Trends of phosphorus, nitrogen and chlorophyll a concentrations in Finnish rivers and lakes in 1975-2000.

During recent decades the amounts of nutrients discharged to Finnish surface waters have markedly decreased. This has been achieved by considerable investments in water protection, which were made mainly to improve municipal and industrial wastewater purification. We investigated whether these water protection measures have decreased phosphorus and nitrogen concentrations in Finnish rivers and lakes. In addition, possible trends in chlorophyll a concentrations in lakes were studied. The data consisted of a total of over 68000 monitoring results of 22 rivers and 173 lakes (or sub-basins of lakes) with different types of catchment areas. The study period covered the years 1975-2000 and the non-parametric Kendall Tau b and Seasonal Kendall tests were applied for detecting trends. Decreasing nutrient concentration trends were typical in many lakes and rivers earlier polluted by municipal and industrial wastewaters. Increasing nutrient concentration trends were common in smaller rivers and lakes receiving diffuse loading from agriculture. The results show that the investments directed towards wastewater purification have effectively improved the quality of Finnish inland waters. However, no clear effects of decreasing non-point loading were found. Thus, more effective measures should be directed towards decreasing non-point source loading.

Internal nutrient fluxes counteract decreases in external load: the case of the estuarial eastern Gulf of Finland, Baltic Sea.

Despite a decrease of about 30% in the external nutrient inputs, no corresponding improvement has been observed in the state of the open Gulf of Finland. At the end of the 1990s the external nutrient load to the Gulf totalled 120,000 tonnes (t) yr-1 of nitrogen (N) and 7000 t yr-1 of phosphorus (P). Relative to its surface area, the nutrient load of the Gulf is 2 to 3 times the average of the Baltic Sea. Despite the decrease in loading, an increase in the phosphate-P concentration was observed both in the surface and near-bottom layers around the mid-1990s. The reason for this development was most probably the acceleration of internal loading, triggered by poor oxygen conditions at the sediment-water interface of the eastern Gulf, where the oxygen conditions weakened during the 1990s, after being relatively good in the 1980s and the early 1990s. On the basis of experimental data from the coastal Gulf of Finland, the phosphate-P flux from the reduced surface sediment to water averaged 13 kg km-2 d-1. This corresponds to total amounts which can explain the observed trends of P in the open Gulf. The low N:P ratio of the sediment efflux can partly explain the N limitation of primary production in the Gulf.