Chemical control of algae by submerged plants

Posted on October 15, 2015

The appearance and ecology of lakes can be strongly affected by competition between large submerged plants (macrophytes) and algae. Macrophyte-dominated lakes tend to have a limited nutrient supply and clear water, while algal dominance is usually associated with nutrient enrichment and turbid water. Although macrophytes and algae compete mainly for nutrients and light, Belgian experiments suggest that the contest can also be affected by the ability of submerged plants to release chemical compounds that impair the growth and survival of phytoplankton. A series of 200 litre polyethylene tanks were filled with water and inoculated with algae from local lakes. Western pondweed (Elodea nuttallii) plants were added to some of the tanks. Depletion of algae by zooplankton was negligible, and nutrient competition between Elodea and phytoplankton was prevented by regularly adding nitrogen and phosphorus to the tanks. Experiments ran for 4-8 weeks, during which time levels of algal chlorophyll were significantly lower (on average about 3.5 times less) in the tanks containing waterweed. Because the controlling effect of the Elodea remained strong throughout the duration of the experiments, there was no sign that phytoplankton species evolved resistance to the inhibiting chemicals. These results help to explain why algae have sometimes been observed to stay at a low abundance in lakes even when nutrients are plentiful and zooplankton biomass is low.

Reference: Vanderstukken, M. et al. 2014. Long-term allelopathic control of phytoplankton by the submerged macrophyte Elodea nuttallii. Freshwater Biology 59, 930–941.