Invasion of the Lakes

Eighty years ago, a naturalist surveyed the Lake District's aquatic plant life. Ecologists recently repeated his work. The changes they found, says Simon Hadlington, provide a stark warning

It is the summer of 1920. A lone figure in a rowing boat heads out from the shore of Ullswater in the Lake District. Over the side of the boat he drops a three-pronged hook attached to a rope, and hauls out a clump of water plant, which he puts into a jar. He rows a little further out and repeats the exercise, until the water is too deep to support plant life on the bed of the lake.

The man in the boat is the naturalist WH Pearsall, a luminary among this country's early ecologists and a founder member of the Freshwater Biological Association. Pearsall conducted the first systematic survey of aquatic plant life in the English lakes, showing the variation of species between different types of lake. He published his findings as "The Aquatic Vegetation of the English Lakes", in the Journal of Ecology.

Eighty years on, and ecologists from the University of Hertfordshire have returned to the lakes where Pearsall did his seminal work, to see how things have changed.

And things have changed. Species have completely disappeared from some lakes, and in others, invasion by alien water plants is becoming a serious threat. The new survey has been carried out by Professor Max Wade, who now works for Ecoscope Applied Ecologists, and Angela Darwell, a consultant aquatic botanist.

"What Pearsall has given us is a wonderful snapshot of the lakes before any significant interference by man," says Professor Wade. "It is as close as we will get to knowing the natural plant communities in the lakes."

Pearsall was interested in classifying the lakes according to how their geological characteristics were reflected in the plant communities that the lakes supported. The plant life in a lake is largely dictated by the nutrients in the water, which in turn are governed by the geology of the catchment area of the lake – the type of soil that water drains through to reach the lake.

Pearsall selected a range of the larger lakes that fell into the spectrum of geological types. In the bays of these lakes, he took a "transect", a straight line running from the shore out into the water, and sampled plants along this line. From these samples, he produced a series of intricate maps, showing the plant communities along these sections.

"Pearsall showed that lakes in hard rock, with barren catchments, would support primitive plants that thrived in conditions of low nutrient, such as stoneworts and quillworts, whereas those lakes surrounded by fertile soil were much richer in nutrients, with the flora characterised by a wider variety of plants, including pondweeds and water-milfoil," says Professor Wade. A lake such as Wast Water is in the former category, while Esthwaite Water is in the latter.

Wade and Darwell carried out a more thorough survey, snorkel diving to identify plant species – and using scuba divers where the water was too turbid, something that Pearsall would not have needed to do (even if he could have) because the water would have been much clearer then. The new survey concentrated on bays – as Pearsall's had done – where plants are more protected and can become established more easily. Like Pearsall, the modern-day researchers produced maps showing the distribution of the submerged plants.

The new research, which was sponsored by the Environment Agency, United Utilities and English Nature, has shown that Pearsall's classification essentially still holds true. A lake such as Wast Water remains low in nutrients – it is termed oligotrophic – and continues to support mainly primitive plant species. However, the nutrient-rich lakes – termed eutrophic – have become much richer in nutrients over the years, and this is having a significant effect on the plant life.

"One general finding in the eutrophic waters is the change in the depth of water at which plants no longer grow," says Professor Wade. "This is a key indicator of change. The clearer the water, the deeper light will penetrate to allow plant communities to become established. Nutrient-rich waters contain a lot of algae, which make the water turbid and allow less penetration of light. Overall, we found much less vegetation on the bottom of the lakes."

One of the lakes that has suffered nutrient enrichment is Esthwaite Water, largely due to agricultural activity in the surrounding catchment, phosphates in sewage effluent, and the presence of a fish farm on the lake. "One plant, Najas flexilis, or the slender naiad, has become extinct, almost certainly because of nutrient enrichment," says Darwell. Another plant, discovered and named by Pearsall, the Esthwaite waterweed (Hydrilla verticillata) has also disappeared from the lake, although its demise cannot be categorically blamed on nutrient enrichment, as the plant does thrive in eutrophic waters in other parts of the world. The quillwort, however, has clearly declined because of nutrient enrichment.

In Lake Windermere, the researchers identified the horned pondweed, Zannichellia palustris, which is known to prefer nutrient-rich waters, and which received no mention in Pearsall's survey.

Lake Windermere, too, has shown a deterioration in the diversity of plant life. "In some parts, this loss is dramatic," says Professor Wade. "For example, there is an island in the lake – Grassholme – where there used to be large beds of quillwort, stonewort and pondweed. Now it is virtually barren."

Apart from problems caused by the increase of nutrients in the water, invasion by alien species is a serious issue. In itself, this is not a new phenomenon, and Pearsall himself noted the presence of Canadian waterweed.

"But the biggest cause for concern is Crassula helmsii, also called the New Zealand pygmy weed or Australian stonecrop," says Professor Wade. "This has come from garden centres. It is a devil of a plant in terms of its vigour. In Derwent Water, you can see acres of it under the water and there is little doubt that it is displacing natural species." One species known to be under threat is the rare Luronium natans, or floating water-plantain.

Crassula was first recorded in Derwent Water in 1996; Darwell then discovered it in Bassenthwaite Lake the following year; in Coniston Water in 1998; and in Grasmere last year. In one bay in Derwent Water, which has a large Crassula population, eight or nine native species have been displaced.

But do these changes matter? Professor Wade believes that they are providing an important warning. "What all this tells us is that we need to work hard to protect the quality of the water in the lakes," he says. "These plant communities are a barometer of the long-term changes in this ecosystem. When native species disappear, it tells you that something fundamental is occurring. Given the special nature of the Lake District, it is important that we hang on to these aquatic plant communities. They are part of the food chain, which must therefore undergo change also."

While the oligotrophic lakes such as Wast Water seem to have escaped these problems so far, it is important to ensure that they remain pristine – plant communities there are rarer and more susceptible to change.

"Phosphate-stripping has been introduced at most sewage-treatment plants, which could help to recover the water quality of the lakes," says Darwell. "However, it is clear that the spectrum between nutrient-poor and nutrient-rich lakes has become greater, and this is something that needs to be addressed if we are to protect our natural heritage."