Impact of Wetland Decline on Decreasing Dissolved Organic Carbon Concentrations along the Mississippi River Continuum

Abstract

Prior to discharging to the ocean, large rivers constantly receive inputs of dissolved organic carbon (DOC) from tributaries or fringing floodplains and lose DOC via continuous in situ processing along distances that span thousands of kilometers. Current concepts predicting longitudinal changes in DOC mainly focus on in situ processing or exchange with fringing floodplain wetlands, while effects of heterogeneous watershed characteristics are generally ignored. We analyzed results from a 17-year time-series of DOC measurements made at seven sites and three expeditions along the entire Mississippi River main channel with DOC measurements made every 17 km. The results show a clear downstream decrease in DOC concentrations that was consistent throughout the entire study period. Downstream DOC decreases were primarily (∼63-71%) a result of constant dilutions by low-DOC tributary water controlled by watershed wetland distribution, while in situ processing played a secondary role. We estimate that from 1780 to 1980 wetland loss due to land-use alterations caused a ca. 58% decrease in in DOC concentrations in the tributaries of the Mississippi River. DOC reductions caused by watershed wetland loss likely impacted the capacity for the river to effectively remove nitrogen via denitrification, which can further exacerbate coastal hypoxia. These findings highlight the importance of watershed wetlands in regulating DOC longitudinally along the headland to ocean continuum of major rivers.

Spatial patterns of DOC concentration and DOM optical properties in a Brazilian tropical river‐wetland system

“…Although our wet season sampling was conducted after the peak in the annual hydrograph, our data shows a longitudinal increase in DOC concentrations (Table 2 and Figure 5), partly due to the increase in hydrological connectivity that promotes the longitudinal input of DOC from the tributaries [Singh et al, 2015;Lambert et al, 2016b]. The importance of wetlands as a constant source of DOC is documented by Duan et al [2017], who found that downstream decreases in DOC for the Mississippi River were due to the loss of wetlands caused by changes in land use.…”

“…The importance of wetlands as a constant source of DOC is documented by Duan et al . [], who found that downstream decreases in DOC for the Mississippi River were due to the loss of wetlands caused by changes in land use.…”

Where Carbon Goes When Water Flows: Carbon Cycling across the Aquatic Continuum

“…For example, alterations to floodplain networks can reduce aquatic DOC levels, which limits the presence of environments that promote a reduction in nutrient concentrations via denitrification (Philip and Townsend, 2010). Anthropogenic hypoxia is often attributed to agricultural inputs of fertilizers (Rabotyagov et al, 2014), but wetland loss in managed watersheds such as the Mississippi River is likely another important factor to consider (Mitsch et al, 2005;Schramm et al, 2009;Duan et al, 2017). Likewise, increased export of terrestrially-derived OM due to watershed perturbations and its subsequent decomposition in coastal waters is not currently considered to be a factor driving hypoxia, but may play an important role.…”

Environmental Dynamics of Dissolved Organic Matter and Dissolved Black Carbon in Fluvial Systems: Effects of Biogeochemistry and Land Use

“…While DOM processing through photochemistry is expected to decrease in the lower watershed due to increasing turbidity, the association of C4 with higher stream order compared to land use indicates to a small extent that photochemical alteration of DOM in the lower Altamaha River watershed might still be pertinent. We were able to show with RDA that a combination of both watershed land use and the traditional RCC could explain roughly 50% of the spatial variability of DOM composition in the Altamaha River watershed as described by both EEM-PARAFAC and FTICR-MS. Just as watershed land use has been described as good predictors of DOM composition in other systems [83][84][85] , it also explained a higher degree of variability in the Altamaha River watershed compared stream order, highlighting the need to further consider the controls of land use on DOM composition in future adaptations of the RCC.…”

“…Longitudinal shifts in DOM quality have been linked to clear shifts in watershed land cover80 , but more specifically to watershed connectivity with wetlands[81][82] . In the Mississippi River, loss of wetlands downstream generally resulted DOM becoming less aromatic in character83 . In fact, wetlands are generally considered good predictors of DOC concentration[84][85] as well as DOM composition where a higher proportion of aromatic, humic like material are sourced from areas with high degrees of wetland cover29,86 .…”