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We collected this data to better understand the timing of peak benthic cyanobacterial mat occurrence (specifically taxa associated with anatoxin production, Microcoleus and Anabaena) and mat anatoxin concentrations in rivers. We sampled in northern California on the South Fork Eel, Salmon, and Russian Rivers biweekly in 2022, and the Salmon River biweekly and South Fork Eel weekly in 2023. During each sampling event, we conducted benthic cover surveys, measured in-situ water quality parameters (temperature, pH, dissolved oxygen, conductivity), and collected surface water samples and targeted cyanobacteria samples. In 2022 on all rivers and in 2023 at the Salmon River, we also collected distributed non-targeted periphyton samples to characterize full-reach community compositions. All sampling was completed in 150-m reaches upstream of sensors recording continuous dissolved oxygen, conductivity, and temperature data. We analyzed surface water samples for nitrate, ammonium, soluble reactive phosphate, total dissolved carbon, and dissolved organic carbon. We also analyzed surface water samples from 2022 for major anions (Cl, SO4, Br) and cations (Na, K, Mg, Ca). Targeted-cyanobacteria and non-target periphyton samples were analyzed for anatoxins, relative abundance of algal taxa (via microscopy), ash-free dry mass, and chlorophyll-a. To estimate mean river depth within the dissolved oxygen footprint upstream of sensors, we kayaked portions of the river and collected river depth measurements. We also measured discharge at each river excluding the Salmon River (due to high discharge) and completed pebble counts at the South Fork Eel River to obtain sediment grain size distributions. 

Abstract Nearshore (littoral) habitats of clear lakes with high water quality are increasingly experiencing unexplained proliferations of filamentous algae that grow on submerged surfaces. These filamentous algal blooms (FABs) are sometimes associated with nutrient pollution in groundwater, but complex changes in climate, nutrient transport, lake hydrodynamics, and food web structure may also facilitate this emerging threat to clear lakes. A coordinated effort among members of the public, managers, and scientists is needed to document the occurrence of FABs, to standardize methods for measuring their severity, to adapt existing data collection networks to include nearshore habitats, and to mitigate and reverse this profound structural change in lake ecosystems. Current models of lake eutrophication do not explain this littoral greening. However, a cohesive response to it is essential for protecting some of the world's most valued lakes and the flora, fauna, and ecosystem services they sustain.