Fal & Helford SAC subtidal sediment data analysis report 2017.
Abstract
The aim of this project was to analyse sediment infauna data to quantify any changes in community composition at the Fal and Helford Special Area of Conservation (SAC). This will contribute towards Natural England (NE) condition assessment using conservation advice for each site and sub-feature. MarineSpace Ltd (MarineSpace) was commissioned by NE to analyse and report on subtidal sediment grab data collected by the Environment Agency (EA) as part of its Water Framework Directive (WFD) and monitoring from the Fal and Helford. Data were provided from surveys conducted in 2001, 2009, 2010, 2013 and 2016. Biodiversity indices (including total number of species in each sample (S), total number of Individuals in each sample (N), Pielou's Evenness Index (J'), Shannon-Weiner Diversity Index (H'), and Simpson index (1-λ')) were tested using a Kruskall-Wallis test by ranks to see if there were significant differences over time. All univariate tests were conducted in the R statistical computing environment. Community data were examined using the PRIMER v7 software package. ANOSIM was used to test for differences in species composition between groups. SIMPER analysis was then utilised to see which species contributed to similarities and dissimilarities between groups. Site 1 was situated in the Fal River in the upper reaches of the Fal Estuary. Site 1 had low to moderate richness and diversity and there were no significant differences in biodiversity indices between years. Community composition was significantly different between years, but was due to small changes in the relative abundance of common species. Sediments were sandy Mud. Overall, the community was characterised as a typical SS. SMu. SMuVS. AphTubi community (Aphelochaeta marioni and Tubificoides spp. in variable salinity infralittoral mud). Site 2 was situated in the upper reaches of the Carrick Roads in the Fal Estuary. Site 2 had low to moderate richness and diversity and there were no significant differences in biodiversity indices between years. Community composition was significantly different between years, but was due to small changes in the relative abundance of common species. Sediments were typically sandy or gravelly Mud. Overall, the site was best characterised as SS. SMu. SMuVS. AphTubi, though there were elements of SS. SMu. ISaMu. MelMagThy in 2001. Site 3 was situated towards the upper reaches of the Carrick Roads in the Fal Estuary. Heavy metal concentrations in the sediment are known to be especially high here due to extensive historical mining in the Fal Estuary, though no chemical samples were analysed as part of this work. Site 3 had low to high richness and diversity and there were no significant differences in biodiversity indices between years. Community composition was significantly different between years, but was due to small changes in the relative abundance of common species. Sediments were typically either muddy Gravel or Gravel. Overall, the site was best characterised as SS. SMu. SMuVS. AphTubi (Aphelochaeta marioni and Tubificoides spp. in variable salinity infralittoral mud) though there were elements of SS. SMx. IMx. VsenAsquAps (Venerupis senegalensis, Amphipholis squamata and Apseudes latreilli in infralittoral mixed sediment) in 2001. Although the most common assemblage at Sites 1-3 was characteristic of SS. SMu. SMuVS. AphTubi, this biotope is comprised of predominately of r-strategists or opportunistic species, characterised by high fecundity, small body size, and short generation time. This biotope is known to have low sensitivity to chemical pressures and representative taxa can recover rapidly from impacts (<1 year). Site 4 was situated in the mid region of the Carrick Roads in the Fal Estuary. Site 4 had moderate to high richness and diversity and there were no significant differences in biodiversity indices between years. Community composition was significantly different between years, but was due to small changes in the relative abundance of common species. Sediments were predominantly gravelly Mud, but were variable and also included Mud, muddy Sand and Sand. Overall, the biotope is best represented as a slightly gravelly variant of the Mellina biotope SS. SMu. ISaMu. MelMagThy (Melinna palmata with Magelona spp. and Thyasira spp. in infralittoral sandy mud). Site 5 was situated in the outer region of the Carrick Roads in the Fal Estuary. Site 5 had moderate richness and diversity and there were no significant differences in biodiversity indices between years. Community composition was significantly different between years, but was due to small changes in the relative abundance of common species. Sediments were typically Sand or muddy Sand. Across the 2001 and 2013 surveys, the biotope is best represented as SS. SSa. IMuSa. FfabMag (Fabulina fabula and Magelona mirabilis with venerid bivalves and amphipods in infralittoral compacted fine muddy sand). In 2001, there were high levels of Capitella spp. at Site 5 that indicated possible organic enrichment, but these were not present in 2013. Site 6 was situated in the outer region of the Carrick Roads in Falmouth Bay. Site 6 had low to moderate richness and diversity and there were no significant differences in biodiversity indices between years. Community composition was significantly different between years, but was due to small changes in the relative abundance of common species. Sediments were typically gravelly Sand or sandy Gravel. Overall, the community was best characterised as a classic SS. SCS. CCS. MedLumVen (Mediomastus fragilis, Lumbrineris spp. and venerid bivalves in circalittoral coarse sand or gravel). This biotope has previously been described as the 'Deep Venus Community' and the 'Boreal Off-Shore Gravel Association'. Site 7 was situated in the outer region of the Helford River. Site 7 had moderate richness and diversity and there were no significant differences in biodiversity indices between years. Community composition was significantly different between years, but was due to small changes in the relative abundance of common species. Sediments were typically gravelly Sand or sandy Gravel, though there was a larger component of silt in 2010. Overall, the biotope was best characterised as SS. SCS. CCS. Pkef (Protodorvillea kefersteini and other polychaetes in impoverished circalittoral mixed gravelly sand), though in 2010 and 2013 there were additional elements of SS. SSa. IMuSa. FfabMag (Fabulina fabula and Magelona mirabilis with venerid bivalves and amphipods in infralittoral compacted fine muddy sand) associated with elevated silt content. Site 9 was located in the inner region of the Helford River. Site 9 had moderate richness and diversity and there were no significant differences in biodiversity indices between years. Community composition was significantly different between years, but was due to small changes in the relative abundance of common species. Sediments were typically sandy Mud, but there was a higher component of gravel in 2010 (gravelly Mud). Across all surveys, the most consistent characterising taxa were Protodorvillea kefersteini, Caulleriella bioculata, Nemertea and Nematoda, which bears a very strong resemblance to the community SS. SCS. CCS. Pkef (Protodorvillea kefersteini and other polychaetes in impoverished circalittoral mixed gravelly sand). Though there were significant changes in relative species composition observed during the survey period, these were not sufficient to lead to changes in biotope classification, which have not deviated significantly from the 2001 survey. Based upon the findings of this study and acknowledging limitations with subsequent monitoring surveys, there is no evidence that feature presence or distribution, the presence of typical species, sediment composition and distribution, or species composition of component communities have changed since the 2001 survey, outside of what might be expected due to natural change in such a dynamic environment.