Contrasting interactions mediate dissolved organic matter decomposition in tropical aquatic ecosystems

The interaction between photochemical and microbial degradation processes can have important effects on dissolved organic matter (DOM) decomposition in aquatic systems. Photochemical processes can stimulate or inhibit biological DOM degradation while biological processes often stimulate photochemical degradation. The aim of the present study was to evaluate whether these 2 degradation processes compete for the same organic substrates or use complementary components in 2 tropical systems with contrasting DOM sources (one dominated by humic, mostly terrigenous DOM and the other dominated by autochthonous phytoplankton production). We performed sequential exposures of DOM from both systems to photochemical or biological degradation. We then measured bacterial growth and respiration and photochemical oxygen consumption in addition to changes in DOM optical properties. In the humic lagoon, photochemical degradation stimulated bacterial degradation up to 500% with little complementary photochemical stimulation by bacterial degradation (an increase of only 13%). In the eutrophic lagoon, we found that photochemical degradation inhibited bacterial degradation 17%, suggesting competition with microbial substrates, while bacterial degradation had no effect on photochemical degradation. The net effect of these eutrophic lagoon interactions was a ca. 2% reduction in dissolved organic carbon degradation. Thus, we conclude that there was net complementary behavior between photochemical and bacterial processes in the humic lagoon in DOM degradation, while we observed little net competitive behavior in the eutrophic lagoon.