The ecological effects of physical habitat restoration in English chalk streams

Abstract

Chalk streams are globally rare and unique systems that have been extensively subjected to physical modification. This has contributed to the widespread degradation of ecological communities in chalk streams and failures to adhere to legislation (e.g. Water Framework Directive). Physical restoration has emerged as a key strategy to improve the condition of chalk streams, but a current lack of evidence for its effectiveness constrains the development of sound practice. This thesis aimed to develop understanding of the effects of restoration on physical habitat and ecology in English chalk streams. A series of case study appraisals were undertaken to understand: (1) the physical and ecological effects of different techniques (i.e. weir removal and gravel augmentation); (2) the influence of time since restoration; (3) the effects of restoration on different ecological groups. Additionally, (4) a methodological approach for non-invasively evaluating fish populations was developed to help improve monitoring capabilities in chalk streams. Weir removal rapidly altered habitat and ecological communities, especially directly upstream which became more lotic. Little evidence of sediment-pulse related impacts downstream of the weir was found, possibly due to additional silt-management methods which facilitated recovery. Gravel augmentation desirably altered habitat and ecology over the timescale studied (e.g. enhanced macroinvertebrate diversity). However, variability between sites, time periods and ecological groups signifies widespread uncertainties in restoration outcomes and the need to develop a better understanding of the drivers behind these. Time was a key factor influencing the observed effects of restoration (e.g. due to lag-effects). Furthermore, responses varied considerably between ecological groups, where changes in macroinvertebrates (e.g. increased diversity) and fish (e.g. increased brown trout sightings) were not reciprocated in macrophytes. These findings highlight the need for appraisals to take place at commensurate temporal scales and ideally using a multi-taxa approach to understand responses more accurately. Remote underwater video proved a useful tool for assessing fish communities and population size. Given the adaptability and utility of the technique, as well as its potential application as a citizen science methodology, it may prove useful for monitoring at chalk stream restoration projects and requires further investigation. Overall, this thesis contributes valuable evidence suggesting restoration can be an effective tool for desirably altering habitat and ecological communities in chalk streams. However, the need to conduct more robust appraisals to consolidate knowledge is highlighted and emphasises the need to fund the development of flagship case studies to guide and inspire future restoration efforts.

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Identifiers

ORCID for Paul Kemp: orcid.org/0000-0003-4470-0589

ORCID for Andrew Vowles: orcid.org/0000-0001-8253-5938

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