Benthic cyanobacterial blooms (cyanoHABs) have become more common in oligotrophic systems, but their cause is unclear. Higher temperatures and higher pH levels may increase benthic cyanoHAB formation in systems lacking sufficient surface water phosphorus (P) because high temperatures decrease gas solubility and increase microbial respiration. Resultant anoxic conditions can also stimulate P release by reducing iron and manganese, which then releases sorbed P. Benthic photosynthesis can increase pH in pore water and stimulate the release of aluminum-bound P, particularly along shallow river margins with abundant solar radiation.
To test the hypothesis that sediment P release drives benthic cyanoHABs in oligotrophic rivers, we measured sediment P on the Hanford Reach of the Columbia River, where recent benthic cyanoHABs killed dogs and sickened humans. We collected pore water and surface water from several sites along the river reach. We also collected littoral sediment cores and performed sequential P extractions to determine the conditions that could lead to P release. These P fractions included loosely bound, redox-sensitive, pH-sensitive, organically bound, and calcium bound P. Our aim was to identify dominant pools of sediment P to determine if higher temperatures and pH might cause elevated P flux to pore water, potentially providing a nutrient source for benthic cyanobacteria.
Surface water P concentrations were generally low (mean = 0.018 mgP/L, range = 0.002 - 0.125 mgP/L), whereas pore water P concentrations were much higher (mean = 0.484 mgP/L, range = 0.012 - 4.943 mgP/L), often exceeding EPA safe water guidelines (0.100 mgP/L). Releasable P fractions (by percentage of totals) in the Hanford Reach varied by site, but in general, pH-sensitive and organically bound P dominated (redox-sensitive range = 8% - 28%, pH-sensitive range = 31% - 65%, organically bound range = 18% - 61%). Our fractionation data suggests that a high proportion of sediment P in the in the Hanford Reach is releasable under increased temperature and pH, potentially causing elevated pore water P concentrations. Our results also suggest that low-P surface water can overlie high-P pore water, with pore water P potentially fueling benthic cyanobacteria blooms in oligotrophic surface waters.