Characterization of Legacy Labile Organic Carbon Reserves

1. Characterization of Legacy Labile Organic Carbon Reserves RasikaGawde,Phillip A. DePetro, Kenneth J. Windsand, and Martin T. Auer Department of Civil and Environmental Engineering, Michigan Technological University, Houghton, MI 49931 Objectives & Approach Decadal Scale Changes in Total Organic Carbon Study Site Onondaga Lake is a dimictic, eutrophic system located in Syracuse, NY. Historically, more than 75% of the phosphorus (P) loading to the lake has originated from the Syracuse Metropolitan Treatment Plant (METRO). P-removal technologies, implemented over the past four decades have reduced the METRO effluent P concentration from >10 mgP∙L-1 in 1970 to its present concentration of <0.25 mgP∙L-1 (figure at right, top panel). This reduction in P loading has been accompanied by an almost two-fold reduction in the areal hypolimnetic oxygen demand (AHOD, figure at right, bottom panel). The time course and extent of further recovery of Onondaga Lake, as manifested to date in changes in AHOD and, by association, in sediment oxygen demand, will be governed by the reservoir of labile organic carbon present as legacy deposits. Total Organic Carbon (%DW) Total Organic Carbon (%DW) Total Organic Carbon (%DW) TOC (%DW) The susceptibility of the particulate organic carbon (POC) pool to diagenesis covers a broad range (labile → refractory), but for practical purposes may be divided into two or more fractions (cf. Berner 1980). A 2-G approach is often applied, i.e. and the total organic carbon content at any time after deposition is given as, Yielding a sediment profile as represented by the figure at the left. Depth in Sediment (cm) Depth in Sediment (mm) Core collected in 2008 Core collected in 1995 adjusted to 2008 datum Overlay of 1995 and 2008 cores. Exposed yellow reflects diagenesis. The results of a comparison of TOC profiles from 1995 and 2008 stands in marked contrast to the results of the labile organic carbon assays. Here, reductions in carbon content occurred largely over the depth interval 0-30 cm, with sediment at depths below 30 cm having a similar carbon content. This suggests that diagenesis at depths >30 cm, mediated solely by methanogenesis, is less capable of mineralizing organic carbon than that at shallower depths. Organic Carbon Diagenesis Evidence for Depletion of Legacy Deposits The profile presented above is for a constant rate of POC deposition. Where those deposition rates have changed, i.e. in response to reductions in phosphorus loading and attendant primary production, the profile and its interpretation become more complex. In the panel at right, TOC is plotted as a function of depth with two domains evident: a near-surface, modern domain (green) that exhibits the exponential decay expected for a constant rate of deposition and a deeper, legacy domain (yellow) that reflects major fluctuations in the rate of POC deposition. In the Onondaga Lake sediment profile, the peak in TOC observed at ~30 cm corresponds to maximum in nutrient loading and primary production observed in this system in the mid-1970s. External Loads Limit of ETSA Sedimented particulate organic carbon consists of a labile fraction that participates in biogeochemical reactions, collectively termed diagenesis, and a refractory fraction that is ultimately buried. The diagenetic reactions include oxidations mediated by oxygen, nitrate, iron, manganese and sulfate and a fermentation, methanogenesis. These reactions consume oxygen directly or indirectly (oxidation of reduced species end products) leading to a sediment oxygen demand. The consumption of oxygen by the sediment decomposition processes places a demand on the oxygen resources of the hypolimnion. The onset of anoxia may trigger the flux of a variety of chemical species (e.g. phosphorus, methylmercury, see figure at right) from the sediment to the water. Objective: This research seeks to quantify the contribution of legacy deposits of POC to present day oxygen and nitrate demand, applying that result in resolving the when and to what extent question. External Loads Labile Organic Carbon Profile Peak in methanogenesis The labile organic carbon profile, as determined using aerobic bioassays, mirrors the pattern presented for TOC (see figure at extreme lower left). The amount of TOC present in the labile form ranged from 60-70% at depths > 60 cm to 80-100% above that. These results would suggest that legacy deposits account for very large fraction of the labile organic carbon and that a long approach to equilibrium may be expected. The response of a lake to management of external pollutant loads may be retarded by the presence of legacy pollutants in the sediments. As Cooke et al. (2005) point out, the question is not whether lakes will improve following external loading reductions, but when and to what extent. Our research group, studying Onondaga Lake in Syracuse, New York, has estimated that it takes ~20 years for sediment nitrogen (Wickman 1996) and 19-26 years for sediment phosphorus (Lewis et al. 2007) to reach steady state with new external loads. Here, we turn our attention to organic carbon and the role of legacy sediment deposits in mediating the extent and duration of hypolimnetic anoxia. Other measures of sediment biogeochemistry stand in agreement with the 1995-2008 profile comparison. Electron transport system activity, a measure of oxidative diagenesis (i.e. excluding methanogenesis) reaches an asymptote at ~10 cm. Similarly, methane production reaches its peak value at ~20 cm. Taken together, this information suggests that legacy deposits are essentially exhausted at a depth of 30 cm. We can, thus, consider the organic carbon below a depth of 30 m to be preserved. Therefore, the response time of sediment organic carbon in Onondaga Lake is on the order of 35 years, longer than that determined for nitrogen (~20 years) or phosphorus (20-25 years).