Research will uncover impacts of reducing sewage pollution on freshwater wildlife
16th December 2025
New Forest Programme Manager Gemma Stride explains how her PhD research will uncover the impacts of pollution – and help us understand whether reducing combined sewage overflows will help biodiversity recover.
Freshwaters occupy less than one percent of the planet’s surface, yet they support nearly ten percent of all known species. These ecosystems are astonishingly rich and vital, but are also threatened. Globally, freshwater species populations have declined more rapidly than those of terrestrial or marine environments.
The pressures are complex and ubiquitous: land-use change, invasive species, water abstraction, and – of course – pollution. Here in the UK, most rivers and lakes fail to meet ‘Good’ ecological status under the Water Framework Directive. Despite decades of investment, water pollution remains one of the biggest challenges facing our rivers today.
Over recent years, public concern about water quality and the scale of untreated sewage discharges (diluted by rainfall or groundwater) into our rivers has increased. In part, this awareness has been driven by the availability of storm overflow data from Event Duration Monitors installed and published by water companies (a legal requirement) over recent years. In 2024, the Environment Agency reported an astonishing 3.6 billion hours of combined sewer overflow (CSO) spills into English rivers. These CSOs were designed as emergency relief points in our sewer systems. As a result of complex factors, they now operate far more frequently than intended, discharging untreated wastewater into rivers and streams during heavy rainfall.
Despite this, we still lack data on the impacts of pollution on freshwater species and the difference wastewater management improvements will make, particularly in landscapes of high ecological value. That’s where my PhD research comes in.
- Lymington River and floodplain ponds
The New Forest: a unique setting for this study
My research, a collaboration between Bournemouth University, Freshwater Habitats Trust, Southern Water, Wild New Forest and Harper Adams University, focuses on the New Forest National Park, which is one of the most ecologically significant freshwater landscapes in the UK. It’s a place that I cherish – and where I have spent my career with Freshwater Habitats Trust.
A freshwater biodiversity hotspot, the New Forest boasts the highest proportion of land designated for nature conservation of any National Park in England and Wales. It supports over 30% of the UK’s freshwater invertebrate species, and an extraordinary diversity of rare freshwater plants – including Pillwort, Hampshire Purslane, and Marsh Clubmoss – which is astonishing considering it is the second smallest National Park. Unlike many lowland areas, its rivers and floodplains still retain a high degree of naturalness. Clean waters and traditional grazing practices help to maintain open habitats where freshwater plants and animals – including some of our rarest species – can thrive.
The New Forest’s water environment, with its lower levels of diffuse pollution, makes an ideal location for this study. This is because although other sources of pollution – such as those from agriculture – exist, they are less intense and lower than in a typical English landscape. Therefore, it is easier to isolate and study the impact of storm overflows.
- Pillwort in the New Forest.
Yet even here in the New Forest, pollution poses a threat. Fifteen CSO outfalls discharge into 10 Water Framework Directive waterbodies within the National Park. Around 75% of these rivers currently fail to achieve Good Ecological Status for a variety of known and unknown reasons.
What we’re aiming to find out
This research asks a deceptively simple question: What happens to freshwater biodiversity and ecosystem functioning when pollution inputs from combined sewer overflows are reduced?
To answer this, I’ll be studying four river systems in the New Forest: Avon Water, the River Blackwater, the Beaulieu River, and the Lymington River. By comparing these rivers – each of which represents a different pollution and management scenario – we can begin to tease apart how water quality improvements influence biodiversity and ecosystem function. In particular, we will examine how reductions in CSO discharges may affect both the ecological (plants and invertebrates) and functional aspects, such as decomposition, which underpin overall river health.
Looking beyond the river channel
Water quality research has traditionally focused on the main river channel, but we’re considering the whole freshwater landscape. For example, many of the ponds in the surrounding floodplains may also be impacted by CSO discharges during flood events when the river spills onto its floodplain.
Ponds are particularly important for biodiversity in the New Forest, providing a refuge for many rare species. My project will examine 32 ponds across two catchments, comparing those upstream and downstream of CSO outfalls. By examining these alongside river habitats, we’ll gain a more complete, catchment-wide view of how pollution – and its management – affects the freshwater landscape as a whole.
Taking the pulse of river life
Over the next three years, I’ll be carrying out intensive fieldwork across six study areas on each river. This will involve sampling:
- Macroinvertebrates (such as mayflies, caddisflies, and beetles) using standardised three-minute kick sampling methods, to reveal ecological community structure and pollution sensitivity.
- Macrophytes (aquatic plants) surveyed in summer to assess longer-term water quality conditions.
- Decomposition rates, measured using cotton strips as a proxy for how organic matter breaks down. This will indicate the underlying biological functioning of the system.
- Water quality and chemistry, measured through a combination of high-frequency in-situ monitoring (and laboratory analysis of water chemistry).
Why it matters
Across England and Wales, there is evidence that some rivers have shown biological recovery following improvements to wastewater treatment. However, the recovery has often been partial and uneven and has now potentially plateaued. Sensitive species are still not found in many catchments, and smaller headwater streams – where biodiversity recovery potential is greatest – are still underrepresented in national monitoring programmes.
By focusing on the New Forest, we can address two key knowledge gaps:
- How do pollution and management interventions affect freshwater biodiversity in ecologically sensitive, low-impact landscapes?
- Does improving water quality translate into tangible recovery of ecosystem processes, not just species diversity?
We’re aiming to generate data that can directly inform catchment-scale management recommendations. The results will help guide Southern Water’s investment decisions, shape conservation priorities in the New Forest, and feed into broader national strategies for freshwater restoration.
Fieldwork in the New Forest brings its own challenges, from this summer’s drought to the autumn deluge in rainfall, and the inevitable race against time. But it also offers enormous opportunities for learning, collaboration, and community engagement.
Looking ahead
My PhD research will run until 2030, and I’ll share initial results at local and national forums, including the New Forest Biodiversity Forum. In the longer term, the findings will contribute to our shared mission at the Freshwater Habitats Trust: to protect the best, restore the rest, and reconnect people with freshwaters.
The results could help inform the way we create the Freshwater Network, supporting our strategy of protecting the best remaining habitats, which are under threat, so we can protect and build out from biodiversity hotspots to reverse the decline of freshwater biodiversity.
This project brings together expertise from academia, conservation, and industry and is a partnership between Bournemouth University, Freshwater Habitats Trust, Southern Water and Wild New Forest, and Harper Adams University with additional input from the New Forest National Park Authority, Natural England, Environment Agency, New Forest Biodiversity Forum and local landowners including Forestry England.
- Ponies in stream - Avon Water
Our shared goal is to create credible, independent evidence that will help drive more effective management decisions – not just for the New Forest, but across other protected landscapes across the UK.