Aquatic food webs: stoichiometric regulation of flux and fate of carbon

A key determinant of C-flux in aquatic ecosystems is the supply of elements relative to the demands of producers and consumers. Uptake of C is commonly in excess relative to C or N in both autotrophs and het- erotrophs, yet they may have different ways of coping with excess C. Stoichiometric demands thus govern C-use efficiency in individual organisms, and in food webs these stoichiometric principles will affect C- transfer efficiency across trophic levels. If a high rate of C-fixation via photosynthesis is not met by corresponding increased uptake rates of N and P, this deficiency will yield plant biomass with low nutrient value (high C: N or C: P). Normally, plants and detritus have far higher C: N or C: P ratios than that of the heterotrophs (bacteria and zooplankton), which may lead to P or N-limited growth of consumers. This limitation will also affect population dynamics of the consumer and food web interactions. The excess C may enter the detritus pathway, it may be buried in sediments, or it may be oxidized to CO2. Thus the balance or mismatch of elemental ratios in individual organisms and food webs will add up to a major determinant for the overall C-cycle and production:respiration ratio at the ecosystem level. This surplus of C is especially pronounced in many freshwater systems receiving high inputs of allochthonous C that will shift the balance from autotrophic to heterotrophic processes, thus reinforcing the net export of CO2 from water to atmosphere. Based on a large database of lakes, this paper will explore and review these stoichiometric aspects of C-metabolism and trophic transfer efficiency in lakes.