Sources, degradation and recycling of organic matter associated with sinking particles in Lake Michigan

Sediment trap material collected at five depths from two locations in Lake Michigan has been studied to determine the sources of particulate organic matter and the early diagenctic changes which occur during sinking of particles. Aquatic material dominates in shallower depths whereas land-derived material is more important in near-bottom depths. Organic carbon concentrations decrease with depth as a result of remineralization of organic matter by microbial activity and dilution by nsuspended bottom sediments. Preferential remineralization of algal organic matter and downslope transport of particulate matter from coastal areas create differences in the lipid biomarker characteristics of settling panicles found at different dcpths in the water column. Calculations of apparent decomposition rate constants from lipid diitributions at different depths show that shorter chainlength n-alkanoic acids are degraded faster than longer components. Carbon and nitrogen isotopic compositions of total organic matter are nearly unaffected by large-scale amounts of remineralization during sinking. Key words--sediment traps, carbon isotopes, nitrogen isotopes, n-alkanes, n-alkanoic acids, decomposition rate constants, lake sediments Organic matter is an especially dynarpic component of the nutritional, geochemical, and microbial cycles of lakes and oceans. Organic matter associated with sinking particles is subject to a variety of transformational and degradational processes in the relatively short time required to settle to bottom sediments. Sediment trap studies in lakes and oceans have shown that major decreases in the amount of organic matter accompany the sinking of particles. Eadie et al. (1984) estimate that 96% of the organic carbon produced in the photic zone of Lake Michigan is recycled before i t can sink to the bottom only 100 m below, a value nearly as great as the 99% or more reported in deeper marine areas (e.g., Cobler and Dymond, 1980; Knauer et al., 1979; Sues, 1980; Knauer and Martin, 1981; Martin er al., 1987). The different components comprising organic matter have different susceptibilities towards degradation, which can result in progressive alteration of the composition of particulate organic matter during sinking. Plant pigments are exceptionally vulnerable to destruction, with as little as 1% of the original amount becoming buried in the sediments of Dabob Bay, Washington (Furlong and Carpenter, 1988). Sugars and amino compounds are also lost during sinking only 110 m to the bottom at this location, although some lignin components of vascular land plants experience essentially no changes (Hedges et al., 1988). Studies of lipid compositions of sediment trap contents from a variety of environments, including Lake Michigan (Meyers et al., 1980, 1984a; Gardner et al., 1989; Pamsh et al., 1992). Dabob Bay (Prahl et al., 1980). and open-ocean areas (Wakeham et al., 1980; Wakeham, 1982; Wakeham and Lee, 1989). have shown, furthermore, that lipid material contributed from aquatic organisms is remineralized more quickly than is total organic matter, whereas some land-derived lipid components appear to survive better. The results of our earlier studies in Lake Michigan have suggested a complex interaction of multiple sources, remineralization within the water column, and resuspension of bottom sediment which control the organic matter contents of particles sinking below the epilimnion (Meyers et al., 1980, 1984a; Eadie et al., 1984). Our earlier data have been from samples collected by sediment traps moored at two depths, one in the metalimnion and one in the hypolimnion, and they indicate that important changes occur to particulate organic matter during the early stages of sinking. Study of the epilimnion-the upper part of the water column-is therefore especially warranted. We also were concerned about the significance of resuspension of bottom sediment in the hypolimnion. We report here our results from study of the organic matter collected in sediment traps positioned at five depths throughout the water column at two locations in Lake Michigan. 48 PHILIP A. MEYERS and BRIAN J. EADIE SAMPLES AND PROCEDURES