International Summer Water Resources Research School Dept. of Water Resources Engineering, Lund University

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1 International Summer Water Resources Research School Dept. of Water Resources Engineering, Lund University The effect of warming on sediment nutrient fluxes and N-removal By

2 International Water Summer Resources Research School Abstract The river sediment can act as a source or sink of nutrients. There are several factors influencing the nutrient dynamics in the sediment-water interface, this study focuses on the multiple impact of temperature change and dam construction on nutrient cycling and downriver export. Many previous studies has shown that an increase in temperature will lead to increased release of nutrients from the sediment to the water body. This implies that the increasing temperature due to global climate change may increase the nutrient load in rivers, which can aggravate eutrophication. The first effort of the study is to setup an incubation system and test the signal of nutrient exchange and gaseous nitrogen production (dissolved N2). Two core sediments were collected from a lake in Xiamen University, Xiang an campus. The sediment samples were incubated at 28 C, 20 C and 10 C using a continuous water flow system. Two controls without sediment were incubated simultaneously. The outflow water was collected at an interval of 24 hours. Water samples were analyzed for concentrations of nutrients, dissolved reactive phosphorous, nitrate, nitrite and ammonium and net increase of dissolved N2. Preliminary result shows that the concentration tends to become stable as time went by. This work provided some important knowledge about how the system behaves and how stability can be obtained and maintained, which is useful for improving the incubation experiment and doing research on the Jiulong river in near future. Key words: Sediments, nutrients, nitrogen removal, global warming 1

3 International Water Summer Resources Research School Table of Contents 1. INTRODUCTION 3 2. BACKGROUND SEDIMENT NUTRIENT DYNAMICS NITROGEN PHOSPHOROUS PREVIOUS STUDIES 4 3. METHODS PRE STUDY SAMPLING EXPERIMENTAL SET UP 7 CONTINUOUS WATER FLOW TECHNIQUE CHEMICAL ANALYSIS 7 NUTRIENTS 7 N CALCULATION OF FLUXES 8 4. RESULTS SAMPLING OCCATIONS NUTRIENT CONCENTRATIONS NUTRIENT FLUXES N N 2 FLUX FLUXES OF NITROGEN COMPOUNDS DISCUSSION CONCLUSION ACKNOWLEDGEMENTS 19 REFERENCE LIST 20 2

4 International Water Summer Resources Research School 1. Introduction Global climate change and changes in land use alters the conditions for rivers and aquatic ecosystems. Changes in land use such as construction of dams changes the channel flow. In the dams the flow velocity is dramatically decreased and the vertical mixing of the water decreases due to less turbulent flow. Construction of dams also leads to large areas being flooded. Combined with increasing temperatures due to global warming, this can have large effect on the river ecosystem dynamics. The nutrient load is a very important parameter affecting the balance of the river ecosystem. Today many rivers are being subject to an increase in nutrient load due to use of fertilizers in agriculture in the river catchment area. Increased nutrient load can lead to eutrophication. The river sediment can act as a source or sink of nutrient, as organic matter is deposited and decomposed in the sediment. This study will focus on the fluxes of nutrient between the sediment and the water body. The aim is to investigate how the nutrient flux is affected by changes in temperature and land use. The studied river is Jiulong river in the Fuijan province in southeast China. This river is being subject to many changes in its morphology due to the construction of step hydropower stations. By studying sediment samples from representative sites along the Jiulong river and incubating these at different temperatures, the temperature dependency of sediment dynamics across land use can be investigated. The study focuses on the cycling of nitrogen and phosphorous and the permanent removal of nitrogen though denitrification. To understand how these cycles are affected by the changing climate and land use is very important for predicting how the river ecosystems will respond to these changes. In this study, sediments from the Xiamen University Xing an Campus lake will be used for a pre-study of the laboratory equipment. The temperature dependency of the sediment nutrient dynamics will be analyzed and the experimental set-up will be evaluated. This study will provide knowledge for further studies of how the changing climate and the human impact due to dam construction will affect the Jiulong river. 3

