Investigations of growth metrics and δ15N values of water hyacinth (Eichhornia crassipes, (Mart.) Solms-Laub) in relation to biological control

• Stable isotope values of water hyacinth reflect environmental N-loading.
• Water hyacinth has equilibration rates of ∼16 days for δ15N, C/N ratios and %N.
• Links between δ15N and petiole length suggest a positive correlation to N loading.
• Estimates of plant health and quality based on growth metrics should be avoided.
• δ15N, C/N ratios and petiole length describe sites which may complicate biocontrol.

The success of biological control for Eichhornia crassipes (Mart) Solms-Laub has been varied, with failure attributed to various factors including increased eutrophication in freshwater systems. Studies have shown that high N-loads are associated with enriched δ15N values of aquatic biota. Stable isotope analyses may help to assess the nutrient status of invaded ecosystems and predict the success of classical biological control. A combination of controlled greenhouse experiments and in situ observations were used to quantify the δ15N equilibration rate of E. crassipes leaf tissue (i.e. the time span of the information on environmental N-loading), confirm the ability of plant tissue to reflect ecosystem N-loading and to compare standard water hyacinth growth metrics with δ15N and C/N ratios to evaluate how well they described eutrophication. Using both δ15N values and C/N ratios, it was possible to clearly distinguish between manure, fertilizer and control regimes and plant tissue provided information on ecosystem N loading over approximately 16 days. Interpretations were complicated by the plant's capacity for N storage. Strong correlations between petiole length of E. crassipes and δ15N suggest petiole length may be linked to N loading. Weaker (or a lack of) correlations between δ15N or C/N ratios and remaining growth metrics suggest that generalisations and/or predictions of eutrophication based on growth metrics should be avoided. A combination approach using plant tissue δ15N and C/N ratios along with maximum petiole length of E. crassipes plants will provide a time integrated assessment of ecosystem eutrophication and identify areas where classical biological control may fail to regulate populations.