Nitrogen removal in constructed marshes at sites protected from and exposed to waves

Abstract

Nutrient removal is among the most valuable ecosystem services provided by marshes and is often a stated goal of coastal restoration projects. However, the removal capacity of constructed marshes is potentially affected by several site-specific and design factors, such as marsh platform elevation, slope, sediment type, initial planting density and wave climate. Here, the main and interactive effects of these factors on the capacity of constructed marshes to remove nitrate from runoff was explored in field experiments at sites protected from and exposed to waves. At both sites, three experimental blocks were established, each with 24 treatment combinations of factors in experimental flumes: two platform elevations (high and low), two slopes (steep and shallow), two sediment types (coarse and fine grain), and three initial planting densities (0%, 50% and 100% cover). Nutrient rich (KNO3) groundwater solution was fed through the marsh rhizosphere using subsurface diffusers. The relative effects of treatment combinations were then assessed by analyzing porewater NOx concentrations with ANOVA models. None of the treatment combinations had any observable effect on porewater NOx concentrations at the exposed site. However, both sediment type and planting density were significant main effects at the protected site with the lowest NOx concentrations found in flumes with fine sediments and initially planted. These results confirm that design factors can have large implications on nutrient dynamics of constructed marshes in areas protected from waves and that wave energy can substantially reduce the influence of these design factors.