California gulls, which stopped nesting on Mono Lake’s Negit Island about the time low water exposed a land bridge to the island a few years ago, have returned there this spring, researchers said. The environmentalist Mono Lake Committee said the return of the gulls shows that diversion of water by the Los Angeles Department of Water and Power was responsible for driving the gulls away. But, a DWP spokesman said there is no proof for that thesis. The Mono Lake Committee claimed that the birds left in 1979, when the lowering lake level exposed the bridge that allowed coyotes to reach the island. The lake’s volume began to rise again with the heavy snow runoff in 1982, and abundant water that season and the next meant the DWP did not need to divert water to the aqueduct.