...My dives this past weekend revealed great progress in its disappearance. The ocean floor is now covered in juvenile blades of our native giant kelp (Macrocystis) that have sprouted over the last week as sunlight finally penetrates down to the tiny spores waiting on the bottom. Starting last fall, the Sargassum had effectively out competed them for substrate to attach to, light to photosynthesize and even nutrients to grow. The incredible density of the Sargassum needs to be viewed as a "sink" or trap for many of the nutrients that our giant kelp needed to grow during winter and spring. The Sargassum appears to be more efficient in capturing the "tasty" nitrates, phosphates and other nutrients to build its tissues, leaving less for our native species to utilize.
This means that some of our giant kelp will begin its growth cycle a few months late, and in what is probably a nutrient depleted ocean. Instead of developing into the magnificent forests over the winter and spring as it should, many stands of giant kelp are starting to grow at a time the water is warming up. Giant kelp, like Sargassum, is a cold water species and does not do well in warmer periods. One major reason is that the warm water in late spring, summer and early fall contains fewer nutrients. So our native giant kelp is beginning its life cycle at a very bad time and may not reach its full glory. In addition, plankton blooms usually start in the spring and the phytoplankton or plant plankton also consumes a great deal of nutrients. Our Macrocystis has survived one competitor only to begin a new duel with another.
A second observation also bodes poorly for our own kelp forests. I was surprised to see that many of the very young giant kelp blades have attached to the dying stipes ("stems") of the Sargassum, making them look like an odd terrestrial shrub! Of course the Macrocystis juveniles are looking for any substrate they can attach to, but by adhering to the Sargassum in large numbers they will simply "drift away" (to the tune of Dobie Gray) when the Sargassum finally expires. Thus the nasty Asian kelp keeps tormenting our own giant kelp even in death. To my shock, I spotted a number of juvenile Sargassum on the bottom after the mature ones started dying off. I feel pretty confident that they will die in the warmer water of summer, but if there is the possibility of two "generations" of this nasty stuff in a single year this is not good news.
Another interesting conclusion from my dives this weekend relates to the fact that I am seeing lots of late season algae growing on the rocks left largely bare by the death of the Sargassum. These are species that I normally don't see until summer. They are often much better at utilizing the low nutrient levels common when the water is warmer. Their presence this early in the year seems to establish additional evidence that our waters are currently somewhat nutrient poor. If my hypothesis is correct, the Sargassum has altered the normal seasonal cycle of both nutrients and of the native algae that depend on them. By shifting much of the giant kelp growth to later in the year, and creating conditions suitable for normally late summer species to appear in spring, the devil weed has radically altered our marine environment not only during the eight months it is actually present, but also in the four months it is not...