AME 13:29-35 (1997)  -  doi:10.3354/ame013029

Many highly eutrophic waters are dominated by Cyanobacteria non-edible for zooplankton. This dominance of large phytoplankton and its exudation as well as the low biomass of small phytoplankton lead to a high ratio of exudation to edible primary production. As bacteria can effectively use exudates, they could be an important alternative food source for zooplankton. We investigated primary production (PP) in various size fractions and bacterial production (BP) in the hypertrophic Bautzen reservoir (Germany) in order to test whether (1) high PP can promote high BP and (2) BP can amount to similar values as particulate PP of the edible fraction. The main part of PP was produced by non-edible colonies of Microcystis spp. in summer and fall. The dynamics of BP and PP showing maximum activities in July and October were closely coupled. The strong relationship between BP and PP (r² = 0.98) indicates that allochthonous DOC (dissolved organic carbon) should be of little importance for bacteria. Consequently, PP was their main DOC source, but exudation could not cover bacterial organic carbon demand at the maxima of BP. The mean ratio of BP to PP was 3.5%, a low value typical for hypertrophic waters. However, BP amounted to similar or higher values than particulate PP of the edible fraction with respect to carbon production in summer and fall. Since the dominating daphnids are considered to be able to graze effectively on bacteria, it must be assumed that bacteria should be an equally important food source for Daphnia as edible phytoplankton.

Bacterial production · Exudation · Primary production · Hypertrophic lake · Biomanipulation