The petroleum does not stick to the kelp because of the mucilage that kelp produces. We refer to the mucilage as an "antifouling" agent, which means it prevents a lot of organisms, especially bacteria, but also oil products, from adhering to the blades. Some kelp, like Macrocystis, produce a lot of mucilage, while other kelp do not. That is an interesting finding that you observed bryozoans on kelp from petroleum areas but not the non-petroleum; you might have the basis for some new research!