A permit approved Thursday by the California Coastal Commission will allow the Carlsbad desalination plant to modify its seawater intakes and discharge structures to better protect marine life and create a “stand-alone” system. The desalination plant takes ocean water from the Agua Hedionda Lagoon through intakes formerly used for the cooling system at the now demolished Encina power plant, built by San Diego Gas & Electric Co. in the 1950s. The old power plant was replaced by a more efficient plant that does not use seawater for cooling.