Fifty years of primary production measurements in the Baltic entrance region, trends and variability in relation to land-based input of nutrients

Inter-annual variations and long-term trends in phytoplankton primary production (PP) within the Baltic entrance region (the Kattegat and the Belt Sea) are presented and discussed. The study employs the core of Danish monitoring data, with measurements at 6–8 different sites from the past 20–50 years. Temporal development of the annual PP is compared with changes and variations in the land-based nutrient inputs and to other, independent, Swedish and Danish PP data. Spatial and seasonal variations based on annual and monthly PP, respectively, are evaluated. There are large variations on all scales; annual PP ranges from 50 to 500 g C m, with maximum values in some Danish fjords and minimum values in open waters during the 1950s. The Kattegat and the Sound have a lower mean production (135–165 g C m ; 1981–2000) than the Great and Little Belts (185–220 g C m ). Compared to 1950s and 1960s, the daily PP has changed from being almost constant betweenMarch and October, to having two more distinct maxima, one in March and one between July and September. It is obvious that annual mean production has increased considerably since the 1950s, but also that this increase took place before 1980. For data after 1980, we find a co-variation between annual nutrient loads and regional mean PP; years of low total nitrogen (TN) and total phosphorus (TP) input to the region coincide with low PP and vice versa. Waste-water treatment and measures in agriculture have reduced the land-based input of TN by about 1/3 and input of TP by about 2/ 3 since the 1980s, enough to cause a substantial decrease in the surface water nutrient concentrations. A simultaneous, but weak downward trend in the regional mean PP can be seen up to 1997, after which the trend is broken. Higher production from 1998 is most likely an effect of changes in the method used for determination of PP. © 2006 Elsevier B.V. All rights reserved.