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

Unprecedented agricultural intensification and increased crop yield will be necessary to feed the burgeoning world population, whose global food demand is projected to double in the next 50 years. Although grain production has doubled in the past four decades, largely because of the widespread use of synthetic nitrogenous fertilizers, pesticides, and irrigation promoted by the ''Green Revolution,'' this rate of increased agricultural output is unsustainable because of declining crop yields and environmental impacts of modern agricultural practices. The last 20 years have seen diminishing returns in crop yield in response to increased application of fertilizers, which cannot be completely explained by current ecological models. A common strategy to reduce dependence on nitrogenous fertilizers is the production of leguminous crops, which fix atmospheric nitrogen via symbiosis with nitrogenfixing rhizobia bacteria, in rotation with nonleguminous crops. Here we show previously undescribed in vivo evidence that a subset of organochlorine pesticides, agrichemicals, and environmental contaminants induces a symbiotic phenotype of inhibited or delayed recruitment of rhizobia bacteria to host plant roots, fewer root nodules produced, lower rates of nitrogenase activity, and a reduction in overall plant yield at time of harvest. The environmental consequences of synthetic chemicals compromising symbiotic nitrogen fixation are increased dependence on synthetic nitrogenous fertilizer, reduced soil fertility, and unsustainable long-term crop yields.legume ͉ nitrogen fixation ͉ symbiosis ͉ Sinorhizobium meliloti

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The composition and cycling of high-molecular-weight dissolved organic carbon (HMW-DOC) were examined in a tidal stream (Bayou Trepagnier) with seasonally high DOC concentrations (1.0-5.6 mM).13 C nuclear magnetic resonance ( 13 C NMR) was used to examine the bulk chemical composition of natural HMW-DOC from two field sites over 1.5 yr. The HMW-DOC at both sites was dominated by aliphatic (41%), carbohydrate (33%), and carboxyl (16%) carbon, with relatively low aromatic carbon abundance (10%). A comparison of 13 C NMR signatures of bayou HMW-DOC and leachate HMW-DOC collected from leaf litter and soils revealed that plant litter leachate appears to be a more important source of HMW-DOM than soil. Dominant sources of HMW-DOC were likely allochthonous inputs of terrestrial plant litter with periodic inputs of soil organic matter during flooding events. The low aromaticity of bayou HMW-DOC may reflect the influence of low-oxygen conditions, which inhibit the decomposition of particulate aromatic macromolecules such as lignin and humic material to HMW-DOC. Ligninphenol biomarker concentrations (⌳ 6 , in mg/ 100 mg OC) were much lower in HMW-DOC (1.2) than in plant (5.2) and soil (6.8) organic matter, indicating that a significant fraction of this highly aromatic material was not degraded to HMW-DOC. Finally, this study demonstrated that lignin and other compounds from terrestrially derived organic matter in sediments and adjacent soils are not a significant source of more soluble moieties that enter the HMW-DOC pool of the bayou.

-Long-term (1977-Long-term ( -2000 ecological biomass data for total phytoplankton, diatoms, and cyanobacteria, based on plankton samples from Himmerfjärden Bay (Baltic proper), were compared to the historical sediment record using diagnostic plant pigment biomarkers. Radionuclides ( 210 Pb and 137 Cs) were used to determine chronology, sedimentation, and carbon burial rate. Despite high resolution sampling, using crust-freeze sampling, of sediment layers representing annual varves, no significant correlations between pigments preserved in sediments and phytoplankton biomass in plankton samples were detected for periods of 1-4 yr. This lack of correspondence was probably at least partly due to the importance of resuspension events in Himmerfjärden. When sedimentary pigments were averaged over longer time intervals (5 yr) averages of annual diatom biomass in the Himmerfjärd inlet were positively correlated to down-core concentrations of fucoxanthin (r 2 ϭ 0.98) and diatoxanthin (r 2 ϭ 0.62). This indicates that pigment biomarkers can still be used to interpret longer term development of eutrophication-related blooms in such estuarine systems. In contrast, zeaxanthin concentrations were not significantly correlated (p Ͼ 0.05, r 2 ϭ 0.05) to cyanobacterial biomass as a 5-yr average. Using fossil pigments to determine relative differences in phytoplankton biomass composition in the absence of historical ecological patterns of phytoplankton composition can be misleading due to selective losses of pigments such as the epoxy-carotenoids. However, while the use of fossil pigments in laminated sediments alone may not allow for detailed interpretations of past phytoplankton communities, it does allow for the simple determination of the presence of significant biomass of phytoplankton classes, for which unique biomarker pigments exist.