Baseline isotope data for Spirodela sp.: nutrient differentiation in aquatic systems

Abstract

The excessive addition of nitrogen to watersheds is recognized as one of the main causes of the global deterioration of aquatic ecosystems and an increasing number of studies have shown that δ¹⁵N signatures of macrophytes may reflect the N-loading of the system under investigation. This study investigated isotopic equilibration rates and concentration level effects of KNO₃ and cow manure nutrient solutions on the δ¹⁵N and δ¹³C signatures, C/N ratios, % N and % C of Spirodela sp. over time, to determine the feasibility of their use in monitoring anthropogenic N-loading in freshwater systems. Spirodela δ¹⁵N signatures clearly distinguished between nutrient types within 2 days of introduction, with plants grown in KNO₃ showing extremely depleted δ¹⁵N values (-15.00 to -12.00‰) compared to those growing in cow manure (14.00-18.00‰). Isotopic equilibration rates could not be determined with certainty, but plant isotopic differentiation between nutrient regimes became apparent after 2 days and started to equilibrate by day 4. Concentration level effects were also apparent, with Spirodela tissue displaying more depleted and enriched δ¹⁵N values in higher concentrations of KNO₃ and cow manure respectively. δ¹³C signatures of some plants grown in manure were more enriched than plants grown in KNO₃ and reverse osmosis (RO) water. However, nutrient induced differences in δ¹³C were small and are likely to be obscured in the natural environment. Decreased C/N ratios and increases in plant % N in zero N concentration treatments confirmed the presence of a commensal cyanobacterial-duckweed association within Spirodela sp., reducing its effectiveness as an in-situ incubator in low nutrient environments. However, indications are that Spirodela may make a useful isotope monitoring tool under conditions of long-term, continuous nutrient inputs such as systems impacted by sewage outfalls and/or wastewater inputs.