Assessing how flow regime changes affect biotic indicators of ecosystem health in UK chalk streams

Abstract

The overall aim of this thesis was to assess the effects of changes in low flows on biotic indicators of environmental quality on chalk streams in the context of water trading. Water trading is likely to go ahead in England in the upcoming years with little assessment of the likely impacts on the freshwater environment and subsequently the ecosystem services provided. Using the river Nar in Norfolk (UK) as a case study, a method using habitat models and data collection was used to investigate this. The methods were derived into three research questions: Firstly data collected in the field and EA data were used to investigate the impact of low flows on the three indicator species: Fish (brown trout), Macrophytes (Crowfoot) and benthic macro-invertebrate (Mayfly) (RQ1). Secondly hydraulic and habitat models (Flood modeller, TUFLOW, CASiMiR) were built to show how flows affected habitat availability (RQ2). Finally a trading model was developed by a team at Manchester University to show how water trading influences flow. These flow scenarios were run through the habitat models to show how water trading affects the habitat availability of the indicator species (RQ3). The results showed how the indicator species are generally more affected by the antecedent flow conditions as opposed to the daily flows. Furthermore the difference in habitat between the typologies was highlighted during the first research question where the fen reach generally provided poorer habitat than the chalk reach. A key finding in RQ1 was that brown trout numbers increase when BMI have better quality and therefore when the brown trout have more food sources. This led onto a key area of research which investigated the interconnectedness of species. It was found that in order to use habitat models to fully assess habitat availability, not only do the hydraulic components need to be addressed but a species’ biotic dependants i.e. food sources and refugia also need to be taken into account. RQ2 described a novel approach to do this and showed how spawning brown trout’s habitat availability changes when their biotic dependants are included. Finally RQ3 showed how water trading does affect habitat availability but the small impact should be weighed up against the benefits to water resources. Three main conclusions could be drawn from this research; firstly how there is a site specific nature of habitats and management should reflect these differences, secondly there is much uncertainty around habitat model and finally water trading does not impact habitat availability if HOF is activated. These findings and methods can be taken forward with the increase in water trading in England which would enable environmentally efficient water trades whilst being beneficial to effective water resource management.