Geodiversity and biodiversity interactions: how natural rocky shore microhabitats can inform the ecological enhancement of engineered coastal structures

MacArthur, Mairi (2019) Geodiversity and biodiversity interactions: how natural rocky shore microhabitats can inform the ecological enhancement of engineered coastal structures. PhD thesis, University of Glasgow.

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The primary aims of this thesis were two-fold. First, this thesis examined the interplay between geodiversity and biodiversity on natural rocky shores from regional – site – microhabitat scales exploring how rock material and rock mass properties influence geomorphological and biogeomorphological processes that shape microhabitats, which in turn exerts an influence over biota. A key finding from this part of the thesis was that lithology is not an important determinant of species richness and abundance at the national or regional scale, being more a modifier of patterns than a driver. At the site scale, lithology exerts more of an influence, with complex lithologies (such as limestone) and associated microhabitats more likely to have higher species richness and abundance than adjacent areas of the shore platform that lack this geomorphic complexity.

All the work in this thesis was conducted in the mid-upper intertidal zone, between Mean Tide Level (MTL) and Mean High Water Springs (MHWS), as this zone has high exposure during low tide and most coastal defences are built at this level, meaning interventions at this tidal height hold the most biodiversity potential. Natural shore surveys were also conducted at this height to directly inform engineering design and to contribute to biogeomorphological theory as more species were found to congregate in microhabitats for refuge in the mid-upper intertidal zone.

Results of surveying the mid-upper intertidal zone of several rocky shores across the UK highlight that it is critical to account for both lithology and the presence of geomorphic features (microhabitats) to better understand the distribution of species and their habitat requirements. The location of species within quadrats on each shore was recorded relative to their position on either the shore platform surface or within microhabitats, such as pools, cracks, pits and ledges. Results showed that microhabitat type is a key driver of species distribution within shores, while lithology modifies patterns between shores (10s of km scale). Although the most suitable habitat varied with location, deep pools (2.8-24 cm deep) were significant in increasing species richness and abundance where they were present while crevices and ledges facilitated significantly greater mobile species abundance, particularly compared to the adjacent shore platform. These findings improve the understanding on the interrelationships between geodiversity and biodiversity and highlight the need for ecological and biogeomorphological surveys to incorporate species distributions within geomorphic features and processes in greater depth.

The second aim drew from this new understanding of the rock and microhabitat preferences of intertidal species on natural rocky shores to evaluate the effectiveness and further the evidence base for ecological enhancement, i.e. improving the quantity and quality of available habitat for species on artificial structures, on new and existing artificial coastal defences.

With the construction and expansion of coastal defences in the intertidal zone globally, ecologically enhanced designs are needed to mitigate some of the impacts of construction on the intertidal zone, such as disturbance from the addition of artificial substrate and habitat loss. To do this, an ecological enhancement trial using 160 artificial concrete tiles of 8 different designs and 24 cleared natural surfaces was conducted at three UK sites over an 18-month period, representing the largest (to date) UK enhancement trial of this kind. Key findings from this trial showed that intermediate complexity in the form of mm-scale grooves was statistically significant in increasing the abundance of early-colonising species (i.e. barnacles) from 2 months onwards compared to plain-cast control tiles and designs of higher complexity. Additionally, the design with the highest level of habitat complexity (up to 30 mm deep pits) significantly increased humidity and reduced temperature compared to lower complexity designs. Species richness and abundance was greatest in the microhabitats of the most complex design during monitoring from 2-18 months after installation. These designs highlight the value of ecological enhancement from the mm-cm scale in providing habitat that would otherwise be absent on plain-cast artificial coastal structures, such as seawalls.

A secondary ecological enhancement trial was conducted on passively enhanced (passive positioning and optimised material choice for ecology) rock armour boulders at a live coastal defence scheme in Hartlepool. From laboratory testing, Portland limestone and Carboniferous limestones (Hartlepool and Welsh) were optimal boulder material choices for rock armour revetments, combining ecological and engineering suitability. Adding in field survey results showed the importance of considering ecological suitability from the mm-dm (decimetre) scale and including rock material and rock mass properties in making engineering recommendations. Ledges at the dm-scale in field surveys on the revetment at Hartlepool were the optimal geomorphic feature in significantly increasing limpet abundance on the boulder surface. To conclude, the findings from natural shores were utilised to suggest improvements to future ecological and biogeomorphological survey techniques on natural and artificial shores. These in-depth surveys were coupled with findings from ecological enhancement trials to provide detailed recommendations on the design of future enhancements on artificial shores, with specific design parameters delineated.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Biogeomorphology, ecological enhancement, rocky shores, rock coast, ecological engineering, biodiversity, coastal defences.
Colleges/Schools: College of Science and Engineering > School of Geographical and Earth Sciences
Funder's Name: University of St Andrews (HEI-STAN)
Supervisor's Name: Naylor, Dr. Larissa, Hansom, Dr. Jim and Burrows, Professor Michael
Date of Award: 2019
Depositing User: Dr Mairi MacArthur
Unique ID: glathesis:2019-80246
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 09 Mar 2020 14:49
Last Modified: 03 Apr 2023 08:01
Thesis DOI: 10.5525/gla.thesis.80246
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