Trophic Structure and Carbon Flow Dynamics in the Pelagic Community of a Large Lake

Abstract

Understanding the flow and cycling of matter within food webs is a major objective of contemporary ecosystem research (e.g., Wulff et al. (1989) and literature cited therein). A number of investigations on carbon (C) cycling in pelagic ecosystems focused on the role of the microbial loop (e.g., Ducklow et al. (1986), Jackson and Eldridge (1992), and Stone et al. (1993)). The present case study from large and deep Lake Constance aims to contribute to this partially controversial issue by evaluating separately the contribution of the microbial community to the nutrition of large zooplankton (i.e., the link-sink issue), and to the overall C flow dynamics. This is done by aggregating all species from the entire food web into two separate chains, one based directly on phytoplankton (called grazing chain in the following), and one relying energetically on dead organic matter taken up by osmotrophic bacteria (detritus chain). The organismal composition and the relative quantitative significance of both chains will be compared. Perceiving the food web as being composed of these two chains approximates the concept of distinguishing between a classical pelagic food chain (going from phytoplankton via crustaceans to fish) and a microbial loop. However, in contrast to the latter concept, we account explicitly for flows into and from the pool of dead organic material. Second, we do not allocate each plankton group to one of the chains entirely but compute the fractional contribution of each group to both chains according to its diet composition. For example, ciliates belong mostly to the grazing chain as they are predominantly herbivorous. However, additional grazing on bacteria and flagellates results in the allocation of some ciliate fraction to the detritus chain.