Summer temperatures, although near optimal for ammonium oxidation, may indirectly cause a decrease in nitrifying activity in eutrophic systems. As heterotrophic metabolism increases as a result of a seasonal increase in temperature and increased availability of nutrients, the concentrations of dissolved oxygen may decrease. To examine the effect of in situ low dissolved oxygen concentrations on nitrifying bacteria, water column rates of nitrification and nitrifier densities were documented in 3 eutrophic systems that experience differing degrees of low oxygen conditions: a tidal salt marsh creek, a high marsh pool, and a coastal lagoon. Fluctuations in dissolved oxygen concentrations have been observed at each, but the magnitude and duration of these fluctuations differs among systems. Additionally, 2 experiments were conducted in the laboratory to examine how nitrifying bacteria respond to controlled oxygen concentrations over a week long incubation. Nitrification rates were estimated using the N-Serve sensitive 14C bicarbonate uptake technique (Billen, 1975) and nitrifier densities were estimated by the multiple tube dilution method with Most Probable Number statistical analysis.
No discernible in situ low oxygen effects on the rates or densities of nitrifying bacteria are observed through several analyses. No correlation was observed between dissolved oxygen concentrations and rates or densities. No difference was observed between rates or densities from times when oxygen concentrations were high and low. In addition, no difference in rates or densities was found between two sites that experience different degrees of low oxygen conditions. Lastly, no difference was found between rates from three different temperature ranges that reflect less than optimal, greater than optimal, and optimal temperatures for nitrifying bacteria. In the 2 laboratory experiments, rates appeared to be limited by oxygen concentrations below 0.5 mg/l.
There are several possible reasons for the observed lack of response of nitrifiers to in situ low oxygen conditions. First, in situ dissolved oxygen concentrations may not be low enough, or present for a long enough duration to illicit a response from nitrifying organisms. Second, there is a high degree of variability associated with the measurement techniques. Several factors may be responsible for this variability, including the tendency of nitrifiers to form aggregates, the presence of other autotrophic organisms, as well as the modifications made in the technique to ensure in situ dissolved oxygen conditions during incubations. Lastly, small rate changes that are indicative of a response may be lost because the method is not sensitive enough.