Drugs in surface water – a hidden threat to ecosystems

The findings of rivers and lakes are increasingly confirming the presence of not only chemical pollutants, detected there for years, but also trace amounts of drugs. The latest analysis, described in the journal Current Biology, indicates that one of the most frequently labeled psychoactive substances in rivers and lakes is cocaine. Scientists have shown its negative effects on wild salmon populations.

Cocaine in water

According to the study authors behind the 2024 World Drug Report, some 292 million people worldwide were using illicit psychoactive substances in 2022. Drug metabolites flow with urine into wastewater, and most wastewater treatment plants cannot cope with their complete elimination. As a result, compounds enter rivers and lakes in trace amounts.

A meta-analysis of previous studies shows that cocaine concentrations in surface waters can average about 105 ng/l, and in extreme cases several thousand nanograms per liter. These are extremely small amounts, but nonetheless biologically active. Importantly, the aquatic environment contains not only cocaine itself, but also its metabolite, benzoylecgonine, which persists in the environment for longer and in higher concentrations than the drug itself.

How can drugs affect fish?

Cocaine is one of the so-called psychomotor stimulants, or substances that stimulate the nervous system. It affects neurotransmitters – chemical compounds in the brain responsible for transmitting signals between nerve cells. One of these is dopamine, which in animals (including fish) is responsible for movement, motivation and responses to stimuli, among other things.

Previous laboratory studies have shown that even very low concentrations of cocaine can increase the locomotor activity of aquatic organisms, altering their swimming rates or foraging behavior. However, most of these studies were conducted under artificial conditions, and therefore in a simplified environment that does not reflect the reality of lakes or rivers.

Experiment in a natural lake

To test how psychoactive substances work in natural ecosystems, scientists conducted an experiment in Vättern, a Swedish lake with an area of nearly 1,912 square ^kilometers and a depth of 128 meters. The study involved juvenile Atlantic salmon, which are in the process of preparing to migrate from freshwater to the sea, and thus particularly sensitive to environmental changes.

The experiment used 105 individuals. The fish were divided into three groups: one exposed to cocaine, another to its metabolite, and a third – a control, not exposed to the drug. Each fish received an implant that slowly released the interacting substance – a technique that allows the compound to be delivered to the body gradually, mimicking natural absorption from the environment. After the salmon were released into the lake, their movement was tracked using acoustic telemetry, a system of underwater transmitters and receivers.

The results of the observations did not answer the question of the effect of drugs present in the water on the survival of the fish. However, clear differences in behavior were found – all groups became less active over time, but the rate of decline in activity varied. Individuals in the control group reduced their activity after returning to the wild faster than those exposed to psychoactive substances. This means that cocaine and its metabolite promoted increased mobility.

Differences can also be seen in the distance traveled by the fish. The control salmon reduced their weekly score by about 30 km, while the fish exposed to cocaine and the metabolite reduced their weekly score by only 20-21 km. In the final phase of the experiment, fish in the group given the metabolite swam a distance that was as much as 13-14 km longer per week, and traveled more than 12 km farther from the release site than fish in the control group.

Major discovery: a metabolite more potent than cocaine

Analysis of fish tissues showed that drugs penetrate their bodies and accumulate in the brain. In the case of cocaine, the average concentration was about 42.85 ng/g, while benzoylgonine reached a concentration of about 33.74 ng/g in the metabolite group. By comparison, in the group exposed to cocaine alone, the metabolite was present in the brain in much smaller amounts. This means that the fish actually absorbed the psychoactive substances in a manner comparable to organisms living in heavily polluted sewage environments.

One of the most surprising results was that benzoylgonine had a stronger effect on fish behavior than cocaine alone. This is important because the breakdown products of drugs and medicines are often overlooked in environmental risk assessments. The authors suggest that this may be due to the fact that the metabolite persists longer in the body and environment than the drug itself, and also affects physiological processes such as energy management and stress responses.

The results of the study show that even very low concentrations of psychoactive substances in the aquatic environment can affect the behavior of fish under natural conditions, and even subtle changes in the movement of organisms and their use of space can have serious ecological consequences, shaping the ability to migrate, acquire food, avoid predators and reproduce.

Source:

Brand J, Palm D, Cerveny D et al, Cocaine pollution alters the movement and space use of Atlantic salmon(Salmo salar) in a large natural lake, Current Biology, 36, 2018-2027.e4