Does the stoichiometric carbon:phosphorus knife edge apply for predaceous copepods?

Area of Science:

• Ecology
• Aquatic Ecology
• Consumer-Resource Interactions

Background:

• Stoichiometric food quality, particularly the carbon:phosphorus (C:P) ratio, influences consumer physiology and growth.
• Previous research focused on autotroph-herbivore trophic transfer, leaving higher trophic levels understudied regarding stoichiometric effects.
• Carnivores, with body compositions typically matching their prey, may be particularly susceptible to pronounced stoichiometric imbalances.

Purpose of the Study:

• To investigate the impact of dietary C:P ratio on a predatory copepod, Parabroteas sarsi.
• To determine if stoichiometric imbalances affect predators at higher trophic levels, similar to herbivores.
• To assess the vulnerability of predators to prey elemental composition fluctuations.

Main Methods:

• Parabroteas sarsi were fed monospecific diets varying in C:P ratio (high and low).
• Predator's elemental composition, growth, prey selection, excretion, egestion, and respiration rates were measured.
• Analysis focused on the direct effects of dietary stoichiometry on predator performance.

Main Results:

• Dietary C:P ratio significantly altered the elemental composition and growth of Parabroteas sarsi.
• Prey selection, excretion, egestion, and respiration rates remained unaffected by dietary treatments.
• Predators exhibited high vulnerability to both nutrient deficits and excesses in their prey's C:P ratio.

Conclusions:

• Predatory copepods like P. sarsi operate on a narrow 'stoichiometric knife edge' and are highly sensitive to prey C:P ratios.
• Stoichiometric effects extend beyond herbivores to higher trophic levels, impacting predator secondary production.
• Fluctuations in prey quality pose significant risks to predators with specific elemental requirements.