Abstract
Tidal saline wetlands include mangrove swamps, seagrass beds, and tidal salt marshes situated on coastal margins receive both terrestrial and oceanic energy subsidies. These wetlands are increasingly being credited for their ability to produce and sequester large amounts of organic carbon, despite their small spatial extent compared to the vast open ocean. This study presents research on carbon sequestration rates along two tidal mangrove creeks near Naples Bay in Southwest Florida. One tidal creek is hydrologically disturbed and the other is a relatively pristine reference creek. Urban development, dredge spoils, diverted freshwater, and upstream land use changes have altered the hydrology and sediment dynamics of the hydrologically disturbed creek. As a result, it was hypothesized that the disturbed tidal creek sequesters less carbon than our relatively pristine reference creek. Three soil cores were collected in a basin, dwarf, fringe, and riverine hydrogeomorphic settings and analyzed for total organic carbon and inorganic carbon profiles. Detection of radionuclides 137Cs and 210Pb in the soil cores was used to estimate recent sediment accretion and carbon sequestration. The majority of carbon found within our study was organic (89 - 99%). The mean carbon sequestration rates for the tidal creeks on the Naples Bay (98 ± 49 g C m-2 yr-1 (n = 18)) is low compared to published global means for mangrove wetlands. The mean carbon sequestration rates in the reference riverine setting were highest (162 ± 9.6 g C m-2 yr-1), followed by rates in the reference fringe and disturbed riverine settings (127 ± 58 g C m-2 yr-1 and 125 ± 10 g C m-2 yr-1, respectively). The disturbed fringe exhibited a sequestration rate of 73 ± 18 g C m-2 yr-1, while rates within the basin settings were 50 ± 7 and 47 ± 7, for the reference and disturbed creeks, respectively. Our hypothesis, that the hydrologically disturbed creek has sequestered less carbon than the reference creek proved correct; mean values of 113 and 82 g C m-2 yr-1, for the reference and disturbed creeks, respectively, were significantly different. [f (1) = 6.39, p = 0.0265]. In addition, using the 210Pb-calculated rates of sedimentation estimated in this study, only two of the seven sampling sites that we investigated may be able to persist, given the current rates of sea-level rise rates for this region in Southwest Florida of 2.4 mm yr-1. This study has shown that hydrologic disturbance and location within the landscape of mangrove wetlands significantly affects the amount of carbon sequestered.