Cascading effects of predators on algal size structure

Area of Science:

• Ecology
• Aquatic Ecology
• Nutrient Cycling

Background:

• Boreal peatlands are experiencing climate change, leading to wetter, nutrient-rich conditions.
• These changes expand consumer access to aquatic habitats and promote algae susceptible to trophic regulation.
• Food web dynamics, including nutrient availability and herbivory, influence primary production.

Purpose of the Study:

• To evaluate the effects of nutrient enrichment and food web manipulation on algal assemblage structure in an Alaskan fen.
• To determine how herbivore presence and predator exclusion impact algal cell size and composition.
• To test the hypothesis that the proportion of inedible algae influences top-down and bottom-up effects.

Main Methods:

• An in situ mesocosm experiment was conducted in an Alaskan fen.
• Manipulations included enhanced nutrient availability and exclusion of herbivores and predators.
• Algal assemblage structure, including cell size and type, was analyzed under different food web conditions.

Main Results:

• Algal cell size increased in the presence of herbivores (two-tiered food web) but not in the presence of a trophic cascade (three-tiered food web).
• Predators indirectly prevented changes in algal morphology by limiting herbivory.
• Increases in algal size were attributed to a higher proportion of filamentous green algae and nitrogen-fixing cyanobacteria, which are less susceptible to consumption.

Conclusions:

• Consumer-driven shifts in algal assemblage structure are significant and influenced by food web interactions.
• Increasing wet periods in peatlands will enhance trophic regulation of algal production through direct consumption and trophic cascades.
• Algal morphology, particularly cell size, plays a key role in mediating the effects of herbivores and nutrient availability on primary production.