Isles of Scilly Zostera Marina Monitoring 2002 Expedition Report

Isles of Scilly Zostera marina monitoring

2002 Expedition Report

Prepared by Kevan Cook

Supported by

Full Reference: Cook, K., 2002. Report on 2002 Isles of Scilly Zostera marina survey. Acknowledgements

The author would like to thank the following for their help and support:

Jeremy Pontin Duchy of Cornwall' s Office

Geoff Penhaligon, Harbourmaster St Mary's, Isles of Scilly

Liz Butterworth The Isles of Scilly Steamship Company

Cyril Nicholas English Nature

Jim Heslin St Mary's Underwater Centre Underwater

Miles Hoskins English Nature

And to all the volunteers, 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 2002...... 11 2.2 Analysis of Time Series Data ...... 12 2.21 Leaf Infection ...... 12 2.22 Plant Size Distribution and Plant Density...... 14 Seagrass Bed...... 15 Source of Variation...... 15 2.23 Mean Plant Density ...... 17 2.24 Mean Biomass Index ...... 18 2.3 Individual Bed Reports...... 19 2.31 Old Grimsby Harbour, Tresco ...... 19 2.32 Higher Town Bay, St. Martin’s ...... 20 2.33 Broad Ledge, Tresco...... 22 2.34 West Broad Ledge, St Martin’s ...... 23 2.35 Little Arthur, Eastern Isles...... 24 2.36 Bar Point, St. Mary’s ...... 25 2.37 Rushy Bay, Bryher ...... 26 2.4 Correlation of variables recorded ...... 27 2.5 Discussion and Future Work ...... 29 2.6 References ...... 30

3 Abstract

A total of 6 people took part in the 2002 expedition to survey the Isles of Scilly Zostera marina beds during the week period 27 July – 3 August 2002. During this time the volunteer divers carried out Zostera marina monitoring dives on the seven sites surveyed in previous expeditions.

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 at Old Grimsby Harbour remains highly patchy in nature and evidence of damage to the rhizome systems is 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. Evidence of a different but equally destructive type of anthropogenic input was observed at the Higher Town Bay, St. Martin’s site. However it is the bed at West Broad Ledge that had the lowest mean plant density though no cause for this dramatic decrease is given.

Infection by the parasitic slime mould Labrynthula remains at similar levels to those recorded in 2001 though there was a noticeable reduction in the amount of infection in the bed at Little Arthur. Only one bed, West Broad Ledge, St. Martin’s, had a greater mean percentage score than the previous year, the remaining beds all having a reduced level. The bed with the lowest mean percentage leaf infection was Higher Town Bay, St. Martin’s.

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 2002 expedition was the eleventh consecutive year in which volunteers have 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 2002 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 Claire Hinton 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 part in the 2002 Isles of Expedition:

Kevan Cook Jim Bull Ben Thompson Danny Edmunds Claire Hinton Steve Pooley

Also part of the team was Cyril Nicholas who acted as Cox’n of the RIB EVA.

1.13 Expedition Finance The expedition was part funded by volunteer contributions and part by a grant from English Nature to whom the author would like to acknowledge the part they have played in maintaining the surveys. Assistance from the Duchy of Cornwall, in the form of accommodation and the Isles of Scilly Steamship Company, in the form of subsidised transport for personnel, also helped the expedition achieve its objectives.

6 2 Zostera marina Monitoring

Seagrass beds were identified under Agenda 21 as being of vital importance and worthy of preservation. They are ecosystems of high productivity and are found in sheltered shallow marine conditions. They are also environmentally susceptible to natural and human related activities, such as climatic changes, sea level rise, dredging and fill activities, sedimentation, sewage discharge and shoreline development. Zostera marina is an important species both ecologically and economically because it provides critical habitat for a range of invertebrates and fish, it protects coastlines from erosion, it causes increased sedimentation leading to enhanced recycling of nutrients and improves water clarity (IGC, 1996).

Zostera marina is a marine angiosperm and together with Z. angustifolia and Z. noltii are the only marine flowering plants found in UK waters. The flowering plants appear markedly different from the non-flowering ones in that they have extended stems containing the flowers growing from the rhizomes with small leaves off the main stem. The leaves of the plants die back at the end of each summer / autumn and then regrow in the spring with the rhizomes, the root system, remain in place. However, although Zostera can reproduce both sexually and vegetatively Grassle and Grassle (1978) suggest that due to the vigorous rhizome growth of this and other seagrasses, and their wide distributions and ecological success, vegetative reproduction generally prevails. Flowering stems have been noted on several Zostera plants within the beds surveyed, however it is not known whether any of the resulting seeds are viable.

