Responses of microbial food web to increased allochthonous DOM in an oligotrophic subarctic lake

Abstract

Climate-induced changes in catchment area vegetation and runoff alter the quality and quantity of carbon that enters lakes, with implications for food webs in recipient water bodies. The effect of dissolved organic matter (DOM) on the ratio between heterotrophic and autotrophic biomass and productivity was studied in a subarctic, clear water lake in northern Finland. In a mesocosm experiment, natural DOM from a subarctic bog and a boreal lake was added to the lake water, doubling the initial dissolved organic carbon (DOC) concentration. Optical indices suggested that the subarctic DOM addition was more bioavailable, which was in line with the greater increase in bacterial biomass and production observed in this treatment. Both DOM additions increased the abundance of heterotrophic nanoflagellates (HNF) and decreased primary productivity. They also led to lower ratios of primary to bacterial production, autotrophic to mixotrophic algae and pigmented nanoflagellates (PNF) to HNF relative to the control samples, indicating a shift from a primary production-based food web towards one based on bacterial production. A comparable increase in DOM in the natural environment would lead to a considerable decrease in the euphotic layer and loss of areas available for primary production, resulting in a shift towards a heterotrophic production based food web. KEY WORDS: Dissolved organic matter · Dissolved organic carbon · Subarctic · Microbial food webResale or republication not permitted without written consent of the publisher

Oxygen depletion in subarctic peatland thaw lakes

“…Although DOC concentration has been identified as a main driver of interannual variations in oxygen content in other aquatic ecosystems, bacterial biomass and production in these peatland thaw lakes do not seem to be limited by organic carbon availability (Palmer et al 2014;Deshpande et al 2016). In humic lakes, it has been shown that high concentrations of DOC encourage a dominance towards chemoorganotrophic metabolic processes and away from autotrophic metabolic processes that would increase oxygen concentrations (Forsström et al 2013). This phenomenon has been consistently observed in humic lakes of northern Sweden when DOC concentrations were equal to or greater than 10 mg L −1 (Jansson et al 2000), as is the case for the peatland thaw lakes studied here (Table 1).…”

“…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 .…”

Bacterial production in subarctic peatland lakes enriched by thawing permafrost

“…Other studies have shown that DOM-enrichment increased heterotrophic metabolism, sometimes at the expense of autotrophic metabolic activity (Forsström et al, 2013). In an Arctic tundra lake of the Northwest Territories, addition of permafrost soil resulted in decreased bacterial production in the pelagic zone, but increased rates in the benthic zone (Moquin and Wrona, 2015).…”

Permafrost thaw lakes and ponds as habitats for abundant rotifer populations

“…The SAS waters lie directly on a peatland with palsas and therefore receive large quantities of organic matter as the palsas collapse into the ponds . In Scandinavian subarctic waters, addition of allochthonous DOC can stimulate bacterial production and abundance (Forsström et al 2013), which in turn could influence the rotifer growth and diversity. However, the dominant taxon of the SAS waters, P. euryptera, is known to be a selective feeder on larger prey items (Pourriot 1977).…”