Abstract
The Mississippi River experienced historic flooding during 2019, inducing >150 days of floodplain wetland inundation. We evaluated flood effects using repeated measures of hydrogeomorphic (HGM) wetland assessment variables prior to the flood (October 2018), immediately post-flood (August 2019) and one year after initial assessment (October 2019). The flood had little/no impact on 11 of 13 assessment variables, but altered the abundance of woody debris and forest floor litter. Immediately after the flood, these changes decreased the functional capacity of wetlands to 1) detain floodwater (mean − 9.7% reduction) and 2) precipitation (−17.3%); 3) cycle nutrients (−7.5%); and export organic carbon (−23.8%). Subsequent sampling documented the detain precipitation function returning to pre-flood conditions. The export organic carbon function also improved, yet remained below pre-flood levels. Other functions will likely require additional recovery time due to the persistence of accumulated excess woody debris. Across all sample intervals, floodplain wetlands displayed high wetland function capacities and appear resilient to surface water inundation. This analysis highlights the utility of the HGM assessment to detect responses to changing environmental conditions over short time intervals. The study also emphasizes the need to incorporate metrics with appropriate impact-response characteristics when developing and implementing ecological assessments.
Highlights
The Mississippi River watershed conveys water from 41% of the conterminous United States, representing the world’s fourth largest river system
Forested wetlands in the Mississippi River valley provide a wide variety of ecological functions related to hydrology, habitat, and biogeochemical cycling that in turn benefit society through regulation of flooding, provide economic and recreational value, and improve water quality (Smith and Klimas 2002)
The HGM wetland functional assessment developed by Murray and Klimas (2013) was applied at 35 locations within the active floodplain of the mainline Mississippi river levee system
Summary
The Mississippi River watershed conveys water from 41% of the conterminous United States, representing the world’s fourth largest river system. The constriction of the river to a narrow floodplain results in more erratic flow regimes, more frequent major floods, and fewer years with stable water levels (Sparks et al 1998) These altered flood regimes have important implications for faunal populations, vegetation communities and soil characteristics (Jones et al 2019). We applied the hydrogeomorphic (HGM) wetland functional assessment approach prior to the onset of 2019 flooding (October 2018) and at two sampling intervals following floodwater recession (August 2019 and October 2019). The objectives of the study included 1) determine if the HGM approach detected flood effects and identify associated assessment variables, 2) document implications for wetland functions, and 3) evaluate potential functional recovery during short (< 1 year) time frames. Post-hoc testing using Wilcoxon signed-rank test with Bonferroni-adjustment (α < 0.017) identified differences between sample intervals (preflood vs. post-flood; pre-flood vs. post-flood; post-flood vs. post-flood2)
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