Reminder about EEOB 698.02
• Still spaces available on the Lake Guardian for
EEOB 698.02 – Great Lakes Limnology
• Week-long cruise on Lake Erie; June 23-30
• See course website for more information
Finishing up Inorganic Carbon
• Hall Lake Free CO2,
pH, HCO3
• Distribution in time
and space
• Effect of turnover
and stratification
• Changes in pH
associated with
photosynthesis and
reduction
Nitrogen in Lakes and Streams
Wetzel Chapter 12
pp. 205-237
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 Nfixation
• 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 hypolimnions
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 concentrations can have an effect on
algal productivity but it is more likely
the phosphorus is the limiting factor
• 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
–
–
–
–
–
End product of deamination of organic material
Present in non-oxygenated areas – highly reduced
Used rapidly in trophogenic zone
Sorbs quickly to particles and can sediment out
Can be higher at sediment interface due to reduced
adsorptive properties of sediments under anoxic conditions
or due to excretion products of benthic heterotrophs
Variation by lake
status
Generation and Distribution continued
• Nitrification –
biological conversion
of nitrogen from a
reduced state to a
more oxidized state
NH4++3/2O22H++NO2-+H20
G0=-66kcalmol-1
- Nitrosomonas also
methane-oxidizing
bacteria capable of
producing this reaction
Generation and Distribution continued
• Nitrification cont.
• NO2-+1/2O2NO3G0=-18kcalmol-1
• Nitrobacter
responsible – NOTE:
less energy is given
off by this oxidation
• Overall:
NH4++2O2NO3-+H20+2H+
Need oxygen for this
reaction
• 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=-46kcalmol-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