Aggregated filter-feeding consumers alter nutrient limitation: consequences for ecosystem and community dynamics

Area of Science:

• Ecology
• Aquatic Ecosystems
• Nutrient Cycling

Background:

• Nutrient cycling is fundamental to ecosystem function, particularly in streams where nutrients move downstream.
• Ecological stoichiometry posits interdependence of elemental cycles due to organismal nutrient requirements.
• Animals, especially dense consumer aggregations, significantly influence biogeochemical cycles by creating hotspots.

Purpose of the Study:

• To investigate the role of filter-feeding freshwater mussels in creating biogeochemical hotspots in streams.
• To determine if mussel aggregations alter nutrient limitation and algal community composition.
• To assess the impact of mussel excretion stoichiometry on stream nutrient dynamics.

Main Methods:

• Field study manipulating nitrogen and phosphorus using nutrient-diffusing substrates in areas with high and low mussel abundance.
• Monitoring algal growth and community composition.
• Measuring in situ mussel excretion stoichiometry across 18 sites in three rivers.

Main Results:

• Mussel presence significantly modified nutrient limitation: sites without mussels were nitrogen-limited, while sites with high mussel densities were co-limited by nitrogen and phosphorus.
• Algal communities shifted from N-fixing blue-green algae in N-limited sites to diatoms in co-limited sites.
• Mussel excretion, with its high N:P ratio, strongly influenced stream water column N:P, alleviating strict N-limitation.

Conclusions:

• Dense freshwater mussel aggregations act as biogeochemical hotspots, altering nutrient dynamics and primary productivity in streams.
• Mussel translocation of nutrients is critical for regulating nutrient limitation and algal species composition.
• This study underscores the ecological significance of consumers, particularly imperiled mussel populations, in aquatic nutrient cycling.