Warning signs are still up at two Russian River beaches after health officials detect low levels of a harmful algae. County health officials said yesterday samples collected found levels of a naturally produced neurotoxin at Johnson’s Beach in Guerneville and Steelhead Beach in Forestville. In every case, the measurable levels were extremely minimal, but the state requires posting warnings at any level of detection. Though beaches remain open, visitors are urged to take precautions to reduce the threat of accidental ingestion.