Benthic flux of dissolved organic matter from lake sediment at different redox conditions and the possible effects of biogeochemical processes
Graphical abstract
Introduction
Inland waters such as lakes and rivers play an important role in the global carbon cycle. It is known that the estimated amount of carbon burial in the inland sediments is comparable with that in the ocean (Cole et al., 2007, Tranvik et al., 2009). Human disturbances such as dam construction may accelerate the deposition of suspended solids and the associated organic carbon at the bottom of watersheds around the world (Syvitski et al., 2005). It was reported that the sedimentary organic carbon can be released into the pore water as dissolved organic carbon (DOC), upon the reduction and dissolution of redox-sensitive minerals (e.g., Skoog et al., 1996, O'Loughlin and Chin, 2004, Skoog and Arias-Esquivel, 2009). The concentration gradient of DOC between pore water and overlying water may finally produce the efflux of DOC across the sediment–water interface. Determining the benthic flux of DOC and the chemical composition of efflux DOC is critical for understanding both the fate of the massive organic carbon deposited in the inland waters, and the biogeochemical and ecological effects of efflux DOC in the bottom water.
Several studies have demonstrated much higher concentrations of dissolved organic matter (DOM) for lake sediment pore waters, as compared to those for the overlying water column (Chin and Gschwend, 1991, Mozeto et al., 2001, O'Loughlin and Chin, 2004, Fu et al., 2006, Ziegelgruber et al., 2013). These results suggest that there is likely to be a notable upward benthic flux of DOM. To our knowledge, however, this benthic flux of DOM has not been directly quantified using benthic chamber experiments for lake sediments. The chamber flux measurement is direct and it can overcome some problems with estimating the flux based on pore water gradients because of the difficulty in measuring possible steep gradients in the surface sediments (Berelson et al., 1987). A few studies have been carried out for the coastal sediments, which have revealed a significant benthic flux of DOM across the sediment–water interface (Burdige and Homstead, 1994, Skoog et al., 1996, Burdige et al., 2004, Skoog and Arias-Esquivel, 2009). Furthermore, many lakes are subject to eutrophication and the subsequent seasonal oxygen depletion in the bottom water, imposing the necessity of the investigation on the effects of the redox condition on benthic flux. In addition, considering the structural heterogeneity of DOM, it is critical to compare the flux behaviors of DOM with respect to its various components whose transport characteristics and biogeochemical reactivities may be very different.
Absorption and fluorescence spectroscopy are now popular optical techniques for studying the dynamics of DOM in aquatic environments, particularly for chromophoric and fluorescent DOM (CDOM and FDOM). Based on these techniques, there are a series of optical parameters suggested for estimating the concentration and chemical composition of DOM, which include absorption coefficient and fluorescence intensity, as quantitative parameters, and absorption spectral slope (S), fluorescence index (FI), humification index (HIX), and biological index (BIX) as DOM quality indices (McKnight et al., 2001, Helms et al., 2008, Huguet et al., 2009, Fellman et al., 2010). In particular, the combined use of fluorescence excitation emission matrices and parallel factor analysis (EEM-PARAFAC) provides a powerful tool for identifying individual fluorescent components as well as for tracing their source, the transformation processes and the fate (Stedmon et al., 2003, Jaffé et al., 2008, Ishii and Boyer, 2012, Andrade-Eiroa et al., 2013).
Most previous studies have applied the optical techniques primarily to characterize CDOM and FDOM in the lake sediment pore waters (Fu et al., 2006, Ziegelgruber et al., 2013). However, the benthic fluxes of CDOM and FDOM have not been quantified from benthic chamber experiments using the lake sediments. Much is to be learned about the benthic flux of DOM using spectroscopic methods. Therefore, our study aimed to (1) estimate the benthic fluxes of DOM from artificial lake sediment (the Uiam Lake, Korea) under oxic and hypoxic conditions based on benthic chamber experiments and (2) discuss the possible influences of biogeochemical transformations on the apparent benthic flux in the overlying water. The results will have implications for better constraining the carbon cycle in inland waters and for inferring the influences of benthic flux of DOM on bottom water ecosystems.
Section snippets
Field sampling and laboratory incubation
The sampling site was located in the Uiam Lake (37°51ʹ43.4″N, 127°41ʹ19.4″E), an artificial lake in the middle reach of the North Han River, where green algal blooms often occur. The bottom water was collected and filtered through pre-combusted GF/F filters, and sediment cores were obtained by a diver using a self-made arcylic sampler on September 5, 2013. The sediments were transferred carefully into 15-cm-diameter cylinders for our benthic chamber experiment under oxic and hypoxic conditions
Fluorescent components identified by PARAFAC
Four fluorescent components were identified from the EEM dataset of the DOM samples using PARAFAC (Fig. 1), which were well comparable with those in the OpenFluor database (Murphy et al., 2014). Component C1 had two excitation maxima at 250 and 330 nm and one emission maximum at 418.5 nm, resembling a terrestrial humic-like component in the Florida Coastal Everglades (C3 in Yamashita et al., 2010) and in Central European headwater streams (C1 in Graeber et al., 2012). Component C2 had
Characteristics of DOM in pore water
The pore water DOC values in the sediment of the Uiam Lake (97 ± 20 mg L−1) were comparable with those of two North Dakota prairie pothole lakes (26–183 mg L−1, Ziegelgruber et al., 2013), the Yangtze Estuary, China (20–448 mg L−1, Wang et al., 2013) and marine sediments (12–155 mg L−1, Chen and Bada, 1994), but they were higher than those reported for the Guarapiranga reservoir, Brazil (4.7–12 mg L−1, Mozeto et al., 2001), the Green Bay of Lake Michigan (7.7–15 mg L−1, O'Loughlin and Chin, 2004
Conclusions
The rough estimate of the benthic flux of DOC in global inland waters (∼0.1 Pg year−1) was comparable with those in coastal and marginal sediments, and other major components of inland water carbon cycle. This suggests that DOC benthic flux may be an overlooked, important component of the inland water carbon cycle model. The finding has another environmental implication in that the flux can be accompanied with the transport of associated nutrients, trace metals and xenobiotics across the
Acknowledgments
This study was supported by a National Research Foundation of Korea grant funded by the Korean Government (MEST) (NRF-2011-0029028). We thank Ms. Bo-Min Ki for her assistance in the experimental work. We are also grateful to the editor and two anonymous reviewers for their valuable comments and suggestions.
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