1. The Nitrogen Cycle Sources of Nitrogen
N is abundant on earth, but only about 2% is available to organisms as reactive nitrogen (NOx, NHx, Org N)
N is made available by Nitrogen-fixation and by fertilizer production
Gaseous N2  NO3
Reactive N can be recycled through the biota until it is eventually lost to the atmosphere through denitrification

2. K. Schulz

3. Nitrogen inputs to lakes
Atmospheric deposition from combustion of fossil fuels [dryfall] (NO3,NH4, Organic N)
Atmospheric deposition has doubled every 34 yrs
Watershed inputs
Terrestrial systems are generally N-limited, therefore most N is retained on land and not exported to via streams to lakes

4. Nitrogen Cycle in Lakes

5. Nitrogen Cycle in Lakes

6. Nitrogen Transformations
NH4+ (ammonium) uptake by algae.
NH4+  PON (Particulate Organic Nitrogen)
No change in oxidation state - not a redox reaction.
takes least amount of energy, therefore preferred by algae.
High concentrations of NH4+ in aerobic aquatic conditions are usually an indication of pollution by sewage or feedlot runoff.
Most other reactions are mediated by bacteria
Ammonification NH4+ production
decomposition of PON  NH4+

7. Nitrogen Transformations
Nitrification
NH4+ (ammonium) + 3/2 O2  NO2– (nitrite) + 2H+ + H2O then,
NO2– + 1/2 O2  NO3– (nitrate)
NO2– usually does not build up
NO3 is the final product of nitrification. It may build up in conditions where there is much NH4+ being produced, oxygen is present, but there is little vegetation to take up NO3
Assimilative nitrate reduction
NO3- uptake by algae
algal uptake of N to make more cells. NO3– ORG-N (NH3)
needs light, done with O2 present.

8. Nitrogen Transformations
Denitrification (dissimilatory nitrate reduction)
CHO + NO3- + H+  CO2 + N2 + H2O
must be anaerobic –
sediments, anoxic hypolimnia.
Nitrogen fixation
N2  ORG-N (NH3).
Difficult to break triple bond of N2 therefore energetically expensive
May be conducted by Cyanobacteria (Blue-green algae) under bright sunlight
Or by bacteria in sediments, coupled with other reactions.
Both cases require anoxic conditions for reaction to occur

9. Nitrogen fixation in Cyanobacteria
Nitrogen fixation occurs in special cells called heterocysts, but
Not all cyanobacteria have heterocysts or can fix nitrogen
Some cyanobacteria can fix nitrogen without heterocysts

10. Redfield ratios of ocean phytoplankton (by number of atoms)
C       H      O      N     P    S Fe
106    263   110   16    1    
Hecky et al. compiled data from lakes around the world to see if the ratios held true for lakes (as well as the ocean)

11. N:P ratio
Lakes with N:P ratio > 22 are considered to be P-limited
Note also C:N and C:P ratios. If they are higher than the Redfield ratio it means that algal cells are making do with less N or P than they would like.
Mean N:P ratio = 24

12. Some large tropical lakes can be severely nitrogen limited
N:P = 13, 11 for Lakes Victoria and Albert
Some large tropical lakes can be severely nitrogen limited

13. Most lakes are P deficient relative to N

14. What about C:P ratios?
C:P is usually above the Redfield ratio meaning that algae is usually P-limited relative to C

15. What about N:C ratios?
N:C is also usually above the Redfield ratio meaning that algae is usually N-limited relative to C
Therefore, freshwater phytoplankton is usually both N and P limited relative to C. Ie. The cells are making do with less than optimal N and P.
But, P is usually more limiting than N

16. Finally, recent studies show that the Redfield N:P ratio of 16 is not a universal biochemical optimum for phytoplankton, but rather an average of ratios for many different species.
Christopher A. Klausmeier, Elena Litchman, Tanguy Daufresne and Simon A. Levin
Nature 429, (13 May 2004)