Assessing the ecological status of the Taw catchment through surveying diatom populations

Diatoms represent one of the key biological quality elements that can be used in the assessment of surface water status. As primary producers in many food chains, diatoms are sensitive to an array of environmental factors and in particular change such as nutrient levels like phosphorus. During this study, high spatial resolution diatom samples were collected during late winter and summer from the Taw catchment and two of its sub-catchments; the Little Dart and Lapford Yeo. Samples were collected in triplicate from five sites along each catchment yielding a total of forty-five samples per season. Field Preparation, production of slides and microscope analysis were carried out following the standard Environment Agency protocols (Kelly et al, 2001) permitting comparisons within comparisons within and between the Taw and its sub-catchments to be made as well as seasonal comparisons. Through taking replicate samples, the uncertainty on individual scores could be assessed. The Trophic diatom Index (TDI) data suggest that phosphorus levels in the upper reaches of the Taw were low but downstream the composition of phosphorus tolerant versus phosphorus intolerant species was reversed leading to higher TDI values. However, when looking at the TDI values in the two sub-catchments, the transition of phosphorus tolerant vs. phosphorus intolerant was not as distinguished from the upper reaches and downstream. This suggests that the upper reaches of the Little Dart and Lapford Yeo may not be as pristine as the Taw, resulting in a higher abundance of phosphorus tolerant diatoms