A tale of two floods

By Kallie Townsend

The Goulburn River at McCoys Bridge. Credit: Kallie Townsend

For a shrimp, 2016 to 2017 was an exciting, if not terrifying, time to live in the Goulburn River. To borrow and change the words of Charles Dickens: it was the best of times, it was the worst of times. It was the age of prosperity, it was the age of poverty. It was the epoch of success, it was the epoch of recession. It was the season of dreams, it was the season of nightmares. It was the spring of life, it was the summer of death.

As a researcher studying how environmental water affects invertebrates, it was a confusing time and it was an informative time. It began with record breaking rainfall in winter and spring. The Goulburn River broke its banks and experienced moderate flooding. Usually environmental water was delivered at this time as a spring fresh to promote fish spawning and vegetative growth. This year the spring fresh was cancelled. Instead of monitoring macroinvertebrates before and after the spring fresh, we were left scratching our heads. Then we decided to monitor after the flood and see if we could use this information to build on what we know about environmental flows.

What was the flood providing for these shrimp that the spring fresh wasn’t?

Some macroinvertebrates showed very strong responses to the flood. One of these were shrimp, Paratya australiensis. Previously, we found shrimp populations grew slightly bigger after spring freshes. In comparison, their population seemed to explode after the spring flood. What was the flood providing for these shrimp that the spring fresh wasn’t? Although it wasn’t clear, the flood may have been providing more food. Unlike a spring fresh, a flood isn’t confined to the river banks. Parts of the floodplain are inundated and wetlands are reconnected to the river channel. As a result, a lot of organic matter is flushed into the river channel and becomes a part of the food web, increasing the amount of food available for macroinvertebrates like shrimp. Flooding can also change the availability of habitats within the river channel. Shrimp tend to love areas with aquatic plants and organic matter. Places where water is slow-moving and shallow are especially important for shrimp larvae. The flood may have increased the availability and quality of these habitats, enabling more shrimp to survive and thrive in the lower Goulburn River.

Crustaceans including shrimp, prawns and crayfish were seen climbing out of the water.

However, the best of times was short-lived. Just like in spring, another flood happened in summer. On the last day of the year, a massive storm event in the catchment caused flooding in the Goulburn River and some of its tributaries, including Seven Creeks. This flood was smaller than the spring flood, but it was devastating. Many of the wetlands that were flooded during spring were now filled with stagnant, organic rich water. The summer flood washed the stagnant water back into the creeks and rivers, causing dissolved oxygen concentrations to drop dangerously low in what is known as a blackwater event. In Shepparton and further down the Goulburn River, reports started coming in about fish dying. Crustaceans including shrimp, prawns and crayfish were seen climbing out of the water. Over the next few days, environmental water was released to improve water quality and disperse the blackwater. It worked, but the blackwater event had already left its mark on the aquatic organisms of the lower Goulburn River. The once-thriving shrimp population was reduced. Although shrimp showed signs of recovery in February, the survivors were smaller individuals that probably drifted into the area from unimpacted sites upstream.

Two floods had two very different effects on shrimp in the Goulburn River. It serves as a reminder that ecosystems are complex and we need to be cautious in how we manage our ecosystems. It would be very easy to say we need to change how we deliver environmental water so that we can achieve the same positive effects we saw from the first flood. We do that at the risk of replicating the second flood. For now, it is wiser to continue to monitor the smaller, positive effects of spring freshes, while learning what we can from the large spectacular events nature occasionally throws our way.

Dr Kallie Townsend can be contacted at tok@unimelb.edu.au.