, and their short, medium and long-term evolution trends. The present work is important for water quality assessment to detect changes in water quality under human pressures and climate change and to protect and conserve these natural water resources