We tested the hypothesis that reservoirs with low water residence time and autochthonous production influence river biogeochemistry in eutrophied river systems draining cultivated watersheds. The effect of a single artificial water reservoir and consecutive reservoirs on silica (Si) river fluxes is exemplified by the moderately dammed Vistula River and the heavily regulated Daugava River that are compared with the practically undammed Oder River. The sum of the discharge weighted annual mean biogenic silica (BSi) and dissolved silicate (DSi) concentrations in the rivers Oder, Vistula and Daugava were about 160 lM (40 + 120 lM), 150 lM (20 + 130 lM) and 88 lM (6 + 82 lM), respectively. Assuming BSi and DSi concentrations as observed in the Oder River as typical for eutrophied but undammed rivers, complete trapping of this BSi could have lowered Si fluxes to the Baltic Sea from rivers with cultivated watersheds by 25%. The superimposed effect of hydrological alterations on reduced Si land-sea fluxes is demonstrated by studies in the boreal/subarctic and oligotrophic rivers Kalixa¨lven and Luea¨lven. The DSi yield of the heavily dammed Lulea¨lven (793 kg km À2 yr À1 ) constituted only 63% of that was found in the unregulated Kalixa¨lven (1261 kg km À2 yr À1 ), despite the specific runoff of the Lulea¨lven (672 mm m À2 yr À1 ) being 19% higher than that of theKalixa¨lven (563 mm m À2 yr À1 ); runoff normalized DSi yield of the former, regulated watershed, was only half the DSi yield of the latter, unperturbed watershed. Based on these findings, it is hypothesized here that perturbed surface water-groundwater interactions are the major reasons for the reduced annual fluctuations in DSi concentrations as also seen in the heavily dammed and eutrophic river systems such as the Daugava and Danube.

Abstract. Nitrate input to a river is largely controlled by land use in its catchment. We compared the information carried by the isotopic signatures of nitrate in 12 Baltic rivers, in relation to the vegetation cover, land use, and fertilization of agricultural land of their catchments. We found isotope values in nitrate ranging from −2 to 14‰ for δ 15 N and 8 to 25‰ for δ 18 O. The annual variability of riverine nitrate isotope signatures is presented in detail for one Nordic, the Kemijoki, and two southern rivers, the Vistula and Oder. Nordic rivers with relatively pristine vegetation in their catchments show not only low δ 15 N values and high δ 18 O-NO − 3 but also lower annual variability than rivers draining densely populated land. Seasonal signals were found in all the rivers. We used load weighted nitrate isotope data and data from the three major N sources (farmland/sewage, atmospheric deposition and from runoff of pristine soils) to theoretically estimate the shares of nitrate from these sources. The results of an isotope mixing model (IMM-1) agree reasonably well with the same estimates for agricultural land derived from a Global Land Cover (GLC) data base, with a deviation varying from −16% to +26%. The comparison with an emission model (EM) reveals relatively good agreements for intensively used catchments (−18 to +18% deviation). Rather unsatisfactory agreement was found between the IMM-1 and GLC calculations for pristine catchments (−36 to +50% deviation). Advantages and limitations of the tested model are discussed.

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In t h~s study we estimated the amount and fate of phytoplankton primary product~on in the coastal zone of the Gulf of Gdansk, Poland, an area exposed to nutnent enrichment from the Vistuld R~v e r and nearby inunlcipal agglomeration The ~n v e s t i g a t~o n s were carned out at 2 sites d u r~n g 5 months in 1993 (February, April, May, August and October). A prolonged bloom pei-iod occurred in the coastal zone, as compared to the open Gulf and the open sea waters. From Aprll until October most values of gross primary production in the near-surface layer were in the range 100 to 500 m g C m-'' d 1 Phytoplankton net exudate release constituted on average 5 % of the gross prlmai-y production, total exudate release was estimated to be about 2 times h~gher. Bacterial production in the growth season was relatively low (the mean value l y~n g between 5 and 9 % of gloss primary production), nevertheless, the microbial community (bactena and protozoans) u t~l~z e d a large proportion of primary production (from about 50% in April and May to 16'%, in October). Usually direct protozoan grazing on phytoplankton exceeded bacterial uptake of the phytoplankton exudates. In winter, summer and autumn communlty respiration exceeded depth-averaged primary production, ~n d~c a t i n g that external energy sources ( s e d~m e n t resuspension, allochthonous organic matter) play a substantial role in communlty metabol~sm.

Eutrophication of the Baltic Sea is considered a major threat to its ecological status. We present and discuss Polish riverine flow normalized loads of total nitrogen (TN) and total phosphorus (TP) discharged into the Baltic Sea in (i)1988–2014, (ii) periods of maximum TN (1992–1994), TP (1988–1991) emission, (iii) the reference period (1997–2003) established by the Helsinki Commission (HELCOM), (iv) 2012–2014, last years of our study. Despite considerable nutrient load reductions prior to the HELCOM reference period, Poland is expected to reduce TN and TP loads by 30% and 66%, respectively. In the light of our historical and up-to-date findings defining ecological status of the Baltic Sea, we suggest that the proposed TP load reduction is overestimated and its realization may lead to (i) undesirable consequences for the Baltic ecosystem, (ii) would require a decline in TP concentrations to 0.067 mg P dm-3 (the Vistula River) and 0.083 mg P dm-3 (the Oder River), values reported for pre-industrial times. The current nutrient concentrations in the Vistula and Oder safely comply with the requirements of the Water Framework Directive. We also comment on the top-down and bottom-up effect resulting in quantitative and qualitative reorganization of the Baltic ecosystem, a phenomenon already observed in the Baltic Sea.