BS5 Visibility depth

>> Background information

Development

Visibility depth is a measure of water transparency and clarity. In general, it is correlated with the abundance of phytoplankton in the water. Visibility depth has overall decreased in most parts of the Baltic Sea troughout the last century. In Gulf of Finland, Bothnian Sea and Bothnian Bay the trends seem to have been even stronger than the average. Especially in the northern sea areas, the decrease is probably due to humic substances and inorganic particles leaching from the catchment area and colouring the water, rather than just eutrophication.

Impact on biodiversity

Similar trends between visibility depth and abundance of algae have been observed since the late 1970s when the monitoring of chlorophyll-a begun (see also BS6). Monitoring data for visibility depth covers a lot longer time period, possibly indicating that the increase in the concentration of chlorophyll-a begun already before the 1970s. However, the relationship between water transparency and phytoplankton biomass can not be interpreted as straightforward for the Finnish sea areas.

All photosynthetising organisms need light from the sun to produce biomass. Eutrophication increases biological production and thus the amount of living, dead and decaying organic matter in the water and on the seabed. As water transparency is reduced, less light reaches the deeper water columns. Low amount of available light decreases the abundance of submerged vegetation, for example macroalgae and eelgrass growing on shallow bottoms.

Submerged vegetation is important for a well functioning ecosystem since it creates a variety of microhabitats for other organisms. As vegetation of the seabed disappear, the communities it supports start to change. Several species are lost and eventually the whole ecosystem may be damaged.