Maren Voss

Impacts of Atmospheric Anthropogenic Nitrogen on the Open Ocean

Increasing quantities of atmospheric anthropogenic fixed nitrogen entering the open ocean could account for up to about a third of the oceans external (nonrecycled) nitrogen supply and up to ∼3% of the annual new marine biological production, ∼0.3 petagram of carbon per year. This input could account for the production of up to ∼1.6 teragrams of nitrous oxide (N2O) per year. Although ∼10% of the oceans drawdown of atmospheric anthropogenic carbon dioxide may result from this atmospheric nitrogen fertilization, leading to a decrease in radiative forcing, up to about two-thirds of this amount may be offset by the increase in N2O emissions. The effects of increasing atmospheric nitrogen deposition are expected to continue to grow in the future.

The global nitrogen cycle in the twenty- first century

Global nitrogen fixation contributes 413 Tg of reactive nitrogen (Nr) to terrestrial and marine ecosystems annually of which anthropogenic activities are responsible for half, 210 Tg N. The majority of the transformations of anthropogenic Nr are on land (240 Tg N yr−1) within soils and vegetation where reduced Nr contributes most of the input through the use of fertilizer nitrogen in agriculture. Leakages from the use of fertilizer Nr contribute to nitrate (NO3−) in drainage waters from agricultural land and emissions of trace Nr compounds to the atmosphere. Emissions, mainly of ammonia (NH3) from land together with combustion related emissions of nitrogen oxides (NOx), contribute 100 Tg N yr−1 to the atmosphere, which are transported between countries and processed within the atmosphere, generating secondary pollutants, including ozone and other photochemical oxidants and aerosols, especially ammonium nitrate (NH4NO3) and ammonium sulfate (NH4)2SO4. Leaching and riverine transport of NO3 contribute 40–70 Tg N yr−1 to coastal waters and the open ocean, which together with the 30 Tg input to oceans from atmospheric deposition combine with marine biological nitrogen fixation (140 Tg N yr−1) to double the ocean processing of Nr. Some of the marine Nr is buried in sediments, the remainder being denitrified back to the atmosphere as N2 or N2O. The marine processing is of a similar magnitude to that in terrestrial soils and vegetation, but has a larger fraction of natural origin. The lifetime of Nr in the atmosphere, with the exception of N2O, is only a few weeks, while in terrestrial ecosystems, with the exception of peatlands (where it can be 102–103 years), the lifetime is a few decades. In the ocean, the lifetime of Nr is less well known but seems to be longer than in terrestrial ecosystems and may represent an important long-term source of N2O that will respond very slowly to control measures on the sources of Nr from which it is produced.

Internal ecosystem feedbacks enhance nitrogen-fixing cyanobacteria blooms and complicate management in the Baltic Sea

Abstract Eutrophication of the Baltic Sea has potentially increased the frequency and magnitude of cyanobacteria blooms. Eutrophication leads to increased sedimentation of organic material, increasing the extent of anoxic bottoms and subsequently increasing the internal phosphorus loading. In addition, the hypoxic water volume displays a negative relationship with the total dissolved inorganic nitrogen pool, suggesting greater overall nitrogen removal with increased hypoxia. Enhanced internal loading of phosphorus and the removal of dissolved inorganic nitrogen leads to lower nitrogen to phosphorus ratios, which are one of the main factors promoting nitrogen-fixing cyanobacteria blooms. Because cyanobacteria blooms in the open waters of the Baltic Sea seem to be strongly regulated by internal processes, the effects of external nutrient reductions are scale-dependent. During longer time scales, reductions in external phosphorus load may reduce cyanobacteria blooms; however, on shorter time scales the internal phosphorus loading can counteract external phosphorus reductions. The coupled processes inducing internal loading, nitrogen removal, and the prevalence of nitrogen-fixing cyanobacteria can qualitatively be described as a potentially self-sustaining “vicious circle.” To effectively reduce cyanobacteria blooms and overall signs of eutrophication, reductions in both nitrogen and phosphorus external loads appear essential.

