From carp to chemical fate–the cascading effects of invasive fish species in our waterways

Environmental fate specialists and ecotoxicologists are worried about the effects of invasive fish species and chemical residues on the ecological balance of our waterways.

5 March 2020

New Zealand is struggling with the ecological aftermath of exotic species that turned invasive. It is not always just about whom is eating who, or what. Invasive species can change the ecosystem, and by doing so, the environmental fate and bioavailability of chemicals. A good example is the situation in New Zealand involving invasive carp which was thought to arrive in the 1960’s in a goldfish consignment.

Read Marieke’s and Laura’s biography

The impact of invasive carp on the ecosystem

So a carp swam into a stream in New Zealand, big deal? Yes, it is! We already know about possums and the effects they have, but invasive carp are something to be worried about too. The effects start with signs we all can see - floating plants because of carp digging, changes in water quality due to nutrients, and murky waters from suspended sediments and so on. Over a longer period of time a beautiful waterbody full of macrophytes (aquatic plants) can be transformed into a green soup.

To top it off, if you are really out of luck, some toxic cyanobacteria join the party too. So here you have it, from beautiful aquatic ecosystem to ecological disaster because of a fish, and from this point turning back time isn’t easy. The system has a feedback loop preventing it from going back to the “good old times”. But why are the environmental fate specialists and ecotoxicologists worried?

Chemicals – locking them up and letting them go

Carp swimming in a stream.

Carp swimming in a stream

In lakes with a history of contamination, it is especially concerning. The effects of the carp invasion doesn’t stop with “just” a changed ecosystem structure and a murky swimming spot. From an environmental fate perspective, sediments (the bottom of waterways and lakes) are known to be a sink for persistent chemicals such as some pesticides. This is particularly true for those substances that have a tendency to prefer to bind to particles (organic matter) over water, this includes pesticides like chlorpyrifos and paraquat.

These pesticides stick to particles in the sediment, and become “locked up” in the environment. How much is locked up and how much is in the water around it depends on the equilibrium, a kind of ratio. If the sediment particles become suspended in the water column it doesn’t only result in a murky water column but the “locked up” (sorbed) chemicals are now surrounded by relatively clean water, and as a result some of the chemicals will be released (desorb) into the water to restore the equilibrium that the chemical likes.

Chemicals and the living world

Chemicals dissolved in the water are bad news for the organisms living in them, as now their bodies can more easily take them in. Fish, for example, filter a large volume of water through their gills where membranes are thin and blood flow is high. Chemicals hitchhiking on particles can also be ingested and absorbed via the gut. A lot of the chemicals that like to stick to sediments also like to stick to fatty tissues, so they have a tendency to move into organisms.

Once inside an organism there is a potential for bioaccumulation (the build-up of the chemical within an animal) to . The rate at which this occurs depends on several characteristics of the chemical and the host. There might even be accumulation up the food chain too, when a bigger fish, for example, eats the contaminated animal. When organisms are exposed to chemicals they can show signs of intoxication (with a worst-case scenario of death). Who survives, the native or invasive species, depends on the species specific sensitivity as well as body burden.

So there you have it, a fish, an ecological change that impacts on the environmental fate of chemicals, that in turn can result in ecotoxicological effects. At this point in time the EPA is not involved with the management of invasive species and legacy contamination. The EPA is, however, involved in the approval process to import or manufacture a wide range of dispersive chemicals. During this process the EPA staff provide advice to the decision makers, like our independent decision making committees. This advice contains a detailed risk assessment (for new active ingredients) considering the environmental fate as well as the impact of the substance on aquatic organisms.

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Author biographies

Marieke Soeter holds a Bachelor in Biology and a Master in Biological Sciences at the University of Amsterdam focussing on Ecotoxicology. She has collaborated with several institutes (IBED, Alterra, NIOO) with a key interest in the effects of biotic interactions on the distribution of pollutants. After gaining regulatory experience in a research lab in the Netherlands as a Study Director – Ecotoxicology she moved to New Zealand to work for the EPA as a senior advisor in Ecotoxicology.

Laura Suddaby is an Environmental Fate Specialist. She has a PhD from the University of York (UK, 2012), where she investigated the irreversibility of pesticide sorption to soils by laboratory experiments and mathematical modelling. She has worked as a research scientist in industry, and as a Regulatory Environmental Consultant (Environmental Fate Specialist). Laura joined the EPA in 2016 as an advisor in the Hazardous Substance Application Team.