Residents living along the Thornapple River in the greater Grand Rapids, MI area have reported nuisance levels of the filamentous algae Pithophora over the past several summers in their section of the river between two dams in the cities of Ada and Cascade. The Thornapple River Association turned to AWRI for help investigating potential causes behind the recurring algae blooms.

In 2025, summer intern Mya Harmer and the Steinman Lab conducted a nutrient bioassay, using flasks of Thornapple River water with pre-measured clumps of Pithophora algae. Nitrogen and phosphorus amendments were added to subsets of the flasks, spiked to 10x the ambient river concentration of each nutrient based on a prior study. All flasks were stored on a shaker table and incubated at a consistent temperature, while half the flasks were exposed to either light or dark conditions.

Mya's study found significant growth differences under changing light irradiance, but only in P-added treatments. Questions raised by this study for future research on Pithophora growth include:

• Light levels throughout the mat
• How “lower level” Pithophora mat responds to high light levels
• How important is “internal” nutrient cycling vs. “external” (river) nutrients for Pithophora growth; Gordon and McComb (1989) found that the nutrient concentrations of SRP and ammonia were much higher inside the mats of Cladophora (closely related to Pithophora) than in the surface water where they were growing
• Is phosphorus being stored internally (and if so, where) or adsorbed to the Pithophora mat
• The influence of water velocity on Pithophora growth
• The influence of benthic barriers on Pithophora growth

The Thornapple Association asked whether airport deicing fluids (ADF) being applied upstream from their homes at the Gerald R. Ford International Airport (GRR) could be having an effect on Pithophora growth. Multiple formulas of ADF products are manufactured and sold airports and the unknown recipes are proprietary. Thus, the Steinman Lab proposed to conduct a laboratory bioassay using varying concentrations of Thornapple River water and ADF supplied by GRR to incubate and measure the growth of Pithophora algae collected from the river. Field and lab work is anticipated to occur in summer 2026.

Additionally, M.S. graduate student Ashley Suttner will investigate the impacts of extreme temperature and light intensities on Pithophora growth. Monthly field sampling will collect or measure algae biomass, water chemistry, river flow, light irradiance, and macroinvertebrate community structure.

During the late summer algae bloom, additional biomass samples will be collected and used for a biomass and metabolism experiments using laboratory incubators, lamps, and river water in flasks to measure how biomass responds to three varying temperature regimes and two light treatments.

Identifying the limiting factor(s) of Pithophora growth in the Thornapple River (2025)

Al Steinman: [email protected]
Ashley Suttner: [email protected]