Freshwater fish: investigating how multiple stressors alter freshwater communities

For the Trinity Term 2021 Alumni Newsletter, Michelle Jackson explains how her research group is looking into the incredible diversity of freshwater environments, and considering how miltiple stressors impact them. 

Earth is a blue planet, with 70% of the surface covered by salty water. Freshwater, in comparison, covers less than 1% of our planet’s surface, yet accommodates more than 50% of described fish species! To me, it is astonishing that these rare, and often overlooked, habitats are so disproportionately diverse. This, along with the fact that freshwater is essential for life, suggests that these ecosystems should be highly valued and protected. Yet humans continue to degrade and pollute freshwater environments. In 1858, an event known as The Great Stink occurred in London’s River Thames from a combination of heatwaves and sewage pollution, causing cholera outbreaks. This led to the construction of the sewage system, much of which is still in use today. However, wartime bombings damaged this infrastructure and, by 1957, the Thames was declared “biologically dead” – no fish or insects could survive the foul conditions. With post-war recovery, and increasing environmental awareness and regulation, the River Thames again teems with life – there are now 125 species of fish, rare invertebrates, and sometimes even seals!

Despite this improvement, freshwater environments are again increasingly at risk from human activity as our population increases rapidly. Sewage pollution should be a thing of the past, yet our infrastructure is drastically outdated and cannot keep up with population growth. Water companies regularly release raw sewage into our rivers, including the Thames. On top of this, rivers are subject to a cocktail of other stressors – chemical pollution, habitat degradation, biological invasions, and climate change. Globally, these stressors have driven many species to extinction, and others are in serious decline. A recent report from the World Wildlife Fund found that, on average, freshwater vertebrate populations have declined by 84% since 1970.

The Aquatic Ecology Lab in the Department of Zoology is investigating how these stressors combine to alter freshwater communities. Using a combination of field sampling and experiments, we aim to disentangle how stressors (such as heatwaves and nutrient pulses from sewage overflow) interact – potentially having an effect which is larger than the sum of their parts. Experimental ponds and streams, known as mesocosms, allow us to manipulate stressor magnitude in a semi-natural environment, and quantify how the community changes. A major focus is on the complex feeding interactions between members of the food web, from microalga, through herbivores, to predatory insects and fish. We use our knowledge of these networks to track how the impacts of stressors combine and ripple through the food web, sometimes having far reaching and unexpected results.

The plight of freshwater ecosystems is receiving more and more attention from the media, regulators, and scientists alike. Yet much more needs to be done to improve our understanding of multiple stressors. With the aim of inspiring freshwater ecologists of the future, this year the first-year biologists will, for the first time, take part in freshwater field trip!