Organic matter mineralization rates in sediments: A within- and among-lake study

Organic matter mineralization rates were measured by the accumulation of DIC + CH4 in the water overlying intact cores taken from littoral and profundal sediments of nine Quebec lakes. The variability in areal carbon mineralization is much greater within lakes than among lakes varying in trophic richness. Organic matter mineralization in littoral sediments is more variable and, on average, threefold higher than in the profundal sediments. Sixty percent of the variation in mean summer mineralization rates is explained by site depth, a surrogate variable that incorporates the effect of temperature and may also be reflecting substrate quality and(or) supply. The lake specific characteristics most strongly correlated to the residuals of the regression with depth are catchment area-to-lake area ratio (CA:LA) and water residence time. In lakes with a larger CA:LA and a shorter residence time, the amount and(or) the quality of organic matter settling to the sediments at a given depth may be reduced, resulting in the lower observed mineralization rates. Total mineralization in the sediments is, not surprisingly, greater in larger lakes but the rate per unit area is smaller, reflecting the decreased importance of the littoral zone. More than half (54-100%) of the DIC + CH4 produced in the sediments is from the littoral zone. Yet, because of the large biomass in epilimnetic waters, the littoral sediments account for <20% of the sum of metabolism in the epilimnetic water column and underlying sediments. The relative importance of the sediments in metabolism in lakes is a function of both the trophy and lake morphometry. In deep lakes a smaller proportion of total respiration occurs in the sediments than in shallow lakes, and in eutrophic lakes the sediments account for a smaller proportion of total respiration than in oligotrophic lakes.