![\"Lake](\"https:\/\/www.europesays.com\/us\/wp-content\/uploads\/2025\/07\/Lake-Anza.jpg\")
Swimmers enter Lake Anza in Tilden Monday afternoon. Credit: Sara Martin\/Berkeleyside<\/p>\nSwimmers enter Lake Anza in Tilden Monday afternoon. Credit: Sara Martin\/Berkeleyside<\/p>\n
The swim season this year at Lake Anza<\/a>, which began on April 26, has been busy.\u00a0<\/p>\n On a hot, sunny day, \u201c700 people will come through the gate,\u201d said Luke Beals, a lifeguard at the Tilden lake. \u201cWe\u2019ll see about 100 people in the water at once.\u201d\u00a0<\/p>\n But until recently, it wasn\u2019t certain whether there would be a swim season at all.<\/p>\n For a decade, harmful algae blooms have been spotted at Lake Anza and other East Bay lakes, closing them periodically to swimmers. Lake Anza was entirely closed<\/a> to swimmers from 2020 to 2022 due, mostly, to the toxic blue-green algae. Last year, an algae bloom resulted in danger advisories that cut the usually April-to-September swimming season to 77 days, ending June 17.\u00a0<\/p>\n After treatments by the East Bay Regional Park District (EBRPD) earlier this year, the lake is currently safe for swimmers, with no dangerous toxin levels detected<\/a>. According to the latest weekly water quality status report, E. Coli bacteria levels are currently below the state\u2019s health standards and there\u2019s a \u201clow health risk for water contact<\/a>.\u201d<\/p>\n \u201cWe\u2019re hopeful that the treatments we did earlier this spring will keep it open through most of the year,\u201d said Matthew Graul, chief of stewardship at the park district. \u201cThere\u2019s no guarantee, so I would encourage people to look at our website before they come out (to swim).\u201d<\/p>\n But the underlying problem that the park district believes led to last year\u2019s algae bloom still remains in need of a permanent solution, which the district won\u2019t attempt until at least 2026.\u00a0<\/p>\n What causes Lake Anza to turn toxic?<\/p>\n Cyanobacteria<\/a>, commonly called blue-green algae, is present in ocean and freshwater around the world. It tends to bloom when certain conditions are met: low water levels, limited circulation and warm temperatures.<\/p>\n Cyanobacteria can be found naturally growing in oceans and water bodies across the world. But man-made lakes like Lake Anza (which was created in 1938 when the Tilden Park dam was built) tend to become more eutrophic, or excessively filled with nutrients like phosphorus and nitrogen that feed cyanobacteria. That\u2019s because when a stream channel flows into an open water body, the velocity of water slows down, depositing sediment \u2014 and nutrients \u2014 into the lake.\u00a0<\/p>\n Climate change, which drives extreme weather, makes harmful algae blooms more likely, and they\u2019re increasing worldwide. Heavy storms dump nutrient-filled sediment on the bottom of the lake. When followed by periods of drought, the stagnant water and warm temperatures create the perfect conditions for a harmful algal bloom.\u00a0<\/p>\n Eutrophication sets off a chain reaction in the ecosystem. The excessive loads of nutrients result in algal blooms, which in large amounts deplete oxygen levels in the water and can result in mass marine die-offs like the one that impacted Lake Merritt in 2022<\/a>.<\/p>\n What went wrong last year and what is the park district doing?\u00a0<\/strong><\/p>\n To reduce the frequency and duration of harmful algal blooms, the district in July 2020 installed a hypolimnetic oxygenation system in Lake Anza to bring dissolved oxygen to the depths of the lake (important to maintain its rainbow trout population).<\/p>\n But the oxygenation system \u201cappears to be too strong for Lake Anza,\u201d according to a 2024 EBRPD report on harmful algal blooms<\/a>.\u00a0<\/p>\n Last summer, bubbles from the system caused the colder, denser and nutrient-rich water at the bottom of the lake to float up and \u201cmix the lake,\u201d fueling the harmful algae growth. (Graul said harm to marine life in the lake was minimal.)\u00a0\u00a0<\/p>\n Oxygen levels in pipes buried at the bottom of the lake have been dropped to prevent this mixing from happening again this year, Graul said.\u00a0<\/p>\n An upgraded model, called oxygen saturation technology<\/a>, that uses a pump system to allow the park district to more easily moderate oxygen levels without creating any bubbles is being tested in Lake Temescal in Oakland<\/a>, which also faces cyanobacterial blooms, Graul said.\u00a0<\/p>\n \u201cThey take oxygen gas from nearby on the shore and saturate the water and release it into the lake,\u201d Eli Kersh, whose company LakeTech, works to improve water quality in the East Bay, has said<\/a> \u2014 describing the technology as expensive but \u201cfoolproof.\u201d<\/p>\n If the experiment is successful at Lake Temescal, Graul said the park district \u201cmay consider options to retrofit or replace the system at Lake Anza\u201d \u2014 likely in 2026.\u00a0\u00a0<\/p>\n This year for the first time, the park district is also using a water coagulant called aluminum chlorohydrate \u2014 which acts as a binding agent \u2014 that it hopes will help mitigate the release of sentimental nutrients like phosphorus and nitrogen-ammonia both from the surface and bottom of the lake, thereby improving water clarity and quality.