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 CO2 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 CO2 (pCO2). Our results indicated that the seasonal and spatial variability of chlorophyll concentrations (Chl) and pCO2 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 hydro\\-dynamic 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 CO2 fluxes throughout the reservoir. During periods of high retention, the average CO2 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 CO2 fluxes results in the reservoir acting as a CO2 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 pCO2 spatial variability than the in-system ecological factors.
Highlights
Over the last two decades, hydropower reservoirs have been identified as potentially important sources of greenhouse gas (GHG) emissions (St Louis et al, 2000; Rosa et al, 2004; Demarty et al, 2011)
We investigated the relationships between phytoplanktonic biomass and water–air CO2 fluxes in an old, stratified tropical reservoir (Funil, state of Rio de Janeiro, Brazil) where intense phytoplankton blooms and low pressure of CO2 (pCO2) are observed in the water
The riverine zone (RZ) has a high input of nutrients coming from terrestrial systems and human activities, but primary production is limited by high turbidity and turbulence
Summary
Over the last two decades, hydropower reservoirs have been identified as potentially important sources of greenhouse gas (GHG) emissions (St Louis et al, 2000; Rosa et al, 2004; Demarty et al, 2011). High temperatures and forest flooding have intensified GHG emissions (Abril et al, 2005; Fearnside and Pueyo, 2012). Emissions are larger in tropical Amazonian (Abril et al, 2013) than in tropical non-Amazonian reservoirs (Ometto et al, 2011) and are larger in younger than in older reservoirs (Barros et al, 2011). Those created by impounded rivers, are morphometrically complex. Pacheco et al.: The effects of river inflow and retention time
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