Nitrogen in Lakes and Streams Wetzel Chapter 12 pp Joe Conroy 12 April 2004

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

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

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

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

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 – mgL -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

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

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

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

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 CO 2 +6H 2 0  G 0 =-460kcalmol -1

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

Seasonal Distribution continued

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

Nitrogen Cycle

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)

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