Density-dependent positive feedbacks buffer aquatic plants from interactive effects of eutrophication and predator...

Area of Science:

• Ecology
• Aquatic Botany
• Conservation Biology

Background:

• Coastal ecosystems face synergistic impacts from eutrophication and loss of predatory fish.
• Habitat-forming aquatic plants are vulnerable to these disturbances, which can promote ephemeral algae growth.
• Intraspecific plant facilitation, where individuals of the same species benefit each other, may mitigate stress.

Purpose of the Study:

• To investigate how shoot density of Myriophyllum spicatum affects individual plant responses to eutrophication and the exclusion of large predatory fish.
• To determine the conditions under which self-facilitation in aquatic plants can counteract negative impacts of multiple stressors.

Main Methods:

• A 12-week field experiment using cage enclosures was conducted.
• Manipulated factors included the presence/absence of large predatory fish and nutrient levels (eutrophication).
• Measured plant performance metrics (biomass, branching) and epiphytic algal biomass at varying Myriophyllum spicatum densities.

Main Results:

• High plant density improved individual plant performance, but only under combined stress (fish exclusion + fertilization).
• Excluding fish led to a doubling of epiphytic algae, indicating a trophic cascade.
• Under fish exclusion and fertilization, plant biomass and branching increased with density; under ambient nutrients, they decreased.
• In open cages (fish present), epiphytic algae were low, and plant density/eutrophication had no effect.

Conclusions:

• Intraspecific plant facilitation in Myriophyllum spicatum was observed only when fish exclusion (high epiphyte load) coincided with fertilization.
• Intraspecific competition dominated when nutrients were not added.
• Maintaining high plant density is crucial for ecosystem-based management in coastal areas facing multiple stressors.