Abstract
Abstract. The sequestration of nutrients from surface waters by aquatic macrophytes and sediments provides an important service to both natural and constructed wetlands. While emergent species take up nutrients from the sediment, submerged and floating macrophytes filter nutrients directly from the surface water, which may be more efficient in constructed wetlands. It remains unclear, however, whether their efficiency is sufficient for wastewater purification and how plant species and nutrient loading affects nutrient distribution over plants, water and sediment. We therefore determined nutrient removal efficiencies of different vegetation (Azolla filiculoides, Ceratophyllum demersum and Myriophyllum spicatum) and sediment types (clay, peaty clay and peat) at three nutrient input rates, in a full factorial, outdoor mesocosm experiment. At low loading (0.43 mg P m−2 d−1), plant uptake was the main pathway (100 %) for phosphorus (P) removal, while sediments showed a net P release. A. filiculoides and M. spicatum showed the highest biomass production and could be harvested regularly for nutrient recycling, whereas C. demersum was outcompeted by spontaneously developing macrophytes and algae. Higher nutrient loading only stimulated A. filiculoides growth. At higher rates ( ≥ 21.4 mg P m−2 d−1), 50–90 % of added P ended up in sediments, with peat sediments becoming more easily saturated. For nitrogen (N), 45–90 % was either taken up by the sediment or lost to the atmosphere at loadings ≥ 62 mg N m−2 d−1. This shows that aquatic macrophytes can indeed function as an efficient nutrient filter but only for low loading rates (polishing) and not for high rates (purification). The outcome of this controlled study not only contributes to our understanding of nutrient dynamics in constructed wetlands but also shows the differential effects of wetland sediment types and plant species. Furthermore, the acquired knowledge may benefit the application of macrophyte harvesting to remove and recycle nutrients from both constructed wetlands and nutrient-loaded natural wetlands.
Highlights
Excess loading of phosphorus (P) and nitrogen (N) from domestic, agricultural and industrial wastewaters is the main cause of eutrophication of aquatic ecosystems, damaging their ecological quality and functioning (Kronvang et al, 2005; Kantawanichkul et al, 2009)
Using a full-factorial outdoor mesocosm experiment, we studied the nutrient uptake rates of three different aquatic macrophytes with contrasting growth forms (Azolla filiculoides, Ceratophyllum demersum and Myriophyllum spicatum), growing on peat, peaty clay or clay sediments
When macrophytes were growing on peat or peaty clay sediments, P concentrations in the surface water increased with increasing external P loading (X2 = 99.80, P < 0.001 and X2 = 59.40, P < 0.001 for peat and peaty clay sediments, respectively)
Summary
Excess loading of phosphorus (P) and nitrogen (N) from domestic, agricultural and industrial wastewaters is the main cause of eutrophication of aquatic ecosystems, damaging their ecological quality and functioning (Kronvang et al, 2005; Kantawanichkul et al, 2009). Wetlands have been constructed to mitigate eutrophication of watercourses, lakes and seas by reducing the nutrient loads in discharge water of wastewater treatment plants, farmlands, households or industries (Brix and Arias, 2005; Mitsch et al, 2005). Constructed wetland systems (CWSs) use macrophytes or a combination of macrophytes and sediment to remove nutrients from the water (Brix, 1994; Vymazal, 2007). As a result of low maintenance, these systems become saturated with P and other nutrients, which decreases their nutrient-binding capacity. As a result, they only work efficiently for a limited amount of time (Drizo et al, 2002). At higher latitudes seasonality is an important factor for these systems because additional energy will be needed during cold seasons (see, e.g., the use of warmed greenhouse facilities) to remove nutrients by macrophyte growth year-round (Wittgren and Mæhlum, 1997)
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