Carbon dynamics in highly heterotrophic subarctic thaw ponds

Abstract

Abstract. Global warming has accelerated the formation of permafrost thaw ponds in several subarctic and arctic regions. These ponds are net heterotrophic as evidenced by their greenhouse gas (GHG) supersaturation levels (CO 2 and CH 4 ), and generally receive large terrestrial carbon inputs from the thawing and eroding permafrost. We measured seasonal and vertical variations in the concentration and type of dissolved organic matter (DOM) in five subarctic thaw (thermokarst) ponds in northern Quebec, and explored how environmental gradients influenced heterotrophic and phototrophic biomass and productivity. Late winter DOM had low aromaticity indicating reduced inputs of terrestrial carbon, while the high concentration of dissolved organic carbon (DOC) suggests that some production of nonchromophoric dissolved compounds by the microbial food web took place under the ice cover. Summer DOM had a strong terrestrial signature, but was also characterized with significant inputs of algal-derived carbon, especially at the pond surface. During late winter, bacterial production was low (maximum of 0.8 mg C m −3 d −1 ) and was largely based on free-living bacterioplankton (58 %). Bacterial production in summer was high (up to 58 mg C m −3 d −1 ), dominated by particle-attached bacteria (67 %), and strongly correlated with the amount of terrestrial carbon. Primary production was restricted to summer surface waters due to strong light limitation deeper in the water column or in winter. The phototrophic biomass was equal to the heterotrophic biomass, but as the algae were mostly composed of mixotrophic species, most probably they used bacteria rather than solar energy in such shaded ponds. Our results point to a strong heterotrophic energy pathway in these thaw pond ecosystems, where bacterioplankton dominates the production of new carbon biomass in both summer and winter.

Bacterial production in subarctic peatland lakes enriched by thawing permafrost

“…This was not unique to the organic-rich SAS waters, but was found in several of the other lakes. These results are consistent with a previous study of five lakes in the mineral-rich KWK area, which showed a substantially higher proportion of particle-attached bacteria (62 ± 30 % in the > 3 µm sizefraction) and high BP (59 ± 30 %) by these groups (Roiha et al, 2015). The BG patterns of some of these lakes are also consistent with data elsewhere; for example, Kirchman and Mitchell (1982) found that particle-attached bacteria ac- Giorgio et al (1997) counted for < 10 % of the total bacterial populations but contributed substantially to total microbial activity.…”

“…Net bacterial C production was estimated using a conversion factor of 3.1 kg C per mole of leucine (Kirchman et al, 1993;Iriberri et al, 1990). Bacterial community specific growth rates (BG, d −1 ) were determined (as described in Kirchman, 2001) via BG = BP/B, where BP are bacterial production rates (µg C L −1 h −1 ), and B is the bacterial biomass (fg C mL −1 ) that was converted from bacterial cellular counts (cells mL −1 ) to units of carbon using an average cellular carbon content of 18 fg C cell −1 , as measured in the nearby KWK lakes (Roiha et al, 2015).…”

“…In the Mackenzie River Delta region, a mesocosm assay showed decreased planktonic bacterial production rates in response to permafrost soil enrichment, but a strong increase in benthic bacterial production rates (Moquin and Wrona, 2015). In a set of subarctic thermokarst ponds, late winter bacterial production was low and dominated by free-living bacterioplankton, while much higher production rates in summer were dominated by particle-attached bacteria, correlated with terrestrial carbon concentrations (Roiha et al, 2015). Permafrost peatland lakes are known to be strong emitters of greenhouse gases with diverse microbiota (Liebner et al, 2015), but little is known about the controls on their bacterial populations and productivity.…”

Oxygen depletion in subarctic peatland thaw lakes

“…This provides substrates for bacterial heterotrophic production and also includes high concentrations of coloured dissolved organic matter (CDOM) that strongly attenuate light throughout the water column (Watanabe et al 2011), thereby reducing autotrophic phytoplankton production at depth (Shirokova et al 2009;Forsström et al 2013;Roiha et al 2015). Subarctic permafrost peatland lakes are favorable to both particle-based and free-living aquatic bacterial communities due to their high carbon and nutrient availability .…”

Periphytic diatom community structure in thermokarst ecosystems of Nunavik (Québec, Canada)

“…Thermokarst lakes show distinct limnological and environmental properties depending on their ecoclimatic setting Roiha et al 2015;Deshpande et al 2016;Bouchard et al 2017). They are generally surrounded by peat deposits with sitespecific shoreline vegetation.…”