Scientific Theme: Advanced Modeling and Observing Systems

Nitrogen Fixation in Lakes
William M. Lewis, Jr.
Complementary Research: Faculty Fellows Research
All lakes contain phytoplankton, a diverse community of unicellular algae that are able to live
suspended in the water column because of their very low sinking rates. Phytoplankton are not
visible individually because they grow as unicells or small colonies with a median size of
approximately 100 μm. In lakes that receive small amounts of plant nutrients – phosphorous
(P) and nitrogen (N) – phytoplankton are collectively invisible. Where plant nutrients are
abundant, phytoplankton pigments bring visible color to the water. At high concentrations of P
and N, phytoplankton become so abundant that they cause a very strong green color and reduce
transparency in a lake. Strong growth (―blooms‖) of algae are responsible for many undesirable
changes in lakes, including anoxia in deep water, formation of scums, production of toxic substances or taste and odor,
and interference with filtration processes used by water-treatment plants.
Lakes containing high concentrations of anthropogenic P often show depletion of inorganic N because the human
sources have a much higher ratio of P:N than natural sources. Under these conditions, N-fixing algae often become
dominant. While other algae are held back by a lack of inorganic N, the N fixers convert N2 (which is biologically
inert) to NH 4+ , which allows their continued growth. N-fixing algae are potent scum formers and sources of toxins.
Blue-green algae are in fact cyanobacteria, which separates them from the other algae in phytoplankton, which are
eukaryotic (non-microbial). The N fixers have growing filaments containing one or more specialized cells (heterocysts)
that are specialized for N fixation. Within the heterocyst, photosystem II does not function, and no oxygen is released.
Therefore, the anoxic conditions necessary for N fixation are available inside the heterocyst.
Aphanizomenon, showing the N-fixation cell
(heterocyst, center).
for most of the cells in the water column, and its ability to store reductant
during brief exposure to light near the surface for later N fixation at night
or when the cell is located beyond the reach of downwelling irradiance.
Bradburn‘s research suggests that forced mixing might be quite effective
in controlling Aphanizomenon. The effect of forced mixing would be to
cause Aphanizomenon to exhaust its stored reductant at a pace faster than
the reductant could be restored by exposure to strong light.
Mark has a position with NOAA‘s Cooperative Institute for Limnology
and Ecosystem Research program (CILER) that we hope will be the first
step toward his participation in NOAA‘s very active and important
program on nuisance algal species, which plague not only freshwater
lakes but also estuaries throughout the world.
One particularly potent N fixer in Colorado and throughout the
west is Aphanizomenon, which forms blooms in many local lakes,
such as Cherry Creek Reservoir, and national lakes, such as Upper
Klamath Lake, Oregon. It has not been clear why Aphanizomenon
is more successful than some other genera of N fixers. Research
led by Mark Bradburn, CIRES graduate student, has provided
recent insight into the success of Aphanizomenon. Using an
acetylene reduction method to measure N fixation rates, Bradburn
conducted experiments leading to the construction of lightresponse
curves for Aphanizomenon and, uniquely to his work,
experiments with neon dark fixation. Mark showed that
Aphanizomenon is extraordinarily efficient at using weak sources
of light in fixing N, and that it has a remarkable capability to
conduct fixation in the absence of light. The secret to
Aphanizomenon’s success is its ability to fix N in water that is
thickly crowded with algal cells, thus providing only weak light
112
Nitrogen Fixation (nmol N 10 -6 heterocysts hr -1 )
90
80
70
60
50
40
30
20
10
0
Pelican Pond
31-Aug
0 200 400 600 800 1000 1200
Light (umol m -2 s -1 )