Oral Presentation Society for Freshwater Science 2026 Annual Meeting

Groundwater-surface water exchange influences on the longitudinal spatial distribution of toxigenic cyanobacterial proliferations in the South Fork Eel River, California, USA (135576)

Andrea Garcia Jimenez 1 , Janette L Davidson 1 , Robert Padan 1 , Donovan Davis 1 , Danielle Hare 2 , Mark B Hausner 3 , Christopher B Kratt 4 , Sydney Brown 5 , Jacob K Mormando 5 , Ramon C Naranjo 6 , Gregory L Boyer 7 , Zac Triumph 7 , Rosalina Christova 5 , Ramesh Goel 8 , Joanna Blaszczak 1
  1. Department of Natural Resources, University of Nevada, Reno, NV, Washoe
  2. Earth, Geographic, and Climate Sciences, University of Massachusetts, Amherst, MA
  3. Division of Hydrologic Sciences, Desert Research Institute, Reno, NV, Washoe
  4. Center for Transformative Environmental Monitoring Programs, University of Nevada, Reno, NV, Washoe
  5. Environmental Science and Policy, George Mason University, Woodbridge, VA
  6. Nevada Water Science Center, US Geological Survey, Carson City, NV
  7. Department of Chemistry, SUNY- College of Environmental Science and Forestry, Syracuse, NY
  8. Civil and Environmental Engineering, University of Utah, Salt Lake City, UT

Benthic algal assemblages, including toxin-producing cyanobacteria, are increasingly detected in rivers globally and threaten aquatic ecosystems and public health. Anatoxins are a neurotoxic cyanotoxin produced by the cyanobacterium Microcoleus, which can dominate benthic habitats with low dissolved reactive phosphorus and low dissolved inorganic nitrogen, indicating that water-column nutrients alone cannot explain bloom formation or toxicity. Other environmental conditions commonly associated with Microcoleus proliferation include elevated water temperature, stable summer discharge, and moderate to high specific conductance, some of which are typically indicative of groundwater influence in rivers. Groundwater-surface water exchanges in rivers vary longitudinally and can influence nutrient and dissolved ion concentrations, water temperature, and benthic algal assemblages. We hypothesized that longitudinal variation in groundwater-surface water exchanges along a river influences the distribution of Microcoleus growth and anatoxin production through its effects on nutrient availability and water temperature. To test this, we investigated the role of groundwater influences on Microcoleus proliferation and anatoxin production in the South Fork Eel River, northern California, where recurrent proliferations have been observed for over a decade. Groundwater or hyporheic inflows were identified using fiber-optic distributed temperature sensing (FO-DTS) and continuous temperature loggers. We installed monitoring wells to characterize the groundwater chemistry and temperature. We estimated percent cover along 57 transects within a 2 km reach and collected surface water velocity, substrate size, and Microcoleus mats, which we analyzed for anatoxins using liquid chromatography with tandem mass spectrometry (LC-MS/MS). Preliminary field observations indicate that side channels with warmer temperatures had a more extensive Microcoleus mat cover, which appeared to be at a more advanced successional stage than the main channel. This suggests that channel complexity and groundwater-surface water exchange may influence the longitudinal spatial distribution of Microcoleus by creating warmer water temperatures. Our study provides insights into how groundwater processes may influence the ecology and spatial distribution of toxigenic benthic cyanobacteria in rivers.