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

摘要

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 P 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 het-erotrophs (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 CO_2. 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:res-piration 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 het-erotrophic processes, thus reinforcing the net export of CO_2 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.