Seasonal isotopic shifts in fish of the Pantanal wetland, Brazil

Abstract. Seasonal inundations shape the floodplain characteristics of the Pantanal, a large wetland in Central South America. In the first study combining stable carbon and nitrogen isotope analysis with classical stomach content analysis in this region, we investigated the influence of the annual inundation on diet and isotopic composition of floodplain fish. Apart from potential food items, 33 fish species from the Coqueiro Lake were analyzed, 10 of which were present during both the wet and dry season 1999. A δ13C and δ15N plot of the floodplain ecosystem allowed us to assess a foodchain of 3–4 trophic levels. However, the wide overlap of nitrogen values suggested that the organisms act on trophic continua rather than on distinct levels. The foodweb was based mainly on C3-plant carbon. However, fish species capable of feeding on terrestrial invertebrates (e.g., Brycon microlepis) had δ13C values above –25‰, indicating 13–30% intake of C4-plant based carbon during the flooding period. The novel use of vector coordinates and 2-dimensional ANOVA showed that the seasonal isotopic shifts of δ13C and δ15N were highly significant for some feeding guilds. δ15N values increased from the wet to dry season in most fish species, and these shifts were highly significant for omnivores (Astyanax bimaculatus, Triportheus nematurus, Tetragonopterus argenteus and Moenkhausia dichroura), and significant for invertivores (Gymnogeophagus balzanii and Poptella paraguayensis) and carnivores (Serrasalmus spilopleura). Average carbon isotope ratios decreased at the same time in the herbivores (Methynnis mola: 3.4‰) and detritivores (Psectrogaster curviventris: 5.3‰), but they did not change in the hypostomatic herbivore Sturisoma robustum. We explain these shifts by abundant and variable food sources during the inundation period and increasing carnivory and starvation during the dry season when the lake is confined to its central basin. Isotopic shifts between seasons were more prominent in less specialized species of omnivores, invertivores and some carnivores, whereas more specialized herbivores and detritivores appeared to be more influenced by changes in the carbon isotope ratio of the diet affected by biogeochemical processes such as respiration and methanogenesis. A general model for the interpretation of isotope data of floodplain fish considering different time-scales is given.

Stable isotope signals of eutrophication in Baltic Sea sediments

Abstract Increasing δ 15 N and δ 13 C values in coastal Baltic marine sediments are evaluated as indicator of changes in the trophic status of the ecosystem. The influence of eutrophication on the δ 15 N values was found to be so dominant that it even overprints the usually observed mixing gradient from terrestrial (low isotope values) to the marine environment (high isotope values). A distinct gradient in stable nitrogen isotope values from eutrophic coastal areas to open more oligotrophic waters in the central Baltic Sea and Gulf of Bothnia was found. Our data show high δ 15 N values in surface sediments: 13‰ in the Oder Lagoon and the Pomeranian Bight, over 9‰ in the Gulf of Riga (Daugava River), 7‰ in the inner Gulf of Finland (Neva River), 6.5‰ in the Curonian Lagoon (Nemunas River), and 5.7‰ in the Gdansk Deep (Vistula River). In the Baltic Sea Proper, significantly lower δ 15 N values of 3–5‰ are found. A decrease in δ 15 N values with depth/age of the sediment was indicated in some cores that were analyzed down to 15–40 cm depth in 1-cm steps. There is a great overall difference between pre-industrial δ 15 N values in coastal sediments and recent ones of 2.3–10‰. As explanations for this increase are suggested, elevated nutrient δ 15 N values of waste water in combination with fractionation processes like nutrient uptake by phytoplankton and denitrification and nitrification processes in rivers discharging into the coastal water. Delta 13 C values in sediment surfaces off the river estuaries primarily indicate differences between the inorganic carbon signatures of the rivers. However, since the δ 13 C values also decrease downcore, we contribute this change to increased primary production caused by the enhanced nutrient load. Since both stable isotope values in sediments ( δ 13 C and δ 15 N) correlate downcore, this strongly suggests that the anthropogenic nutrient loads in the rivers might be the reason for the changes of stable isotope values.

