Mineralization of Sediment Organic Matter under Anoxic Conditions

Organic matter steadily accumulates in eutrophic lakes as a result of deposition of detrital tissue from algae and aquatic macrophytes and the slow rate of anaerobic decomposition. The rate of organic matter decomposition is affected by the nature of the sediment C (electron donor) and supply of electron acceptors. Batch incubation experiments were conducted to determine the rate and extent of organic matter decomposition in bottom sediments under anoxic conditions. Sediment samples from three distinct horizons were collected from a hypereutrophic lake (Lake Apopka, located in central Florida), which included the surface unconsolidated flocculent material (UCF), an underlying consolidated flocculent material (CF) and native pe at sediment. Sediment samples were incubated in serum bottles at 15, 25, and 35 °C in the dark for 534 d. Periodically, the serum bottle head space was analyzed for COz and CH4 produced during decomposition. At selected time intervals, the sediments were analyzed for water soluble organic C, pH, exchangeable NH4-N, and soluble reactive P. First-order rate constants for C mineralization (defined as the sum of gaseous C evolved and soluble C species produced) ranged from 6.7 × 10-4 d-~ for the UCF sediment incubated at 35 °C to 8.2 × 10-6 d-~ for the peat sediments incubated at 15 °C. Two phases of organic C decomposition were observed in the UCF and CF sediments while, decomposition in the peat sediments was characterized by only one phase. The UCF sediment, composed of recently deposited detrital organic matter, was found to be the most labile. However, under current experimental conditions, only 8.6% of the UCF sediment organic C was mineralized to CO2 and CU4o The biodegradability of sediment organic C ranked in the order UCF > CF > peat. Net mineralization of N and P was observed in all of the sediment samples (with the exception of the CF samples incubated at 15 °C). The N and P mineralized during the decomposition of UCF organic matter may contribute to the nutrient load of the overlying water column. T EUTROPHICATION of a lake often involves increased inputs of nutrients that results in an increase in primary production (i.e., fixation of CO2) within the system (Henderson-Sellers and Markland, 1987). If the increases in primary production are not balanced by organic matter degradation and outflow, an accumulation of organic-rich sediments can occur. These conditions have been observed in hypereutrophic lakes, such as Lake Apopka in Florida (USEPA, 1979a) and Lake Balaton in Hungary (Somlyody and van Straten, 1986). Although nutrient loading from external sources contributes to the eutrophication of lakes, it is also known that organic-rich sediments can release nutrients to overlying waters (Gardner et al., 1989). lake sediments, N and P occur in organic and inorganic forms with the latter often predominating. Thus, the transformations regulating the breakdown of organic N and P can be critical in supplying nutrients to phytoplankton and other aquatic biota. Seitzinger (1988) estimated that between 76 to 100% of freshSoil Science, Univ. of Florida, Inst. of Food and Agricultural Sciences, Gainesville, FL 32611. Contribution to the Florida Agfic. Exp. Stn. Journal Ser. no. R-01591. Received 16 May 1991. *Corresponding author. Published in J. Environ. Qual. 21:394-400 (1992). water N transformations are microbially mediated within the sediments of the system. Soluble P concentrations are not only a function of the organic matter mineralization process but are also controlled by dissolution, sorption, and precipitation reactions that occur within the sediments (Elderfield et al., 1981). Several studies have shown the irnportance of phytoplankton and bacteria in the cycling of C in aquatic systems (Adams and van Eck, 1988; Boers and Boon, 1988). In eutrophic lake systems, the production and degradation of particulate organic matter are regulated by physical and biogeochemical processes (Boers and Boon, 1988). Of the organic C deposited at the sediment surface, Adams and van Eck (1988) found that 60% was decomposed aerobically, 15% decomposed anaerobically, and 25% was buried. In shallow lakes such as Lake Apopka, FL, aerobic decomposition in the water column can play a major role, especially during the periods of wind-driven sediment resuspension. Under quiescent conditions, facultative anaerobic and obligate anaerobic respiration control the decomposition of organic matter in the sediments. The rate and extent of organic matter decomposition in sediments is governed by the quantity and quality of organic matter. Organic matter decomposition under anaerobic conditions can be thought of as a stepwise process where cellulose is converted to simple sugars, simple sugars to organic acids, and organic acids to CO2 and CH4 (Neue and Sharpenseel, 1984). Any one of these steps can be rate limiting in anaerobic environments. Environmental factors such as temperature have been shown to influence decomposition rates, with rate constants doubling for every 10 °C rise in temperature (Atlas, 1984). Although vast amount of information is available on kinetics of organic matter decomposition for upland soils (Paul and van Veen, 1978), a very limited amount of data has been reported for lake sediments and wetlands (Vogels et al., 1988). Decomposition rate coefficients are useful in simulation models and nutrient budgets to describe quantitatively the nutrient fluxes from sediment to the water column. Billen (1982) has indicated that an evaluation of the rates of microbial processes in an aquatic system are necessary for a complete description of the ecosystem. The cycling of depositional C is important in determining sediment accumulation and is inherently linked to other nutrient transformations. Work by Ali et al. (1988) found significant correlations between nutrient concentrations in the overlying water and sediment organic C levels in Lake Monroe, an eutrophic central Florida lake. The objective of this study was to determine the rate of organic matter decomposition under anaerobic conditions as influenced by sediment type and temperature. This was accomplished by simultaneously Abbreviations: UCF, unconsolidated flocculent material; CF, consolidated flocculent material; TCD, thermal conductivity detector; FID, flame ionization detector; WSOC, water soluble organic C; SRP, soluble reactive P; DIC, dissolved inorganic C.