Each year, as winter shifts toward spring, visitors to Lake Merritt sometimes spot a bright yellow film drifting across the water’s surface. The sight can be unsettling. Is it sewage? Paint? Chemical runoff?
In most cases, the explanation is far less alarming: it’s pollen. Acacia pollen grains are relatively large — about 30–70 micrometers in size — and often appear in clustered groupings called polyads, containing 16 to 32 grains. Unlike many tree pollens that disperse widely on the wind, acacia pollen is primarily transferred by bees, which limits its constant presence in the air. However, during peak blooming season, fallen blossoms and localized air movement can still carry enough pollen into nearby water to create visible yellow streaks or slicks.
Although the film can look artificial or even oily, it is a natural and generally non-toxic occurrence. These seasonal blooms do not typically pose a long-term threat to aquatic ecosystems and usually fade as flowering ends and weather conditions change.
Because several acacia trees line the lake’s shoreline, pollen and flower debris have a direct path into the water each year. The result can be visually dramatic but short-lived.
So when a yellow sheen appears on the lake, there is usually little reason for alarm. More often than not, it is simply a seasonal signal — pollen marking the arrival of spring rather than evidence of pollution.
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