Effect of interannual variation in winter vertical mixing on CH₄ dynamics in a subtropical reservoir

Abstract

Although freshwaters are considered to be substantial natural sources of atmospheric methane (CH 4 ), in situ processes of CH 4 production and consumption in freshwater ecosystems are poorly understood, especially in subtropical areas, leading to uncertainties in the estimation of global CH 4 emissions. To improve our understanding of physical and biogeochemical factors affecting CH 4 dynamics in subtropical lakes, we examined vertical and seasonal profiles of dissolved CH 4 and its carbon isotope ratio (δ 13 C) and conducted incubation experiments to assess CH 4 production and oxidation in the deep subtropical Fei-Tsui Reservoir (FTR; Taiwan). The mixing pattern of the FTR is essentially monomixis, but the intensity of winter vertical mixing changes with climatic conditions. In years with incomplete vertical mixing (does not reach the bottom) and subsequent strong thermal stratification resulting in profundal hypoxia, we observed increases in sedimentary CH 4 production and thus profundal CH 4 storage with the development of reducing conditions. In contrast, in years with strong winter vertical mixing to the bottom of the reservoir, CH 4 production was suppressed under NO 3 À -rich conditions, during which denitrifiers have the competitive advantage over methanogens. Diffusive emission from profundal CH 4 storage appeared to be negligible due to the efficiency of CH 4 oxidation during ascent through methane-oxidizing bacteria (MOB) activity. Most of the profundal CH 4 was rapidly oxidized by MOB in both oxic and anoxic layers, as characterized by its carbon isotope signature. In contrast, aerobic CH 4 production in the subsurface layer, which may be enhanced under high temperatures in summer, may account for a large portion of atmospheric CH 4 emissions from this reservoir. Our CH 4 profiling results provide valuable information for future studies predicting CH 4 emissions from subtropical lakes with the progress of global warming.

Food web properties of the recently constructed, deep subtropical Fei-Tsui Reservoir in comparison with the ancient Lake Biwa

“…Considering such emerging environmental issues, long-term ecosystem monitoring was launched in 2004. The physical and chemical environmental data we obtained have been published (Itoh et al, 2015;Chow et al, 2016).…”

“…A recent concern is the frequent release of typhoon-induced suspension interflows from the main tributary, increasing phosphorous loading and affecting water quality (Chen et al, 2006). Moreover, strong summer stratification and incomplete winter mixing due to warming have often caused hypoxia in profundal waters (Itoh et al, 2015), which may have non-linear and lethal effects on profundal communities. Considering such emerging environmental issues, long-term ecosystem monitoring was launched in 2004.…”

“…In the FTR, it has been reported that MOBs dominate profundal bacterial communities, showing remarkable vertical and seasonal variations in community compositions (Kojima et al, 2014;Kobayashi et al, 2016). Itoh et al (2015) experimentally demonstrated that the methane oxidation activity is highest in deep sub-oxic layers, in which both dissolved methane and oxygen are available to MOBs. In the deep FTR, methanotrophic food webs can be mediated through zooplankton vertical migration to feed on profundal MOBs, leading to the coupling of pelagic and profundal food webs.…”

“…Since original zoobenthos faunas were living in shallow and lotic environments before the construction of this reservoir, they may not be tolerant to deep anoxic conditions. The long-term monitoring revealed that the profundal habitat interannually alternates between anoxic and hypoxic conditions, depending on the intensity of winter vertical mixing under changing climates (Itoh et al, 2015). If there are some scopes for hypoxia-tolerant species to colonise populations from adjacent lakes, trophic carbon flows in profundal food webs may be modified over time.…”

Abundance of planktonic methane-oxidizing bacteria in a subtropical reservoir

“…Its surface area is 10.2 km 2 , and it can store 406 10 6 m 3 of water, when the water surface elevation is at 170 m above sea level. During the current study period, the elevation of the water surface ranged from 158 to 169 m above sea level, and the water depth at the sampling point was around 100 m. Water samples were collected with a 5-liter Go-Flo bottle 130 (General Oceanics, Miami, FL, USA) at the same sampling point (121°34′E, 24°54′N) that was used in previous studies (Kojima et al 2014, Itoh et al 2015. The samples for catalyzed reporter deposition-fluorescence in situ hybridization (CARD-FISH) were taken from 6 depths (0, 10, 30, 50, 70, 90 m) during October 2012 to February 2014 and additional water samples were obtained from depths of 80 m and 100 m. The water samples were fixed with 2% (w/v) paraformaldehyde and were stored in a cooler box until filtration (within 6 hours).…”