Biogeochemical response of organic-rich freshwater marshes in the Louisiana delta plain to chronic river water influx

Abstract

To help evaluate effects of Mississippi River inputs to sustainability of coastal Louisiana ecosystems, we compared porewater and substrate quality of organic-rich Panicum hemitomon freshwater marshes inundated by river water annually for more than 30 years (Penchant basin, PB) or not during the same time (Barataria basin, BB). In the marshes receiving river water the soil environment was more reduced, the organic substrate was more decomposed and accumulated more sulfur. The porewater dissolved ammonium and orthophosphate concentrations were an order of magnitude higher and sulfide and alkalinity concentrations were more than twice as high in PB compared with BB marshes. The pH was higher and dissolved iron concentrations were more than an order of magnitude lower in PB marshes than in BB marshes. The influx of nutrientrich river water did not enhance end-of-year aboveground standing biomass or vertical accretion rates of the shallow substrate. The differences in porewater chemistry and substrate quality are reasonably linked to the long-term influx of river water through biogeochemical processes and transformations involving alkalinity, nitrate and sulfate. The key factor is the continual replenishment of alkalinity, nitrate and sulfate via overland flow during high river stage each year for several weeks to more than 6 months. This leads to a reducing soil environment, pooling of the phytotoxin sulfide and inorganic nutrients in porewater, and internally generated alkalinity. Organic matter decomposition is enhanced under these conditions and root mats degraded. The more decomposed root mat makes these marshes more susceptible to erosion during infrequent highenergy events (for example hurricanes) and regular low-energy events, such as tides and the passage of weather fronts. Our findings were unexpected and, if generally applicable, suggest that river diversions may not be the beneficial mitigating agent of wetland restoration and conservation that they are anticipated to be.

Freshwater river diversions for marsh restoration in Louisiana: Twenty-six years of changing vegetative cover and marsh area

“…[19] We believe that the three freshwater diversions failed to increase vegetation health or area because of the physiological consequences of high nutrient flux and greater flooding of marsh plants [Swarzenski et al, 2008]. The agricultural literature has established that N loadings as little as 30 kg ha −1 can cause severe lodging (stem collapse) and low root growth in cereals (i.e., graminoids [Mulder, 1954]).…”

Hurricane-induced failure of low salinity wetlands

“…In upper Breton Sound, a region that is heavily impacted by a freshwater diversion, we find that low salinity marshes have shallower rooting and significantly weaker soils. These conditions could be exacerbated by low inorganic sediment content (14) and high nutrient inputs (15,16).…”

Discussion of: Olea, R.A. and Coleman, J.L., Jr., 2014. A Synoptic Examination of Causes of Land Loss in Southern Louisiana as Related to the Exploitation of Subsurface Geological Resources.Journal of Coastal Research, 30(5), 1025–1044.

“…It is the dredging of canals that has affected the green ''toupee'' overlying the sediments, not the geological structure beneath the emergent vegetation. It is this organic layer that can be compromised when nutrient supply increases sufficiently to stimulate its decomposition and reduce live, belowground biomass, reducing soil strength and making it susceptible to storm-induced erosion (Deegan et al, 2012;Swarzenski et al, 2008). The health of the plants may become compromised with a higher rate of sea-level rise this century, erode at the edge, and migrate inland.…”