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The impact of wastewater effluent on accretion and decomposition in a subsiding forested wetland

Insufficient sedimentation, coupled with high rates of relative sea-level rise (subsidence plus eustatic sea-level rise), are two important factors contributing to wetland loss in coastal Louisiana, USA. We hypothesized that adding nutrient-rich, secondarily treated wastewater effluent to subsiding wetlands in Louisiana could promote vertical accretion in these systems through increased organic matter production and subsequent deposition and allow accretion to keep pace with estimated rates of relative sea-level rise (RSLR). However, we also hypothesized that nutrient enrichment could stimulate the decomposition of organic matter, thus negating any increase in accretion due to increased organic matter accumulation. To test these hypotheses, we measured leaf-litter decomposition, litter nutrient dynamics, and sediment accretion in a permanently flooded and subsiding forested wetland receiving wastewater effluent and in an adjacent control site, both before and after effluent applications began. We also measured organic and mineral matter accumulation in the treatment site before and after effluent applications began. A Before-After-Control-Impact (BACI) statistical analysis revealed that neither leaf-litter decomposition rates nor initial leaf-litter N and P concentration were affected by wastewater effluent. A similar analysis revealed that final N and P leaf-litter concentrations did significantly increase in the treatment site relative to the control after effluent was applied. Total pre-effluent accretion, measured 34 months after feldspar horizon markers were laid down, averaged (±SE) 22.3±3.2 mm and 14.9±4.6 mm in the treatment and control sites, respecitvely, and were not significantly different. However, total accretion measured 68 months after the markers were installed and 29 months after effluent additions began in the treatment site averaged 54.6±1.5 mm in the treatment site and 19.0±3.2 mm in the control site and were significantly different. Additionally, after wastewater applications began, the estimated rate of accretion in the treatment site (11.4 mm yr−1) approached the estimated rate of RSLR (12.3 mm yr−1). Most of this increase in accretion was attributed to organic matter inputs, as organic matter accumulation increased significantly in the treatment site after effluent application began, while mineral accumulation rates remained constant. These findings indicate that there is a potential for using wastewater to balance accretion deficits in subsiding wetland systems.