Community metabolism of aquatic Closed Ecological Systems: Effects of nitrogen sources

To investigate the effect of nitrogen sources on Closed Ecological Systems (CESs), three nitrogen sources (NaNO₃, sodium nitrate; NH₄Cl, ammonium chloride; and NH₄NO₃, ammonium nitrate) were each tested in freshwater CESs consisting of a chemically defined medium, three species of green algae (Ankistrodesmus, Scenedesmus, and Selenastrum), the grazer Daphnia magna, and associated microbes, under 12 h light/12 h dark cycles. It had been hypothesized that the development of high pH in earlier CESs was the result of nitrate utilization, and that ammonium might result in acid conditions, while ammonium nitrate might result in more moderate pH. The three nitrogen sources supported similar densities of algae (estimated by in vivo fluorescence) and similar Daphnia populations. The experiments showed that pH levels rapidly increased when grazers were absent or at low abundances irrespective of the nitrogen source. Consequently, it is hypothesized that carbon cycles, rather than nitrogen sources, are responsible for the pH dynamics. Oxygen diurnal (light:dark) cycles tended to come into balance more quickly than pH. It may be more feasible to convert O₂ data to energy units (using “oxycalorific” values) than CO₂ data since CO₂ dynamics may include other chemical reactions than just photosynthesis and respiration. The feasibility of sustaining grazer populations for at least several weeks in small, simple CESs was demonstrated, along with the ability to monitor algae-grazer dynamics, and the recording of O₂ and pH measurements.