Abstract
dNutrient 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: 19 19-l 939 and 1969-l 99 1. 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 -0.05 m yr-’ during both periods. Calculations of changes in chlorophyll concentrations suggest a yearly increase of - 1%. Extending the calculations to represent primary production indicates an increase of slightly < 1% yr- l. These calculations, however, are unreliable due to substantial uncertainty regarding the relationships between Secchi depth and chlorophyll concentration and chlorophyll and primary produclion.
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