The effect of lake browning and respiration mode on the burial and fate of carbon and mercury in the sediment of two boreal lakes

Abstract

In many northern temperate regions, the water color of lakes has increased over the past decades ("lake browning"), probably caused by an increased export of dissolved organic matter from soils. We investigated if the increase in water color in two lakes in Norway has resulted in increased burial of organic carbon (OC) and mercury (Hg) in the sediments and if the Hg was prone to methylation. Lake Solbergvann experienced a threefold water color increase, and OC burial increased approximately twofold concomitant to the water color increase. This lake had prolonged periods of anoxic bottom water, and anoxic OC mineralization rates were only about half of the oxic OC mineralization rates (7.7 and 17.5 g C m À2 yr À1 , respectively), contributing to an efficient OC burial. In Lake Elvåga, where water color increase was only approximately twofold and bottom water was oxygenated, no recent increase in OC burial could be observed. Hg burial increased strongly in both lakes (threefold and 1.6-fold in Lake Solbergvann and Lake Elvåga, respectively), again concomitant to the recent water color increase. The proportion of methylated Hg (MeHg) in surficial sediment was 1 order of magnitude higher in Lake Elvåga (up to 6% MeHg) than in Lake Solbergvann (0.2-0.6% MeHg), probably related to the different oxygenation regimes. We conclude that lake browning can result in increased OC and Hg burial in lake sediments, but the extent of browning and the dominating mode of sediment respiration (aerobic or anaerobic) strongly affect burial and fate of OC and Hg in sediments.

Reduced Mineralization of Terrestrial OC in Anoxic Sediment Suggests Enhanced Burial Efficiency in Reservoirs Compared to Other Depositional Environments

“…Our experiment showed that anoxia reduced the sediment mineralization rates of predominantly terrestrial OC by about half compared to oxic treatments. Similar results have been reported for sediments from natural lakes (Isidorova et al, ; Peter et al, ). In the following, we give an example of the possible effect anoxia could have on the OC burial efficiency in a Brazilian reservoir (Mascarenhas de Morais; (Mendonça et al, )).…”

Investigating molecular changes in organic matter composition in two Holocene lake‐sediment records from central Sweden using pyrolysis‐GC/MS

“…These processes have been previously discussed by Meyer‐Jacob et al [] based on observations of an alteration of the diatom community structure and a decline in diatom production at circa A.D. 200 in Lång‐Älgsjön. Expanding peatlands may increase OM freshness through a greater supply of organic plant residues, leading to more anoxic conditions at the sediment‐water interface [ Kleeberg , ] and subsequently lower rates of OM degradation [ Bastviken et al , ; Isidorova et al , ]. The abundances of plant (lignin oligomers and isopropenylphenol) and nondegraded OM (levosugars and proteins) increase further from A.D. 1260 to 1960, while the proportion of degraded OM continues to decline.…”

Feasting on terrestrial organic matter: Dining in a dark lake changes microbial decomposition

“…As future influxes of t-OM increase across boreal lakes (Creed et al, 2018;Sobek et al, 2009), these water clarity-dependent responses suggest CO 2 release may be greater in clear lakes with high levels of photo-oxidation at the sediment surface (Lapierre, Guillemette, Breggren, & del Giorgio, 2013). Dark lakes may instead experience a decrease in primary production and shift toward retaining rather than mineralizing terrestrial carbon, such as by burying it in sediment (Gudasz et al, 2017;Isidorova et al, 2015;Seekell et al, 2015). However, over the longer term, the darkening of clear lakes will reduce light exposure on the sediment surface, increasing OM burial and encouraging bacterial communities to develop in sediment pore water that can utilize this material (Judd, Crump, & Kling, 2007;Rofner et al, 2017).…”