Extraordinary methane levels in a small bar‐built tropical estuary

Abstract Small estuaries link uplands and nearshore marine environments but are understudied in many regions. We quantified spatial and temporal variability in CH 4 , CO 2 , 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 μ M at three stations. Surface methane flux was 17.1 mM/m 2 /day. Methane concentrations were high throughout the water column, declined temporarily following estuarine flushing that increased salinity, and were correlated with CO 2 ( r 2 = 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.