Green carpets in a blue-ribbon fishery: predicting nuisance algal blooms using river metabolism

Many freshwaters across the Mountain West are experiencing nutrient enrichment, leading to the excessive growth of algae and aquatic plants. In healthy rivers, these primary producers perform critical functions, providing energy for macroinvertebrates and fish, determining water quality, and directing carbon and nutrient cycling for the entire ecosystem. Nutrient enrichment profoundly alters these functions, motivating research to predict and prevent its impacts. Since 2018, nuisance algal blooms in the Gallatin River (Montana) have roused concern. The Gallatin blooms are hypothesized to occur in response to changing nutrient dynamics, likely due to human development in tributary streams. Fortuitously, continuous dissolved oxygen measurements provide estimates of gross primary production (GPP) and ecosystem respiration (ER). When fit to models, these estimates allow us to characterize a river’s metabolism regime. My research develops a metabolism metric for the Gallatin, providing the foundation to assess the effects of algal blooms in nutrient enriched rivers. State agencies and watershed management coalitions currently point sample nutrients and measure levels of dissolved oxygen in the Gallatin. However, dissolved oxygen instruments are not deployed throughout the river, and models of metabolism have not been fit to these limited data. My dual-element study addresses this gap with improved study design. Collecting oxygen data before spring runoff will record early deviations or spikes in gross primary production, which could be predictive of a seasonal algal bloom. First, I will estimate metabolism at sites with and without blooms by fitting models to dissolved oxygen data, generating daily estimates of GPP and ER. Second, I will use novel drone-based hyperspectral imaging to estimate algal biomass, which will improve model estimates. I will also hand-sample algae to corroborate the aerial estimates. My metabolism metric will provide agencies with a useful management solution and ensure we all enjoy clear, clean rivers well into the future.