Over this wet summer, Melburnians and Sydneysiders have had to think twice about cooling off at their local beach. Heavy rainfall has swollen rivers and pumped pollutants, nutrients and murky fresh water far out to sea. Swimmers at Port Phillip Bay beaches are emerging coated in brown goo, while Sydney's seas were contaminated last week.

During 2022, floods repeatedly hit Australia's eastern seaboard, causing an estimated A$3.5 billion in damage and tragic loss of life. In Sydney, it was the wettest year on record, with 2.2 meters of rain falling in the year, twice as much as usual. The heavy rainfall event in March–April dropped more than 600 millimeters of rain alone.

We don't normally think about what happens to floodwater once it pulses out to sea. But we should. Floodwater is fresh. When it hits the sea in large volumes, it lowers the coastal ocean's salinity. In our new research published in Nature Communications, we found floodwaters in 2022 led to 116 extreme low salinity days off Sydney—ten times more than the annual average. Extreme low salinity days are those that fall into the bottom 5% of salinity values ever measured at this location.

Normally, this effect clears within six days. But in 2022, extreme low salinity persisted for months in the coastal ocean. These plumes of freshwater extended as far as 70 kilometers offshore—five times further than original estimates. You could see them from space. For fish, this is confusing and dangerous. For kelp forests or sponge gardens, it can be lethal.

Unprecedented floodwaters, unprecedented impact

Why do we care about very low salt levels in our coastal seas?

First, changing salinity levels let us track where floodwaters are headed. This is important, as floodwater often carries pollutants, sediment and other contaminants from the land into the ocean.

Second, when large volumes of freshwater arrive, it can actually change the density of the ocean. Saltwater is heavier (more dense) than freshwater, which is why some seabirds can find a layer of drinking water far out at sea when it rains heavily.

The ocean's density depends on a combination of water temperature and salinity. Off Australia's east coast, this density is usually influenced more by temperature. But during 2022, we saw something change. For the first time, we saw the density of seawater was becoming controlled by salinity.

Rather than the hottest temperatures always being seen at the surface, the heat could be anywhere in the water column, as the weight (or density) of the water was mostly being controlled by how much salt it contains, not how warm it was.

You might look at the sea and imagine it's the same all the way down. But in fact, there are very real changes as you go down the water column, and there are distinct layers of water.

What this pulse of floodwater did was change the structure and layering of the water column in unusual ways. In this coastal ocean, there's usually a light layer of warm water at the top and colder water below it. During 2022, the normal ocean water was replaced by two additional layers of fresher water from successive floods.

The 50-meter deep layer of fresh water didn't simply mix with salt. Instead, the floodwaters remained off our coastline for months, trapped between the land and the warm, swiftly flowing waters of the East Australian Current.

This article is republished from The Conversation under a Creative Commons license. Read the original article.