Greenhouse gas dynamics in tropical montane streams of Puerto Rico and the role of watershed lithology

The major greenhouse gases in streams and rivers, carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), can contribute significantly to regional greenhouse gas (GHG) budgets, and each appears to be responding to multiple drivers. Recent work suggests that tropical water bodies may be hot spots of GHG emissions due to high primary productivity in their watersheds, but tropical streams and rivers have historically been underrepresented in studies of GHG concentration and emissions. We use a five-year record of weekly water chemistry and dissolved gas data from eight tropical watersheds of varying lithology and redox conditions in the Luquillo Mountains of Puerto Rico to examine controls on GHG variability and estimate gas flux. Streams were frequently supersaturated in all three gases indicating that streams in this tropical landscape are sources of GHGs to the atmosphere. Concentrations of CO2 and N2O were associated with lateral inputs from the surrounding landscape, whereas CH4 concentrations correlated with in-stream oxygen availability and lithology. Our results underscore the importance of including tropical sites in global syntheses and budgets and the role of both in-stream biological and physical processes as well as landscape attributes that contribute to the export of gases to the fluvial network and atmosphere.