Tidally Driven Export of Dissolved Organic Carbon, Total Mercury, and Methylmercury from a Mangrove-Dominated Estuary

Abstract

The flux of dissolved organic carbon (DOC) from mangrove swamps accounts for 10% of the global terrestrial flux of DOC to coastal oceans. Recent findings of high concentrations of mercury (Hg) and methylmercury (MeHg) in mangroves, in conjunction with the common co-occurrence of DOC and Hg species, have raised concerns that mercury fluxes may also be large. We used a novel approach to estimate export of DOC, Hg, and MeHg to coastal waters from a mangrove-dominated estuary in Everglades National Park (Florida, USA). Using in situ measurements of fluorescent dissolved organic matter as a proxy for DOC, filtered total Hg, and filtered MeHg, we estimated the DOC yield to be 180 (±12.6) g C m–2 yr–1, which is in the range of previously reported values. Although Hg and MeHg yields from tidal mangrove swamps have not been previously measured, our estimated yields of Hg species (28 ± 4.5 μg total Hg m–2 yr–1 and 3.1 ± 0.4 μg methyl Hg m–2 yr–1) were five times greater than is typically reported for terrestrial wetlands. These results indicate that in addition to the well documented contributions of DOC, tidally driven export from mangroves represents a significant potential source of Hg and MeHg to nearby coastal waters.

Dissolved carbon biogeochemistry and export in mangrove-dominated rivers of the Florida Everglades

“…There are no previously published DIC inventories or fluxes for the Shark and Harney rivers, so comparison with previous studies is focused on the DOC results. The DOC flux from the Shark River to the coastal ocean in SharkTREx 1 (7.5 ± 0.2 × 10 5 mol d −1 ) is in very good agreement to that estimated by Bergamaschi et al (2011) in an experiment conducted in the Shark River from 20 to 30 September 2010 (7.6 ± 0.5 × 10 5 mol d −1 ). However, the net discharge during the Bergamaschi et al (2011) study was higher than SharkTREx 1 (mean ± SD: 9.1 ± 7.1 vs. 6.9 ± 5.3 m 3 s −1 ), which would lead to a shorter residence time of 4.6 days using a relationship presented in Ho et al (2016).…”

“…Calculations using the shorter residence time and higher DOC inventory yields a DOC flux of 9.7 ± 0.2 × 10 5 mol d −1 , which is ca. 30 % higher than the estimates of Bergamaschi et al (2011).…”

“…However, the net discharge during the Bergamaschi et al (2011) study was higher than SharkTREx 1 (mean ± SD: 9.1 ± 7.1 vs. 6.9 ± 5.3 m 3 s −1 ), which would lead to a shorter residence time of 4.6 days using a relationship presented in Ho et al (2016). Using the DOC concentration data presented in Bergamaschi et al (2011) yields an inventory that is ca. 3 % higher than the DOC inventory in Shark River during SharkTREx 1.…”

“…These values indicate the export of dissolved carbon from flooded portions of the forest during the discontinuous inundation periods should be significantly greater than the dissolved carbon lateral export rate derived above in order to produce the observed inventories of mangrovederived dissolved carbon in the main channel. Bergamaschi et al (2011) proposed an annual total DOC export from the forest surrounding Shark River of 15.1 ± 1.1 mol m −2 yr −1 and describe their method of calculating contributing area using a model based on the relationship between discharge volume and changes in water levels during tidal cycles. They do not provide a contributing area, but this can be calculated from their results.…”

“…5 % of the total longitudinal DOC fluxes were of mangrove origin, with an average mangrovederived DIC to DOC flux ratio of 10.5. Using this information, the Bergamaschi et al (2011) results were recalculated to yield a wet season dissolved carbon lateral export rate of 46.5 ± 4.4 mmol m −2 d −1 (as DIC and DOC) from the forest.…”