River biogeochemistry and source identification of nitrate by means of isotopic tracers in the Baltic Sea catchments

International audienceNitrate 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 of their catchments. We found isotope values in nitrate ranging from ?2 to 14 for ?15N and 8 to 25 per mil for ?18O. Seasonal signals were evident in all rivers. The annual variability of riverine isotope signatures is presented in detail for one Nordic, the Kemijoki, and two southern rivers, Vistula and Oder. Nordic rivers with relatively pristine vegetation in its catchments show not only low ?15N values and high ?18O-NO3? but also lower annual variability than rivers draining densely populated land. Seasonal signals could be found in all of the rivers. We used load weighted nitrate isotope data and data from the three major sources (farmland/sewage, atmospheric deposition and from runoff of pristine soils) to theoretically estimate the shares of nitrate from these sources. The results agree well with same estimates derived from a Global Land Cover data base. The comparison with an emission model (EM) reveals good agreements for intensively used catchments and rather bad ones for pristine catchments. Advantages and limitations of the tested model types are discussed

A comparative study of responses in planktonic food web structure and function in contrasting European coastal waters exposed to experimental nutrient addition

We quantify, compare, and generalize responses of experimental nutrient loadings (LN) on planktonic community structure and function in coastal waters. Data were derived from three mesocosm experiments undertaken in Baltic (BAL), Mediterranean (MED), and Norwegian (NOR) coastal waters. A planktonic model with seven functional compartments and 30-32 different carbon flows fit to all three experiments was used as a framework for flow-rate estimation and comparison. Flows were estimated on the basis of time series of measured biomass, some measured flows, and inverse modeling. Biomass and gross uptake rate of carbon of most groups increased linearly with increasing LN in the nutrient input range of 0-1 µmol N L-1 d-1 at all locations. The fate of the gross primary production (GPP) was similar in all systems. Autotrophic biomass varied by two orders of magnitude among locations, with the lowest biomass and response to nutrient addition in MED waters. The variation of GPP among sites was less than one order of magnitude. Mesozooplankton dominated by doliolids (Tunicata), but not those dominated by copepods, presumably exerted efficient control of the autotrophic biomass, thereby buffering responses of autotrophs to high nutrient input. Among the many factors that can modify the responses of autotrophs to nutrients, the time scale over which the enrichment is made and the precise mode of nutrient enrichment are important. We suggest a general concept that may contribute to a scientific basis for understanding and managing coastal eutrophicatio