The status of eutrophication is assessed using several core indicators. Each indicator focuses on one important aspect of the complex issue. In addition to providing an indicator-based evaluation of total nitrogen, this indicator also contributes to the overall eutrophication assessment along with the other core indicators.
Eutrophication is one of the four thematic segments of the HELCOM Baltic Sea Action Plan (BSAP) with the strategic goal of having a Baltic Sea unaffected by eutrophication (HELCOM 2007). Eutrophication is defined in the BSAP as a condition in an aquatic ecosystem where high nutrient concentrations stimulate the growth of algae, which leads to imbalanced functioning of the system. The goal for eutrophication is broken down into five ecological objectives, of which one is "Concentrations of nutrients close to natural levels". Increase in nutrient concentrations can be assessed using measurements of all suspended and dissolved nutrients.
The EU Marine Strategy Framework Directive (Anonymous 2008) requires that "human-induced eutrophication is minimized, especially adverse effects thereof, such as losses in biodiversity, ecosystem degradation, harmful algal blooms and oxygen deficiency in bottom waters" (Descriptor 5). 'Total Nitrogen (TN)' is identified as a criteria element to be assessed using the criterion D5C1 'Nutrient concentrations are not at levels that indicate adverse eutrophication effects' in the Commission Decision on criteria and methodological standards on good environmental status of marine waters (Anonymous 2017).
The EU Water Framework Directive (Anonymous 2000) requires good ecological and chemical status in the European coastal waters. Good ecological status is defined in Annex V of the Water Framework Directive, in terms of the quality of the biological community including phytoplankton biomass (usually measured as chlorophyll-a), the hydromorphological/hydrological characteristics and the chemical characteristics. Nutrient concentrations, measured as total or inorganic nutrients, is one of the indicators listed in Annex V.
Marine eutrophication is mainly caused by nutrient enrichment leading to increased production of organic matter in the Baltic Sea with subsequent effects on water transparency, phytoplankton communities, benthic fauna and vegetation as well as oxygen conditions. Phytoplankton and benthic vegetation need nutrients, mainly nitrate, ammonia and phosphorus, for growth.
Adding total nutrients alongside inorganic nutrients as core indicators strengthens the link from nutrient concentrations in the sea to nutrient enrichment. In particular these parameters allow to take account of climate change in the eutrophication assessment since higher temperatures will lead to year-round phytoplankton proliferation and / or possible changes in zooplankton communities. To illustrate this point, the concentration of the total and the dissolved inorganic fractions of nutrients have been compared, and diverging trends have been observed in some sub-basins. For example, a decrease in winter DIN concentrations has been identified in the Bornholm Basin since the 1990's, but TN concentrations have remained high (Relevance figure 1). A possible reason for this observation could be that in winter more nutrients are bound in the phytoplankton due to the higher water temperatures. In such a situation, assessing only dissolved inorganic concentrations gives the wrong impression that nutrient concentrations seem to be declining, while, in fact, they are stable or increasing as can be seen when also assessing total concentrations (Relevance figure 2). In conclusion, to get a good understanding of the trend in nutrient concentrations in the marine environment monitoring and assessing both, total and dissolved nutrients, is important.
Relevance figure 1. Time series of annual TN (black line and dots) and winter DIN (gray line and dots) in the Bornholm Basin. The decrease in winter DIN since the 1990's is not expressed by annual TN. The figure is modified from BSEP 133.
Relevance figure 2. Monthly values of total nitrogen concentration (in µmol l-1) in the surface layer (0- 10m) during 2007-2011.
Substances, litter and energy
- Input of nutrients – diffuse sources, point sources, atmospheric deposition
- Input of organic matter – diffuse sources and point sources