The Fates of Nitrogen in an Experimental Wetland Food Web: a Stable Isotope Study

Abstract

Human activities have increased the availability of reactive nitrogen in many freshwater ecosystems, leading to negative impacts on the health and biodiversity of lakes, rivers and wetlands. Yet, understanding the pathways of nutrient cycling in wetlands is limited. We conducted a nitrogen stable isotope (¹⁵N) tracer study to assess nitrogenous nutrient transfers within the biota community in a small experimental wetland between August 13, 2016, and September 12, 2016using a. Results showed rapid use of nitrogen by various aquatic plants and subsequent transfer to the consumer community. ¹⁵N enrichments were found in all plant samples from the first collection event. The fastest nutrient uptake was found in the simplest form of plants (periphyton and Lemna minor), followed by submerged macrophytes (Ceratophyllum demersum and Utricularia vulgaris). Emergent plants displayed the lowest ¹⁵N enrichment, likely due to their inability to directly assimilate nutrients from the water column and their initial large biomass. ¹⁵N enrichments in consumers were also pronounced, indicating that the nitrogen added to the wetland surface water was quickly transferred to various trophic levels. Primary consumers were more strongly labeled than secondary consumers; water column feeders were typically more enriched in ¹⁵N than benthic consumers. This experiment revealed different uses of nitrogen among the plant and consumer communities, and the results may provide useful information on community secession and ecosystem restoration under anthropogenic influence.