Integrating nitrogen fluxes at the European scale

Executive summary Nature of the problem • Environmental problems related to nitrogen concern all economic sectors and impact all media: atmosphere, pedosphere, hydrosphere and anthroposphere. • Therefore, the integration of fluxes allows an overall coverage of problems related to reactive nitrogen (Nr) in the environment, which is not accessible from sectoral approaches or by focusing on specific media. Approaches • This chapter presents a set of high resolution maps showing key elements of the N flux budget across Europe, including N2 and Nr fluxes. • Comparative nitrogen budgets are also presented for a range of European countries, highlighting the most efficient strategies for mitigating Nr problems at a national scale. A new European Nitrogen Budget (EU-27) is presented on the basis of state-of-the-art Europe-wide models and databases focusing on different segments of Europe’s society. Key findings • From c. 18 Tg Nr yr −1 input to agriculture in the EU-27, only about 7 Tg Nr yr− 1 find their way to the consumer or are further processed by industry. • Some 3.7 Tg Nr yr−1 is released by the burning of fossil fuels in the EU-27, whereby the contribution of the industry and energy sectors is equal to that of the transport sector. More than 8 Tg Nr yr−1 are disposed of to the hydrosphere, while the EU-27 is a net exporter of reactive nitrogen through atmospheric transport of c. 2.3 Tg Nr yr−1. • The largest single sink for Nr appears to be denitrifi cation to N2 in European coastal shelf regions (potentially as large as the input of mineral fertilizer, about 11 Tg N yr–1 for the EU-27); however, this sink is also the most uncertain, because of the uncertainty of Nr import from the open ocean. Major uncertainties • National nitrogen budgets are diffi cult to compile using a large range of data sources and are currently available only for a limited number of countries. • Modelling approaches have been used to fill in the data gaps in some of these budgets, but it became obvious during this study that further research is needed in order to collect necessary data and make national nitrogen budgets inter-comparable across Europe. • In some countries, due to inconsistent or contradictory information coming from different data sources, closure of the nitrogen budget was not possible. Recommendations • The large variety of problems associated with the excess of Nr in the European environment,including adverse impacts, requires an integrated nitrogen management approach that would allow for creation and closure of N budgets within European environments. • Development of nitrogen budgets nationwide, their assessment and management could become an effective tool to prioritize measures and prevent unwanted side effects.

Stable carbon and nitrogen isotope signatures of decomposing tropical macrophytes

The Pantanal of Mato Grosso, Brazil, is a large, seasonal wetland, which exhibits high macrophyte productivity at the beginning of the rainy season, when the floodplain becomes flooded. During inundation, from December through May, there is rapid turnover of decomposing macrophyte litter, which is subsequently colonized and consumed by various organisms. In this paper, the variation in the carbon and nitrogen isotope signatures of decomposing macrophytes and detritus was determined to provide an isotopic baseline for the elucidation of higher trophic levels. Seven abundant macrophyte species, Cyperaceae sp., Pontederia lanceolata, Cabomba furcata, Salvinia auriculata, Eichhornia crassipes, Nymphaea amazonum and Paspalum repens, were exposed in mesocosm decomposition experiments lasting 21 or 100 days. Stable isotope ratios of carbon and nitrogen and the atomic C/N ratios were determined for decomposing plant material, particulate organic matter (POM), the microbial film, and aquatic invertebrate larvae. The δ13C values for the macrophytes did not change during decomposition. However, the variability of δ15N was high (range of ± 6 ‰) due to microbial activity. There was no consistent difference in the isotopic signatures of macrophytes and POM. C/N ratios decreased from 17 to 50 in macrophytes, to 7 to 12 in POM. The isotopic signatures and C/N ratios of the microbial film were the same as those of POM. We concluded that heterotrophic processes did not fractionate stable carbon isotopes but caused an increase in the variability of stable nitrogen ratios and a change in the C/N ratios in our experimental system. Therefore, it was not possible to distinguish fresh and senescent material or even POM when used as a food source. The δ13C values of the aquatic larvae were closely coupled to those of the carbon source provided.

Nitrogen flows from European regional watersheds to coastal marine waters

Approaches A comprehensive evaluation of net anthropogenic inputs of reactive nitrogen (NANI) through atmospheric deposition, crop N fi xation, • fertiliser use and import of food and feed has been carried out for all European watersheds. A database on N, P and Si fl uxes delivered at the basin outlets has been assembled. A number of modelling approaches based on either statistical regression analysis or mechanistic description of the processes involved • in nitrogen transfer and transformations have been developed for relating N inputs to watersheds to outputs into coastal marine ecosystems.

Nitrogen transformation processes in the Elbe River: Distinguishing between assimilation and denitrification by means of stable isotope ratios in nitrate

Abstract.During a 9-day Lagrangian sampling campaign along the free-flowing section of the Elbe River in July 2005, water from the river as well as from major tributaries and sewage treatment plants was sampled to investigate major nitrogen transformation processes, focussing on denitrification and N-assimilation by phytoplankton. Samples were analysed for