Effects of vegetation state on biodiversity and nitrogen retention in created wetlands: a test of the biodiversity–ecosystem functioning hypothesis

Abstract

1. Nitrogen retention in wetlands provides an example of an ecosystem function that is desired by human society, and is a rationale for the creation of wetlands to decrease nitrogen fluxes from nitrate-loaded river catchments to coastal waters.2. Here, we tested the impact of different vegetation states on species diversity and nitrogen retention during 4 years in surface-flow wetlands receiving nitrate-rich water. Tall emergent vegetation or submerged vegetation was introduced to six experimental wetlands each and six wetlands were left as unplanted controls for free development of vegetation. This resulted in three vegetation states dominated by emergent vegetation, by a mixture of submerged vegetation and filamentous green algae or by filamentous green algae. 3. Species diversity (species richness and Shannon diversity) of plants was initially lowest in free development wetlands, but during the study became lower in the emergent vegetation wetlands than in the other wetlands. Diversity of macroinvertebrates was initially lower in the submerged vegetation wetlands than in the other wetlands, but this difference disappeared during the study. Nitrogen retention was consistently higher in emergent vegetation wetlands than in the other wetlands throughout the study. 4. We conclude that plant diversity in wetlands dominated by tall emergent vegetation gradually became lower than in other wetlands, due to dominant species competitively excluding other plants. However, these wetlands were more efficient at removing nitrogen than those dominated by filamentous algae or submerged macrophytes. 5. Management of wetlands often aims to decrease the dominance of tall emergent vegetation for the benefit of plant species diversity and habitat heterogeneity. Our results demonstrate a biodiversity benefit, but also show that this strategy may decrease the ability of wetlands to remove nitrogen. In this case, there is no support for the hypothesis that biodiversity enhances ecosystem function.

National Large-Scale Wetland Creation in Agricultural Areas—Potential versus Realized Effects on Nutrient Transports

“…This emphasizes that there are also other factors that are important to consider for obtaining high nutrient removal in created wetlands in agricultural catchments. These can be factors related to wetland design and biological composition [33][34][35][36][37][38], as well as concentrations and form of the nutrients in inflowing water or other factors related to catchment characteristics [6,23,27,32]. Further studies are needed to identify and evaluate the important factors affecting the nutrient removal efficiency of created wetlands in agricultural catchments.…”

Tracer behaviour and analysis of hydraulics in experimental free water surface wetlands

“…The types of vegetation targeted were filamentous green algae (FGA), emergent, floating-leaved and submerged vegetation. The vegetation survey followed the methodology outlined by Weisner and Thiere (2010). Results from the vegetation survey are shown in Table 2.…”

Tradeoffs among ecosystem services in restored wetlands

“…Similarly, other recent work conducted in wetlands has contradicted the hypothesized positive relationship between biodiversity and ecosystem functions, and suggests that certain ecosystem functions, such as nutrient retention and productivity may be maximized at lower biodiversity (Hansson et al, 2005;Weisner and Thiere, 2010;. This relationship in wetlands may be driven by the dominance and greater relative productivity of robust, invasive plant species such as Phalaris arundinacea and Phragmites australis (Martina et al, 2014).…”