Isles of Scilly Zostera marina monitoring 2003 Expedition Report Prepared by Kevan Cook Supported by Full Reference: Cook, K., 2004. Report on 2003 Isles of Scilly Zostera marina survey. Acknowledgements The author would like to thank the following for their help and support: Colin Sturmer Duchy of Cornwall's Office Geoff Penhaligon, Harbourmaster St Mary's, Isles of Scilly Cyril Nicholas English Nature Jim Heslin St Mary's Underwater Centre Underwater And to all the volunteers that gave so readily of their time and effort and without whom, none of the survey work would have been possible. 2 Contents Acknowledgements ........................................................................................................................................ 2 Contents.......................................................................................................................................................... 3 Abstract .......................................................................................................................................................... 4 1 INTRODUCTION....................................................................................................................................... 5 1.1 The Expedition ..................................................................................................................................... 5 1.11 Expedition Management................................................................................................................. 6 1.12 Expedition Members....................................................................................................................... 6 1.13 Expedition Finance......................................................................................................................... 6 2 Zostera marina Monitoring ......................................................................................................................... 7 2.01 Aims of the survey.......................................................................................................................... 8 2.02 Locations ........................................................................................................................................ 9 2.03 Method............................................................................................................................................ 9 2.04 Data Analysis................................................................................................................................ 10 2.1 Data Analysis for 2003....................................................................................................................... 12 2.2 Analysis of Time Series Data ............................................................................................................. 14 2.21 Leaf Infection ............................................................................................................................... 14 2.22 Plant Size Distribution and Plant Density..................................................................................... 16 2.23 Mean Plant Density ...................................................................................................................... 19 2.24 Mean Biomass Index .................................................................................................................... 20 2.3 Individual Bed Reports....................................................................................................................... 21 2.31 Old Grimsby Harbour, Tresco ...................................................................................................... 21 2.32 Higher Town Bay, St. Martin’s .................................................................................................... 23 2.33 Broad Ledge, Tresco..................................................................................................................... 25 2.34 West Broad Ledge, St Martin’s .................................................................................................... 26 2.35 Little Arthur, Eastern Isles............................................................................................................ 27 2.36 Bar Point, St. Mary’s .................................................................................................................... 28 2.4 Discussion and Future Work .............................................................................................................. 30 3 Monitoring of Leptopsammia pruvoti at Gap Point................................................................................... 31 3.2 Results ................................................................................................................................................ 31 3 Abstract A total of 13 people took part in the 2003 expedition to survey the Isles of Scilly Zostera marina beds during the week period 26 July – 2 August 2003. During this time the volunteer divers carried out full Zostera marina monitoring dives on six of the sites surveyed in previous expeditions with the seventh, Rushy Bay only partly surveyed. On selected beds a new method of assessing plant density was also tried. Zostera plant densities follow a similar pattern from that seen in previous years with the bed at Little Arthur continuing to have the greatest mean plant density. The bed with the biggest change is that at Old Grimsby Harbour which, although remains highly patchy in nature, had an increase in mean plant density to the highest level since 1994. Evidence of damage to the rhizome systems from the swinging mooring chains is still present. This patchy nature of Zostera growth was observed at a number of sites although the reasons for this are not as obvious as at Old Grimsby Harbour. The bed at West Broad Ledge which had lowest mean plant density in 2002 was somewhat recovered in 2003 but still down on the levels recorded in 2000. Infection by the parasitic slime mould Labrynthula remains at similar levels to those recorded in 2002 though there was a noticeable reduction in the amount of infection in the bed at Old Grimsby Harbour, Tresco which had the lowest mean percentage leaf infection of all the beds surveyed. It is suggested that while natural conditions and factors will have an impact on the health of the Zostera beds, anthropogenic impacts are also important in their long term viability. 4 1 INTRODUCTION The Isles of Scilly, lying some 40km south west of Lands End, are the most westerly land mass of Great Britain and are comprised of approximately 200 granite islands and rocks separated by a shallow sea. The seafloor between the islands is mainly comprised of a mixture of sands and smaller pebbles with a distinct decreasing size gradient in material from west to east. The five main islands; St. Mary’s, St. Martin’s, Tresco, Bryher and St. Agnes are inhabited and support farming, fishing and are a popular tourist destination. The islands are a variety of unique compositions of natural history as a result of the combination of their physical environment and climate. Rare indigenous plants, as well as exotics more characteristic of subtropical climates, thrive on the Islands. The low level of traffic on the roads and plentiful windbreaks made from hedges and pines provide ideal conditions for a variety of birds. The marine environment is also shaped by the extreme physical conditions. The rocky shores of the western islands are so extreme that the scientific classification of shore- types had to be extended to include a special category for Scilly - the 'super-exposed'. Below the low-water mark, only those species that are able to withstand periodic inundations of the full force of the Atlantic are able to survive. On the more protected eastern seaboard by contrast, sheltered, deep-water habitats are home to colonies of various delicate species such as sea fans and branching sponges which often have southerly distributions. At the opposite end of the scale of exposure to the battered western shores, the sea bed inside the archipelago is very sheltered. Sediments derived from the granite bedrock are carried by tides and waves into the sheltered inter-island basin and deposited there. These shallow and sheltered inter island waters provide an ideal marine environment for the growth of Zostera marina, which grows in large beds on the sandy sediments found there. Although deprived of the nutrient sources that are common to the inshore waters of mainland Britain, the waters around Scilly support a wealth of marine life, including numerous commercial species and an important colony of the Grey Seal (Halichoerus grypus). 1.1 The Expedition The 2003 expedition was the twelfth consecutive year in which volunteers visited the Isles of Scilly to carry out surveys of the coastal waters around the islands. The team was a mixture of volunteers with previous experience of surveys in the Scillies and new members. The main objective of the 2003 expedition was to repeat surveys of the seven Zostera marina beds surveyed in previous years also noting the occurrence of any Sargassum muticum within the beds. 5 1.11 Expedition Management The expedition was planned and led by Kevan Cook and assisted by Tom Ried, John Whitaker and Ben Thompson. Other members of the expedition team carried out various other tasks of the associated with the work of the expedition. 1.12 Expedition Members The following people took