Surface water-methane (CH4) and nitrous oxide (N2O) concentrations were measured and diffusive fluxes were estimated in three subtropical freshwater reservoirs (Little Nerang Dam (LND), Lake Wivenhoe (LW) and Lake Baroon (LB)) in southeast Queensland, Australia, during four seasons in 2011-2012. All reservoirs were strong sources of CH4 in all seasons. Surface water CH4 varied between 1350 and 524,000% saturation, and was overall highest in spring and summer, and lowest in winter, however, with no clear patterns common to all reservoirs. In contrast, all reservoirs switched from weak N2O sinks in spring to strong N2O sources for the rest of the year. N2O saturation in all reservoirs varied between 70 and 1230%. There were significant differences for CH4 concentrations and fluxes between the reservoirs. Within each reservoir, there was strong spatial CH4 variability but minimal N2O saturation variability. CH4 saturation was higher in inflow zones than in the main body. Area-weighted average fluxes were estimated using six water-air gas transfer velocity estimation models and resulted in fluxes in the range 4.8-20.5, 2.3-5.4, and 2.3-7.5 mg CH4 m(-2) d(-1), while N2O was 0.07-0.41, 0.09-0.22, and 0.03-0.09 mg N2O m(-2) d(-1) for LND, LW, and LB, respectively. Total emissions, in carbon dioxide equivalents, from all measurement campaigns were CH4 dominated (67-86%). The measured degree of CH4 saturation and fluxes are among the highest reported thus far indicating that subtropical freshwater reservoirs could be significant aquatic greenhouse gas sources. This paper provides a comprehensive assessment of the interplay between biogeochemical processes and the physical forcing driving the water-air gaseous emissions. The high variability coupled with the lack of consensus among estimation models calls for concerted efforts to address uncertainty of measurements for reliable emissions accounting.
Lake sediments are important areas for remineralisation of nutrients involved in phytoplankton blooms. This study simultaneously analysed the microbial community structure and measured the sediment fluxes of inorganic nitrogen, phosphorus and silica from sediment cores collected at 2 different locations within a sub-tropical polymictic reservoir, Lake Wivenhoe. The bacterial and archaeal community structure was determined by amplifying and cloning the 16S rRNA gene from co-extracted DNA and RNA samples. A total of 19 phyla or candidate divisions of bacteria were identified, with sulphur-reducing bacteria within the phylum of Deltaproteobacteria being the most abundant ribotypes in DNA-derived clones libraries. In contrast, Actinobacteria and Acidobacteria were the most abundant ribotypes in RNA-derived clone libraries from the upper and lower sediments, respectively. The archaeal community was dominated by Euryarchaeota, with methanogenic archaea belonging to subdivisions of Methanobacteria and Methanomicrobia accounting for 69 to 98% of the sequenced clones. Comparison of the 16S rDNA and rRNA clone libraries revealed that bacterial groups highly abundant in the sediments were mostly metabolically inactive, whilst those metabolically active were not very abundant. A higher relative abundance of nitrifying ribotypes (Nitrospira sp.) was identified at Site B, which corresponded to a higher efflux of nitrate from the sediments to the water column at this site. At the time of sampling, Lake Wivenhoe was stratified, and sediment cores were collected from the hypolimnion. Our results suggest that water column depth and delivery of dissolved oxygen to the sediments influenced the sediment microbial community structure and the fluxes and speciation of nutrients, which are reported to influence phytoplankton species composition and bloom dynamics.
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