THE INFLUENCE OF HUMIC SUBSTANCES ON LACUSTRINE PLANKTONIC FOOD-CHAINS

Humic substances (HS) might influence planktonic food chains in lakes in two ways: 1) by altering the physical or chemical environment and t hus modifying autotrophic primary production and the dependent food ch ains; 2) by acting as a direct carbon/energy source for food chains. H S compete with phytoplankton for available quanta underwater and this effect is seen in the reduced euphotic zone depth in lakes with high c oncentrations of HS. Thus potential photosynthetic production is lower in the presence of HS. However, this effect can be offset in small la kes in which the depth of mixing is also reduced when HS concentration s are high. Complexation by HS of important nutrients such as iron and phosphorus may also restrict primary production. Evidence is accumula ting that photosynthetic primary production is insufficient to support measured metabolic activity in human lakes, which implies that metabo lism of allochthonous HS underpins much of the observed activity. Stud ies of bacterial abundance and growth in the presence of HS support th e view that bacteria are the most significant utilisers of HS. This us e is apparently facilitated by photolysis of HS, particularly by short wavelength radiation. Bacteria are grazed by both micro-zooplankton ( heterotrophic and mixotrophic flagellates and ciliates) and macrozoopl ankton. It is within this microbial community that the food chains der ived from autotrophic and allotrophic sources interact. These effects of HS on food chains are discussed in relation to possible implication s for the response of different lake types to eutrophication.