East Bay lake treatments, oxygenation dramatically cut toxins and kept beaches open in 2025

Anya Bray, ecological services coordinator at East Bay Regional Park District, told CC Hub members that a 2025 pilot combining phosphorus‑binding treatments and an oxygenation saturation technology produced striking results at two small urban reservoirs.

"So before the treatment, the secchi depth was 1.6 meters. After the treatment, the depth that I could see the secchi depth doubled to 3.5, and then 12 days later, we even had 4.6 meters of clarity," Bray said, summarizing on‑the‑water monitoring after the aluminum‑based treatment and the oxygenation installation.

The park district tested ACH (aluminum chloride) at Lake Anza and a mix of ACH plus Foslox at Lake Temescal, and installed an oxygenation system at Temescal to reduce bottom anoxia that can release phosphorus from sediments. Bray presented surface and deep phosphorus data showing hypolimnetic phosphorus dropped after treatment and remained low until the storm season. Toxin measurements (microcystin) fell to mostly below CCHAB caution thresholds during the monitoring period; Bray reported a December microcystin result of 0.96 µg/L at Temescal as a recent measurement.

The practical result was that, according to Bray, both swim beaches stayed open through the swim season without HAB‑related closures — a first in several years for the district. Bray and collaborators used a multi‑metric monitoring approach including Secchi clarity, chlorophyll, extracted phycocyanin, microscopy, toxin analyses, and targeted phytoplankton counts to document community changes over six months. Phytoplankton composition shifted away from cyanobacteria dominance to diatoms and other groups during the study period, and dissolved oxygen profiles improved after OST installation.

Bray cautioned that increased clarity produced more aquatic‑weed growth (milfoil and pondweed at Anza; filamentous algae at Temescal) and stressed this consequence requires follow‑up management. She said the district will collect sediment cores in the coming season to estimate residual phosphorus and to inform whether maintenance doses of ACH (a lower, periodic application) could be effective rather than full re‑treatment.

The study was carried out with monitoring support from Blue Water Science, Lake Tech, and other consultants; Bray noted the summer was unusually cool in 2025 and that climate variation is one of several factors that could influence outcomes.

The park district expects to repeat a similar, adaptive approach in 2026 while clarifying permitting and weed‑management plans. Bray said partners and technical advisers remain engaged to refine dosing, sediment monitoring and safeguards for sensitive species.