High dissolved-inorganic-carbon uptake in closed ecological systems: Is this why ‘cosms tend to demonstrate carbon limitation, unlike lakes?

Background/Question/Methods The goal is to investigate the carbon dioxide and oxygen dynamics in Closed Ecological Systems (CESs) consisting of 3 species of algae (Scenedesmus, Ankistrodesmus, and Selenastrum), (in some) the grazer Daphnia magna, and associated microbes. CESs are open to energy exchanges (light and heat), but closed to material exchanges (to the extent feasible).  The ultimate goals are (a) to use these measurements to estimate total ecosystem energy exchanges and relate these to population dynamics of primary producers and grazers in the CESs and (b) to compare these model ecosystems to natural ecosystems.

Results/Conclusions In un-grazed CESs, pH increased more each 12 hour lighted period than decreased each 12 hour dark period; as a result pH tended to increase each 24 hours in a step-wise fashion until > 10. The calculated Dissolved Inorganic Carbon (DIC = CO₂, HCO₃^-, and CO₃ ^-2) uptake in un-grazed CESs was approximately three times the Redfield Ratio (based on N as the presumed limiting factor). This is consistent with at least one of the algae (Scenedesmus) being able to vary its protein composition from 60% (close to the Redfield Ratio) to 20%; the other two algae may have similar abilities as many green algae are reported to vary their chemical composition. The algae drew down the DIC (supplied as NaHCO₃) until mainly CO₃^-2 remained.  The DIC was not lost to the CES, because it reappeared during periods of intense grazing in CESs that included the grazer, Daphnia magna.  The presence of grazers had major impact on pH and DIC cycles in these systems. Based on Ca and Mg concentrations, precipitation could have accounted for only ~15% of the loss.  The tendency for ‘cosms (small experimental systems) to display carbon limitation has been noted, whereas in lakes, photosynthetic CO₂ uptake is usually satisfied by atmospheric and sediment inputs and P and/or N are usually the limiting nutrient for algal biomass.  Oxygen concentrations did not always match the DIC patterns; the mechanisms for the mis-matches are under investigation.