| | | 19.3 Variation in riverine loads |
The temporal variation in riverine loads of nutrients to the Gulf of Riga is substantial and characterised by both large interannual and seasonal variations. Most of these variations can be explained by natural fluctuations in runoff [282]. The large interannual variation in the nutrient loads can be exemplified by the nutrient loads for 1984 and 1990, which differed by a factor of two for all major nutrient constituents. The seasonal distribution in load also correlates well with the variability in runoff. Comparatively low loads normally follow a pronounced peak during the spring flood during the low-flow period in summer. For example, the phosphorus load of the Daugava River in April usually accounts for 20% of the total annual load, whereas the corresponding loading during summer is much lower.
The annual loads in the period investigated (1993-1995) were, when compared with a 20-year average, relatively normal. Thus the biological investigations of the present study were carried out in a runoff scenario that resembles the average over the last 20 years. However, in recent years increased nitrate concentrations were observed in the Daugava River (P. Stålnacke, pers. comm.). No definite and clear evidence of downward trends in the nutrient loadings related to the rapid economical recession (particular agriculture: fertiliser application decreased to 20%) in the catchment area in the former republics of the Soviet Union (i.e. Baltic Republics and Belarus) can be noted. The decrease in annual loadings that can be observed in certain periods is almost solely due to a decrease from the high runoff observed other periods.
| | | 19.3 Variation in riverine loads |