Mesotrophic lakes have moderate trophic or nutrient conditions, being neither 'eutrophic' (very rich) nor 'oligotrophic' (nutrient poor, like many of the Lake District lakes and tarns). Lakes in the middle of the trophic range are relatively infrequent in the UK and largely confined to the margins of upland areas in the north and west.
The particular geological history of the Yorkshire Dales has resulted in there being very few lakes in the area. As rare habitats, the few tarns that do exist are important and highly valued. The two main ones, Malham Tarn in the South and Semer Water in the North of the Yorkshire Dales National Park, both lie in catchments that have some limestone rocks and they can be classified as mesotrophic.
Malham Tarn is a National Nature Reserve (NNR). It is a marl lake because of the light coloured limestone sediments that have been deposited over the centuries and is the highest known marl lake in the UK. Notable features include its stonewort algae (Chara and Nitella species) a range of pondweeds including the long-stalked pondweed and its invertebrate species including the extremely rare flightless caddis-fly and two rare copepods. Other interesting species include white-clawed crayfish, bullhead, otter and breeding great-crested grebe. It has been very well studied by university researchers including those based at the Malham Tarn Field Studies Centre.
Semer Water by contrast is more nutrient-rich and work is underway through the Catchment Sensitive Farming Scheme, to reduce agricultural and sewage effluent inputs from the catchment as well as excessive silt and sand deposition. There are few submerged plants (for example, sparse Potamogeton pectinatus) but some marginal stands of yellow water-lily, bulrush and bottle-sedge. Amongst the animals of the lake, the variety of mayflies is notable and the large cladoceran (Leptodora kindti) is otherwise rare in Yorkshire.
Lakes concentrate nutrients and pollutants and are seriously affected by agricultural practices in the catchment. These factors can gradually cause mesotrophic lakes to become eutrophic and of less ecological value. They are also susceptible to introduced species, such as the signal crayfish bringing disease that has killed many populations of our native crayfish.