<\/p>\n \u201cIt doesn\u2019t work to completely eliminate them, but it reduces the frequency and magnitude of the algal blooms,\u201d Graul said.\u00a0<\/p>\n Another organism affecting swimmers at Lake Anza in recent years has been the Azolla fern, a squishy, green and brown plant that quickly took over much of the lake\u2019s surface in 2022. The fern isn\u2019t toxic, and actually helped to reduce the algae blooms, but creates an obstacle for swimmers. But it disappeared in 2023, likely washed away<\/a> by heavy winter storms, and didn\u2019t need treatment this year. The park district now has an aquatic harvester machine, purchased in 2023, that can cull the fern if needed.<\/p>\n This year\u2019s treatments, including beach maintenance, said Graul, have cost the park district $84,500. The amount surpassed the previous year\u2019s expenditures on the lake, according to Graul.<\/p>\n The work has included weekly water quality testing, which continues, and manual labor with \u201chand tools and rakes\u201d to clear aquatic vegetation in the swim area \u201cwhenever the plants accumulate.\u201d\u00a0<\/p>\n Lifeguard says lake is cleanest it\u2019s been in years<\/p>\n Beals, the Lake Anza lifeguard, said he and his colleagues swim 550 yards as a warm-up exercise in the morning whenever they are on duty and are amazed by the lake\u2019s cleanliness.\u00a0<\/p>\n \u201cI\u2019ve been a lifeguard here for three years now, and this year is the best year I\u2019ve ever seen the lake,\u201d he said.\u00a0<\/p>\n The lake opens for swimming at 11 a.m. and closes at 6 p.m. on weekdays and weekends.\u00a0<\/p>\n People are still cautioned<\/a> to avoid algae and scum while swimming, keep pets out of the lake and clean fish caught there with tap or bottled water. Several dog deaths in California have been attributed to toxic algae poisoning in re<\/a>cent years. And people should monitor the EBRPD website<\/a> for water quality updates.<\/p>\n There were 3,212 swim attendances at Lake Anza in 2024, according to EBRPD spokesperson Dave Mason. That number could double this year with a full swim season.\u00a0<\/p>\n While optimism is high, Mason struck a cautionary tone. Lake Anza and other bodies of water in the East Bay will continue to face water quality issues, he said.<\/p>\n \u201cThe changing climate has created challenges for water quality, which can limit recreation access, including open water swimming,\u201d he said.<\/p>\n Iris Kwok and Zac Farber contributed reporting to this story.\u00a0<\/p>\n “*” indicates required fields<\/p>\n","protected":false},"excerpt":{"rendered":"Swimmers enter Lake Anza in Tilden Monday afternoon. Credit: Sara Martin\/Berkeleyside The swim season this year at Lake…\n","protected":false},"author":3,"featured_media":31243,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[23],"tags":[746,26433,26434,26435,159,26436,67,132,68],"class_list":{"0":"post-31242","1":"post","2":"type-post","3":"status-publish","4":"format-standard","5":"has-post-thumbnail","7":"category-environment","8":"tag-environment","9":"tag-home-highlight","10":"tag-home-lead","11":"tag-lake-anza","12":"tag-science","13":"tag-tilden-regional-park","14":"tag-united-states","15":"tag-unitedstates","16":"tag-us"},"share_on_mastodon":{"url":"https:\/\/pubeurope.com\/@us\/114780887032126608","error":""},"_links":{"self":[{"href":"https:\/\/www.europesays.com\/us\/wp-json\/wp\/v2\/posts\/31242","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.europesays.com\/us\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.europesays.com\/us\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.europesays.com\/us\/wp-json\/wp\/v2\/users\/3"}],"replies":[{"embeddable":true,"href":"https:\/\/www.europesays.com\/us\/wp-json\/wp\/v2\/comments?post=31242"}],"version-history":[{"count":0,"href":"https:\/\/www.europesays.com\/us\/wp-json\/wp\/v2\/posts\/31242\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.europesays.com\/us\/wp-json\/wp\/v2\/media\/31243"}],"wp:attachment":[{"href":"https:\/\/www.europesays.com\/us\/wp-json\/wp\/v2\/media?parent=31242"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.europesays.com\/us\/wp-json\/wp\/v2\/categories?post=31242"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.europesays.com\/us\/wp-json\/wp\/v2\/tags?post=31242"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}![\"Lake](\"https:\/\/www.europesays.com\/us\/wp-content\/uploads\/2025\/07\/1751417232_105_Lake-Anza.jpg\")
A caution sign posted at Lake Anza this week. Credit: Sara Martin\/Berkeleyside<\/p>\n![\"Lake](\"https:\/\/www.europesays.com\/us\/wp-content\/uploads\/2025\/07\/1751417232_250_Lake-Anza.jpg\")
Lake Anza is a man-made lake that came into existence when the Tilden Park dam was built in 1938. Credit: Sara Martin\/Berkeleyside<\/p>\n![\"Lake](\"https:\/\/www.europesays.com\/us\/wp-content\/uploads\/2025\/07\/1751417233_215_Lake-Anza.jpg\")
Park district officials are hopeful the swim season will stretch until mid-September. Credit: Sara Martin\/Berkeleyside<\/p>\n