Sea Sawdust, Comb Jellies, Salps and Rapid Responders – Tony Isaacson
When I moved from South Australia to the Sunshine Coast in 2003 a bright orange slick on the ocean was described to me as coral spawn. Sometimes called “sea sawdust”, this stuff even confused Captain Cook in 1770 as he charted shoals along the Queensland coast. Neither shifting sands nor coral spawn, it is a kind of cyanobacteria (blue–green algae) that turns enormous areas of the ocean bright orange from the oxidation of iron (rust) in the water column.
The tiny photosynthetic cells clump together and the oxygen that they produce carries them to the surface in such quantities that they can reduce the light available to ecosystems below them. Trichodesmium is of interest because up to 40% of all nitrogen fixation in the ocean can be attributed to the genus. Some blooms or “red tides” can have a toxic effect on invertebrates and humans because of bacteria associated with the blue-green algae.
When students from a Sunshine Coast Grammar School study group snorkelled through one of these blooms they said that it felt like swimming through jelly. On closer inspection, the water below the surface was full of comb jellies or Ctenophores. While tempting to assume that they were feeding on the Trichodesmium, it’s more likely that these shimmering, cilia propelled predators were feeding on copepods, the tiny crustaceans that are targeted by Southern Right Whales. On even closer inspection different comb jellies that looked like attack craft from Star Wars were hunting their cousins and swallowing them whole.
I write of these events at the front end of the food chain because I am fascinated by what I see during the safety stops at the end of many dives on ex-HMAS Brisbane. When weather events prevent us from enjoying the usual sights on a scuba dive, the algae blooms stimulate a rapid response by plankton that includes some of the fastest growing and weird life forms on our planet. Several baited shark dives were postponed by life affecting floods when I was in Africa and when we eventually got visibility good enough to see the sharks, I remember being distracted by a long gelatinous “snake” that swam between me and my dive buddy. Not a comb jelly this time but a colonial tunicate (sea squirt) called a salp. The collaborative tunicates are embedded in a gelatinous inward facing wall. Each tunicate pumps nutrient-carrying sea water into the tube to produce a strong current of sea water to jet-propel a beast that has been known to grow up to 15 metres long and a metre in diameter. Perhaps one of the sea monsters from stories of the unexplained.
Salps respond very quickly to algae blooms and can grow much faster than other multicellular animals. We can thank them for rapidly turning the green – blue haze on our dive sites into relatively clean, diver friendly conditions after major storm events. The bloom ends when the salps filter out most of the phytoplankton and as their life cycle ends, we get to enjoy those days of great visibility that keep us coming back for more.