Effects of flocculated humic matter on free and attached pelagic microorganisms

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Abstract
The effect of dissolved and particulate humic matter on pelagic microorganisms was studied in laboratory experiments. A humic concentrate was added to cultures of natural bacterial populations in either dissolved or partly flocculated form. Due to the occurrence of epibacteria attached to particles, a markedly higher bacterial yield was obtained when a fraction of the added carbon was flocculated. Attached bacteria apparently utilized flocs as well as dissolved organic carbon surrounding the particles. Both free and attached bacteria were grazed by phagotrophic flagellates. In addition to grazing attached bacteria, attached flagellates could ingest free bacteria. In 11 oligotrophic lakes, ranging from those with clear water to those heavily colored with humic substances, a linearly increasing amount of both particulate material and particle‐associated epibacteria was found in the pelagial with increasing humic content. We suggest that suspended detritus particles (humic flocculates) have a positive influence on the secondary productivity of humic lakes due to an enrichment of the heterotrophic epibacterial‐bacterivorous flagellate segment of the microbial food web.