Eutrophication – a marine management perspective

The goal of marine management is to achieve a good status in the marine environment. The definitions set out for good status must first be met in order to speak of the good status of the marine environment.

The status of the sea is assessed using various indicators. These indicators monitor how far from a good status the sea is at any given time.

The various indicators can be used to form broader parameter groups. One such group focuses on measuring specific areas of eutrophication. A group of parameters may include several indicators.

Eutrophication status is measured using three groups of parameters

The status of eutrophication is assessed by examining the nutrient content of waterbodies, as well as the direct and indirect effects of eutrophication. Together, the results of these three groups of parameters form the final assessment of the eutrophication state of the sea.

The first group of parameters measures nitrogen and phosphorous levels in the water

The first group of descriptors monitors the nutrient levels of seawater. Measurements are made of both nitrogen and phosphorus concentrations from the surface water layers. In coastal areas, the total nitrogen and phosphorus are used as indicators during the summer period. There are four indicators used in the open sea areas. Besides the year-round total nitrogen and phosphorus, the winter levels of both inorganic nitrogen and phosphorus are also measured.

The second group of parameters focuses on the concrete effects of eutrophication

The second group of parameters includes indicators that measure the direct effects of eutrophication. In coastal areas, direct eutrophication is characterised by the concentrations of chlorophyll-a, the phytoplankton content, aquatic plants and algae, and water transparency. For open sea areas offshore, the parameter group has three indicators, i.e. chlorophyll-a, water transparency, and the extent of cyanobacterial blooms.

Plankton sampling.

The third group of parameters charts the indirect effects of eutrophication

The third group of parameters identifies the indirect effects of eutrophication and consists of different indicators, depending on the sea area in question.

The status of benthic faunal communities is the main indicator for the coastline and open sea areas of the Bay of Bothnia, as well as the open sea areas of the Kvarken area, the Bothnian Sea, and the Sea of Åland. In addition, the status of macrophyte communities is used as an indicator along the coasts of the Kvarken, as well as the Bothnian, Åland, and Archipelago Seas, and the Gulf of Finland.

The only indicator of indirect eutrophication used in the open sea areas of the Gulf of Finland and the Northern Baltic Sea is the oxygen debt of the seabed. Here, the term oxygen debt refers to the amount of oxygen needed to take the seafloor out of its anoxic, i.e. oxygen-depleted, state.