Top, the bull kelp found off Northern California Josh Russo, left, organized a group of divers and boaters to remove purple urchins from Albion Cove.Credit...Gabriella Angotti-Jones/The New York Times That was before the urchins mowed down Northern California’s kelp forests. The underwater forests — huge, sprawling tangles of brown seaweed — are in many ways just as important to the oceans as tree