Carbon Dynamics on Floodplains of the Yangtze and Mekong Rivers

Abstract

The lateral expansion and contraction of rivers across their floodplains inextricably links aquatic and terrestrial ecosystem processes for part of each year, yet our understanding of the ecological responses to this seasonal hydrologic forcing is distinctly incomplete. The Flood Pulse Concept (FPC) predicts how in-situ primary production and respiration respond to this forcing. Although many of its predictions remain untested, the FPC is highly-cited and continues to guide hypotheses of ecosystem studies in tropical and subtropical flood-pulse rivers. In Chapter 1 of this dissertation, I reviewed the literature published from 1989 to 2019 on tropical rivers to provide an updated narrative of how primary production and respiration change in response to the seasonal flood-pulse. The literature shows that the in-situ respiration of a combination of aquatic and terrestrial organic carbon (C) exceeds primary production in tropical and subtropical flood-pulse rivers (i.e.,ecosystems are net heterotrophic). For the remainder of the dissertation, I propose that this net heterotrophy changes in response to flood-pulse hydrology and is sustained by both aerobic and anaerobic metabolism, contributing to the composition of dissolved C gases in water, atmospheric emissions of C gases, and the energetic base of aquatic food webs. I further propose that such cycling of C in flood-pulse rivers is fundamentally changed by hydropower development, which alters the magnitude and timing of the seasonal flood-pulse. Net heterotrophy within inland waters sustained by terrestrial-aquatic transfers of organic C and its subsequent, in-situ aerobic respiration constitutes one of the most significant findings by aquatic ecologists over the past thirty years (Cole et al., 1994; del Giorgio and Peters, 1994). One of the defining features of net heterotrophy within rivers and lakes is the oversaturation of dissolved carbon dioxide (CO2) relative to atmospheric levels as a byproduct of aerobic respiration. When “plumbing” the global C cycle, Cole et al. (2007) posited a unidirectional transfer of organic C from terrestrial to aquatic ecosystems, and assigned only three fates to this C: burial within sediments, aerobic respiration and diffusive loss of CO2 to the atmosphere, and export to the oceans. Recently, Abril and Borges (2019) proposed the bidirectional expansion and contraction of floodwaters over the terrestrial landscape as a major revision to the model presented by Cole et al. (2007). In Chapter 2, I hypothesized that the bidirectional expansion and contraction of rivers and lakes over the terrestrial landscape for months each year during the flood-pulse would create anaerobic floodplain habitat conducive to a fourth fate for organic C and additional cause of CO2 oversaturation within inland waters: methane (CH4) production and oxidation. I further hypothesized that net heterotrophy, dissolved CO2 and CH4, and their diffusive fluxes to the atmosphere would increase with water levels and floodplain inundation. I tested these hypotheses in Tonle Sap Lake (TSL), on a tributary of the Lower Mekong River in Southeast Asia. Each June through September, monsoonal rains in this river basin increase TSL levels by up to 7 m and lake surface area by up to 12,000 km2 (Holtgrieve et al., 2013). I measured the concentrations of respiratory gases (O2, CO2, CH4) and isotopic composition of CO2 and CH4 during different flood stages in the TSL and used these data to model aerobic ecosystem metabolism and diffusive fluxes of CO2 and CH4 from TSL to the atmosphere. Stable C isotopes demonstrate that 67-97 % of dissolved CO2 was derived from CH4 oxidation. Dissolved CO2 and CH4 were unrelated to aerobic respiration and inundation time, which shared a strong, negative relationship. The flood-pulse increased net heterotrophy and diffusion, from 3,700 ±500 mg C-CO2 m-2 d-1 and 0.5 ±0.2 mg C-CH4 m-2 d-1 during the rising-water stage to 10,000 ±1,000 mg C-CO2 m-2 d-1 and 110 ±40 mg C-CH4 m-2 d-1 during the high-water stage. These data highlight that anaerobic metabolism can play a key role in the oversaturation and diffusive flux of CO2 within rivers and lakes, particularly when these