Guidelines for monitoring of water transparency (Secchi depth).

Abstract

Water transparency serves as an index for the trophic state of a water body. It reflects eutrophication through changes in the phytoplankton abundance; increase in the ambient nutrient status in the water leads to higher phytoplankton biomass that diminishes the propagation of light in the water. Water transparency is approached by Secchi depth (Cialdi and Secchi 1865, Whipple 1899). Secchi depth is influenced by dissolved and/or colloidal inorganic and organic substances as well as total suspended solids and resident seston. It is thus affected by substances unrelated to eutrophication as well. This source of error has to be taken into consideration whenever eutrophication state is assessed using Secchi depth in the Baltic Sea that is optically classified as a Case II water body (Morel and Prieur 1977), i.e., the body where concentrations of colour producing substances (e.g. phytoplankton, inorganic particles and CDOM) vary independently from each other. Those Secchi depth estimations should be treated with special caution that are collected in the sub-basins possessing high absorption by chromophoric dissolved organic matter (the Gulf of Riga, the Gulf of Bothnia). Secchi depth relates to primary production by being a proxy for the thickness of the euphotic zone wherein the large bulk of the gross production takes place. In principle, the euphotic depth is twice Secchi depth, but this relation varies largely in practice (French et al. 1982). 1.2 Purpose and aims Monitoring of Secchi depth provides information of water transparency that is used for assessing direct effects of eutrophication (with certain limitations shown above). The aim is to provide spatiotemporal information for detection of short-term status and long-term trends and to ensure that the data is compatible for the HELCOM core indicator ‘Water transparency‘. The indicator description, including its monitoring requirements, is given in the HELCOM core indicator web site: [http://helcom.fi/baltic-sea-trends/indicators/water-clarity].