The coastal seawater quality evaluation based on organic carbon, nitrogen and phosphorus data of Tallinn Bay, Estonia

It is generally known that one of the important objectives of EU countries is to improve the quality of water in water bodies.  The quality of the water in the Gulf of Tallinn is poor. Nitrogen and phosphorus compounds in coastal water and stormwater discharges have been studied, but very little is known about the role of dissolved organic matter, chemical properties and relationship with pollutants. It is important to be aware of the role of carbon compounds as nutrients, the high content and inflows into the coastal sea can lead to the proliferation of algae and bacteria, the reduction of dissolved oxygen in water, etc. The impact of algae on the water quality in the Tallinn Bay is a significant problem and can also worsen the water quality of Pirita beach. The main concern of bad water quality has been considered eutrophication, which causes algal bloom near coastline of Tallinn Bay.   

In recent years, stormwaters from Tallinn are believed to be the main cause of high nutrient levels. In present study the intention was to investigate different factors by measuring the concentrations of organic carbon, total, inorganic and organic phosphorus and nitrates in different locations of the coastal seawater. The concentrations of phosphorus and nitrate were determined by spectrophotometry, organic carbon by HPLC.  Detailed characterization of dissolved organic matter was carried out in order to identify sources of organic matter that has entered the water. As a result, it should become clear whether, in addition to the study of nitrogen and phosphorus compounds in coastal water, it would be expedient and necessary to monitor and characterize natural organic matter.

The aims of present study were:  to determine the organic carbon, phosphorus and nitrate in coastal seawater near the stormwater discharge outlets; to investigate the climatic factors (rainfall, temperature), and freshwater inflow (River Pirita); to compare the results with average nutrient levels in the Gulf of Finland; to assess the condition of Tallinn Bay according to legislation.

The study results indicated that nutrient levels in the coastal seawater of the Tallinn Bay area were remarkably higher than average nutrient levels in the Gulf of Finland. According to legislation, the status class of Tallinn Bay is mainly poor, based on total phosphorus data and bad or even worse, based on nitrate data. Stormwaters did not increase nitrate and total phosphorus contents substantially and they mainly affected total phosphorus concentrations near the discharge outlets. River Pirita was identified as the major source of nitrates, but not of phosphorus. Further studies are required to obtain a complete picture about nutrient flows to Tallinn Bay.