COMPONENTS, STRUCTURE AND FLUXES OF THE MICROBIAL FOOD-WEB IN A SMALL, STRATIFIED LAKE

We studied the planktonic community established in Lake Ciso (Girona, Spain) during summer stratification, with the aim of describing the fo od web of a system as completely as possible. The lake was sampled 19 times during 1990 and 1991. We first determined which populations cont ributed 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 w eb obtained indicated that Me structure of Me food web in Lake Ciso wa s similar to that found in other systems. Finally, we quantified Me tr ophic fluxes among populations using a simple algorithm which consider s the vertical distribution of organisms and the functional responses of the different predators. The trophic food web obtained revealed 2 i nteresting properties. First, the compartments with larger biomass wer e relatively stable during stratification and presented slow growth an d low predatory losses. Second, there was a very inefficient transfer of organic matter from the lower levels (bacteria, algae and protozoan s) to the higher levels (rotifers and zooplankton) of the food web. Bo th properties could be explained by the fact that most biomass of the system accumulated in the metalimnion, along opposite gradients of oxy gen and sulfide, which determined an environment with reduced competit ion and predation. We postulate that metalimnetic communities above an aerobic hypolimnia can be regarded as sinks of organic matter off the epilimnion.