Algal assemblage drives patterns in ecosystem structure but not metabolism in a productive river

Area of Science:

• Ecology
• Limnology
• Riverine Ecosystems

Background:

• Algal dynamics are key drivers of lotic (river) ecosystem processes.
• Rivers are typically heterotrophic, but filamentous algae can form nuisance blooms, altering autotrophic structure and impacting ecosystem function variably.

Purpose of the Study:

• To investigate the structural and functional contributions of filamentous and epilithic algae in a river system.
• To link algal biomass to primary production fluxes and assess the influence of blooms on ecosystem processes.

Main Methods:

• Conducted a two-year study across six sites on the Upper Clark Fork River, Montana.
• Measured algal biomass and partitioned daily productivity estimates across algal groups.
• Utilized reach-scale metabolism estimates to assess in situ functional rates of different algal groups.

Main Results:

• Observed high ecosystem productivity fluxes and spatially variable filamentous algal blooms.
• Filamentous algal blooms dictated ecosystem structure (biomass, turnover times) but not ecosystem functions (gross primary productivity, respiration, net production).
• Epilithic algae exhibited rapid growth (0.30 day⁻¹) and turnover, dominating biomass production and driving ecosystem function, while filamentous algae grew slowly (0.026 day⁻¹) and accumulated biomass.

Conclusions:

• Filamentous algal blooms shape river ecosystem structure but do not significantly alter core ecosystem functions.
• Epilithic algae are the primary drivers of ecosystem function in this snow-melt river system due to their rapid growth and turnover rates.