For most of the past decade, toxic cyanobacteria kept Oakland's Temescal swim beach closed more days than open. In 2025 it stayed open all season — because of a machine on the lake floor, not the rain gauge.

In 2025, Lake Temescal's swim beach stayed open the entire season without a single harmful algae closure — the first such stretch in years, according to the East Bay Regional Park District. The difference isn't weather or the end of drought. EBRPD this year installed an Oxygen Saturation Technology system in the lake and applied phosphorus-capping treatments that interrupted a bloom cycle that had been building since at least 2014.

The mechanism behind those closures ran like this: Lake Temescal is a shallow reservoir whose sediment holds a chronic surplus of phosphorus. When the water warms and stratifies in summer, the bottom layer goes oxygen-depleted. That anoxia releases stored phosphorus back into the water column, feeding Microcystis — the dominant bloom-forming cyanobacterium, commonly called "blue-green algae." EBRPD posts a red Danger/Closure sign at the swim beach when microcystin toxin concentrations hit 6 micrograms per liter and keeps it there until the count drops.

Between 2014 — when EBRPD documented its first toxic harmful algal bloom at Temescal — and 2022, that process produced 670 total closure days and 955 more days of caution-level advisories, per district records. Drought conditions through the 2010s accelerated the problem: warmer, lower water hastens stratification and deepens anoxia. But drought was an accelerant onto a pre-existing structural phosphorus surplus, not its cause.

EBRPD tried chemical routes across several seasons. A 2019 alum treatment temporarily bound phosphorus in the sediment — "while the treatment was temporarily effective, a toxic bloom was detected the following summer," Ecological Services Manager Becky Tuden told Oakland North in November 2023. In 2023, four separate chemical treatments, including hydrogen peroxide and EutroSORB, a lanthanum-modified clay designed to cap sediment phosphorus, produced about two weeks of swimming. In 2024, a Danger/Closure Advisory ran from March 29 to July 18; the swim season didn't open at all until August 3. A 2019 feasibility study had put full lake dredging and sediment management at $25 million with a multi-year closure — an option the district has not pursued.

The OST system now running in Temescal addresses the problem at its root: by mechanically oxygenating the deep water, it prevents the anoxic bottom conditions that trigger phosphorus release in the first place. EBRPD applied similar logic at Lake Anza in Tilden Regional Park, where it installed a Hypolimnetic Oxygenation System in July 2020; the Temescal installation follows five years later. The district reports both beaches "stayed open the entire swim season without HAB-related closures" in 2025.

Current monitoring: weekly HAB and E. coli sampling April through October, real-time dissolved oxygen and temperature loggers at multiple depths, and a "stoplight" signage system — green, yellow, red — at the swim beach, with current readings posted at ebparks.org. Alameda County Environmental Health co-determines advisories. The closure threshold remains ≥6 µg/L microcystins.

One note for anyone bringing a dog: EBRPD flags that even brief water contact during a bloom can be lethal for animals. The live stoplight is at the lake entrance and online before you leave the house.