Plymouth sound & estuaries SAC subtidal sediment data analysis report 2017.
The aim of this project was to analyse sediment infauna data to measure any changes in community composition at the Plymouth Sound and Estuaries Special Area of Conservation (SAC). This will contribute towards Natural England (NE) condition assessment using conservation advice for each site and sub-feature. 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 Plymouth Sound and Estuaries. Data were provided from surveys conducted in 1998, 1999, 2011, and 2015. The scope of the analysis covered site features - estuaries, large shallow inlets and bays, subtidal sandbanks - and sub-features - subtidal coarse, mixed, mud and sand sediments. 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. The benthic survey data used in the site assessment came from a number of different surveys that had utilised different survey techniques, processing methods, and experimental designs. As such, the analyses undertaken reflected the quality of the data and any results should be interpreted with caution. Cawsand Bay had low to moderate species richness and total abundance and there were no significant differences in biodiversity indices between years, except for Pielou's evenness. Evenness decreased with survey year and sampling effort and it is thought to be due to the variable benthic assemblages and low sampling effort in later years (2011 and 2015). Sediments predominantly consisted of sands, but there was a variable quantity of silt and gravel present at the stations, and there was a general north-east to south-west (onshore-offshore) gradient with somewhat finer sediments in the inshore areas and coarser, poorly sorted gravelly sediments offshore. Community composition was statistically significantly different between some years - between 1998 and 1999 and also between 2015 and both 2008 and 2009 - though sampling effort and hence possible ANOSIM permutations varied dramatically between surveys. The most abundant taxa at Cawsand Bay were Chaetozone gibber, Magelona filiformis, Melinna palmata and Galathowenia oculata. Overall, the benthic assemblages were best characterised by two biotopes: SS. SMu. ISaMu. MelMagThy (Melinna palmata with Magelona spp. and Thyasira spp. in infralittoral sandy mud) and SS. SCS. ICS. MoeVen (Moerella spp. with venerid bivalves in infralittoral gravelly sand). Community composition remained similar from 1998 to 1999. Though there were changes in composition in 2015, this was primarily due to changes of species within genus (Magelona spp., Bathyporeia spp., Chaetozone spp., and Chamelea spp.) and could be due to analytical reasons rather than any genuine shift. Torpoint had moderate to high species richness and total abundance and there were no significant differences in biodiversity indices between years, though variability was far higher in 1998 where sampling effort was highest. The seabed at Torpoint was observed to be very heterogeneous with a high proportion of gravel, pebbles, cobbles, boulders and shell with variable amounts of sand and mud. Community composition was statistically significantly different between all years, but largest between 2011 and both 1998 and 1999. The most abundant taxa at Torpoint were Aphelochaeta marioni, Melinna palmata, Chaetozone gibber and Mediomastus fragilis. The dominant biotope in all surveys was SS. SCS. CCS. MedLumVen (Mediomastus fragilis, Lumbrineris spp. and venerid bivalves in circalittoral coarse sand or gravel), but with apparent elements of other biotopes. In 1998 and 1999, this was SS. SMx. IMx. VsenAsquAps (Venerupis senegalensis, Amphipholis squamata and Apseudes latreilli in infralittoral mixed sediment), but in 2011 was SS. SMu. ISaMu. MelMagThy (Melinna palmata with Magelona spp. and Thyasira spp. in infralittoral sandy mud). The change in composition from 1998-9 and 2011 was mainly due to a reduction in Aphelochaeta marioni, Amphicteis gunneri, or Apseudopsis latreillii and an increase in the bivalve Kurtiella bidentata. This change could be linked to a slight increase in silt observed in sediment samples due to sample location in 2011 being predominantly in the outer edge of the Torpoint site or the fact that the reduced sampling effort sampled fewer habitats. The River Yealm had low to moderate species richness and total abundance and there were no significant differences in biodiversity indices between years, though variability was higher in 1998 and 1999 where sampling effort was highest. The sediment composition at the River Yealm site was heterogeneous with stony gravelly sand along the northern side of the Inner Yealm, fine-medium sand and silty sands in the central inner Yealm, and predominantly fine sand in the outer Yealm. Community composition was statistically significantly different between 1998 and all other years and between 1999 and 2011. The biggest difference was between the benthic community in 1998 and that in 2015. The most abundant taxa at River Yealm were Apseudopsis latreilliid, Gammarella fucicola, Amphipholis squamata, and Calyptraea chinensis. The average similarity within year at River