Dependencies of food web and nutrient cycling dynamics on dissolved organic matter (DOM) and inorganic nutrient concentrations in lake enclosures.

An autotrophic-allotrophic gradient was established in 12 lake enclosures across a natural DOM concentration gradient. Phytoplankton were co-regulated by solar irradiance and inorganic nutrient concentrations, whereas bacterioplankton were strongly dependent on DOM in the reference enclosures. Nutrient scavenging in the reference enclosures was limited by efficient biotic incorporation and recycling, across the full DOM gradient. Nutrient enrichment stimulated a strong autotrophic response across the autotrophic-allotrophic gradient due to increased phytoplankton productivity. Bacterioplankton productivity was still strongly dependent on DOM, but bacterioplankton productivity also increased either as a direct or indirect result of nutrient enrichment. Carbon, nitrogen, and phosphorus were effectively scavenged from the water column by incorporation into biomass at high rates and then deposited in the sediments in the nutrient enriched enclosures, producing nutrient-rich sediments. The data further suggest that at DOM concentrations greater than 14 mg L -1, allotrophy would dominate regardless of inorganic nutrient enrichment.