Spotted a jellyfish bloom recently? Here’s what may have triggered it
- Written by Lisa-ann Gershwin, Research Scientist in Marine Biology, University of Tasmania
On a calm summer morning in southern Australia, the water can look deceptively clear, until you see thousands of gelatinous shapes washing ashore.
In January, thousands of pink lion’s mane jellyfish washed into Port Phillip Bay, prompting beach warnings and startling swimmers more accustomed to cold water than the shock of stinging tentacles.
The same month, unusually high numbers of moon jellyfish were reported across southern Tasmanian coastal waters.
If you’re swimming in southern seas and have an encounter with a jellyfish swarm, you may well wonder what led to it. Could a cold spike, a marine heatwave or other changes in the ocean have triggered the bloom?
Are these blooms normal?
Many Australians associate jellyfish with the dangerous stings of northern species such as box jellyfish and Irukandji. But jellyfish aren’t just confined to tropical waters. In southern Australia, species such as lions mane and moon jellyfish are more common, particularly this summer.
In fact, hundreds of species of jellyfish are found in Australian waters, ranging in size from a mere speck to nearly two metres wide. And it’s natural for jellyfish numbers to boom and bust. When conditions are favourable, they can bloom into superabundance. Their millions of mouths can strip every particle of food out of the water, from fish fry to fish eggs and plankton.
That said, there is a clear pattern visible globally. More disturbed marine ecosystems tend to experience larger and more persistent jellyfish blooms.
Jellyfish blooms can act as a visible indicator that something is out of balance in the ocean. For example, when oceans are overfished, polluted or suffer from other environmental degradation, this can trigger highly visible jellyfish blooms. Overfishing can remove predators and competitors, leaving jellyfish to thrive unchecked.
Coastal impacts from urbanisation add further pressure. When rivers dump huge amounts of nitrogen and other nutrients from farms and cities into the seas, this and other forms of pollution can favour jellyfish growth. For oceans affected by several of these issues, jellyfish blooms often grow larger and last longer.
Researchers have found jellyfish are becoming more abundant in many areas. Warmer waters favour jellyfish. But it is too simplistic to say that all jellyfish are are taking over everywhere. Different species respond to different conditions in their ecosystems.
Warmer water is good for jellyfish
As climate change heats up the oceans, many species are struggling to cope. Warmer water holds less dissolved oxygen than cooler water, making life harder for species with higher oxygen demands, like crustaceans and fish.
Jellyfish are highly responsive to their environment. Warmer water gives their metabolism a boost, so longer warm periods or sudden marine heatwaves can create conditions highly favourable for rapid population growth. They grow faster, eat more, reproduce faster and live longer.
Surprisingly, cold matters too. Researchers have found moon jellyfish can also get a boost from cold water. In their polyp stage, young moon jellies attach themselves to rocks or coral. When a cold period is followed by a return to normal temperatures, the polyps get a cue to begin budding off larval jellyfish (a process known as strobilation).
Interestingly, the size of the temperature change – not just whether it is warm or cool – can make jellyfish blooms more intense.
For example, in the eastern Bering Sea off Alaska and in the waters off Peru, long-term monitoring has shown jellyfish numbers closely tracked warming and cooling periods associated with the El Niño climate driver.
Ocean warming can be a threat multiplier, amplifying pressures on marine ecosystems, creating conditions in which jellyfish populations can expand dramatically.
Less oxygen is bad for fish but jellies don’t care
Heavy breathing marine species such as fish struggle to survive in warmer, less oxygenated water. Here, too, jellyfish do well. They can even survive periods in oxygen-free habitats, as they can store oxygen in their jelly.
As I put it in my TEDx Talk, when marine ecosystems face pressures on several fronts, jellyfish are often advantaged – while the predators normally keeping them in check suffer.
What to do if stung
While life-threatening box jellyfish and Irukandji stings have occurred in subtropical and even temperate Australian waters, both types are commoner in the tropics. For this reason, the Australian Resuscitation Council recommends different treatments for stung swimmers in southern and northern waters.
In the tropics, the priority is saving a life. Douse the sting liberally with vinegar to inactivate stinging cells, seek medical care if appropriate and be prepared to commence CPR if necessary.
Outside the tropics, the priority is pain relief. Rinse the sting well with seawater to wash away undischarged stinging cells, then use hot water or ice for the pain. In either case, seek immediate medical care if breathing difficulties or other systemic symptoms develop.
Blooms are beautiful
Many people who marvel at the beauty of jellyfish in an aquarium might shy away from them in the sea. But while it can feel daunting to think about being stung, jellyfish blooms in southern waters more often present a splendid opportunity to see an unusual natural phenomenon.
Authors: Lisa-ann Gershwin, Research Scientist in Marine Biology, University of Tasmania
