Centennial changes in water clarity of the Baltic Sea and the North Sea

Secchi depth is a valuable proxy for detecting long term changes in the water clarity of oceanic and coastal ecosystems. We analyse approximately 40 000 observations, which are available from ICES, from the Baltic Sea and the North Sea in the 20th century. Our results suggest pronounced effects of bottom depth and distance to coast on Secchi depth, and we account for this topographical effect in an assessment of the long term change in water clarity. Our results suggest a centennial Secchi depth shoaling of 3.2 ± 0.2 and 5.8 ± 0.6 m in areas that are shallower and deeper than 100 m in the Baltic Sea. For the North Sea the corresponding numbers were 1.8 ± 0.3 and 5.2 ± 0.9 m. We discuss potential ecosystem effects involving pronounced reductions in photic habitats and reduced visibility for visual predators. We suggest that the role of long term variations in colour dissolved organic matter (CDOM) on the transparency in the Baltic Sea and North Sea deserves future attention.