waters expand and contract across the terrestrial landscape. The apparent magnitude of CH4 production and oxidation in TSL suggested that this anaerobic energetic pathway may—at the same time—sustain net heterotrophy and introduce significant amounts of C to the base of the lake’s food web. Traces of methane-derived C within the biomass of chironomids (Jones et al., 1999; Bunn and Boon, 2003), zooplankton (Bastviken et al., 2003; Kankaala et al., 2006), and fish (Sanseverino et al., 2013) have been detected in other lake food webs using stable C isotopes. Yet, metabolism measured at the base of lake food webs has traditionally focused on photosynthesis and whole-ecosystem aerobic respiration (ER). In Chapter 3, I hypothesized that CH4 production would introduce C to the TSL food web at rates comparable to photosynthetic gross primary production (GPP), and that CH4 oxidation would transfer this C to higher trophic levels at rates comparable to net ecosystem production (NPP) until it was detectable in the food web using stable isotopes. I further hypothesized that rates of CH4 production, oxidation, and their contribution to ecosystem metabolism in TSL would increase with lake level and floodplain duration. CH4 production comprised up to 36 ±7 % of C introduced to the TSL food web through GPP and CH4 oxidation comprised up to 11 ±3 % of NPP. Rates of both CH4 production and oxidation were small compared to ER, suggesting that net heterotrophy in TSL is sustained by ER, despite sizeable contributions from CH4. CH4 production and oxidation were greatest during full flooding, but neither were correlated with floodplain duration as expected. Certain invertebrates and fish showed unambiguous methane-derived C within their biomass. Collectively, these results demonstrate that CH4 production and oxidation contribute to the overall energetic base of TSL. Flood-pulse rivers like the Lower Mekong are being altered by hydropower development at rates higher than anywhere else (Arias et al., 2013; Zarfl et al., 2015). Some of the highest rates of impoundment (>100 dams currently planned) are in China’s Yangtze River basin (Zarfl et al., 2015). The Three Gorges Reservoir (TGR) has notably altered the Yangtze River flood-pulse; water levels on the TGR floodplain now fluctuate by up to 30 m annually (Chen et al., 2009). During low reservoir storage, water levels near Kai Xian are much as they were before the TGR, presenting a proxy for pre-dam C cycling on the TGR floodplain. In Chapter 4, I measured this C cycling in terms of atmospheric CO2 and CH4 fluxes from aquatic environments (i.e., ponds) on the TGR floodplain during low reservoir storage, and from the reservoir, itself, following inundation and high reservoir storage. I hypothesized that the magnitudes and ecosystem drivers of these fluxes would change with hydrology between low reservoir storage and high reservoir storage. Like other studies (Del Sontro et al., 2016), we found that CH4 ebullition comprised a majority of CH4 fluxes (60-68 %) during low reservoir storage, but was two orders of magnitude lower during high reservoir storage. We also found that floodplain inundation by the TGR significantly moderated areal atmospheric CO2 and CH4 fluxes (diffusion and ebullition). Linear mixed effects modeling indicated that in-situ respiration was the dominant ecosystem driver of fluxes during both low and high reservoir storage. Thus, these data show that the magnitudes, but not the drivers, of atmospheric CO2 and CH4 fluxes have been altered along with Yangtze River flood-pulse hydrology by the TGR. A central theme that has emerged from the dissertation research presented here is the importance of anaerobic metabolism, specifically CH4 production and oxidation, within inland waters. Anaerobic metabolism has been largely ignored in studies of aquatic C cycling and ecosystem metabolism, with implications for C accounting in other flood-pulse and anaerobic ecosystems, worldwide. Collectively, this work demonstrates that CH4 production and oxidation contribute significantly to CO2 oversaturation, atmospheric C fluxes, and aquatic biota, challenging existing assumptions about terrestrial-aquatic transfers, net heterotrophy, and food web support within flood-pulse rivers and lakes.