Methane emissions from mangrove soils in hydrologically disturbed and reference mangrove tidal creeks in southwest Florida

Studies have suggested that some mangrove soils might contribute to wetland methane (CH4) production and emissions, especially when the mangroves are disturbed. CH4 emissions were measured seasonally from nine locations on two mangrove creeks on Naples Bay in southwest Florida, USA. One of the tidal creeks has been impacted in the past few decades with alteration of upstream watersheds and freshwater inflows; the other creek was an un-impacted reference tidal creek. Our study sites were established along a longitudinal gradient (distance to the cree.s mouth) with different plant communities and freshwater influences, which were further evaluated by measurements during the dry and wet seasons. CH4 emission measurements were also performed during the flood and ebb tides (n = 6) to incorporate the influence of water level fluctuations on CH4 emissions. Additionally, hydroperiods and soil pore water electrical conductivity (EC) as a measure of salinity were measured along the study creeks. Our study showed very small, if not negligible, CH4 emissions from mangrove soils in this southwest Florida location at all sampling locations and sampling times. Despite our collecting over 1900 methane samples from methane emission chambers, most analyses of rates of change in the chambers showed no methane emissions. Seasonal averages ranged from 0.24 to 1.68 mg CH4-C m(-2) d(-1) (annual average of 0.32 g CH4-C m(-2) y(-1)). Ironically, the lowest methane emissions were at the end (December-January) of a typical wet season of daily rainfall and were highest in the dry season (March-April) when freshwater inputs from watersheds and precipitation were negligible. Water level fluctuations, freshwater inputs, and plant species composition did not play a significant role in CH4 emissions. There appeared to be a slight pattern of methane emissions versus air temperature but the relationship was not linear. Combining our results with carbon sequestration rates in a companion study suggested that mangroves in southwest Florida are clearly net sinks of both carbon and radiative forcing and therefore beneficial for mitigating climate warming.