Image Bioluminescence © L. Gershwin
The Beauty and the Beast
Phytoplankton, that is, single-celled drifting algae, are enjoying a Renaissance in Tasmania like never before. Two major types are taking advantage of the excess nutrients (a polite way of saying "too much excrement"), such as those emanating from salmon farms . Diatoms are single-celled algae with each cell encased in a microscopic glass case, like both sides of a petri dish. Dinoflagellates are similar to diatoms, but naked. Dinoflagellates are the more opportunistic of the two.
Locally, the wondrous nightime displays of bioluminescence are primarily due to an introduced species of dinoflagellate with the melodic scientific name, Noctiluca scintillans (Latin for 'sparkling night light'). This is where wonderment turns to bewilderment. Many dinoflagellates, including Noctiluca, are highly toxic to fish... and many are lethal to humans.
The biology and ecology underlying toxic algae blooms are fairly straightforward. Nutrients act like fertiliser in grandma's garden, stimulating everything to grow more vigorously. And then things go badly from there [1, 2].
Red tides (which aren't necessarily red) often produce highly noxious toxins. These bioaccumulate and can be toxic to us or to other species higher on the food chain. Some types of phytoplankton don't even need to be ingested to be toxic, their toxins are in the air or the water.
Any time there's a bloom of phytoplankton, there is a risk of an hypoxic, or low oxygen, situation developing. Most phytoplankton produce oxygen (but not all!), but they all also consume it, especially at night. Phytoplankton can also be a major driver of a type of hypoxia known as eutrophication, where bacteria involved in the breakdown of dead phytoplankton deplete the dissolved oxygen in the water. Regardless of the cause, hypoxia almost always means many animals will asphyxiate.
All phytoplankton, whether they are toxic or not, can injure delicate gills. They are physical particles, and if you've ever had a small pebble in your shoe or even a tiny sand grain in your eye, you can imagine how this would affect organisms when there are dense blooms of plankton in the water.
While poisons, hypoxia, and grittiness cause immediate injury or death, a longer-term problem is the cascade effect. Phytoplankton are stimulated by excess nutrients --> excess phytoplankton offer a lush banquet for grazers like small-bodied invertebrates and larvae (zooplankton, or animal plankton) --> abundant zooplankton means abundant food for jellyfish --> then too many jellyfish eat the eggs and larvae of other species. Ecosystems shift to being dominated by jellyfish in this way, and these very stable simplified ecosystems make it hard for fish and other complex species to return. Simply, any larvae or plankton are eaten by jellyfish before they can re-establish.