5 International Water Summer Resources Research School 2. Background 2.1 Sediment nutrient dynamics The river sediment can act as a source or sink of nutrients. Nutrients can be transferred from the water to the sediment through biotic uptake, or they can be released to the water through diffusion. The nutrients that are released back to the water can be the product of the microbial decomposition of sediment organic matter or the product of redox reactions occurring in the sediments. The dynamics in the sediment water interface is dependent on many factors. The temperature affects the organic matter decomposition rate, which can affect the nutrient cycling. The redox conditions in the sediment are also an important factor in determining which reactions that will occur, since the redox conditions determines which compound that will serve as the dominant electron acceptor.(duan and Kaushal 2013) In this study sediment samples will be incubated at different temperatures to examine the impact of warming on the fluxes of nitrogen and phosphorous in the sediment-water interface Nitrogen Nitrogen is the limiting nutrient in many freshwater ecosystem. Nitrogen gas, N2, is the main component of the atmosphere, through nitrogen fixation nitrogen gas can be fixed from the atmosphere to the ecosystem. In the nitrification process nitrate is produced via the oxidation of ammonium, NH4. Nitrate can be removed from the aquatic system through denitrification, which is the reduction of nitrate via a series of intermediates. The product of denitrification is N2 or N2O gas. Since nitrification is an aerobic process, it usually occurs in upper parts of the sediment where oxygen is available. Deeper in the sediment the conditions are anaerobic, which makes denitrification possible. The two processes hence occurs simultaneously in the sediment. The denitrification rate is limited by the availability of nitrate, which makes the denitrification dependent on the nitrification rate. The removal of nitrogen from through denitrification in hence dependent on the oxygen availability and the redox potential in the sediment. (Duan and Kaushal 2013) Phosphorous Phosphorous can also be limiting in freshwater ecosystems. Phosphorous is usually present in the form of phosphate PO4 3-. Phosphate generally cannot be reduced to a gaseous compound, removal of phosphate from freshwater ecosystems occurs through runoff to the oceans. Phosphorous enters the ecosystem through weathering of bedrock. 2.2 Previous studies Many previous studies on the sediment nutrient fluxes and the impact of temperate and land use has been made. Studies on the effect of warming on the sediment nutrient fluxes has shown that the nutrient release increases with temperature (Silvenoinnen et al 2008:A) In this section some results from previous studies on sediment nutrient dynamics will be presented. These results can be used to understand and interpret the results that will be obtained in this study. A study conducted by S.-W. Duan and S.S. Kaushal at University of Maryland 2013 investigated the effect of temperature and land use change on the sediment nutrient fluxes. Sediment samples 4

6 International Water Summer Resources Research School from different sites with catchment characteristics were collected and incubated at different temperatures. The results showed an increase in the release of DOC, phosphorous and nitrogen across land use, an increase in sulfate release at rural and suburban sites and a decrease in sulfate release at urban sites(duan and Kaushal 2013). Another study provides a technical review of a laboratory microcosm with continuous flow technique which was used to analyze sediment nutrient dynamics. The study was conducted by A. Liikanen, H. Tanskanen, T. Murtoniemi and P. J. Martikainen in Sediment samples from lake Kevatön were incubated and analyzed for nutrient and greenhouse gas fluxes. The results showed that the microcosm using the continuous water flow was a promising tool for analyzing gas and nutrient dynamics in the sediment. This study will use a similar experimental set-up as the one used to analyze the samples from lake Kevatön. This report will provide an evaluation of the potential of using this technique in this specific project with the samples from Jiulong river. (Liikanen et al 2002:A) Several studies on how the oxygen concentration effects the sediment nutrient fluxes has also been performed. Since the redox conditions in the sediments determines what reductions and oxidation reactions that will occur, the oxygen availability will have a large impact on the chemical dynamics in the sediment. A study on the denitrification rate and N2O effluxes on river sediments as affected by oxygen availability and temperature shows an increase in denitrification rate with increasing temperature. (Silvennoinen et al 2002:B). Another study by H. Silvennoinen, A. Liikanen, J. Torssonen, C. F. Stange and P. J. Martikainen from 2008 investigates the denitrification rate and fluxes of N2O under increasing nitrate load. The study showed that the fluxes of nitrogen gas as well as nitrous oxide increased with increasing nitrate load. The ratio of N2O to N2 also increased, which means that higher amounts of the potent greenhouse gas N2O was released from the sediment. The study also showed that the microbial activity in the sediment increased with the increasing nitrate load (Silvenoinen et al 2008: B) 5

