Lake Merritt, in the center of Oakland, California, is a tidal estuary connected to the Pacific Ocean. It usually teems with life, both human and marine. In early September, its 3-mile shoreline was bustling with joggers. But in the sunset-blackened waters, the gleaming white corpses of thousands of decaying fish bobbed along in the gentle tide and piled up in mounds along the lagoon’s edges. In late July, an algae bloom began spreading in San Francisco Bay, which stretches 60 miles north to south.