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 food w e b of a system a s completely a s 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 w e b in Lake Ciso was similar to that found in other systems Finally, we quantified the trophic fluxes among populations using a simple algorithm w h ~c h considers the vertical distribution of organisms and the funct~onal responses of the different predators. The trophic food web obtained revealed 2 interesting properties. First, the compartments with larger b~omass were relatively stable during ~tratific~ition and presented slow growth and low predatory losses Second, there was a very inefficient transfer of organic matter from the lower levels (bactena, 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 accun~ulated in the metalimnion, along opposite gradients of oxygen and sulfide, which determined a n environment with reduced competition and predation. We postulate that m e t a l ~m n e t ~c communities above anaerobic hypolimnia can be regarded a s sinks of organic matter off the epilimnion.