At first, it looked like there was just one. And then there was another — and another, and another, and another.   The ripples, which suddenly appeared on the surface of Lake Merritt in October, have been observed for weeks now — and multiple experts in the Bay Area still can’t discern what’s causing them.  Damon Tighe, a local biotech educator who’s been studying Lake Merritt’s organisms for the past 20 years, says there are a few theories that could explain the mysterious ripples: one, that the lake has low oxygen, forcing small fish to rise to the surface, and two, that sardines or inland silversides are preying on zooplankton, resulting in a mass feeding event.