Abstract: Materially closed, aquatic ecosystems provide an opportunity to test lower trophic level interactions without the uncertainties introduced by atmospheric exchange, mixing, migration, and predation. Their limitations also indicate the degree to which exchanges with other meta-communities are necessary. We have used Closed Ecological Systems (CESs) to measure the net photosynthetic (P) and respiratory (R) cycles of algae, grazers, and undefined microbes. The CESs were composed of 3 green algae (Ankistrodesmus, Scenedesmus, and Selenastrum), Daphnia magna, and unidentified microbes. Containers were either 250 ml bottles (200 ml of liquid and 50 ml of gas) or ~ 1 liter (800 ml liquid and 200 ml of gas), some with implanted pH, O2, pressure, and conductivity sensors. Fluorescence was used to estimate algae populations and Daphnia were counted visually. Starting with an initial nutrient source, 20° C, and a 12:12 Light-Dark cycle, the CESs demonstrated a period of “gain” (P >R, an increase in biomass), “maintain” (P~R, biomass sustained) and eventually a gradual “loss” (P< R, biomass reduced). Although environmental conditions were constant, the systems behaved like the “Plankton Ecology Model” (spring maximum of algae followed by a clear phase) or the “Holling Panarchy Symbol” of exploitation, conservation, and release … but we did not get to reorganization. This contrasts with earlier theories that laboratory and natural ecosystems would move toward climax or steady-state conditions. Our systems demonstrated more rapid P and R rates just after the light changes, as has been demonstrated by others, for which varying mechanisms have been proposed. Similar P/R cycles have been reported for other CESs based on algae and a wide variety of soil microbes and were proposed to be a universal property. The data rejected our initial hypothesis that high N and P concentrations would benefit the grazer population; rather, grazers were eliminated when algal populations were high with associated high pH and O2. The earth’s biosphere is essentially a materially Closed Ecological System as will be the initial extraterrestrial Life Support Systems. Although our little bottles cannot include such complexity, their simplicity allows us to gain an understanding of some complex processes.
