Recent studies have demonstrated the importance of whether aquatic ecosystems act as a net source or sink of carbon dioxide (CO_2, Del Giorgio Peters 1994, Williams 1998). The balance between production and respiration is the basis for understanding carbon flow and food web structure in marine and freshwater ecosystems. Aquatic ecosystems in which photosynthesis exceeds total respiration are net au-totrophic and sinks for CO_2. Conversely, ecosystems in which respiration exceeds photosynthesis are net heterotrophic and sources of CO_2 (Carignan et al. 2000). Among the large number of lakes studied by Cole et al. (1994), 87 percent were supersaturated with CO_2, indicating most lakes are sources rather than sinks of atmospheric CO_2. Lakes are a small but potentially important conduit for carbon from terrestrial sources to the atmosphere (Cole et al. 1994). Some ecologists have considered oligotrophic lakes as net sources of CO_2, whereas highly productive ecosystems tend to be net autotrophic (e.g. Duarte Agusti 1998). However, our knowledge of the production-respiration balance in eutrophic lakes is still limited. In this study, monthly variations of phytoplankton photosynthesis and plankton community respiration from October 2002 to September 2003 were analyzed from large, shallow, eutrophic Lake Taihu, China. Earlier research results from 1991-1999 were also used to determine the plankton community production-respiration balance in the lake.
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