DONKEY(110607)：驴的占20（110607）.docVIP

Final report forDONKEY - Dissolved Organic Nitrogen as a Key Nutrient in the Baltic SeaStiig Markager, Colin Stedmon, Lars Tranvik, Leif Kronberg og Maaret Kulovaara 1 IntroductionThis report summaries the activities, main results and products of the project “Dissolved Organic Nitrogen as a Key Nutrient for Marine Plankton (DONKEY). The project was funded by The Nordic Council of Ministers from 2004 to 2006.2 Activities2.1 MeetingsA kick-off meeting was held in Uppsala on the 5 and 6. of May 2004. Next meeting was in bo, January 25-25, 2005. The last meeting was held in Roskilde in January 2006. 2.2 Experiments2.2.1 SamplingSampling was done once in 2004 at three stations: Oulu in the Bothnian Bay, Kotka in Gulf of Finland and in the Arkona Sea. A seasonal study at the same stations was performed in 2005/2006 where sampling was done during summer (May or August), fall (September or November) and winter (February, only Arkona due to sea ice at the other stations). Sampling for bioavailability of DON was also done at five stations in November 2004 in Gulf of Gdansk in corporation with the Polish Institute of Oceanology. Additional sampling was performed from lakes (see 2.2.4).2.2.2 Bioavailability of DON in degradations experimentsIn total 31 experiments with bioavailability of dissolved organic matter (DOM) was performed during the project. Briefly, seawater was filtered through 0.2 m filters and pre-treated for 6 hours in the dark or under natural levels of UV-light. Then sub samples where incubated in the dark for up to 150 days. Concentrations of nutrients, dissolved organic carbon, ph, optical properties of DOM and oxygen consumption was measured over time. The procedure and results are described in a manuscript under preparation for publication in the international journal Estuaries.The main results are shown on Fig. 1 and 2. Concentrations of total nitrogen (TN) varied from 12.0 to 34.5 mmol N m-3 with an average value of 21.4. The organic fraction (dissolved organic nitrogen, DON) constituted on average 81% with the remaining 19% as nitrate, nitrite and ammonium (DIN). The DON-fraction varied between 56 and 99% so in most samples DON was clearly the dominating fraction of TN. No clear pattern was found in the distribution between DIN and DON with respect to location or season. Only the samples from Kotka showed a tendency to high fractions of DIN.Fig. 1 The total pool of dissolved nitrogen divided into three fraction: dissolved inorganic compounds (nitrate, nitrite and ammonia), bioavailable dissolved organic nitrogen and refractive dissolved organic nitrogen. Each bar show the results from one degradation experiment without pre-treatment with UV-light. Sampling month and location is indicated below each bar.The bioavailability of DON in samples not pre-treated with UV-light, varied from 2.0 to 60% with an average value of 21% (Fig. 2). The effect of a pre-treatment with was initially a faster degradation of DON, but the net effect over 150 days was minimal in most cases. The bioavailable fraction was high at Arkona in February (42%) and in Gulf of Gdansk in November (24-60%). In all other samples the fraction was below 17%. The results indicate a seasonal pattern in the quality of the DON fraction. During winter, or close to a large river like the Vistula River flowing into Gulf of Gdansk, a considerable fraction of DON can be utilised. During summer, where the phytoplankton population is actively growing and often limited by access to nutrients, the DON-pool is depleted for biodegradable compounds. Fig. 2 The fraction of the pool of dissolved organic nitrogen the is degraded to nitrate, nitrite or ammonium over up to 150 days versus sampling month and location.The result show a rather complex pattern for bioavailability of nitrogen bound in DOM. On one hand the fraction available during the productive season is low, less then 20% and in many cases less than 10%. One the other hand, during fall and winter up to 40-60% is degradable. The latter is comparable to the findings in Stepanauska et al. (2002). there showed the about 30% of the nitrogen in DON was bioavailable.2.2.3 Photochemical production of ammonium from DOMThe photochemical production of ammonium from DOM was measured on samples from the three study sites on summer samples. Additionally, changes in optical characteristics and molecular size following exposure to UV-light were measured. The results are published in Marine Chemistry (Stedmon et al. 2007, attachment 1).Briefly, the study shows that there is a release of ammonium when DOM is exposed to UV-light at levels found in the upper layers of the water column in the Baltic Sea during summer. The DOM from Oulu and Kotka resembled that of terrestrial DOM with high C:N ratios, a large fraction of high molecular weight molecules and dominated by humic compounds. DOM from Arkona displayed a lower C:N ration and a larger content of proteins. Ammonium production was significant, particularly at the Arkona station. Estimation of daily areal rates ranged from 37 mol N m-2 d-1 in the Bothnian Bay to 237 mol N m-2 d-1 in the Arkona Basin. This is similar to rates for atmospheric deposition where annual averages vary between 40 and 196 mol N m-2 d-1. However, where atmospheric deposition is highest in winter, when precipitation is high, photochemical production of ammonium depends on UV-light and is therefore most important during summer. Thus, during much of the growth season for phytoplankton, photochemical production of ammonium is probably at the same level or higher than the nitrogen input from atmospheric deposition. We believe that this finding is new and has important implications for the understanding of eutrophication in the Baltic Sea, as it means that there is a lower limit to how much that nitrogen input to surface layers in summer can reduced by addressing nitrogen emissions to the atmosphere.2.2.4 Release of ammonium in sea water due to ion exchangeBinding of ammonium on dissolved organic matter, and its dependence on salinity was tested in laboratory experiments.We hypothesized that ammonium bound to dissolved organic matter is subject to ion exchange, with the major cations of seawater (Na+) binding more strongly than NH4+ to dissolved organic compounds. Hence, ammonium may be bound to humic substances and other organic molecules in the freshwater