Dissolved greenhouse gases in streams and their response to Hurricanes Irma and María in a tropical forest in Puerto Rico

Large-scale disturbances such as hurricanes can drastically change many ecosystem dynamics in forested watersheds. Hurricane-force winds can deposit a large amount of organic matter into stream channels, open up the canopy above the riparian zone, and deliver sufficient precipitation to scour primary producer communities from the stream bed. Surface water samples were taken from tropical montane streams at weekly intervals across 8 sites in the Luquillo Experimenal Forest (LEF) in Puerto Rico before and after Hurricanes Irma and Maria. Samples were analyzed for carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) as well as dissolved organic carbon (DOC), nitrate (NO3) and other major cations and anions. Additionally, a set of sensors and gaging stations provided data for discharge, dissolved oxygen, conductance, pH, NO3, and fluorescent dissolved organic matter (fDOM) at 15-minute intervals. Results show that CH4 nearly doubled in concentration immediately after the hurricanes at two of our sites, while concentrations were unchanged at the others. CO2 concentrations showed a slight increase in the weeks following the hurricanes but returned to pre-hurricane levels soon after. Sites with the strongest diel pattern for dissolved oxygen post-hurricane had the largest responses in greenhouse gases. N2O showed a strong negative correlation with NO3 concentrations post-hurricane, which we had not seen in pre-hurricane data. Our site-specific results suggest that while CH4 and N2O responses could be driven mainly by decomposing and leaching organic matter, CO2 responses could be driven by changes in canopy cover and shifts in primary producer communities after hurricane disturbance in these tropical streams.