Tilden Regional Park’s Lake Anza will reopen after facing a closure due to the COVID-19 pandemic and maintenance issues.

During the COVID-19 pandemic, the lake underwent a hypolimnetic oxygenation system, which was intended to improve the amount of oxygen directed to the lake, according to Becky Tuden, East Bay Regional Park’s ecological services manager.

“The reason we need to provide oxygen to the lake is at the very lower depths of the lake,” Tuden said. “It was becoming anoxic, which means that there was no oxygen available at those lower depths.”

Tuden added this situation harmed Lake Anza’s biota, especially since it is home to the native species: rainbow trout. Due to the lake’s anoxic state, there was potential for the trout to migrate into the larger bay ecosystem. 

In addition, Tuden noted the lake’s anoxic state facilitated the process of harmful algal blooms by holding high-nutrient sediments.

“(Algae) love low-oxygen environments because the chemistry of the lake is such that when there’s no oxygen, the nutrients are released,” Tuden said. “And then we like to call it sort of a smorgasbord for the cyanobacteria on these blue green algae.”

According to Tuden, over the last few years in Lake Anza, a growth of native algae called Azolla covered much of the lake.

However, Tuden said the large growth of Azolla was actually beneficial to the lake’s ecosystem.

“While it looked unattractive, it actually helped keep the lake cool, (as) colder water can have more oxygen,” Tuden said. “It also helped keep the cyanobacteria from being competitive because they couldn’t get any sunlight.”

Not every algal bloom necessarily is toxic. Tuden noted that sometimes a very small amount of algae can be high in toxicity, whereas large blooms may sometimes not be. According to Tuden, scientists are unsure what causes this difference in toxicity.

Toxic algal blooms can cause anything from gastrointestinal problems to skin rashes, Tuden said. Some can even cause neurotoxins.

As the lake opens, Tuden said the park will continue to monitor the lake’s oxygen content and the state of its algae to make sure that residents and their pets are safe from toxins.