Nitrogen in Lakes and Streams

Wetzel Chapter 12pp. 205-237

Joe Conroy

12 April 2004

Introduction

• Where does the Nitrogen come from?– Biological Fixation

• By bacteria and Cyanobacteria

• Lightning Fixation– Reduction of N2 in the atmosphere

• Human Fixation– Crop production– Energy Production

Sources and Forms of N in Water• Forms:

– Dissolved N2

• Oxidation State = 0

– Ammonia NH4+

• Oxdn State = -3

– Nitrate NO3-

• Oxdn State = +6

– Nitrite NO2-

• Oxdn State = +3

– Organic Nitrogen• Various States

• Sources– Precipitation– Fixation– Surface/Groundwater

Drainage

• Losses– Effluent Outflow– Reduction with loss of

gaseous N2

– 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 N2 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

– NH4 – 0-5mgL-1; higher in anaerobic hypolimnion of eutrophic waters

– NO2-N – 0-0.01mgL-1; possibly higher in interstitial waters of deep sediments

– NO3-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:

NH4-N>NO3-N>N2-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

NH4++3/2O22H++NO2

-+H20G0=-66kcalmol-1

- Nitrosomonas bacterium

NO2-+1/2O2NO3

-

G0=-18kcalmol-1

• Nitrobacter bacterium NOTE: less energy is given off by

this oxidation

•Overall:NH4

++2O2NO3-+H20+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:C6H12O6+12NO3

-12NO2-+6CO2+6H20

G0=-460kcalmol-1

Seasonal Distribution

• Interaction of Stratification, Anoxia, and Circulation with Biology control distributions

Seasonal Distribution continued

Seasonal Distribution continued

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

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 (Sw uptake length) + distance traveled within biota until regenerated (SB 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