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Isles of Scilly eelgrass bed voluntary monitoring programme: 2018 Annual Survey (RP2940)

In this report, we 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 the 2018 survey, as well as their place in
continuous time series from 1996, are presented and analysed. This represents twenty- three 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
significant variation in shoot density between survey sites in 2018. Longer-term trends reveal
significant declines in average shoot density at three out of five surveyed sites since Special
Area of Conservation designation in 2005, with Higher Town Bay declining by over 40%.

Canopy height was found to differ between sites but this may simply be a feature of environmental
differences between sites, such as depth. No long term linear trends in canopy height were found at
three sites. At Little Arthur a slight decrease is evident; whereas at Broad Ledges Tresco a slight
increase is observed.

Shoot density and canopy height were combined into a measure of leaf area index (LAI), estimating
total photosynthetic area per unit ground. Significant differences in LAI were observed between the
five survey sites on the 2018 survey, with Little Arthur the most productive and Broad Ledges
Tresco the least. Long term declines in productivity are observed at Higher Town Bay and Little
Arthur, with the other three sites remaining stationary.

The 2018 results also showed differences in eelgrass ‘patchiness’ between survey sites, with
eelgrass being 64% absent at West Broad Ledges and 79% absent at Old Grimsby Harbour. Analysis of
long term trends showed that there have been significant declines in patch occupancy at Higher Town
Bay (25% loss) and Old Grimsby Harbour (64% loss).

Long term changes in wasting disease and epiphyte cover have been observed but without any clear
overall trend. Interestingly, across the whole length of the survey, wasting disease prevalence
differs significantly between survey sites, but epiphyte cover does not. This may indicate that the
relative influence of local versus regional drivers is different for wasting disease and epiphytes
but more research would be needed to explore this.

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 again indicates concerning declines in eelgrass across the Isles of
Scilly and, in particular, at Higher Town Bay and Old Grimsby Harbour, since SAC designation in

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Isles of Scilly eelgrass bed voluntary monitoring programme: 2017 Annual Survey (RP2939)