Long-term trends in Secchi depth in the Baltic Sea

Nutrient loads to the Baltic Sea have increased during the last century, and primary production has probably also risen. However, the evidence of such a rise is circumstantial, and most of the available findings concerning primary production cover only brief time periods. A more appropriate type of data in this area is Secchi disk measurements. We present the results of trend tests applied to Secchi depth values recorded during two discrete time periods: 1919-1939 and 1969-1991. We performed a step trend test to compare the data from the two periods and applied a monotonic trend test to the later series. Both tests showed that Secchi depth decreased by similar to 0.05 m yr(-1) during both periods. Calculations of changes in chlorophyll concentrations suggest a yearly increase of similar to 1%. Extending the calculations to represent primary production indicates an increase of slightly <1% yr(-1). These calculations, however, are unreliable due to substantial uncertainty regarding the relationships between Secchi depth and chlorophyll concentration and chlorophyll and primary production.