Secchi depth in the Baltic Sea an indicator of eutrophication

Secchi depth, a proxy of water clarity, is widely applied as an indicator of eutrophication or water quality both in open-seaand coastal areas. In optically complex waters, such as the Baltic Sea, Secchi depth is known to respond to several components – yet its performance, or possible restrictions, have not been explored. In this study, I investigated long-term changes in Secchi eutrophication in the Baltic Sea. Secchi depth decreased in the open Baltic Sea during the last century (Paper I). The decrease was especially intense in the northern areas, amounting to 3.3 – 4.0 m (averaging 0.033 – 0.040 m y-1), when comparing summer time averages in 2005 – 2009 to those observed one hundred years earlier. The decrease was proposed to be strongly linked with documented simultaneous increase in chlorophyll-a concentration (Papers I, III). A closer look at the Finnish coastal areas, where a national monitoring program has taken place since 1970, revealed clear decreasing trends only in the Archipelago Sea – accompanied by opposing trends in chlorophyll-a (Paper II). Contradictory to this, and to the development in adjacent open sea areas, Secchi depth was observed to increase in the coastal Bothnian Sea, Quark and Bothnian Bay. I suggest the increase was at least partly a consequence of decreased concentrations of dissolved iron in the surface waters near the coast. The relationship between Secchi depth and indicating that a large part of organic carbon was colorless. Unfortunately, the long-term coastal dataset did not allow comparison to suspended inorganic matter, leaving the possible effect of potentially important coastal constituent unrevealed. The effect of the main optical constituents on light attenuation in the open sea were investigated through a bio-optical model setup, in order to resolve how the Secchi depth indicator should be applied in different parts of the Baltic Sea (Paper III). Secchi depth was shown to be highly sensitive to variation in both phytoplankton (by chlorophyll-a as proxy) and colorful dissolved organic matter (CDOM). As expected, based on the high spatial gradients in both optical constituents, the evaluation against monitoring data called for sub-basin-wise adjustments to the model outcome. Secchi depth is often applied together with other indicators, including chlorophyll-a. The modelling exercise revealed, that the environmental targets for Secchi depth, set by the Baltic Sea coastal states via their collaboration through the Baltic Marine Environment Protection Commission (HELCOM), were stricter than those set for chlorophyll-a. To facilitate future management use of the Secchi depth indicator, I made an effort to characterize it in relation to indicators in general. Secchi depth is a commonly applied and well established indicator of eutrophication and water quality in the Baltic Sea. It is technically relatively advanced: quantitative, regularly monitored, and includes ecological targets as well as documented methodology. It is also easily understood by the public. On the other hand, though simple to associate, it Finally, Secchi depth was applied in the Baltic Sea eutrophication status assessment (Paper IV), and alternative ways to apply the indicator were explored. According to the assessment 20072011, all open-sea areas of the Baltic Sea were severely affected by eutrophication. Due to the