7 International Water Summer Resources Research School 3. Methods 3.1 Pre study Before the experiments with the samples from Jiulong river can be conducted, a pre-study has to be made to test the experimental set-up and find the optimum conditions. Sediment samples and incubation water was collected from the Xiamen University Xing an Campus Lake on the 25 th of June. Four sediment cores where collected. During the incubation time two sediment cores where lost, so the results from the pre-study can only be presented for two sediment cores and two control cores without sediment. The samples where incubated for four days at 28 C, outflow water samples were collected twice daily. Thereafter the temperature was decreased to 10 C and the sediment samples were incubated for another five days during which outflow water samples were collected twice daily. The temperature was thereafter increased to 20 C and incubated for eight days, outflow water samples were collected once a day. 3.2 Sampling Sediment samples were collected using a sediment sampler, see picture. The sediment was collected directly into the incubation tube. The tube had an outer diameter of 60 mm, an inner diameter of 56 mm, and a height of 30 cm, and hence a volume of cm 2. The incubation water was collected from the free water in the river or lake. Before water for incubation could be connected to the system it had to be filtered to remove grains and other particles. Figure 1. The sediment sampler Figure 2. An intact sediment core. 6

8 International Water Summer Resources Research School 3.3 Experimental set up Continuous Water Flow technique Figure 3. A simplified schematic sketch of the incubation system A simplified schematic representation of the experimental set up is shown in the picture above. The tubes containing the sediment cores are put in a container with water which is being kept at a constant temperature using a temperature controlling heat exchanger. The tubes are connected to a 25 l bag with incubation water which has been collected from the river. The incubation water is lead from the water bag to the sediment tubes through a small tube, another tube is connected to the sediment tube through which the water can flow out. This incubation set up is called continuous water flow (CWF) technique, since water is continuously flowing through the sediment tubes. To enhance the mixing of the water in the tubes, a magnetic stirrer operating at low intensity is placed in each tube. In this specific set-up, two cores are containing sediment and two cores are used as control, containing no sediment. 3.4 Chemical analysis Samples from the outflow water were collected and tested for nutrient content and concentration of dissolved N2. At each sampling occasion the discharge from each tube was measured in order to be able to calculate the fluxes of nutrients and nitrogen gas from the sediment. Nutrients Preparation 30 ml of outflow water was collected and filtered through a 0.45 m filter. The water samples 7

9 International Water Summer Resources Research School were kept cold during the time between the sampling and the analysis. The water samples were then diluted with distillated water. Analysis The samples where analysed for concentration of PO4-P, NO3-N, NH4-N and NO2,-N. This was done with a SKALAR nutrient auto-analyzer. The maschine measures the nutrient concentrations in the sample direcltly. N 2 Preparation Outflow water was collected into a 20 ml tube. To kill all present microorganisms 6 l of HgCl2 was added to the water sample. The samples were stored in a cold container before the analysis begun. Analysis The samples were analysed using the membrane inlet mass spectrometer Hiden HPR40 (see picture). The spectrometer measures the ratio of N2:Ar present in the sample. The amount of argon that is soluble in water is only dependent on physical conditions such as temperature and salinity, whilst the amount of N2 in the water also depends on biological processes such as denitrification. The ratio of N2:Ar is measured for a series of water samples with different salinity, using this data a calibration curve can be obtained. This relates the measured N2:Ar ratio to the ratio that is expected using Weiss law of gas solubility in water. Thereafter the water samples that are to be analyzed can be measured. The measured N2:Ar ratio in the Figure 4. The Hiden HPR40 membrane inlet mass spectrometer sample is compared to the calibration curve, and thereafter the Weiss equation can be used to calculate the amount of N2 present in the sample. If the amount of N2 has increased this is a sign that nitrate reduction through denitrification has occurred. (Kana et al 1994) 3.5 Calculation of fluxes When data of the nutrient and N2 concentrations in the samples is obtained, the fluxes can be calculated. This is done using the formula below: 8

