1. Nitrogen in Lakes and Streams Wetzel Chapter 12 pp. 205-237 Joe Conroy 12 April 2004

2. Introduction Where does the Nitrogen come from? –Biological Fixation By bacteria and Cyanobacteria Lightning Fixation –Reduction of N 2 in the atmosphere Human Fixation –Crop production –Energy Production

3. Sources and Forms of N in Water Forms: –Dissolved N 2 Oxidation State = 0 –Ammonia NH 4 + Oxdn State = -3 –Nitrate NO 3 - Oxdn State = +6 –Nitrite NO 2 - Oxdn State = +3 –Organic Nitrogen Various States Sources –Precipitation –Fixation –Surface/Groundwater Drainage Losses –Effluent Outflow –Reduction with loss of gaseous N 2 –Adsorption with Sedimentation

4. Nitrogen Fixation Bacterial Cyanobacterial –Only forms with heterocysts are capable of N- fixation N-fixation mainly light-dependent Requires reducing power and ATP –Both of these come from photosynthesis Expensive energetically – 12-15mol ATP: 1mol N 2 reduced Dark rate <10% of light rates

5. Nitrogen Fixation continued N-fixation curve follows the same path as the photosynthesis curve Photosynthetic and Heterotrophic bacteria may also contribute to the fixed N pool Fixation by shrubs on wetland, river, and lake shores can also contribute to N in water

6. Inorganic and Organic Nitrogen Influents bring significant sources of N into lakes and streams Common Amounts in Lakes –NH 4 – 0-5mgL -1 ; higher in anaerobic hypolimnion of eutrophic waters –NO 2 -N – 0-0.01mgL -1 ; possibly higher in interstitial waters of deep sediments –NO 3 -N – 0-10mgL -1 ; highly variable seasonally and spatially –Organic N – up to 50% of Total Dissolved N

7. Inorganic and Organic N continued [N] affect algal productivity but more likely that [P] limits Growth rates for algae are higher with more reduced forms: NH 4 -N>NO 3 -N>N 2 -N

8. Generation and Distribution of Various Forms of Nitrogen Ammonia –Deamination of organic material –Present in non-oxygenated areas –Low concentration in trophogenic zone –Sorbs to particles/sediments out –Higher at sediment interface Adsorptive properties of sediments under anoxic conditions Excretion products of benthic heterotrophs Variation by lake status

9. Generation and Distribution continued Nitrification – biological conversion of N from a reduced to an oxidized state NH 4 + +3/2O 2  2H + +NO 2 - +H 2 0  G 0 =-66kcalmol -1 -Nitrosomonas bacterium NO 2 - +1/2O 2  NO 3 -  G 0 =-18kcalmol -1 Nitrobacter bacterium NOTE: less energy is given off by this oxidation Overall: NH 4 + +2O 2  NO 3 - +H 2 0+2H + Need oxygen for this reaction

10. Generation and Distribution continued Denitrification – biochemical reduction of oxidized nitrogen anions with concomitant oxidation of organic matter Occurs in both aerobic and anaerobic areas but is highly important under anerobic conditions Examples: C 6 H 12 O 6 +12NO 3 -  12NO 2 - +6CO 2 +6H 2 0  G 0 =-460kcalmol -1

11. Seasonal Distribution Interaction of Stratification, Anoxia, and Circulation with Biology control distributions

12. Seasonal Distribution continued

15. Carbon:Nitrogen Ratios Indicative of nutrient availability but also of relative amount of proteins in organic matter Approximate indication of phytoplankton status –C:N >14.6 – nitrogen limitation Nitrogen-Fixing phytoplankton become more abundant –C:N <8.3 – no N-deficiency

16. Nitrogen Cycle

18. Nitrogen Cycle in Streams and Rivers Nutrient Spiraling – net flux downstream of dissolved nutrients that can be recycled over and over while moving downstream Spiraling Length (S) – average distance a nutrient atom travels downstream during one cycle through the water and biotic compartments S = distance traveled until uptake (S w uptake length) + distance traveled within biota until regenerated (S B turnover length)

19. Conclusions Nitrogen is very important to aquatic ecosystem function Different forms occur at different times and depths Occurrence controlled by the interaction between Biology, Chemistry, and Physics