Nutrient Concentrations as Predictors of Nuisance Hydrodictyon reticulatum Populations in New Zealand

Hydrodictyon reticulatum (L.) Lagerh. has spread rapidly since its recent introduction into New Zealand, with persistent nuisance growths in several important recreational waterbodies causing economic problems for local communities. Samples of H. reticulatum and water were collected for nutrient analysis from a wide range of sites in the central North Island where H. reticulatum was present, had been present or where it was likely to have been introduced. The health of the H. reticulatum population was assessed at each site where present. No relationship between health of H. reticulatum populations and dissolved reactive phosphorus or tissue phosphorus concentrations was observed. There was a significant inverse relationship between population health and the tissue C:N ratio. C:N ratios below 18 supported growing populations. Dissolved inorganic nitrogen (DIN) concentrations above 30 mg m -3 were shown to support growing populations, which suggests that nitrogen is likely to be the major limiting nutrient for H. reticulatum populations in this region of New Zealand. In conclusion, healthy natural populations of H. reticulatum in New Zealand are likely to be found where DIN concentrations are above 30 mg m -3