Ambiguous Ecological Control by Dissolved Humic Matter (DHM) and Natural Organic Matter (NOM): Trade-offs between Specific and Non-specific EffectsWe dedicate this paper to Prof. Dr. Fritz H. Frimmel on the occasion of his 60th birthday anniversary

Several papers report on obviously contradictory biological and ecological effects of humic substances (and NOM). For instances, growth promotion as well as growth inhibition of algae and macrophytes, promotion as well as inhibition of net-heterotrophy in non-eutrophic lakes, promotion as well as inhibition of reproduction of the nematode, Caenorhabditis elegans, are reported. If one takes direct effects of humic substances (and NOM) on aquatic organisms into account that are supported by increasing empirical evidence, these obvious contradictions can be solved as trade-offs between specific and non-specific effects. For instance, net-heterotrophy in non-eutrophic lakes can be considered as (weak) inhibition of photosynthesis of algae and macrophytes (specific effect) and simultaneous promotion of heterotrophic growth (non-specific effect). If net-autotrophy predominates, a positive feed-back effect of bacteria on algae, based on growth promoting substances can be observed. These assumptions are based on recent experimental laboratory findings that include even quantitative structure activity relationships. Two more contradictory effects of humic substances (and NOM) from reports are discussed with respect to trade-offs between specific and non-specific effects: reduction/increase of adverse mineral acid effects on invertebrates as well as effects of lipophilic chemicals upon invertebrates and fishes. We present a simple concept model that is based on specific and non-specific effects of humic substances and NOM towards aquatic organisms. Applying this model to hormone-like effects of humic substances and NOM upon the nematode C. elegans, we show that both effects inhibitory and promoting can qualitatively be predicted.