What's in the Merri?

During 2024–25, Merri Paddle — the aquatic ecology working group of Friends of Merri Creek - partnered with Pesticide Watch, Deakin University, to undertake pesticide sampling in Merri Creek.

The results were sobering.

Testing detected a wide range of herbicides and insecticides in the creek, including several chemicals that are banned, or heavily restricted, in the European Union, because of concerns about groundwater contamination, persistence in the environment, and toxicity to aquatic ecosystems and pollinators.

The sampling reminds us that the pressures on the Merri are not limited to litter, or visible pollution. The creek is also carrying a complex chemical burden, generated by urban stormwater, weed management, agriculture and household chemical use across the catchment.

Image credit: Sampling water in the lower Merri Creek, photo by Julia Cirillo, MCMC.

Among the detections was DEET, the active ingredient widely used in personal insect repellents. DEET commonly enters waterways through urban stormwater and wastewater systems, particularly during warmer months when insect repellents are heavily used. Its presence in Merri Creek highlights the extent to which everyday household and consumer chemicals are now part of the pollutant load carried by urban waterways.

The sampling also detected atrazine, a herbicide banned in the European Union since 2004, because of widespread groundwater contamination and concerns about endocrine disruption, meaning it interferes with reproduction and development. It has been linked to a decline in male fertility and may cause cancer.

Another detection was thiamethoxam, an insecticide which was first severely restricted, then withdrawn in the European Union. It is one of a family of pesticides – neonicotinoids — that has been blamed for the dramatic fall in numbers of European honeybees, which threatens pollination of crops. They are also harmful to frogs: bad news for our attempts to protect the endangered Growling Grass Frog. Neonicotinoids are considered by the UNEP's Global Chemicals Outlook to be one of 11 groups of chemicals that "warrant urgent international” action (Guardian 27 September 2022).

Importantly, aquatic ecosystems do not experience these chemicals one at a time. Waterbugs, fish and frogs are exposed to mixtures of pesticides alongside other pressures, including sedimentation, altered flows, warmer water temperatures and habitat loss.

It was not surprising then, that recent waterbug monitoring undertaken by the Merri Creek Management Committee for the Merri Paddle recorded low SIGNAL scores for waterbugs in the Merri, indicating a heavily impacted system with reduced ecological diversity. The pesticide results help explain part of that picture.

Pesticides affect aquatic ecosystems in different ways. Some directly kill insects and other invertebrates,  including the waterbugs that form the base of the food chain for species such as Platypus. Others affect algae and aquatic plants. Some accumulate in sediments, where they can persist long after the original pollution event.

The pollutants detected in the Merri are not usually the result of a single industrial spill, or obvious contamination event. Some are the legacy of our urban lifestyles. For example, the presence of fipronil in the creek is most likely associated with its use as a spot-on or tablet-based flea and tick treatment for dogs.

Programs such as Pesticide Watch are important, not simply because they identify contaminants, but because they help build a clearer understanding of the cumulative pressures affecting urban waterways.

The partnership between Merri Paddle and Pesticide Watch will continue. Future work will likely focus on expanding sampling across different locations and over longer periods of time. That matters because long-term monitoring is one of the few ways to identify trends, understand the sources of pollutants and assess whether restoration and management efforts are actually improving the ecological health of the creek.

For urban waterways like the Merri, chemical pollution is often largely invisible. Community-based monitoring programs help make those pressures visible again — providing both the evidence and public awareness needed to support better protection of the creek in the future.

You can check out Pesticide Watch at: https://blogs.deakin.edu.au/pesticidewatch/about/

Image credit: Brady Hamilton, Pesticide Watch