Extraordinary methane levels in a small bar-built tropical estuary

Small estuaries link uplands and nearshore marine environments but are understudied in many regions. We quantified spatial and temporal variability in CH₄, CO₂, nutrients, and organic matter in a small, deep, bar-built tropical estuary in Fajardo, Puerto Rico. The estuary was typically impounded and strongly stratified at all three sampling locations. Across 5 sampling dates, the pycnocline was sharp, with salinity of 0 in the upper meter and up to 17 ppt at 3–5 m. Methane concentrations at the surface were extraordinarily high for a mangrove-fringed tropical estuary, averaging from 26 to 59 μM at three stations. Surface methane flux was 17.1 mM/m²/day. Methane concentrations were high throughout the water column, declined temporarily following estuarine flushing that increased salinity, and were correlated with CO₂ (r² = 0.48, p < 0.01). Small bar-built estuaries are common in Puerto Rico and other high-wave energy environments and should be included in assessments of greenhouse gas emissions.