Pesticides as a structuring force of planktonic food webs in drinking water reservoirs

The widespread use of agrochemicals in catchments serving drinking water reservoirs has important implications for the water industry. It is well documented that increasing inputs of nitrogen and phosphorus from local catchments are correlated with increasing phytoplankton blooms (pea-soup water) in lakes and reservoirs. Frequently, these blooms are toxic to humans and their removal has increasing cost implications for the water industry. An additional problem comes from the widespread use of pesticides in agricultural catchments. In the UK, limits are drawn up for individual pesticides as specified in the EU Drinking Water Directive. However, less well known are the impacts to the biota within the reservoirs. It is possible that bacteria can rapidly utilise certain pesticides as organic substrates thereby reducing their impact in the water body. Alternatively, they may also break them down into substances that are potentially more toxic. An additional unknown is the impact of herbicides on the autotrophic communities within reservoirs. Herbicide impacts may be selective, promoting growth of the more tolerant members of the phytoplankton. Pesticides are known to cause lethal and sub-lethal effects on zooplankton communities. These organisms can control phytoplankton bloom development by grazing. Reduction in their grazing ability may affect bloom size. Identifying the key pesticides and their interactions with the organisms within reservoirs may lead to alterations in management practices and the potential to reduce the costs of water treatment. A combination of field and laboratory investigations will be undertaken to assess the scale of pesticide inputs and quantify their impacts to the biota both at the single species level and community level within food webs. The study site is Durleigh Reservoir which frequently suffers pollution events from pesticides and herbicides, in addition to nutrients, and is one of a network of drinking water reservoirs within the Wessex catchment experiencing similar impacts from agricultural intensification. The student will benefit from working with a team of scientists, each with expertise in one major group of aquatic organisms, and will receive training in a range of techniques designed to measure the effects of pesticides and/or herbicides on these organisms. Additionally, they will work alongside staff within Wessex Water, both on field site visits and within the laboratory and will gain insight into all aspects of the processing steps involved in the treatment of pesticides from raw water to tap water. Wessex has a regular sampling and analytical programme for water sampled in each of their reservoirs and these data will be provided, at no cost, including all of the pesticide analyses on water samples. The student will receive analytical methodological training whilst working in their laboratories. Results obtained from the intensive survey of one reservoir will have broader application within Wessex Water and will be high utility for other water companies and agencies e.g. the Environment Agency.