Abstract-Massive summer blooms of nitrogen-fixing cyanobacteria have been documented in the Baltic Sea since the 19th century, but are reported to have increased in frequency, biomass, and duration in recent decades-presumably in response to the well-documented anthropogenic eutrophication of the Baltic. Here, we present an 8,000-yr record of fossil cyanobacterial pigments, diatom microfossil assemblages, and ␦ 15

Seasonal cycles of δ 13 C and δ 15 N in dissolved organic carbon and size-fractionated plankton, ranging from bacteria to the jellyfish Aurelia aurita, were studied during a 1 yr cycle at a coastal station in the Baltic Sea. The observed isotopic changes were found with time lags in all size-fractions of plankton. The δ 13 C showed a bimodal cycle with 2 local maxima, the first coinciding with the spring bloom and the second with the autumn bloom. In δ 15 N, the annual cycle was trimodal with 3 local maxima. The first occurred in connection with the spring bloom, the second in mid-summer and the third was a broad autumn-to-winter maximum. The causes of these patterns are discussed in relation to measured oceanographic variables. In the summer, a depleted nitrogen isotopic signal was propagated through all size-classes of plankton, indicating direct or secondary utilisation of fixed nitrogen from cyanobacteria. The strength of the signal indicated that nitrogen-fixing cyanobacteria are more ecologically important as instantaneous nitrogen sources in the Baltic than previously assumed. Enrichment of δ 15 N in size-classes of plankton was found to be a linear function of logarithmic organism size from 20 to 500 µm, reflecting size-related consumption patterns of marine plankton food-webs. The explanatory power of the linear regression and the enrichment per unit size were stronger in spring and autumn than in the summer, reflecting time lags and diversity in the zooplankton community. The sizespecific approach was found to be a simpler and more appropriate way of analysing trophic isotope enrichment in plankton food-webs than the assumption of a general enrichment factor per trophic level. 203: 47-65, 2000 of phytoplankton, but the pattern was variable between years. Zohary et al. (1994) found, in a long-term study of plankton in Lake Kinneret (1971Kinneret ( to 1992, that δ 13 C was most depleted during winter in both phyto-and zooplankton. The available information suggests a characteristic seasonal cycle, but sample variation has often been difficult to separate from seasonal variation. If the seasonal isotopic cycle is not properly described for organisms at a studied site, time lags and memory effects may interfere with attempts to analyse food-web dynamics and material use. KEY WORDS: Stable isotopes · Food web · Baltic · Cyanobacteria · Seasonal cycle · Nitrogen fixation · Trophic enrichment Resale or republication not permitted without written consent of the publisherMar Ecol Prog SerMarine plankton have been found to feed opportunistically in unstructured food webs, with particle size as the most important factor (Isaacs 1972, 1973, Platt & Denman 1977, Sheldon et al. 1977, Platt 1985. Size fractions of plankton are therefore likely to be representative of trophic groups in the marine plankton community. The present study investigated size-dependent changes and an annual cycle of δ 13 C and δ 15 N in sizefractionated Baltic plankton, ranging from <1 µm to the jellyfish Aurelia aurita (40 mm)....

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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.