Enzymatic decomposition and bacterial utilization of various types of particulate and dissolved substrates was studied during spring-summer period in four lakes of Mazurian Lake District (Northern Poland). We found that seston particles, similarly as dissolved organic matter (DOM), undergo intensive decomposition processes in lake water, but only after their previous colonization by bacteria. In lakes of low or moderate trophic status free-living microorganisms predominated. They preferentially utilized low molecular weight, dissolved organic compounds. Increases in particulate organic matter (POM) content in these environments caused rapid change of substrate exploitation strategy and adaptation of these bacteria to live in particle-attached forms. In lakes of POM and colloidal DOM (CDOM) abundant particle-attached microheterotrophs, although less metabolically active than free-living bacteria, were mainly responsible for secondary production and POM mineralization A mechanisms that permit effective POM exploitation by seston-attached bacteria was overproduction of relatively low active (high Km) enzymes (e.g. aminopeptidase) and/or synthesis of the enzymes (e.g. beta-glucosidase or glucosaminidase) that were optimally adapted (low Km) to the environment.