Plant And Soil Responses To Sediment Deposition And Nutrient Enrichment In Healthy, Deteriorating, And Newly Created Coastal Marshes In Barataria Basin, Louisiana: Implications For Mississippi River Sediment Diversions

Abstract

To offset wetland loss in the Mississippi River Delta, sediment diversions that will re-introduce river water and sediment into wetlands are being planned for the lower Mississippi River. River diversions will also deliver high nutrient loads, which may reduce belowground plant productivity, reducing inputs of organic matter important for marsh accretion to keep pace with sea-level rise. However, belowground productivity responses to the combinatory effects of sediment and nutrients are unknown. To test the hypotheses that nutrient enrichment and sediment deposition interact to influence vegetation structure, belowground plant productivity and decomposition, and surface accretion, a field experiment was implemented in three marshes in Barataria Bay, LA (healthy oligohaline [OL], brackish deteriorating [DT] and intermediate created [CR]) across experimental plots receiving one of six treatments: control, low nutrients (LN; 100 g N m-2 yr-1 and 11g P m-2 yr-1), high nutrients (HN; 1000 g N m-2 yr-1 and 50 g P m-2 yr-1), sediment addition (47 kg m-2 [5 cm elevation increase]), and their combination (n = 5). Additionally, these treatments may alter rates of greenhouse gas (GHG) emissions such as CO2 and CH4. Within the oligohaline marsh (n = 3), CO2 and CH4 were measured using the static chamber method to test the hypothesis that GHG production rates differ under sediment deposition and nutrient enrichment as compared to controls. Treatment effects on vegetation structure were marsh specific, influencing species richness in OL and plant height in DT. Belowground productivity of individual root types were affected by treatment with increased dead biomass accumulation across all marshes with LN and LNS treatments, and increased fine root growth with HNS treatments in OL. Decay rates were also increased by LN treatments across marshes, although this did not affect GHG production across treatments or percent material remaining across marshes. Accretion rates across marshes varied, although treatment did not affect surface accretion rates in any of the marshes. These findings demonstrate that sediment, nutrients and their combination differently effect wetland plant and soil dynamics across marsh types, giving insight into how Mississippi River diversions will affect marsh resilience to sea level rise and additional factors.