In situ experimental studies of Eurasian Watermilfoil (Myriophyllum spicatum) downstream from agricultural watersheds: Nutrient loading, foliar uptake, and growth

Recent studies in Conesus Lake, New York, documented significant decreases in the biomass of Eurasian watermilfoil (Myriophyllum spicatum) near the mouths of streams draining sub-watersheds where reductions in nutrient loading occurred as a result of the implementation of agricultural Best Management Practices (BMPs). In situ experiments were conducted to further investigate the relationship between stream loading, foliar uptake, and growth of Eurasian watermilfoil. In two of three experiments, plants cropped to a height of approximately 50 cm had the lowest growth (g/m2) downstream from a sub-watershed where major BMPs had been implemented (80% and 0%). In sub-watersheds where minimal or no BMPs were introduced, plants showed significantly higher growth as biomass increased (216% and 22%). In a second set of experiments, shoots of Eurasian watermilfoil plants were incubated for 24 h in ambient lake water and in lake water with enriched concentrations of nitrate and soluble reactive phosphorus comparable to rain event stream effluent concentrations and then allowed to grow in situ for a 2-week experimental period. For all experiments combined, the shoot biomass increased significantly in the enhanced nutrient treatments when compared to the ambient treatment at the Sand Point macrophyte bed (reduced loading) but not at the Eagle Point macrophyte bed (high loading). Overall, the results indicate that foliar uptake of nutrients in stream effluent can contribute to the growth of Eurasian watermilfoil and reinforce the hypothesis that reductions in stream loading through agricultural BMPs can help reduce macrophyte growth in the lake littoral.