Port Phillip Bay jellyfish, sea lettuce and bioluminescent algae

A few local people have asked me what is going on regarding Port Phillip Bay, jellyfish, sea lettuce and bioluminescent algae at the moment. There seems to have been an unusual increase in these due to high levels of freshwater inputs and nutrientsEnergy and nutrients are the same thing. Plants capture energy from the Sun and store it in chemicals, via the process of photosynthesis. The excess greenery and waste that plants create, contain chemicals that animals can eat, in order to build their own bodies and reproduce. When a chemical is used this way, we call it a nutrient. As we More. To understand a bit more about what you’re seeing, it helps to understand a bit about the current ecology.

La Ninã, freshwater and nutrients

We are currently in a La Ninã which is when a finger of cooler water extends across the Pacific Ocean. This brings much wetter weather to Australia. This year we’ve had strong spring rain. Compare the dam storage levels this season (2021-2022) with 2006, which was peak of the El Ninõ drought. This freshwater flows down the Yarra River and introduces huge amounts of nutrientA substance that contains the raw materials for life. At a chemical level, these are contained inside compounds that are absorbed into the body and essential energy-containing molecules are extracted, so that energy can be transformed into other chemical processes that use the energy for living. More into the bay.

Mixing, salinity fronts, jellyfish and algae

Freshwater has less salt and is therefore less dense. Like the layers in a cocktail, the freshwater flows over the surface. When I was snorkelling in Port Phillip Bay yesterday, I swam through the edge of this layer. This paper has a useful figure (below) which shows how this works.

The furthest edge of the mixing zone in Port Phillip Bay is where you get the highest jellyfish and bioluminescent algae densities. Under natural conditions, this would introduce small amounts of nutrient into Ricketts Point, and drive the ecosystemHow ecosystems function An ecosystem is a community of lifeforms that interact in such an optimal way that how ecosystems function best, is when all components (including humans and other animals) can persist and live alongside each other for the longest time possible. Ecosystems are fuelled by the energy created by plants (primary producers) that convert the Sun's heat energy More. That’s assuming the ecosystem is intact enough to absorb all of this excess.

Now imagine this from above. In summer, the prevailing winds are southerly which pushes a finger of water clockwise around the coast. The fresh water plume extends down towards Ricketts Point where the front is shallowest at its tip. All that biomassThe weight of living organisms. Biomass can be measured in relation to the amount of carbon, the dry weight (with all moisture removed) or living weight. In general it can be used to describe the volume of energy that is contained inside systems, as the size of animals relates to their metabolism and therefore, how much energy they contain and More is concentrated along a line. The wind pushes it towards the shore.

Sal Cooper took the photo in the afternoon (below). In the morning I only saw faint wisps of this pink algae. By the afternoon the wind had pushed it all in against the seawall. At night these algae are bioluminescent.

What we are seeing at Ricketts Point is the very rapid uptake of surplus nutrients from farmland up the Yarra River. Algae grow extremely fast in these conditions, particularly when it is warm. Jellyfish are next in the food chainA single thread in a food web illustrating the chain of animals that eat each other. At the base of the food chain are small high-energy (fast metabolism) animals and at the other end large low metabolism animals. An example would be whales eating krill that eat plankton that eat algae. Or lions that eat gazelles that eat grass. More. So, without them, we’d be overwhelmed with algae and eventually they will die and introduce toxins into the water. Jellies eat the algae and launder this into the food chain. This year, Lions Mane Jellyfish are particularly abundant.

Nutrient settlement and ecosystem resilience

As the winds pick up, surplus the top freshwater layer mixes, whereby nutrients and algae are spread over the reef.

Unused nutrients that reach Ricketts Point and land on the reef create conditions ideal for sea lettuce to grow. Sea urchins love to eat sea lettuce. It grows very fast in nutrient-rich conditions and could perhap explains why these animals do so well. As I discuss here, reducing nutrient input is far more important than killing sea urchins.

A healthier environment would have very little excess nutrient. According to a colleague who monitors the bay, this is not that usual for this location. The La Ninã is likely to have exacerbated conditions this year – though annual freshwater input is essential. Drought has other consequences.

Though fluctuations like this happen naturally, resilienceReferring to an ecosystem’s ability to maintain a steady stable-state. The need to build resilience is entirely anthropocentric and symptomatic of ecosystems that are damaged or declining, leading to loss of ecosystem services on which humans depend. More of systems depends on having adequate biomass of other animals, and making sure these events aren’t too regular. It is likely the sea lettuce will decline over the coming months as nutrient levels decline and the plants senesce and die.

A sign of the future?

Some people are asking how to avoid jellies. The answer is, there is no way to do this, as it’s a natural process addressing a bigger problem. They occur wherever the nutrient front flows and concentrate according to somewhat unpredictable winds and tides.

Some scientists think jellyfish are becoming more common. A warmer and moister climate means greater algal growth and a tipping of systems to a new state. This is certainly something to keep an eye on. We definitely don’t want a jellyfish-dominated ecosystem.

Meanwhile, we would do well to refocus our attention on resilience, which means reducing pressures from fishing and rebuilding animal biomass. We can also do more to reduce nutrient input and climate change.