Abstract
Eutrophication of fresh waters results in increased CO2 uptake by primary production, but at the same time increased emissions of CH4 to the atmosphere. Given the contrasting effects of CO2 uptake and CH4 release, the net effect of eutrophication on the CO2 -equivalent balance of fresh waters is not clear. We measured carbon fluxes (CO2 and CH4 diffusion, CH4 ebullition) and CH4 oxidation in 20 freshwater mesocosms with 10 different nutrient concentrations (total phosphorus range: mesotrophic 39µg/L until hypereutrophic 939µg/L) and planktivorous fish in half of them. We found that the CO2 -equivalent balance had a U-shaped relationship with productivity, up to a threshold in hypereutrophic systems. CO2 -equivalent sinks were confined to a narrow range of net ecosystem production (NEP) between 5 and 19mmolO2 m-3 day-1 . Our findings indicate that eutrophication can shift fresh waters from sources to sinks of CO2 -equivalents due to enhanced CO2 uptake, but continued eutrophication enhances CH4 emission and transforms freshwater ecosystems to net sources of CO2 -equivalents to the atmosphere. Nutrient enrichment but also planktivorous fish presence increased productivity, thereby regulating the resulting CO2 -equivalent balance. Increasing planktivorous fish abundance, often concomitant with eutrophication, will consequently likely affect the CO2 -equivalent balance of fresh waters.
Highlights
Freshwater ecosystems are important components of the global carbon (C) cycle (Cole et al, 2007; Tranvik et al, 2009). They have a significant effect on the atmospheric fluxes of the greenhouse gases (GHGs) carbon dioxide (CO2) and methane (CH4; Bastviken, Tranvik, Downing, Crill, & Enrich-Prast, 2011; Raymond et al, 2013) and bury C in their sediments, which removes C from the active C cycle (Mendonça et al, 2017)
With increasing inorganic nutrient supply and productivity, net ecosystem metabolism turns to become net autotrophic (Hanson, Bade, Carpenter, & Kratz, 2003; Hanson et al, 2004), CO2 emissions are expected to decrease (Balmer & Downing, 2011; Pacheco, Roland, & Downing, 2013; Schindler, Carpenter, Cole, Kitchell, & Pace, 1997), C burial to increase (Anderson, Bennion, & Lotter, 2014; Flanagan, Mccauley, & Wrona, 2006; Heathcote & Downing, 2012) and ecosystems can turn into CO2 sinks (Balmer & Downing, 2011; CO2 emissions < 0)
Recent studies show that the responses to drivers such as temperature and nutrients are different for CH4 production and CH4 oxidation (Fuchs, Lyautey, Montuelle, & Casper, 2016; Sepulveda-Jauregui et al, 2018)
Summary
Freshwater ecosystems are important components of the global carbon (C) cycle (Cole et al, 2007; Tranvik et al, 2009). While several factors drive CO2 emission from fresh waters (Maberly, Barker, Stott, & De Ville, 2013; Marcé et al, 2015), many fresh waters are net heterotrophic ecosystems due to import and subsequent mineralization of terrestrial organic matter They are net sources of atmospheric CO2 (Cole, Pace, Carpenter, & Kitchell, 2000; Duarte & Prairie, 2005; Pace et al, 2004; CO2 emissions > 0). Recent studies show that the responses to drivers such as temperature and nutrients are different for CH4 production and CH4 oxidation (Fuchs, Lyautey, Montuelle, & Casper, 2016; Sepulveda-Jauregui et al, 2018) This implies that the balance between CH4 oxidized and CH4 produced, and the proportion of the produced CH4 that is emitted by diffusion to the atmosphere might change along environmental and climatic gradients. We hypothesized that the presence of fish reduces emissions of CO2 and CH4 due to reduction of zooplankton grazing on phytoplankton and CH4 oxidizers
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