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 the oxygen debt, this indicator also contributes to the overall eutrophication along with the other eutrophication 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 "natural oxygen levels".
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). 'Dissolved oxygen in the bottom of the water column' is listed as a criteria element for assessing the criterium D5C5 'The concentration of dissolved oxygen is not reduced, due to nutrient enrichment, to levels that indicate adverse effects on benthic habitats (including on associated biota and mobile species) or other eutrophication effects.
The EU Water Framework Directive (Anonymous 2000) requires good ecological status in the European coastal waters. Good ecological status is defined in Annex V of the Water Framework Proposal, in terms of the quality of the biological community, the hydrological characteristics and the chemical characteristics, including dissolved oxygen.
Oxygen depletion is a common effect of eutrophication in the bottom waters of coastal marine ecosystems and is becoming increasingly prevalent worldwide (HELCOM, 2002). In the deep basins and other areas of the Baltic Sea which are characterized by vertical stratification and low water exchange, conditions of low oxygen or even anoxia are a natural phenomenon, although enhanced by nutrient loading. Oxygenation of these areas highly depends on inflows of marine water from the North Sea (HELCOM 2017). Oxygen depletion is caused by the consumption of oxygen by the microbial processes responsible for the degradation of organic matter accumulating at the sea floor. Oxygen depletion may result in hypoxia (literally 'low oxygen') or even anoxia (absence of oxygen). These events may be (1) episodic, (2) annually occurring in summer/autumn (most common), or (3) persistent (typical of the deep basins of the Baltic Sea). Oxygen depletion has a clear impact on biogeochemical cycles. Anoxic periods cause the release of phosphorus from sediment. Dissolved inorganic phosphorus (DIP) is significantly negatively correlated with oxygen conditions. The concentration of DIP can vary greatly from year to year depending on the release of phosphorus from sediments under anoxia (Matthäus et al. 2008). Ammonium is also enriched under hypoxic conditions. The DIP and ammonium from the bottom waters can be mixed into the upper water column and enhance algal blooms. Thus, hypoxia results in large changes in the biogeochemical cycle, which may enhance eutrophication.
Substances, litter and energy
- Input of nutrients – diffuse sources, point sources, atmospheric deposition
- Input of organic matter – diffuse sources and point sources
Oxygen depletion is an indirect effect of eutrophication, i.e. increase of organic matter descending to the bottom. It also has an indirect link to anthropogenic pressures, through increased anthropogenic nutrient loads and subsequent increase of organic matter descending to the bottom.