The effects of river inflow and retention time on the spatial heterogeneity of chlorophyll and water–air CO₂ fluxes in a tropical hydropower reservoir

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.