Abstract
Recent studies from temperate lakes indicate that eutrophic systems tend to emit less carbon dioxide (CO2) and bury more organic carbon (OC) than oligotrophic ones, rendering them CO2 sinks in some cases. However, the scarcity of data from tropical systems is critical for a complete understanding of the interplay between eutrophication and aquatic carbon (C) fluxes in warm waters. We test the hypothesis that a warm eutrophic system is a source of both CO2 and CH4 to the atmosphere, and that atmospheric emissions are larger than the burial of OC in sediments. This hypothesis was based on the following assumptions: (i) OC mineralization rates are high in warm water systems, so that water column CO2 production overrides the high C uptake by primary producers, and (ii) increasing trophic status creates favorable conditions for CH4 production. We measured water-air and sediment-water CO2 fluxes, CH4 diffusion, ebullition and oxidation, net ecosystem production (NEP) and sediment OC burial during the dry season in a eutrophic reservoir in the semiarid northeastern Brazil. The reservoir was stratified during daytime and mixed during nighttime. In spite of the high rates of primary production (4858 ± 934 mg C m-2 d-1), net heterotrophy was prevalent due to high ecosystem respiration (5209 ± 992 mg C m-2 d-1). Consequently, the reservoir was a source of atmospheric CO2 (518 ± 182 mg C m-2 d-1). In addition, the reservoir was a source of ebullitive (17 ± 10 mg C m-2 d-1) and diffusive CH4 (11 ± 6 mg C m-2 d-1). OC sedimentation was high (1162 mg C m-2 d-1), but our results suggest that the majority of it is mineralized to CO2 (722 ± 182 mg C m-2 d-1) rather than buried as OC (440 mg C m-2 d-1). Although temporally resolved data would render our findings more conclusive, our results suggest that despite being a primary production and OC burial hotspot, the tropical eutrophic system studied here was a stronger CO2 and CH4 source than a C sink, mainly because of high rates of OC mineralization in the water column and sediments.
Highlights
Lakes and reservoirs occupy only a small fraction of the Earth surface, which contrasts with the large role they play in the global carbon (C) cycle (Cole et al, 2007; Tranvik et al, 2009)
Our findings indicate that the ESEC reservoir has an extremely high primary production, which is corroborated by high concentrations of chlorophyll-a and total phosphorus (∼100 μg L−1; Costa et al, 2015) classifying it as a hypertrophic system (Wetzel, 2001)
Our results demonstrate that the ESEC reservoir is a regional C burial hotspot, the sum of CO2 (510 mg C m−2 d−1)
Summary
Lakes and reservoirs occupy only a small fraction of the Earth surface, which contrasts with the large role they play in the global carbon (C) cycle (Cole et al, 2007; Tranvik et al, 2009). Because of large inputs of terrestrial inorganic and organic carbon (OC), lakes and reservoirs annually emit 0.1 and 0.3 Pg of C as methane (CH4) and carbon dioxide (CO2), respectively (Bastviken et al, 2011; Raymond et al, 2013). Estimates suggest that lakes and reservoirs are 2–3 times larger C sources than sinks (Cole et al, 2007; Tranvik et al, 2009), yet this overall balance is still uncertain because studies on C burial are far more scarce than on C evasion (Mendonça et al, 2012). Age together with latitude and OC inputs explains 40% of CO2 and 54% of CH4 emissions from hydroelectric reservoirs (Barros et al, 2011). External inorganic input may even support CO2 emission from net autotrophic systems as demonstrated for several temperate systems (McDonald et al, 2013)
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