Growth of phytoplankton and vascular aquatic plants at different nutrient levels

Summary

Greenhouse experiments to determine optimal levels of inorganic nitrogen and phosphorous for growth of aquatic plants were conducted as a basis for establishing eutrophic conditions in twenty experimental ponds. Various formulations, levels, and ratios of inorganic N and P were applied to cultures of aquatic vascular plants (Myriophyllum spicatum var. exalbescens, Potamogeton crispus and Elodea canadensis) and freshwater phytoplankton populations (mostly Chlorophyceae) established in battery jars containing natural soil and water. Nutrient levels applied ranged from 0.015–50 mg/l N and 0.005-5.0 mg/l P. The yields of the plants after five weeks growth under experimental conditions were used as indices of response.

The highest nutrient levels produced the best growth of phytoplankton while the lower levels resulted in higher yields of vascular plants. Filamentous algae grew best at the intermediate nutrient levels. High yields of vascular plants never coincided with large phytoplankton populations. At the highest nutrient levels applied, no vascular plant growth occurred. Of two inorganic phosphorous compounds evaluated, CaH₄ (PO₄)₂ produced the greater yields. Smaller differences were apparent amoung three inorganic nitrogen compounds.

This laboratory data suggest that future uncontrolled enrichment of natural bodies of water will result in the growth of larger populations of phytoplankton at the expense of benthic aquatic macrophytes. These conclusions are being examined under natural conditions in experimental ponds.