10 International Water Summer Resources Research School Where, Fx,n is the flux of species x from tube number n and cx,control is the concentration of species x in the outflow from the control tube. Since this experiment had two control cores, the mean value from these two controls is used. cc,n is the concentration in the outflow from sediment tube number n, V is the flow rate of the outflow and A is the surface area of the sediment tube. The unit of the flux is mg m -2 d -1. (Liikanen et al 2002:A) 4. Results 4.1 Sampling occasions The table below shows the date and time of the samplings and at what times the temperature of the incubation system was changed. Table 1. The date and time of all sampling occations Temperature Date Time 28 C 25 june 15:00 21:00 26 june 09:00 21:00 27 june 09:00 21:00 28 june 09:00 Temperature change 20 C to 13:30 10 C 10 C 21:30 29 june 10:00 21:00 30 june 9:30 21:00 1 july 9:00 22:30 2 july 09:00 3 july 09:30 Temperature change 10 C to 20 C 10:30 20 C 22:00 4 july 10:00 5 july 10:00 9

11 International Water Summer Resources Research School 6 july 10:00 7 july 9:30 8 july 9:30 9 july 10:30 10 july 11 july 10: Nutrient concentrations The samples were analyzed for concentration of PO4-P, NH4-N, NO2-N and NO3-N. The graphs below shows the concentration of each nutrient in each of the tubes for the temperatures 10 C and 20 C. The results from the 28 C incubation is not included, since the system was very unstable during this incubation period. The results from this incubation is therefore not reliable. The concentrations of NO2-N and NO3-N for the two temperature is only available for the first core, one of the control cores. Therefore, the results from the other cores only contains the concentrations of phosphate and ammonia. The results are presents as graphs of the nutrient concentration in the outflow water as a function of the incubation time. Core 1 Control core, no sediment Figure 5. The concentration of PO 4 -P, NH 4 -N, NO 3,2 -N and NO 2 -N (mg L -1 ) as a function of time (time interval 12 hr) for the two temperatures, 10 C (blue line) and 20 C (red line) For the first core, concentration data for the two incubation temperatures is available for all of the studied nutrients. The phosphate concentration (upper left) is relatively constant over the 10

12 International Water Summer Resources Research School incubation time for both temperatures, the ammonia (upper right) and nitrate/nitrite (lower right) concentrations approaches a constant level after approximately four days for both temperatures. The nitrite concentration (lower right) approaches a stable concentration after approximately four days for the 20 C temperature, the 10 C the concentration fluctuates. The ammonia concentration is higher for the higher incubation temperature, for the other nutrients there is no clear correlation between temperature and concentration. Core 3 control core, no sediment Figure 6. The concentration of PO 4 -P and NH 4 -N (mg L -1 ) as a function of time (time interval 12 hr) for the two temperatures, 10 C (blue line) and 20 C (red line) The phosphate concentration (left) is constant over the incubation period, and slightly higher when the incubation temperature is 20 C. The concentration of ammonia is increasing with time for both temperatures, the concentration is higher when the incubation temperature is 10 C. Core 4 Figure 7. The concentration of PO 4 -P and NH 4 -N (mg L -1 ) as a function of time (time interval 12 hr) for the two temperatures, 10 C (blue line) and 20 C (red line) 11

13 International Water Summer Resources Research School The phosphate concentration is constant over time and higher for the 20 C temperature. The ammonia concentration is decreasing with time and the concentration is slightly higher when the temperature is 10 C. 12

14 International Water Summer Resources Research School Core 5 The phosphate concentration seems to reach a constant concentration after 3 days for the 10 C incubation temperature, and seems to increase with time for the 20 C temperature. The ammonia concentration fluctuates for both temperatures and does not reach a stable value. 4.3 Nutrient fluxes Figure 8. The concentration of PO 4 -P and NH 4 -N (mg L -1 ) as a function of time (time interval 12 hr) for the two temperatures, 10 C (blue line) and 20 C (red line) The nutrient fluxes were calculated using the method presented in section 3.5. The graphs below shows the fluxes of ammonia (upper left), nitrite (upper right) and phosphate (lower) plotted over the incubation time. All nutrient fluxes varies a lot over time, the fluxes are altering between positive and negative values. The fluxes are higher for the 20 C incubation. 13

15 International Water Summer Resources Research School -1 ) d F NH4-N (mg m -2 d -1 ) F PO4-P (mg m N Time (h) Core ) d F NO2-N (mg m Time (h) Figure 9. The fluxes of nutrients over time for the two incubation temperatures. The net increase or decrease in dissolved N2 plotted over the incubation time is presented in the graph below. The result is plotted only for one control core (core 1) and one sediment core (core 5). The value varies a lot, and the net change alters between being positive and negative over time. Figure 10. The net change in dissolved N 2 over time 14

