Connectivity and habitat typology drive CO₂ and CH₄ fluxes across land–water interfaces in lowland rivers

Abstract

Lowland rivers are assumed to be a net source of carbon dioxide (CO2) and methane (CH4). However, little is known about the contribution of marginal permanent and temporary riverine aquatic systems to river carbon metabolism. Elevation, in relation to flooding, hydro‐morphological, and ecological features of such habitats, can affect the gas fluxes across the water–atmosphere interfaces and the periodically air‐exposed riverbeds. This hypothesis was investigated in the lowland sector of the Po River (Northern Italy) from May to September 2008. Five different aquatic habitats were considered: the main river course; a backwater habitat; and three lateral lentic waterbodies—an oxbow and two quarry lakes of different age. The water mass was always CO2 and CH4 supersaturated, and gas fluxes were from the water to the atmosphere. In the highly dynamic river course and backwaters, CO2 emission rates were nearly one order of magnitude greater than in the lentic and hypoxic oxbow and quarry lakes. By contrast, CH4 fluxes peaked in the lentic, deeper, and permanent water bodies. At sediment–atmosphere interfaces, the CO2 emissions increased along an organic matter, a water saturation, and a chlorophyll a gradient, attaining the maximum rates in the periodically air‐exposed riverbed and marginal sandy sediments. These differences in gas fluxes resulted from either the submersion persistence or the alternation of submersion and emersion phases, which were ultimately due to the gap between elevation and river water variations. All this stresses the critical role of marginal aquatic habitats for river C budget.

Spatial upscaling of CO2 emissions from exposed river sediments of the Elbe River during an extreme drought

“…Table 1 indicates CO 2 flux dependence on land cover and thus is in line with Bolpagni et al (2019). CO 2 fluxes differed depending on prevailing conditions such as sediment temperature, water content, texture, elevation and distance to water (Figure 3).…”

“…However, on a daily scale, even vegetated sites were net CO 2 emitters, indicating that primary production was compensated by respiration and/or CO 2 input from sediment or groundwater. Table 1 indicates CO 2 flux dependence on land cover and thus is in line with Bolpagni et al (2019). CO 2 fluxes differed depending on prevailing conditions such as sediment temperature, water content, texture, elevation and distance to water (Figure 3).…”

“…For the latter, drought effects on greenhouse gas (GHG) emissions are intensively studied (Looman, Maher, Pendall, Bass, & Santos, 2017). Much less is known about GHG emissions from exposed sediments in high‐order rivers, where sediments are typically more fine grained and the effect of exchange with the groundwater and interstitial water is probably less pronounced (Bolpagni, Laini, Mutti, Viaroli, & Bartoli, 2019).…”

“…Much less is known about GHG emissions from exposed sediments in high-order rivers, where sediments are typically more fine grained and the effect of exchange with the groundwater and interstitial water is probably less pronounced (Bolpagni, Laini, Mutti, Viaroli, & Bartoli, 2019).…”

Characteristics, Main Impacts, and Stewardship of Natural and Artificial Freshwater Environments: Consequences for Biodiversity Conservation

“…Indeed, the denitrification efficiency was higher in connected oxbow lakes than isolated ones [290]. On the other hand, oxbow lakes play a relevant role in modulating the river C budget as a function of season and hydrology [291].…”