Annual and long-term water transparency variations and the consequent seafloor illumination dynamics in the Baltic Sea archipelago coast of SW Finland

The availability of photosynthetically active radiation (PAR) is one of the limiting factors of marine primary production. In coastal waters, the proportions of optical constituents vary, causing changes in water transparency, and consequently the rate of seafloor illumination. We illustrate this phenomenon geographically in seasonal and long-term perspectives on the archipelago coast of SW Finland in the Baltic Sea, using data from 21 Secchi measurement stations. The results indicate vast spatial and temporal variation of the proportion of illuminated seafloor area, which, in this study, is defined as the seafloor above the estimated euphotic depth (where over 1% of the surface PAR remains). The seafloor illumination undergoes an annual cycle, during which a quarter of the studied seafloor area is illuminated only for a part of the growing season. Based on long-term Secchi data, we estimate a 50% decrease in the total illuminated seafloor area from 1930 to 2007.