16 International Water Summer Resources Research School 4.5 N2 flux The flux of N2 is presented for core number 5. The flux varies a lot over time. In the beginning of the incubation period the flux is positive. After 120 hr a very low negative value of the flux was measured, and for the two following sampling occations the flux was very low. F N2 (mmol m -2 d -1 ) Core Time (h) Figure 11. The flux of nitrogen gas over time in core number five 15

17 International Water Summer Resources Research School 4.5 Fluxes of nitrogen compounds Below the fluxes of ammonia, nitrite and nitrate in core 5 are presented plotted over time for the two incubation temperatures. The nitrate data is not available for the 20 C incubation. The fluxes of the different nitrogen species are changing over time, nitrite fluxes are low compared to ammonia fluxes ) s F N forms (μg m Temp: 10 Temp: 20 Core 5 NH4-N N03-N N02-N NH4-N NO2-N Time (h) Figure 12. The fluxes of different nitrogen compounds in core 5 plotted over time for the two incubation temperatures 16

18 International Water Summer Resources Research School 5. Discussion Interpretation of the results The preliminary results of the nutrient concentrations in the outflow water (see result section 4.2) shows that the some of the nutrients approaches constant concentrations with time, while others are fluctuating a lot. The nutrient concentration data does thus not imply that the system is becoming stable. The nutrient fluxes (section 4.3) which is calculated based on the nutrient concentrations and the flow rate also does not show that the system is approaching stability. The N2 data, see result section 4.4 and 4.5, shows no trend and does not approach any stable value. One possible explanation is that air was present in the sample bottle, which could have affected the amount of dissolved N2. In the end of the incubation period a new sampling technique was introduced to try to solve this problem. Air was removed using He, thereby no air could pollute the samples. The results in section 4.5 shows the concentration of different nitrogen compounds in the water samples and how these concentrations changes over time. The concentrations varies a lot, sometimes ammonia is present in higher concentrations and sometimes nitrite and nitrate. This is a sign that there are reactions going in the sediment, the nutrients are reacting and nitrogen is present in different forms. Our system is catching this signal, which is good sign that the system is working. Based on the preliminary results it is difficult to draw any conclusions regarding how the temperature affects the nutrient fluxes in the sediment water interface. However, the results can be discussed in relation to previous studies on the same subject. The study from Maryland University by Duan and Kaushal showed an increase in the release of soluble reactive phosphorous and nitrate with increasing temperature. The nitrification and denitrification rate can both increase at higher temperatures, and hence the change in nitrate concentration will be the net result of the change in nitrification and denitrification rate. The nitrification process oxidizes ammonia to nitrate, increasing nitrification rate should hence decrease the present amount of ammonia. Since the biochemical cycles of nutrients are linked it is difficult to predict the effect of an increase in temperature. The microbial activity can increase as a result of increasing temperature, degrading more sediment organic matter and thereby releasing nutrients as a product of the degradation. As the decomposition rate increases the dissolved oxygen is depleted, which changes the redox conditions in the sediment-water interface. The redox conditions determining which compound that will be the terminal electron acceptor, and thereby what reactions that can occur. Problems and improvements There were some problems with the experiment that may have influenced the measurements and contributed to the unstable result. These problems and the way that they were solved during the time period of the study are very valuable for the future of the study. The problems may well occur again, and now the research team has acquired some important knowledge on how to handle these kind of challenges. 17