environment, but as the organic matter is transported via rivers to the Baltic Sea the ammonium may be set free by ion exchange after mixing into more saline water, becoming a readily available nitrogen source for marine primary producers.We tested the mechanisms that are necessary for this scenario in the laboratory. Dissolved organic matter from lakes dominated by terrestrial humic substances, similar to the carbon exported from large parts of the drainage basin of the Baltic Sea, was amended with ammonium (measured with a spectrophotometric and a fluorometric ammonium assay), and we assessed the molecular weight distribution of ammonium and of humic compounds (measured via absorbance) at different salinities. When ammonium is bound to organic matter, its molecular weight distribution should be similar to that of humic compounds. On the other hand, if ammonium is free, it should be of low molecular weight, separate from the high molecular weight organic matter. Two different approaches were tested, 1) gel permeation chromatography, whereby low and high molecular weight compounds can be separated, and 2) dialysis, whereby free ammonium may diffuse through a membrane which has pores that are too small for humic substances to penetrate.Both the dialysis and gel permeation chromatography methods appeared successful from a technical point of view. However, none of them revealed a clear pattern of different behaviour of ammonium at different salinities. Some experiments suggest that ammonium was released from organic matter at enhanced salinity. However, consistent, reproducible results were not obtained. Hence, we can not corroborate the hypothesis that ammonium bound to organic substances under freshwater conditions are released upon mixing into more saline waters.2.2.5 Molecular size distribution of DOMIn the Laboratory of organic chemistry at bo Academy University, the DOM in sea water was characterized by size exclusion chromatography and by mass spectrometric methods. Samples were obtained from the degradation experiments (2.2.2) and were: original sea water stored in the dark, sea water pre-treated with UV-light and thereafter stored in the dark, and sea water incubated at daylight 8 h/day. Samples from each of the three treatments were taken at day 0, 3, 7, 20, 75 and 150 and analysed for changed in the molecular weight distribution of DOM. In all, about 150 samples were analysed.Size exclusion chromatography (SEC) was performed on a TosoHaas TSK gel G3000SW column which was eluted with 10 mM ammonium acetate at a flow rate of 1 mL/min. The column separates the organic material according the molecular size of compounds or aggregates. The technique gives information on the molecular size distribution of DOM in water and is very useful for comparing DOM in samples from various location and for monitoring changes taking place in DOM e.g. a degradation experiments as (2.2.2).The DOM in the samples from the Bothnian Bay and from the Gulf of Finland were very similar and the size exclusion chromatograms resembled those of DOM in brown colored lakes and thus it was concluded that DOM in these samples were mainly of terrestrial origin. The Arkona sample had a different molecular size distribution and the DOM in this sample is most likely produced in the sea. No drastic changes in the DOM were found as a consequence of the UV or daylight irradiation of the samples. On the other hand, it was obvious that during the degradation experiment larger as well as smaller aggregates of DOM were formed, so DOM became more evenly distributed during the experiment.The mass spectrometric analyses were performed on an Agilent Ion Trap mass spectrometer and the samples were infused directly to the electrospray ion (ESI) source. Since ESI is a soft ionization method the ions in the mass spectra represent protonated/deprotonated molecular ions, i.e. the molecular weights of the individual compounds making up the DOM can be observed. Initially, these experiments failed since the salts content in sea water interfered during ionization, producing mass spectra that could not be interpreted. Upon desalination with different techniques, such as dialysis and use of XAD resins, reliable mass spectrometric analyses could be performed. The mass spectra showed that the DOM in the Bothnian Bay sample consisted of material having molecular weights of 200 to 800 mass units with a mean molecular weight of about 500. These mass spectra were very similar to those produced by DOM in brown coloured lakes. Samples from the Arkona Sea could not be analysed by mass spectrometry due to the low concentration of DOM. The desalination procedure was very tedious and could not be applied for all the samples. The work at our Laboratory was mainly performed by Tove Sltis and she compiled the work in her master thesis titled: Analys av naturligt organisk material i stersjn med elektrospayjoniserings masspektrometri (ESI-MS) och storlekskromatografi. (Kemisk-Tekniska Fakulteten vid bo Akademi, 2005.3 ConclusionsThe aim of this project was to provide initial results there could characteristics the role of DOM in the biogeochemical cycling of nitrogen in the Baltic Sea and provide data for modelling of the biogeochemical cycling. The degradation experiments show that the fraction of DON available for primary production is variable. In winter it is about 50%, decreasing to less than 20% during summer and in cases even less. These number, together with DIN and nitrogen bound in phytoplankton and other planktonic organism, can be used as a rough guideline for the amount of nitrogen there has an active role in the planktonic system, e.g. in ecological modelling of the Baltic Sea. The results from photoammonification experiments reveal that this can be a very important process during summer, potentially supporting phytoplankton with the same amount of nitrogen as nitrogen fixation. We believe that this contribute with new knowledge about the role of organic bound nitrogen for the ecology of the Baltic Sea and for our understanding of the eutrophication processes in the ecosystem. 4 ProductsPoster at 10th Nordic IHSS symposium on character of natural organic matter and its role in the environment, Riga, June 1-3, 2005. The use of electrospray - ion trap mass spectrometry in the study of aquatic humic material, Sltis, T., Kulovaara, K. and Kronberg L.Oral presentation at the annual Danish