High terrestrial carbon load via groundwater to a boreal lake dominated by surface water inflow

Abstract

The input of dissolved organic and inorganic carbon (DOC and DIC) via direct groundwater seepage to boreal lakes is often assumed to be small in noncarbonaceous areas. However, measurements are rare. We estimated the terrestrial load of DOC, DIC, and methane (CH4) to a small boreal lake for the open water period, on the basis of measured concentrations of carbon species in near‐shore groundwater wells and inlet streams, and measured area‐specific discharge. The subcatchment directly draining into the lake via groundwater seepage contributed 18% to the total water input during the open water season. Compared to stream and lake water, near‐shore groundwater concentrations of DOC were slightly elevated, and groundwater DIC and CH4 concentrations were highly elevated. Consequently, direct groundwater seepage contributed 27% to the total DOC load, 64% to the total DIC load, and 96% to the total CH4 load from the catchment to the lake. Groundwater DIC import corresponded only to 5–8% of lake carbon dioxide (CO2) emission. In incubation experiments, we observed higher photochemical DOC loss rates in stream and groundwater samples (18–55% DOC loss upon 72 h UV‐A exposure) than in lake water (15% DOC loss) and detected significant DOC flocculation in groundwater samples in both light and dark incubations (2–24% DOC loss). We conclude that even in regions where lake hydrology is dominated by surface water inflow via inlet streams, direct groundwater seepage can represent an important carbon source to boreal lakes, and groundwater DOC may be susceptible to in‐lake removal via degradation and flocculation.

No long‐term trends in pCO2 despite increasing organic carbon concentrations in boreal lakes, streams, and rivers

“…Humic-like DOC is generally derived from the terrestrial environment and is susceptible to within-lake processes and may be rapidly lost from the water column by mineralization, flocculation, or transformation to other DOC by-products [Kothawala et al, 2014]. Flocculation, in particular, is an important transformation process for humic-like DOC components as it has been shown that in lakes, up to 22% of terrestrially derived DOC can be lost from the water column merely from flocculation and subsequent sedimentation [Einarsdottir et al, 2017;von Wachenfeldt and Tranvik, 2008]. The observed increase in DOC in freshwaters in the present study is probably mostly of terrestrial origin, hence consisting of humic-like components [Kothawala et al, 2014].…”

Organic Carbon Processing During Transport Through Boreal Inland Waters: Particles as Important Sites

“…While in transit, the composition of organic matter can be modified by several processes, including photo‐oxidation, respiration, leaching, flocculation, sorption, and settling (Eckard et al, ; Einarsdottir et al, ; Galy et al, ; Spencer et al, ; von Wachenfeldt & Tranvik, ). Most studies on organic C processing by microbes focused on the dissolved fraction as it is the largest contributor to the organic C pool in inland waters (Thurman, ).…”

A Small Temperate Lake in the 21st Century: Dynamics of Water Temperature, Ice Phenology, Dissolved Oxygen, and Chlorophyll a

“…For instance, important cold‐water fish such as salmon and brown trout thrive in a habitat of combining high DO concentrations and low water temperature (Dillon et al, ; Jiang et al, ), typical for the hypolimnion of natural temperate oligotrophic lakes. Importantly, climate change influences temperate lakes through both air‐water interactions and terrestrial‐aquatic interactions (Buffam et al, ; Cardille et al, ; Cole et al, ; Einarsdottir et al, ). For the latter, the transport of water, carbon, and nutrients from catchments to lakes could change considerably under future climate in temperate regions, which would inevitably affect the water and biogeochemical cycles in small temperate lakes (Anderson et al, ; Catalán et al, ; Couture et al, ; Tanentzap et al, ).…”