19 International Water Summer Resources Research School One problem that was challenging during the project period was to maintain a constant water flow from the water bag, through the tube and to the outflow. The unsteady flow rate can affect how much nutrients that are transported away from the sediment core, and hence the affect the concentrations in the outflow water. The flux rates are calculated with the unsteady flow rate taken into account. The discharge from the system was measured at each sampling occasion. The calculations made for each sample were made using the flow rate for that specific sampling occasion. The flux results should therefore not be affected by the changing flow rate. However, the unsteady flow might affect the stability of the system, the mixing of the water will be different depending on the flow rate. This might affect the oxygen concentration in the sediment water-interface and thereby affect the redox conditions. One solution that was tested was to install a peristaltic pump to obtain constant flow. However, the instability remained and the pump was removed. At the end of the pre-study period the flow is still driven by gravity. The incubation water that was used was collected at several different occasions. The water bags used were to small to contain the amount of water required for the incubation, and therefore new water had to be collected every few days. This might affect the results, since the chemical composition of the collected water may vary between the different sampling occasions. One solution that the research team will try is to reuse the incubation water, so that only one water sampling will necessary. The N2 samples can possibly have been polluted by air, which would affect the results of the nitrogen fluxes and net change in dissolved nitrogen in the sample. As mentioned above the research team worked around this problem by introducing a new sampling method where air was removed using helium. Another problem was that water was leaking from the sediment tubes out into the surrounding water. This was solved by isolating the tubes more carefully. When the samples from Jiulong river will be incubated, this will be done more carefully from the beginning to avoid leaking. There were also some problems that occurred which the research team could not affect. For instance, the system stopped at several occasions due to power cuts. There were also problems with the sediment cores, some of them broke and the system therefore had to be stopped so that the tank could be cleaned. However, these problems were not of the kind that can be prevented and should therefore be considered as a source of error. The preliminary results and the acquired knowledge about how the incubation system works provides important knowledge for the future of this research project. The results can be used when analyzing the sediments from Jiulong river. Using the information about the incubation time required to reach stability, the experiments in the later part of the research project can be performed with more precision and efficiency. 6. Conclusion The incubation system has not yet reached stability. A lot of knowledge about what factors that affects the system has been acquired. The system and all its components must be handled with care when preforming the incubation experiments. There are still some improvements of the experimental equipment that needs to be done, such as obtaining a stable flow rate and improving the isolation of the sediment cores. The team has acquired a lot of knowledge about how the experimental set up works that will be very important later in the study when studying the samples from Jiulong river. 18

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21 International Water Summer Resources Research School 7. Acknowledgements I would like to thank Professor Nengwang Chen from Xiamen University who have been very helpful throughout this project period, offering us a lot of advise. I also want to thank my teaching assistant Zhou Xing-peng who has helped us a lot and spend a lot of time working together with us. I want to thank my student partner Siry Shih, who has excellent translating skills. I also want to thank Professor Linus Chang from Lund University, and the people who made this cooperation between Lund and Xiamen possible. I would like to thank our sponsors Thyréns and Sweco for contributing to funding the expenses of this stay. Last but not least I would like to thank all participating students from Sweden and China, for making this a lovely stay at Xiamen University. Thank you! 20

22 International Water Summer Resources Research School Reference list Liikanen et al 2002:A: A. Liikanen, H. Tanskanen, T. Murtoniemi, P. J. Martikainen, 2002, A laboratory microcosm for simultaneous gas and nutrient flux measurements in sediments, Boreal Environment Research 7, Liikanen et al 2002:B A.Liikanen, T. Murtoniemi, H. Tanskanen, T. Väisänen, P. J. Martikainen, 2002, Effects of temperature and oxygen availability on greenhouse gas and nutrient dynamics in sediment of a eutrophic mid-boreal lake, Biogeochemistry 59, Silvennionen et al 2008:A: H. Silvennoinen, A. Liikanen, J. Torssonen, C. F. Stange, P. J. Martikainen, 2008, Denitrification and N2O effluxes in the Bothnian Bay (northern Baltic Sea) river sediments as aff ected by temperature under different oxygen concentrations, Biogeochemistry 2008, 88:63-72 Silvennionen et al 2008:B: H. Silvennoinen, A. Liikanen, J. Torssonen, C. F. Stange, P. J. Martikainen, 2008, Denitrification and nitrous oxide effluxes in boreal eutrophic river sediments under increasing nitrate load: a laboratory microcosm study, Biogeochemistry : Duan and Kaushal 2013: S.W. Duan, S.S. Kaushal, 2013, Warming increases carbon and nutrient fluxes from sediments in streams across land use, Biogeosciences 10, , Chen et al 2013: Chen, N., Peng, B., Hong, H., Turyaheebwa, N., Cui, S., Mo, X, Nutrient enrichment and N:P ratio decline in a coastal bay-river system in southeast China: The need for a dual nutrient (N and P) management strategy. Ocean & Coastal Management. Vol 81, pp Kana et al 1994: Todd M. Kana, Christina Darkangelo, M. Duane Hunt, James B. Oldham, George E. Bennett, and Jeffrey C. Cornwell, 1994, Membrane Inlet Mass Spectrometer for Rapid High-Precision Determination of N2 O2 and Ar in Environmental Water Samples. Analytical Chemistry 1994, 66: