Mapping nitrogen loading in freshwater systems: using aquatic biota to trace nutrients
Motitsoe S.N., Martin P. Hill, Jackie M. Hill
Dept. Zoology & Entomology, Rhodes University
The South African Scoring System (SASS5) is a well-used bio-monitoring tool for the assessment of anthropogenic inputs and ecosystem health in freshwater systems. Increased anthropogenic inputs have numerous effects in aquatic ecosystems, including the facilitation of invasive weeds. There are however a number of ecological challenges associated with SASS5 application, with the primary drawback being that SASS scores only provide “red flags” after ecosystem degradation has taken place. Thus there is a need for new techniques to identify eutrophication before the onset of degradation. Recent work has shown that stable isotope values of the duckweed Spirodela sp. can clearly track different species and concentrations of nitrogen (N) loading in a laboratory setting. This project aims to field test this technique as a bio-monitoring tool in freshwater systems. Previously incubated Spirodela plants with known isotopic values were transplanted in greenhouse cages at ten study sites (with 5 replicates) within the Bushmans-New Year’s and Bloukrans-Kowie river systems,
Eastern Cape. Using δ15N values of these plants, temporal variation in N loading will be monitored monthly over a 13 month period, along with physicochemical variables. Every three months a comprehensive SASS5 assessment will be done at each site in conjunction with micronutrient water quality analysis. Comparisons of N loading as determined by δ15N values and SASS scores will assess the usefulness of stable isotope techniques for identifying N loading and evaluating ecosystem health. Successful field trials will provide an early warning tool for identifying ecosystem eutrophication with the goal of managing and/or preventing further weed invasions.