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, $CH_4$, $NH_4^+$, $PO_4^-3$, reactive silica (RSi), Fe, Mn, and Ca from the sediments of a small, mesotrophic, shield lake (Williams Bay, Jacks Lake, Ontario). The diffusion of $CH_4,$ $\sum CO_2$, $NH_4^+$, 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 $NH_4^+$ is accounted for by diffusion from the sediments. This proportion is 31% for $\sum CO_2$, 62% for $CH_4$, 54% for total P (TP), 36% for RSi, 15% for Fe, 12% for Mn, and 5% for Ca. Significant release of Fe, Mn, and $PO_4^3-$ is limited to the deepest part of the basin. Regeneration of $PO_4^3-$ 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 $(G_0, G_1, and G_2)$ that display well-separated first-order decay constants $(k\ thicksimeq 40, 0.2, and 0.01 yr^-1)$. Most of $G_0$ appears to be decomposed during its descent in the water column. The longer lived sedimentary fractions $(G_1 and G_2)$ show marked focusing in the basin, and most of the regeneration is attributable to decomposition of the less reactive fraction $G_2$. 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.