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 2017 survey, as well as their
place in continuous time series from 1996, are presented and analysed. This represents twentytwo
years of continuous annual monitoring around the Isles of Scilly.
Overall, eelgrass was present at all five survey sites around the Isles of Scilly. We did not find
significant variation in shoot density between survey sites in 2017. However, longer-term trends
reveal significant declines in average shoot density at four out of five surveyed sites since
Special Area of Conservation designation in 2005, with Higher Town Bay declining by over 50%.
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 changes in canopy height were found with the
exception of Little Arthur, where a slight decrease is evident.
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 2017 survey, with Little Arthur the most productive and Old Grimsby
Harbour the least. Long-term declines in productivity are observed at Higher Town Bay, Little
Arthur, and Old Grimsby Harbour.
The 2017 results also showed differences in eelgrass ‘patchiness’ between survey sites, with
eelgrass being 80% absent at Higher Town Bay and 88% absent at Old Grimsby Harbour. Analysis of
long-term trends showed that there have been significant declines in patch occupancy at Higher Town
Bay (30%) and Old Grimsby Harbour (60%).
Long-term changes in wasting disease and epiphyte cover were observed but without any clear overall
trend. Interestingly, both in 2017 and 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 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 2005.