Quantifying rapid spatial and temporal variations of CO2 fluxes from small, lowland freshwater ponds

Abstract

Small ponds comprise a substantial portion of the total area of the Earth's inland waters. They can be powerful carbon sinks or sources, potentially significant processors of organic carbon. Our understanding of their role is constrained by the absence of information regarding their CO 2 fluxes (F CO 2) and how these change with wet or dry phases and across distinct pond plant communities. We monitored the F CO 2 from 26 neighbouring small ponds over a 2-week drying period in late summer in 2014. The mean F CO 2 on day 1 (-641 ± 1490 mg m-2 day-1) represented a net intake across the site. As ponds dried they switched to becoming CO 2 sources resulting in a net site emission of CO 2 by day 12 (3792 ± 2755 mg m-2 day-1) although flux rates did not vary systematically between plant communities. Significant variability in the F CO 2 was observed amongst adjacent ponds on individual sampling days, resulting in marked spatial heterogeneity in CO 2 processing. This large degree of temporal and spatial heterogeneity across short time periods and small distances highlights the variability in the F CO 2 from temporary systems, making it hard to generalize their role in carbon cycle models.

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

“…We never observed a significant flux of CH 4 during our field tests, confirming earlier results which showed very low fluxes of CH 4 from dry sediments (Gomez-Gener et al, 2015). However, if the chamber was sealed with the on-site material, a small CH 4 flux of up to 4.1 mmol m −2 d −1 was detected at the reservoir site.…”

“…The fluxes between 75 and 241 mmol m −2 d −1 are very similar to results obtained in Spain (209 ± 10 mmol m −2 d −1 ; Gomez-Gener et al, 2015), and in Arizona (44 mmol m −2 d −1 ; Gallo et al, 2014). Our measurements in two contrasting temperate habitats confirm that dry sediments emit similar amounts of CO 2 as soils (Raich and Schlesinger, 1992).…”

“…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).…”

“…The most promising option is the use of ductile material. In the past, sediment material collected from the site has been used to seal the chamber (Gomez-Gener et al, 2015). However, such a procedure might produce artefacts because the sealing material is not inert and might produce CO 2 .…”

High carbon burial rates by small ponds in the landscape

“…Previous evaluations of CO 2 flux rates in these study ponds (Gilbert et al . ) showed rapid changes during the transition from holding water (wet phase) to drying out (dry phase) typical of temporary ponds, as well as marked spatial variations among ponds. Mean CO 2 flux varied between an uptake of 641 mg m −2 d −1 to emissions of 3792 mg m −2 d −1 .…”