Isles of Scilly Eelgrass Bed Voluntary Monitoring Programme - 2021 Annual Survey.

Published online
27 May 2024
Published by
Natural England
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Bull, J. & Kenyon, E.

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This report present novel data from an ongoing, spatially replicated, annual study of a comparatively un-impacted, temperate eelgrass habitat, based around the Isles of Scilly, UK. Five sites were assessed: Broad Ledges Tresco, Higher Town Bay, Little Arthur, Old Grimsby Harbour, and West Broad Ledges. Metrics include eelgrass (Zostera marina) shoot density, number of leaves per shoot, maximum shoot length, as well as semi quantitative recording of signs of wasting disease and epiphyte cover on a leaf-by-leaf basis. Findings from this year's survey, as well as their place in continuous time series from 1996, are presented and analysed. This represents 26 years of continuous annual monitoring around the Isles of Scilly. Overall, eelgrass was present at all five survey sites around the Isles of Scilly but we found substantial variation in shoot density between survey sites this year, as is often the case. Canopy height was also found to differ between sites but this may simply be a feature of environmental differences between sites, such as depth or turbidity. Shoot density, number of leaves per shoot, and canopy height were combined into a measure of leaf area index (LAI), estimating total photosynthetic area per unit ground. Substantial differences in LAI were observed between the five survey sites. However, longer-term trends reveal that considerable annual fluctuations in shoot metrics show no consistent changes through time across the Isles of Scilly. At the spatial scale of shoot density within eelgrass patches, there is no evidence to indicate concern. The proportion of pre-determined quadrats located on eelgrass versus bare sand is used as a measure of patchiness. Analysis of long-term trends showed that there have been significant declines in patch occupancy at Old Grimsby Harbour (67.9 %) over the duration of the annual monitoring, while other sites are relatively stable over time. Long-term changes in wasting disease and epiphyte cover have been observed but without any clear, linear trend. Interestingly, across the whole length of the survey, wasting disease prevalence and epiphyte cover both differ substantially between survey sites. However, trends in disease and epiphyte community are not the same and more research would be needed to explore the drivers of these dynamics. Finally, we continue to see Sargassum muticum, an invasive species of brown seaweed known as wireweed, at all surveyed sites in the Isles of Scilly. While this is not formally quantified, no obvious changes in abundance or distribution were evident. The synthesis of these findings indicates concerning declines in eelgrass at Old Grimsby Harbour since the start of monitoring in 1996 and with no obvious change in downward trajectory since SAC designation in 2005. However, analysis of long-term trends at different scales point to patch creation and loss, rather than within-patch dynamics, as the key processes to focus on to identify causes of decline. Elsewhere across our sampling locations within the Isles of Scilly SAC, eelgrass remains in good condition.

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