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

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 CO2 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 CO2 exchange rates at the water- and emergent sand bar-atmosphere interfaces within a lowland large temperate river stretch. CO2 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 CO2 source (between 0.2 and 7.6 mmol CO2 m−2 day−1), whilst the vegetated parafluvial zones acted as a net sink, with assimilation rates peaking at 623.4 mmol CO2 m−2 day−1. Emerging bare sediments were on the contrary net emitting CO2 systems (2.7–60.1 mmol CO2 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 CO2 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 CO2 emitter, should be linked to those from transitional areas in order to better integrate terrestrial and aquatic C budgets.