Hodgekiss and Ho (1997) found that the growth of most red tide algal blooms is optimized at ratios between 6 and 15. Hence, N:P ratios can act as an early warning signal for algal bloom types and frequencies. Based on the N:P ratio Wnt inhibitor trend observed in this study, the N:P ratio should be monitored throughout the BSDB and P input should be

reduced in eastern catchments in order to stop the decreasing trend in the N:P ratio found in this study. From our study we can conclude that the socio-economic changes were most likely responsible for the change in nutrient dynamics in the BSDB. This is because of the steady decrease in TN due to changes in the diffuse sources from agricultural activities mainly in the east (HELCOM, 2011). The transition period brought about improvements in farm management practices, which resulted in reduced

nitrogen loads. In contrast to the changes in diffuse nitrogen DAPT sources, changes in point sources are likely the main driver for the observed trends in TPC presented in this study (consistent with modelling work from Mörth et al., 2007). Negative trends for TPC in the western catchments can be explained by the increasing percentage of wastewater being treated and by the implementation of advanced treatment techniques in municipal and industrial facilities (HELCOM, 2011). Moreover, lifestyle changes such as closure of heavily polluting factories and an increased use of phosphorus-free detergents also helped in reducing phosphorus concentrations in the catchments. However, a substantial increase in TP was found in the eastern catchments. Reduction of P from point-source discharges started only after

the transition period for the eastern countries. Although P loads to the Baltic Sea reduced from 1989 onwards, the major reductions happened after 2005 when Latvia, Lithuania, Estonia and Poland joined the EU (HELCOM, selleck products 2011). The large socio-economic transition in the east was accompanied by a change in land cover that also affected nutrient dynamics. Because no data were available on land cover change, land cover for the year 2000 was used. The first factor shows that cultivated and urban areas both have a positive effect on TNC, TNL and TPC, which is logical as these types of land cover are associated with high input of nitrogen and phosphorus due to anthropogenic activities. Furthermore, wetlands, mixed forest and shrubs and herbs have an adverse effect on TNC, TNL and TPC. This inverse relationship to wetlands confirms that wetlands are important for N-retention (Richardson et al., 1997). It is especially important in the more northern catchments (Fig. 2). Jansson et al. (1998) estimated that wetlands in the BSDB retain approximately 5–13% of the annual total amount of nitrogen entering the BSDB.