Carbon:Nitrogen balance in aquatic communities (closed ecological systems)

Background/Question/Methods

Given the necessary elements, how will an aquatic community regulate its O₂ and CO₂ dynamics? If we could measure ecosystem metabolism precisely, could we learn something new about ecosystem energetics?   To more precisely measure gas exchange, the studies have been done in Closed Ecological Systems (CESs).  The chemical composition of the medium was designed to be N limited, based on the Redfield Ratio and the assumption that only half of the NaHCO₃ carbon would be available for photosynthesis, the C:N ratio was 26.4 (4X the Redfield C:N ratio of 6.625).  The N:P ratio was 12.5, slightly less N than the Redfield Ratio of 16:1.  The biota consisted of 3 algae, (Ankistrodesmus, Scenedesmus, and  Selenastrum), some CESs had grazers (Daphnia magna), and all had microbes associated with their stock cultures.

Results/Conclusions

Experimental results suggest that much more C disappeared from the Dissolved Inorganic Carbon (DIC) pool than expected from the Redfield Ratio, as a result the CESs tended to become C limited with high pH (> 11) in algal-microbe CESs, and >10 in algal-microbe-grazer CESs.  Much of the remaining DIC was CO₃^--, which is not utilized by these algae.  By reducing the N&P concentration of the medium, C uptake was reduced so that the un-grazed systems did not become highly C limited; introducing more N &P again resulted in severe C limitation and high pH.   In the presence of grazers with low N&P, the amount of CO₂ released by the grazers is being balanced by the CO₂ uptake by photosynthesis; this experiment is still in its early stages.  The C uptake is similar to the O₂ production and therefore seems to be photosynthesis.  These  observations could be the result of variable protein in the algae and/or algal excretion of  dissolved organic carbon which may be incorporated into microbes.  These results may partly explain why aquatic communities that are not actively exchanging CO₂ with the atmosphere tend to show carbon limitation.