Benthic Macroinvertebrate Assemblage in relation to Environmental and Hydrological Variables in a Chalk Stream, Upper Colne Catchment

Chalk streams are a rare and valuable river type that are mostly confined to southeast England. They are known for their clear waters flowing over gravel beds and array of flora and fauna, including specially adapted macroinvertebrates that can survive periods of drought. This study aimed to determine the benthic macroinvertebrate community composition in the upper course of Colne Catchment in Hertfordshire and to establish its hydrological regime and water quality status. Previous studies have shown that year-round monitoring is important and that the diversity of macroinvertebrates in chalk streams is related to a number of factors. However, many of these have not been researched in a cohesive manner. Therefore, this study identified the influence the chalk stream environment had on the family richness and diversity of macroinvertebrate taxa. The study was carried out on four sites in the Upper Colne catchment in Hertfordshire over two annual seasons between April 2023 to February 2025.There were significant differences in the macroinvertebrate community composition among the sites, which were indicative of certain environmental conditions. For example, the Ver site, which had the most consistently high flow velocity had greater prevalence of Gammaridae, whereas upstream sites were dominated by Limnephilidae which preferred slower flowing waters. All sites on in the Upper Colne Catchment experienced instances where the water nutrient levels (phosphate, ammonium, nitrates) exceeded the recommended WFD guidelines for surface waters, which suggested that the sites could be at risk from eutrophication, coupled with low flows and high biological oxygen demand levels. In addition, the biometric indices indicated that the sites were mostly of poor environmental quality and associated with low flows. Whilst chalk streams are expected to have lower stream power than other river types, the anthropogenic causes of impeded flow have altered their hydrological functioning. Therefore, with the combination of future warming from climate change and the neglect from authorities and stakeholders, the upper courses of chalk streams could be at a greater risk of negatively being impacted by increasing nutrient levels, pollution events and groundwater abstraction, all of which would alter the macroinvertebrate community and damage the fragile ecosystem along the whole river course. Long-term monitoring of the aquatic macroinvertebrates and environmental parameters will be crucial in preserving the chalk streams ecosystem.