The effect of habitat change on nutrient removal in the Atchafalaya River Basin, Louisiana
The Mississippi and Atchafalaya rivers are the major sources of freshwater and nutrients to the Gulf of Mexico. Increased nutrient loads from these rivers exacerbate eutrophication in coastal receiving waters and contribute to the large area of hypoxia that develops seasonally in the Gulf. Levees along the Mississippi River have reduced contact between the river and the historic floodplain; this limits the ability of floodplain wetlands to naturally mitigate excess nutrients. However, the Atchafalaya River diverges from the Mississippi 217 km from the Gulf and enters a large river floodplain with a widely spaced levee system. This enhances the ability of the Atchafalaya River Basin to remove and sequester nutrients, potentially reducing downstream eutrophication. Overbank flow spreads river-water and sediment across the floodplain. Over time, sedimentation has filled in many of the open water areas on the floodplain, such that lakes are transitioning to baldcypress swamps and bottomland hardwood forests. These habitats differ in their available nutrient reservoirs and the rates at which they transform and store nutrients. This dissertation investigated the major retention and removal mechanisms for carbon, nitrogen, and phosphorus within bottomland hardwood forests, baldcypress swamps, and lakes of the Atchafalaya River Basin. These reservoirs include denitrification, sedimentation, and assimilation by aboveground biomass. Results estimate that nutrient retention and removal within bottomland hardwood forests ranges from 1,177,605—1,561,805 t C yr-1, 46,049—47,603 t N yr-1 and 20,040—20,175 t P yr-1. Within baldcypress swamps, rates range from 493,953—600,180 t C yr-1, 21,821—22,364t N yr-1 and 2,168—2,202 t P yr-1. Rates in the lakes were 57,490 t C yr-1, 5,140—5,390 t N yr-1 and 2,550 t P yr-1. Total retention and removal for the entire basin is on the order of 1,177,605—1,561,805 t C yr-1, 46,049—47,603 t N yr-1, and 20,040—20,175 t P yr-1. Rates varied by habitat, highlighting the need to consider habitat change when developing management strategies to improve water quality. Data from this dissertation can be used to parameterize nutrient models for the Atchafalaya River Basin, as well as for river diversions and floodplains with similar habitat types.
nitrogen, phosphorus, denitrification, carbon, wetlands, floodplain
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