Seasonal and diel variation in greenhouse gas emissions from an urban pond and its major drivers

Abstract

Small water systems are important hotspots of greenhouse gas (GHG) emission, but estimates are poorly constrained as data are scarce. Small ponds are often constructed in urban areas, where they receive large amounts of nutrients and therefore tend to be highly productive. Here, we investigated GHG emissions, seasonal and diel variation, and net ecosystem production (NEP) from an urban pond. In monthly 24‐h field campaigns during 11 months, diffusive water–atmosphere methane (CH4) and carbon dioxide (CO2) fluxes and CH4 ebullition and oxidation were quantified. With oxygen (O2) measurements, NEP was assessed. The pond was a net GHG source the entire year, with an emission of 3.4 kg CO2 eq m−2 yr−1. The dominant GHG emission pathway was CH4 ebullition (bubble flux, 50%), followed by diffusive emissions of CO2 (38%) and CH4 (12%). Sediment CH4 release was primarily driven by temperature and especially ebullition increased exponentially above a temperature threshold of 15°C. The pond's atmospheric CO2 exchange was not related to NEP or temperature but likely to a high allochthonous carbon (C) input via runoff and anaerobic mineralization of C. We expect urban ponds to show a large increase in GHG emission with increasing temperature, which should be considered carefully when constructing ponds in urban areas. Emissions may partly be counteracted by pond management focusing on a reduction of nutrient and organic matter input.

Scientists’ warning to humanity: microorganisms and climate change

“…Non-forested, arid and semiarid regions (47%) are important for the carbon budget and respond differently to anthropogenic climate change than forested regions 132,133 . Lakes make up ~4% of the non-glaciated land area 134 , and shallow lakes emit substantial amounts of CH 4 (rEFS 135,136 ). Peat (decomposed plant litter) covers ~3% of the land surface and, due to plant productivity exceeding decomposition, intact peatlands function as a global carbon sink and contain ~30% of global soil carbon 137,138 .…”

“…A 15-year mesocosm study that simulated freshwater lake environments determined that the combined effects of eutrophication and warming can lead to large increases in CH 4 ebullition (bubbles from accumulated gas) 135 . As small lakes are susceptible to eutrophication and tend to be located in climate-sensitive regions, the role of lake microorganisms in contributing to global greenhouse gas emissions needs to be evaluated 135,136 .…”

Regulation of carbon dioxide and methane in small agricultural reservoirs: optimizing potential for greenhouse gas uptake

“…For example, agricultural reservoirs with the highest WRTs tended to be hydrologically closed systems (E/I > 1) and any watershedderived DIC delivered from previous water sources is likely to be consumed by primary production, which encourages atmospheric CO 2 uptake (Macrae et al, 2004). Additionally, smaller waterbodies with shorter WRT can support higher rates of internal CO 2 production due to higher rates of allochthonous DOC mineralization (Weyhenmeyer et al, 2015;Vachon et al, 2017).…”

“…Lower CO 2 concentrations at higher elevations are common in perched ecosystems with smaller contributing catchment areas (Diem et al, 2012) and low rates of allochthonous carbon influx (Rose et al, 2015). Conversely, waterbodies low in the landscape may receive more watershed C via groundwater influx due to topographical gradient (Winter and LaBaugh, 2003;van der Kamp and Hayashi, 2009). The effect of elevation could also be related to changes in vegetation composition within the local landscape, with the lowest lying catchments exhibiting higher abundance of marginal wetland vegetation (Zhang et al, 2010), which favours higher inputs of terrestrial C (Magnuson et al, 2006;Abril et al, 2014).…”

“…Artificial reservoirs have the potential to be potent sources of CO 2 and CH 4 (Downing et al, 2008;Holgerson and Raymond, 2016). This role can be demonstrated by a carbon budget estimate from an urban pond where carbon emissions (both diffusive and ebullitive for CH 4 ) offset carbon burial by > 1000 % (van Bergen et al, 2019). The recent 2019 IPCC Refinement has assigned a CH 4 emission factor of 183 kg ha −1 yr −1 to constructed waterbodies; however, data are greatly limited, both geographically and in number (n = 68), such that climatic-zone emission factors cannot be estimated (IPCC, 2019).…”