Enhanced transfer of terrestrially derived carbon to the atmosphere in a flooding event

Abstract

Rising CO2 concentration in the atmosphere, global climate change, and the sustainability of the Earth's biosphere are great societal concerns for the 21st century. Global climate change has, in part, resulted in a higher frequency of flooding events, which allow for greater exchange between soil/plant litter and aquatic carbon pools. Here we demonstrate that the summer 2011 flood in the Mississippi River basin, caused by extreme precipitation events, resulted in a “flushing” of terrestrially derived dissolved organic carbon (TDOC) to the northern Gulf of Mexico. Data from the lower Atchafalaya and Mississippi rivers showed that the DOC flux to the northern Gulf of Mexico during this flood was significantly higher than in previous years. We also show that consumption of radiocarbon‐modern TDOC by bacteria in floodwaters in the lower Atchafalaya River and along the adjacent shelf contributed to northern Gulf shelf waters changing from a net sink to a net source of CO2 to the atmosphere in June and August 2011. This work shows that enhanced flooding, which may or may not be caused by climate change, can result in rapid losses of stored carbon in soils to the atmosphere via processes in aquatic ecosystems.

Carbon dioxide dynamics and fluxes in coastal waters influenced by river plumes

“…It ranged from about 1,000 µatm in January to 2,200 µatm in July at the Mississippi river mouth and in the Atchafalaya Bay and other coastal bays (based on unpublished data from more than 10 cruises conducted by the Cai laboratory). Once outside the river channels, salinity increases and pCO 2 decreases rapidly, except in the case of extreme flooding conditions near the Atchafalaya Bay ( Bianchi et al, 2013). pCO 2 can be reduced to below 100 µatm with an increase in pH to above 8.5; these values are close to theoretical predictions for a complete biological use of riverine NO 3 − (Fig.…”

Increases in terrestrially derived carbon stimulate organic carbon processing and CO2 emissions in boreal aquatic ecosystems

“…1.5 Pg y À 1 ), these potential shifts could significantly alter the fine balance that exists between freshwater CO 2 emissions and the estimated continental (2.8 Pg C y À 1 ) and oceanic carbon sinks (2.2 Pg C y À 1 ) 4 . Although more focused measurements should be undertaken to adequately quantify the magnitude of the enhanced aquatic CO 2 emissions at continental or global scales, the scenario that we propose here undoubtedly involves the mobilization and degradation of terrestrial organic C, which, contrary to historical assumptions, appears to be highly reactive in freshwaters but would otherwise experience much slower degradation in more stable environments such as soil or oceanic refuges 1,44,45 . The absence of evidence for generalized long-term declines in discharge, and the positive relationships that have been reported between long-term trends in discharge and DOC concentrations in landscapes where browning is happening 11,12,43 , suggest that greater amounts of DOC are circulating in inland waters today compared with a few decades ago, yet no increase in the export of DOC from continents to the oceans have been observed so far in regions where browning is happening 46 .…”

Riverine DOM

“…The largest storms can be responsible for upwards of half of long-term annual average DOC fluxes (Schiff et al, 1998;Yoon and Raymond, 2012), and due to the bypassing of watershed filters, have impacts on inland and coastal carbon fluxes (Bianchi et al, 2013;Klug et al, 2012). The predicted increased proportion of annual rainfall that will be delivered with large events forced by climate change (IPCC, 2012) should therefore significantly impact these lateral fluxes (Jeong et al, 2012;Sebestyen et al, 2009) and coastal ocean processes.…”