Finding the harvesting frequency to maximize nutrient removal in a constructed wetland dominated by submerged aquatic plants

Abstract Water quality is still poor in many freshwater ecosystems around the world as a result of anthropogenic nutrient loading. Constructed wetlands can be used to remove excess nutrients. In these wetlands, helophytes or free floating aquatic plants are traditionally used to absorb the nutrients. The nutrients are subsequently exported upon harvesting of the plants. However, rooted submerged plants may be more effective to extract nutrients from moderately eutrophicated ecosystems than helophytes or floating species. Here, we tested how the frequency of harvesting affected submerged biomass production, biomass nutrient content and the resulting amount of nutrients removed, as well as the vegetation composition and structure. Two Myriophyllum spicatum dominated shallow ponds, with moderately low surface water nutrient loading (∼5.6 mg N.m −2 .d −1 and ∼1.32 mg P.m −2 .d −1 ) were used. Each pond was subjected to four harvesting treatments: mowing 1x, 2x, 3 x or 5 x between May and September 2015. Harvesting 2 x or 3 x removed most biomass and nutrients, while mowing either 5 x or only once at the end of the growing season removed the lowest amount of nutrients from the system. Furthermore, the dominance of M. spicatum in the vegetation was best maintained in plots mown 2 x, while its cover declined in plots mown more frequently, resulting in an increase of charophyte abundance. We conclude that harvesting at an intermediate frequency is best when aiming to remove the maximum amount of nutrients under a moderately low nutrient loading. Harvesting more frequently may be a suitable management method to reduce dominance of M. spicatum in situations where it causes nuisance problems due to massive growth.