REGENERATION OF DISSOLVED SUBSTANCES IN A SEASONALLY ANOXIC LAKE - THE RELATIVE IMPORTANCE OF PROCESSES OCCURRING IN THE WATER COLUMN AND IN THE SEDIMENTS

We studied the release of inorganic C, CH4, NH4+, PO43-, reactive silica (RSi), Fe, Mn, and Ca from the sediment of a small, mesotrophic, shield lake (Williams Bay, Jacks Lake, Ontario). The diffusion of CH4, SIGMA-CO2, NH4+, and RSi from the sediments, as estimated from pore-water data, increases linearly with depth and sedimentation rate. Release associated with sedimentation rate accounts for 47-84% of the fluxes of these substances to the water column. Regeneration from both the water column and the sediments plays an important role. During summer anoxia, 70% of the hypolimnetic accumulation of NH4+ is accounted for by diffusion from the sediments. This proportion is 31% for SIGMA-CO2, 62% for CH4, 54% for total P (TP), 36% for RSi, 15% for Fe, 12% for Mn, and 5% for Ca. Significant release of Fe, Mn, and PO43- is limited to the deepest part of the basin. Regeneration of PO43- is not well coupled to organic-matter degradation, and undefined anoxic P-immobilization reactions seem to be taking place in the sediments of the littoral and upper hypolimnion. Diagenetic modeling of the sediment pore-water and solid-phase data shows that the layer of sediment involved in nutrient release extends 50-100 cm below the interface. The global recycling of carbon and nitrogen in aquatic systems is attributed to the decomposition of three classes of organic compounds (G0, G1, and G2) that display well-separated first-order decay constants (k congruent-to 40, 0.2, and 0.01 yr-1). Most of G0 appears to be decomposed during its descent in the water column. The longer lived sedimentary fractions (G1 and G2) show marked focusing in the basin, and most of the regeneration is attributable to decomposition of the less reactive fraction G2. The existence of long-lived sedimentary organic-matter fractions is consistent with the observed resilience of sediment catabolism to seasonal or long-term changes in organic matter influx.