The effects of river inflow and retention time on the spatial heterogeneity of chlorophyll and water–air CO<sub>2</sub> fluxes in a tropical hydropower reservoir

Abstract

Abstract. Abundant research has been devoted to understanding the complexity of the biogeochemical and physical processes that are responsible for greenhouse gas (GHG) emissions from hydropower reservoirs. These systems may have spatially complex and heterogeneous GHG emissions due to flooded biomass, river inflows, primary production and dam operation. In this study, we investigated the relationships between the water-air CO 2 fluxes and the phytoplanktonic biomass in the Funil Reservoir, which is an old, stratified tropical reservoir that exhibits intense phytoplankton blooms and a low partial pressure of CO 2 (pCO 2 ). Our results indicated that the seasonal and spatial variability of chlorophyll concentrations (Chl) and pCO 2 in the Funil Reservoir are related more to changes in the river inflow over the year than to environmental factors such as air temperature and solar radiation. Field data and hydrodynamic simulations revealed that river inflow contributes to increased heterogeneity during the dry season due to variations in the reservoir retention time and river temperature. Contradictory conclusions could be drawn if only temporal data collected near the dam were considered without spatial data to represent CO 2 fluxes throughout the reservoir. During periods of high retention, the average CO 2 fluxes were 10.3 mmol m −2 d −1 based on temporal data near the dam versus −7.2 mmol m −2 d −1 with spatial data from along the reservoir surface. In this case, the use of solely temporal data to calculate CO 2 fluxes results in the reservoir acting as a CO 2 source rather than a sink. This finding suggests that the lack of spatial data in reservoir C budget calculations can affect regional and global estimates. Our results support the idea that the Funil Reservoir is a dynamic system where the hydrodynamics represented by changes in the river inflow and retention time are potentially a more important force driving both the Chl and pCO 2 spatial variability than the in-system ecological factors.

Spatiotemporal variability of lake pCO2 and CO2 fluxes in a hemiboreal catchment

“…As few studies have assessed the variability and the key drivers for pCO 2aq and CO 2 fluxes from lakes integrating spatiotemporal variability, there is a great risk that present assessments are biased. For example, previous intensive pCO 2aq measurements were typically designed to either cover large space under a short time (gas equilibrator screening or manual samples in several points [e.g., Borges et al, 2014;Pacheco et al, 2015;Roland et al, 2010]) or long time in specific locations (sensors collecting data or calculated pCO 2aq from one or a few points over time [e.g., Åberg et al, 2010;Huotari et al, 2009;Pacheco et al, 2015]). Therefore, studies including both spatial and temporal variabilities are needed to better understand variability and to develop representative approaches for large-scale assessments of lake CO 2 fluxes.…”

“…While previous studies over short time periods (1 to 6 h) observed gas transfer velocity (k) to be more variable than pCO 2aq , our work shows that over longer time (days to weeks) pCO 2aq variability was greater and affected CO 2 fluxes much more than k. We demonstrate that ≥8 measurement days distributed over multiple seasons in combination with sufficient spatial coverage (≥8 locations during stratification periods and 5 or less in spring and autumn) are a key for representative yearly whole lake flux estimates. This study illustrates the importance of considering spatiotemporal variability in pCO 2aq and CO 2 fluxes to generate representative whole lake estimates.Spatial variability in pCO 2aq and CO 2 fluxes are now increasingly addressed, e.g., in tropical and boreal reservoirs [Pacheco et al, 2015;Roland et al, 2010;Teodoru et al, 2011] and Amazon floodplain lakes [Rudorff et al, 2011], and have been linked to river inputs, littoral zones, and photosynthesis. The variability of k with fetch or NATCHIMUTHU ET AL.SPATIOTEMPORAL VARIABILITY OF LAKE CO 2 30 PUBLICATIONS…”

“…Spatial variability in pCO 2aq and CO 2 fluxes are now increasingly addressed, e.g., in tropical and boreal reservoirs [Pacheco et al, 2015;Roland et al, 2010;Teodoru et al, 2011] and Amazon floodplain lakes [Rudorff et al, 2011], and have been linked to river inputs, littoral zones, and photosynthesis. The variability of k with fetch or NATCHIMUTHU ET AL.…”

A model for the data extrapolation of greenhouse gas emissions in the Brazilian hydroelectric system

“…Studies on greenhouse gases emissions from manmade reservoirs began in the 1990s after the publication of Kelly et al [14], followed by other research [12,13,[18][19][20], and went on to analyze the spatial variations in reservoirs, the sources of organic matter and the processes involved [1,5,10,15,21,24,25].…”

Water quality longitudinal profile of the Paraíba do Sul River, Brazil during an extreme drought event

“…However, considerable variation exists in the relationship between land use and watershed nutrient export (Puckett 1995;Harding et al 1998;Findlay et al 2001). Therefore, assessing the linkage between LULC and water quality is essential for developing watershed management practices (Uuemaa et al 2005;Wan et al 2014) and controlling land based pollution in rivers, reservoirs (Soares et al 2008;Pacheco et al 2015), and coastal bays (Howarth and Marino 2006).…”