The Tocantins River contributes ∼5% of the total flux of water to the Amazon River plume in the Atlantic Ocean. Here, we evaluate monthly variability in the composition and abundance of carbon, nitrogen, and suspended sediment in the lower reaches of the Tocantins River from 2014 to 2016. Dissolved organic carbon concentrations generally increased during periods of high discharge and are ∼1.5 times lower than average concentrations at the mouth of the Amazon River. Dissolved inorganic carbon similarly increased during periods of high discharge. Total dissolved nitrogen and individual nitrogen species followed a similar temporal pattern, increasing during high water.
Torrential rain and extreme flooding caused by Atlantic hurricanes mobilize a large pool of organic matter (OM) from coastal forested watersheds in the southeastern United States. However, the mobilization of toxic metals such as mercury (Hg) that are associated with this vast pool of OM are rarely measured. This study aims to assess the variations of total Hg (THg) and methylmercury (MeHg) levels and the isotopic compositions of Hg in a blackwater river (Waccamaw River, SC, U.S.A.) during two recent extreme flooding events induced by Hurricane Joaquin (October 2015) and Hurricane Matthew (October 2016). We show that extreme flooding considerably increased filtered THg and MeHg concentrations associated with aromatic dissolved organic matter. During a 2‐month sampling window each year (October–November), we estimate that about 27% (2015) and 78% (2016) of the average amount of Hg deposited atmospherically during these 2 months was exported via the river. The isotopic composition of Hg in the river waters was changed only slightly by the substantial inputs of runoff from surrounding landscapes, in which mass‐dependent fractionation (as δ202Hg) decreased from −1.47 to −1.67‰ and mass‐independent fractionation (as ∆199Hg) decreased from −0.15 to −0.37‰. The slight variations in Hg isotopic composition during such extreme flooding events imply that sources of Hg in the river are nearly unchanged even under the very high wet deposition of Hg derived from the intensive rainfall. The majority of Hg exported by the river (74–85%) is estimated to have been derived from dry deposition to the watersheds. An increase in frequency and intensity of Atlantic hurricanes is expected in the next few decades due to further warming of ocean surface waters. We predict that increased hurricanes will mobilize more dry‐deposited Hg and in situ produced MeHg from these coastal watersheds where MeHg can be extensively bioaccumulated and biomagnified in the downstream aquatic food webs.
more » « less- NSF-PAR ID:
- 10456971
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Limnology and Oceanography
- Volume:
- 65
- Issue:
- 9
- ISSN:
- 0024-3590
- Page Range / eLocation ID:
- p. 2158-2169
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract predominated the dissolved inorganic nitrogen pool, followed by , and , characteristic of environments with a relatively low anthropogenic impact. Dissolved fractions represented 92% of the total carbon exported and 78% of the total nitrogen exported. The suspended particulate sediment flux was 2.72 × 106 t yr−1, with fine suspended sediment dominating (71.3%). Concentrations of carbon relative to nitrogen indicate a primarily terrigenous source of organic matter and CO2derived from in situ respiration of this material during the rainy season and a primarily algal/bacterial source of organic matter during the dry season. Considering past estimates of dissolved carbon and nitrogen fluxes from the Amazon River to the Atlantic Ocean, the Tocantins River contributes 3% and 3.7% to total fluxes to the Amazon River plume region, respectively. While this contribution is relatively small, it may be influenced by future changes to the basin's land use and hydrology. -
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