Freshwater bodies dominated by dissolved humic substances (HSs) seem to be the major type of inland waters. HSs are natural environmental chemicals and directly interact with organisms. HSs adversely interfere with photosynthesis and suppress cyanobacteria more than eukaryotic algae. Quinones in the HSs appear to be the effective structure. HSs exert a mild chemical stress upon aquatic organisms in many ways: induction of molecular chaperons (stress proteins), induction and modulation of biotransformation and antioxidant enzymes and lipid peroxidation. Furthermore, HSs can modulate the offspring numbers in the nematode Caenorhabditis elegans and cause feminization of young fish and amphibians. Stronger chemical stresses by HSs lead to birth defects in fish offspring. The nematode C. elegans actively seeks such stressful environments. This behavior is only understandable in the light of new paradigms of the aging mechanisms. The mild HS-mediated stress to organisms expands the individual lifespan. At present the ecological consequences of these findings remain obscure. However, a multiple-stress resistance may be acquired by exposed organisms, which improves the individual fitness in a fluctuating environment. In summary, dissolved HSs have to be considered abiotic ecological driving forces, somewhat less obvious than temperature, nutrients, or light.