FSC | Field Studies Council

Field Studies Council: Bringing Environmental Understanding to All

Freshwater

Slapton Ley ('Ley' is a local term for 'lake') is the largest natural body of freshwater in southwestern England. The 70-hectare expanse of open water is host to a variety of plants, birds, mammals, fish and invertebrates, each contributing to this fragile ecosystem.

However, Slapton Ley is a eutrophic water body, meaning that at times the Ley is prone to algal blooms. These reduce the amount of light entering the Ley and cause many water plants to die. Bacteria breaking down this dead matter use up the oxygen in the water, which has knock on effects for the many animals which rely on the Ley as their home and food larder.

Eutrophication occurs when an area of water is rich in mineral and organic nutrients (in this case Nitrates and Phosphates), providing enough raw materials for spores of algae to 'bloom' into large, plant-like organisms that fill the water. This process is usually at it's worst towards the end of a warm, dry summer, when water levels drop and the water temperature increases, leaving a stagnant, nutrient-rich pool of water - the ideal conditions for algae to thrive.

The summers of 2007 & 2008 saw rainfall levels greatly in excess of the annual average, and while this had an adverse effect on many species, the problem of algal blooms was reduced - with average temperatures lower throughout the summer, and increased rainfall to keep water levels higher, a flow of water was maintained through the Ley and over the weir at Torcross.

However, on the flip side, increased rainfall across the catchment will have undoubtedly led to increased erosion and run-off, sending more nutrient-rich sediment downstream into the Ley. Some of these nutrients will have been 'locked-up' in sediment, and will provide ample resource for algal blooms in subsequent warm, dry summers. So, while high rainfall may reduce algal blooms in the short-term, the long-term effects could be much worse.

The process of erosion across the Slapton catchment leads to something known as DWPA - Diffuse Water Pollution from Agriculture. This phenomenon, caused by steeply-sloped agricultural land draining into rivers and ultimately the Ley, is a difficult problem to try and tackle. A European project called 'Cycleau' attempted to address this and other catchment-based problems at Slapton and 10 other river catchments across North-western Europe (click here for more details).

The Slapton Ley catchment can be split into three main sub-catchments; the Gara river, the Start stream, and the Stokeley stream (see map above). The Gara river catchment is by far the largest of the three, providing the majority of the water that flows into the Ley, entering the nature reserve at the northern end of the Higher Ley.

The Slapton Wood Stream is the final tributary to join the Gara river and is frequently used as a study site, either to sample freshwater invertebrates or to consider the response of a watercourse to a rainfall event.