Relationships Between Turnover
Rates in the Biological Nitrogen Cycle and
Algal Productivity
VERA ALEXANDER, Associate Professor
Institute of Marine Science
University of Alaska
College, Alaska
INTRODUCTION
The problem of accelerated eutrophication is concerning people throughout
large portions of the world. The principle causative factor is the increased release of
nutrient compounds into natural waters. Fundamental information is needed to
evaluate the effects of such additions on the biological behaviour of aquatic systems.
Although the total biomass of organisms which can be produced within a discrete
environment is a function of the total amount of nitrogen and phosphorus available,
without knowing the details of the cycling behaviour of these elements, it is difficult
in practice to correlate nutrient input with biological availability or response.
Baalsrud (1) discussed the role of nutrient input in increasing marine productivity,
and in summing up the published work pointed out that in the case of phosphorus
the lack of a clear correlation is due to the ability of phytoplankton to indulge in
"luxury" consumption, to their ability to vary their cell phosphorus content within
wide limits, and also to the rapid turnover rate of the phosphorus within the biological system. Information on the behaviour of nitrogen is sparse, especially in relation
to turnover rates of the dissolved inorganic fractions. Dugdale (2) gives an estimate
for ammonia turnover in tropical marine surface waters on the order of 40 hr.
Previous work in our laboratory on fresh water systems has indicated a much more
rapid turnover (3).
This paper will deal with a study of the turnover rates of inorganic nitrogen
compounds in fresh water, with emphasis on the role of ammonia. Ammonia was
found the most rapidly used nitrogen source in fresh water lakes (4,5). The uptake
rates as measured by "15]\j tracer experiments were considerably in excess of those
expected on the basis of the ammonia concentrations present in the water (6). In
order to allow these uptakes, a constant source of ammonia supply was necessary.
This suggested a rapid turnover for ammonia, since during a period of time when this
was the major source for the phytoplankton, the concentrations in the water remained low with only minor fluctuations. Nitrification was apparently not an important process at this time, since, although nitrate was not being assimilated by the
micro-organisms, the levels remained low. Nitrification would have produced an increase in the dissolved nitrate under these conditions.
From the considerations discussed above, it appears that ammonia is recycled
rapidly during times of high or moderately high phytoplankton productivity, and
that nitrification cannot compete with the uptake of ammonia by the phytoplankton. In Figure 1, a simplified version of the nitrogen cycle, this implies that the
bottom three compartments are active, and the ammonia is rapidly transformed
between these. According to this scheme, nitrification can be expected to become
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