The kelps in our kelp forest exhibit did the same. We tried adding fertilizer to the exhibit, but it didn't have much of an effect. That doesn't happen here, as even in a strong El Nino we don't get much warmer than 67 degrees F., and the kelps can handle that temperature. We expect everything will be back to normal by late next spring after upwelling starts again in February. Hmm, great question.
Jellies have nerve nets that seem to work fairly well for locomotion.
There is a jelly that I think has a brain - these are the box jellies or cubomedusae. This is an active area of research and a fun one. There may be other types of particles as well, such as mucus, bioluminescent particles, and other byproducts of metabolism. Josh, I doubt that people counted all of the kelp forests along the shores of the cold water around the world, so we probably do not realy know the exact number of kelp forests or kelp bads around the world. But it is well known that to find kelp forests you need to go to cold and nutrient rich ocean sites.