Organic carbon sedimentation dominates over CO2 emission in two net heterotrophic Mediterranean reservoirs during stratification

Abstract

Freshwater reservoirs are significant components of the global carbon cycle. However, relatively few studies address the balance between the atmospheric carbon flux and sedimentation in these ecosystems. We performed a simultaneous analysis of the sedimentation fluxes of particulate organic carbon (SPOC) and atmospheric CO2 fluxes ( $$ {\text{F}}_{{{\text{CO}}_{ 2} }} $$ ) in the epilimnetic zone of two monomictic Mediterranean reservoirs. CO2 partial pressure (pCO2) was monitored hourly, and SPOC was measured at biweekly intervals with sediment traps during the stratification period. Both reservoirs were zones of intense organic carbon sedimentation and CO2 emission to the atmosphere despite having different trophic statuses. The positive relationship observed between SPOC and $$ {\text{F}}_{{{\text{CO}}_{ 2} }} $$ was likely due to the inputs of organic matter and nutrients to the epilimnion, which increased both sedimentation and CO2 emission. Nonetheless, the stronger effect on the sedimentation process led to a net downward flux of carbon in spite of the heterotrophic nature of both reservoirs.