Components, structure and fluxes of the microbial food web in a small, stratified lake

Abstract

We studied the planktonic community established in Lake Cisó (Girona, Spain) during summer stratification, with the aim of describing the food web of a system as completely as possible. The lake was sampled 19 times during 1990 and 1991. We first determined which populations contributed significantly to total summer biomass. Then, we determined the trophic role of these populations by several independent approaches, and aggregated the community into functional groups. The binary food web obtained indicated that the structure of the food web in Lake Cisó was similar to that found in other systems. Finally, we quantified the trophic fluxes among populations using a simple algorithm which considers the vertical distribution of organisms and the functional responses of the different predators. The trophic food web obtained revealed 2 interesting properties. First, the compartments with larger biomass were relatively stable during stratification and presented slow growth and low predatory losses. Second, there was a very inefficient transfer of organic matter from the lower levels (bacteria, algae and protozoans) to the higher levels (rotifers and zooplankton) of the food web. Both properties could be explained by the fact that most biomass of the system accumulated in the metalimnion, along opposite gradients of oxygen and sulfide, which determined an environment with reduced competition and predation. We postulate that metalimnetic communities above anaerobic hypolimnia can be regarded as sinks of organic matter off the epilimnion.