Benthic cyanobacterial blooms (BCBs) have become more common, yet little is known about contributing factors facilitating their formation in oligotrophic systems. One potential driver is higher temperatures, which decrease gas solubility and increase microbial respiration. In hyporheic zones, where dissolved oxygen (DO) is already low, higher temperatures may further lower DO concentrations. Anoxic conditions often stimulate phosphorus (P) release by reducing iron, which then releases sorbed P, potentially fueling BCBs. As water temperatures rise, BCBs may become more frequent, including in systems with low surface water (SW) P concentrations.
We conducted a field study on the Hanford Reach of the Columbia River, where recent BCBs killed dogs and sickened humans. Our aim was to ascertain whether sustained high sediment temperatures (near 21°C and above) might increase P concentrations in hyporheic water, particularly in shallow littoral zones. We deployed fixed temperature loggers in sediments at two locations and measured P concentrations in the SW and the hyporheic zone every two weeks at four locations.
We found that as sustained heating increased sediment temperatures, SW P concentrations often remained below detection, but hyporheic P concentrations rose (SW range = 0.00 to 0.012 mgP/L, hyporheic range = 0.008 to 0.267 mgP/L), exceeding EPA guidelines for safe water (0.100 mgP/L) at times. However, there was a lag between the onset of sustained high temperatures beginning in July (mean sediment temp = 21.2 ± 0.6 °C) and hyporheic P concentration maximums in September through October (mean = 0.098 ± 0.067 mgP/L), suggesting that anoxia and subsequent P release developed slowly. Our preliminary results suggest that sustained high temperatures increase hyporheic P concentrations, potentially fueling BCBs.