Role of ephemeral vegetation of emerging river bottoms in modulating CO2 exchanges across a temperate large lowland river stretch

Abstract

Rivers and marginal wetlands contribute significantly to the carbon (C) exchange rate per unit area compared to adjacent terrestrial ecosystems, due to high C inputs, metabolic activity and CO 2 supersaturation. Within riverscapes, the contribution of emerging bottoms (i.e. parafluvial zones) and ephemeral vegetation (i.e. microphytobenthos or annual vascular plant communities) to the C metabolism is understudied. The aim of this study was to evaluate the CO 2 exchange rates at the water-and emergent sand bar-atmosphere interfaces within a lowland large temperate river stretch. CO 2 fluxes were measured seasonally, in summer (August 2007) and winter (March 2008) via static closed chambers, together with the primary producers' biomass (microphytobenthos and vascular macrophytes). Our results showed that the river was a CO 2 source (between 0.2 and 7.6 mmol CO 2 m -2 day -1 ), whilst the vegetated parafluvial zones acted as a net sink, with assimilation rates peaking at 623.4 mmol CO 2 m -2 day -1 . Emerging bare sediments were on the contrary net emitting CO 2 systems (2.7-60.1 mmol CO 2 m -2 day -1 ). Within lotic environments, seasonally emergent vegetated sand bars may represent important C fixation hot spots, with summer primary production particularly contributing to offsetting CO 2 emissions and counterbalancing the heterotrophic metabolism of the saturated zones. Based on these outcomes, we suggest that studies addressing the strictly aquatic fluvial environment, which is generally a CO 2 emitter, should be linked to those from transitional areas in order to better integrate terrestrial and aquatic C budgets.

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

“…In other words, the present data support the idea that the sectors close to the aquatic boundary interfaces exhibited higher CO 2 emission rates. These results are in agreement with data collected by Bolpagni, Folegot, Laini, & Bartoli, () for a sandbar habitat of the same river. However, there are clear differences among habitats and functional zones investigated, suggesting complex regulation mechanisms that are not exclusively related to the type of functional zone under investigation.…”

“…Additionally, accessory processes as reactions involving carbonates or OM photochemical degradation can have a non‐negligible role in CO 2 release (Rey, ). Hence, periodically air‐exposed sediments exhibit an elevated mineralization capacity as verified for flooded soils (Gawne et al, ), but, depending on the colonization pattern of primary producers, they may turn into temporary but elevated C sinks (see Bolpagni, Folegot, Laini, & Bartoli, ).…”

“…Focusing on the periodically emerging bottoms, the CO 2 exchanges seemed to be strictly controlled by the progressive isolation from fluvial waters. Such condition is related to the development of benthic algal biofilms and the accumulation of OM in the upper sediment layers (Bolpagni, Folegot, Laini, & Bartoli, ; Obrador et al, , and references therein). Accordingly, the Po River and the quarry lake under excavation (BL; which is directly connected with the river waters) exhibited the lowest CO 2 exchange rates—both for SZ and LZ—whereas the CO 2 emissions peaked in backwater and oxbow lake.…”

Carbon Dioxide Emissions from the Littoral Zone of a Chinese Reservoir

“…The decline was caused by the increased accumulation of biomass. The important role that ephemeral vegetation plays in C fixation has been reported in the marginal area of a river [40]. After water recession, new plants sprouted and grew on the exposed soil.…”

A closed-chamber method to measure greenhouse gas fluxes from dry aquatic sediments

“…Recent research indicates that drying and rewetting of freshwater sediments creates hotspots of carbon mineralization and thus CO 2 emissions, which are probably relevant on a global scale (Gomez-Gener et al, 2015;Reverey et al, 2016;Von Schiller et al, 2014). However, existing knowledge is scarce and mainly based on regional studies from, for example, the USA (Gallo et al, 2014), Spain (Gómez-Gener et al, 2016), the UK (Gilbert et al, 2016) or Italy (Bolpagni et al, 2017).…”