Yealm was extremely low, indicating an extremely diverse range of benthic assemblages. The most common biotope at the site was SS. SSa. IFiSa. NcirBat (Nephtys cirrosa and Bathyporeia spp. in infralittoral sand). The other dominant biotope was SS. SMx. IMx. VsenAsquAps (Venerupis senegalensis, Amphipholis squamata and Apseudes latreilli in infralittoral mixed sediment). The presence of Melinna palmata, Magelona spp., Spio filicornis and Galathowenia oculata also indicated elements of SS. SMu. ISaMu. MelMagThy (Melinna palmata with Magelona spp. and Thyasira spp. in infralittoral sandy mud). From 1998 to 1999, there were small changes in the relative abundance of some characteristic species, such as Apseudopsis latreilliid, Magelona filiformis, and Spio filicornis. In 2011, Magelona johnstoni, Bathyporeia elegans and Spio armata were all sampled and were not represented in 1999, though were represented by other species within the genus in 1999 (Magelona filiformis, Bathyporeia guilliamsoniana and Spio filicornis). Sampling effort at River Yealm was highly variable over the surveys, as was the spread of samples, with more stations taken from the estuarine region of the site in 1998 and 1999, with finer sediments and freshwater influence. As such, it is difficult to conclude anything with confidence, but it is suggested that observed changes were primarily caused by changes in survey design. Plymouth Breakwater had low to moderate species richness and total abundance and there was a statistically significant difference in both species number and total abundance with year, though not in any of the other biodiversity indices. Variability in metrics was substantially higher in 1998 compared to other years and associated with higher sampling effort. The Breakwater site had relatively homogenous substratum that is predominantly fine silty sand with small amounts of gravel, though stations closer to the coast were comprised of coarser gravelly sand with low silt content. The most abundant taxa at Breakwater were Melinna palmata, Chaetozone gibber, Ampelisca tenuicornis, Magelona filiformis, and Aphelochaeta marioni. As with the other sites, there were a number of different benthic assemblages present at Breakwater. The most common of these was SS. SMu. ISaMu. MelMagThy (Melinna palmata with Magelona spp. and Thyasira spp. in infralittoral sandy mud). A few sampling stations were characteristic of SS. SMu. SMuVS. AphTubi (Aphelochaeta marioni and Tubificoides spp. in variable salinity infralittoral mud) and SS. SMx. CMx. MysThyMx (Mysella bidentata and Thyasira spp. in circalittoral muddy mixed sediment) respectively. In 1998, 1999 and 2015 the dominant species were similar and there were small changes in the relative abundance of characterising species indicating fluctuations on the transition between SS. SMu. ISaMu. MelMagThy and SS. SMu. SMuVS. AphTubi. In 2011, increased numbers of Eudorella truncatula, Kurtiella bidentata, and Prionospio spp. indicate elements of SS. SMx. CMx. MysThyMx (Mysella bidentata and Thyasira spp. in circalittoral muddy mixed sediment) were becoming more important. This difference appeared to be caused by a reduced sampling effort and a greater proportion of samples being taken closer to the coast and introducing potentially new habitats. There were a number of issues associated with the benthic data supplied that took considerable time to address before analysis could begin. These steps typically involved some form of truncation and standardisation due to the varied survey designs and sampling and processing techniques, but there were also problems with missing data and a lack of supporting information that meant some data were not able to be used. Though there were changes in relative species composition observed during the survey period, it has not been possible to separate these from effects of changes in survey array and sampling effort. In general, though some biodiversity indices changed with survey year, dominant taxa and characteristic biotopes remained similar and were within the envelope of what might be expected due to natural change (e.g. shifts between SS. SMu. ISaMu. MelMagThy and SS. SMu. SMuVS. AphTubi). It is recommended that future surveys replicate the 1998 survey design and array in order to be able to more reliably comment upon changes to the sites sub-features. If other areas of the site need to be monitored, new areas should be defined and a baseline sampled following the principles of Murray (2001). Due to the range of habitats present, it is suggested that sampling effort be increased substantially in order to be able to report on condition of all the subtidal sedimentary sub-features. Based upon the findings of this study and acknowledging limitations with the different sources of survey data, 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 1998-9 surveys outside of what might be expected due to natural change in such a dynamic and heterogenous environment. This assessment is made with low confidence due to the change in survey array and the lack of sampling effort in specific areas of interest in 2011 and 2015.