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 dissolved inorganic phosphorous, this indicator also contributes to the overall eutrophication assessment 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 levels of algal blooms". Increases in phytoplankton abundance and biomass can be assessed using chlorophyll-a as a proxy.
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). "Chlorophyll-a in the water column" is listed as a criteria element for assessing the criterion for D5C2 'Chlorophyll a concentrations are not at levels that indicate adverse effects of nutrient enrichment'.
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 Directive, in terms of the quality of the biological community, the hydromorphological characteristics and the chemical characteristics. Chlorophyll a is used as a proxy for phytoplankton biomass and as such, it was used in the WFD intercalibration exercise.
Chlorophyll-a concentrations are a reliable proxy measurement for phytoplankton biomass, though it is also known that some phytoplankton can control their chlorophyll concentrations in response to certain environmental conditions. Phytoplankton quantity and biomass is a direct proxy of eutrophication as it is linked to the increase of nutrient concentrations. The nutrient load is also supplemented by internal nutrient loading from bottom sediments in some areas, accelerated by oxygen depletion. Phytoplankton increase in turn adds to oxygen depletion, when sedimenting to the bottom, causing a vicious circle of eutrophication. Biotic and abiotic changes, such as climate change or changes in herbivory, also affect the phytoplankton quantity.
Relevance figure 1. Simplified conceptual model for chlorophyll-a.
Substances, litter and energy
- Input of nutrients – diffuse sources, point sources, atmospheric deposition
The increase of chlorophyll a, a proxy of phytoplankton biomass, in the water column is dependent on nutrient concentrations, and thus linked strongly to anthropogenic nutrient loads from land and air. The concentration of chlorophyll a is a proxy for phytoplankton biomass. The amount of phytoplankton in the water depends on the balance between phytoplankton growth and loss factors, such as grazing. As phytoplankton growth is stimulated by nutrients, the chlorophyll-a concentration has a tendency to increase with nutrient inputs. However, a simultaneous increase in zooplankton biomass or other grazers, due to the higher food availability might to some degree counteract this effect.