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 water clarity, 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 "clear water", possible to assess using Secchi depth 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). 'Photic limit (transparency) of the water column' is listed as a criteria element for assessing the secondary criterion D5C4 'The photic limit (transparency) of the water column is not reduced, due to increases in suspended algae, to a level that indicates 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, including water clarity.
Water clarity is affected mainly by the concentration of particles causing scattering of light, therefore enhancing light absorption. Light absorption is mainly due to water itself, chromophoric dissolved organic matter (CDOM), detritus and to phytoplankton. The concentration of detritus particles and CDOM is the result of organic matter accumulated over time due to high nutrient loadings and in particular in the eastern Baltic Sea to a high natural contribution of humic materials from rivers draining peat land and forested areas. Eutrophication increases light attenuation, through nutrients increasing the amount of living organisms. Turbid waters affect the ecosystem through decreases in light availability below the surface.
Relevance figure 1. Simplified conceptual model for water clarity in the Baltic Sea.
Substances, litter and energy
- Input of nutrients – diffuse sources, point sources, atmospheric deposition
- Input of organic matter – diffuse sources and point sources
Water clarity in the Baltic Sea is affected mainly by the concentration of phytoplankton and chromophorous dissolved organic matter (CDOM). Of these, phytoplankton concentration is directly linked to anthropogenic pressures, ie. nutrient increase.