The distribution of Zostera along the south coast extends from the beds in the Scilly Isles eastwards along the channel as far as Studland Bay in Dorset in full marine but sheltered conditions in sand or mud substrates. However, it can also found in some brackish lagoons such as the Fleet in Dorset.

Annual surveys of the Zostera beds around the Scilly Isles were begun in 1984 by the Nature Conservancy Council and were continued until 1988. Following a gap of two year, 1988 and 1989, when no studies were carried out a further limited survey was done in 1991 (Fowler & Pilley, 1992). Volunteer groups have since carried out surveys every year from 1992 to 2001 (Raines et al ., 1993; Cleator et al ., 1996; Mackenzie & Rickards, 1996; Irving et al ., Irving and Mackenzie, 1997 and Irving et al ., 1998, Cook et al ., 2000, Cook et al ., 2001, Cook, 2002). The initial surveys were only carried out on two sites, English Island, East Higher Town Bay, St. Martin's and at Old Grimsby Harbour on the east side of Tresco. Since then addition sites at East Broad Ledge, St Martin's and West Broad Ledge, Tresco were added and following the investigation of a RoxAnn identified site, a further Zostera bed was surveyed at Little Arthur in the Eastern Isles. To complete the set of beds surveyed, the beds at Rushy Bay, Bryher and Bar Point, St. Mary’s were added in 2000 (Cook et al ., 2001).

In addition to the initial concerns relating to the population and concentration of the plants within the beds it was noted in the surveys of the beds between 1988 and 1991 that the plants were infected with the parasitic slime mould of the genus Labrynthula sp. more

7 common known as wasting disease. The infection of the Zostera plants with Labrynthula caused widespread loss in many areas during the 1930s and 1940s including the Isles of Scilly and various other beds in the south-west (den Hartog, 1987). The surveys carried out by the Nature Conservancy Council during the years 1984 to 1988 reported no incidences of Labrynthula within the beds surveyed (Fowler & Pilly, 1992). Although no formal surveys of the beds were carried out in the years 1989 and 1990, Fowler (1992) reported that during the years 1988 to 1991, the health of the beds had shown a marked deterioration. The cause of this was attributed to Labrynthula and this was later confirmed by the Coral Cay Conservation Sub-Aqua Club expedition to the islands in 1992 (Raines et al ., 1993).

It is unclear what caused the reoccurrence of the infection in the beds in the Isles of Scillies although Cleator et al . (1996) suggested a worsening in environmental conditions i.e. changed water temperature water flow and sedimentation or even changes in irradiance. Whatever the cause it was thought, however, thought that the increase in occurrence of Labrynthula was due to metabolic stress making the individual Zostera plants more susceptible to infection by reducing the amount of organic compounds involved in the resistance of disease and the growth of epiphytes on the leaves .

The disease first becomes apparent with the appearance of small black spots on the leaves. These then spread along the length of the leaf until the leaf can become so infected that photosynthesis is no longer possible and the leaf dies. If the infection is not severe the plant will remain viable and continue to grow. However, should the infection continue for a number of years the leaf loss may cause the rhizomes to also become infected and cause the plant to die (den Hartog, 1989). This die back has yet to be confirmed in the beds around the Isles of Scilly.

2.01 Aims of the survey The surveys carried out during the 2002 expedition followed on from those undertaken in earlier years and the aims were:

• To assess the plant density of different beds by surveying randomly selected samples. • To count the number of leaves per plant and measure the maximum leaf length. • To assess the amount of Labrynthula infection within the beds. • To assess the amount of epiphytic cover on each plant • To count the number of flowering plants present • To count the number of plants with eggs present on the leaves.

8 2.02 Locations The positions of the central datum marker for the beds surveys during the 2002 survey were obtained using differential GPS.

Site Latitude °N Longitude °W Date Surveyed Old Grimsby Harbour, Tresco 49 57.621 006 19.784 29th July Higher Town Bay, St. Martin's 49 57.434 006 16.350 29th July Broad Ledge, Tresco 49 56.396 006 19.734 30th July West Broad Ledge, St. Martin's 49 57.646 006 18.465 1st August Little Arthur, Eastern Isles 49 56.921 006 15.860 28th July Bar Point, St. Mary's 49 56.287 006 18.056 2nd August Rushy Bay, Bryher 49 56.606 006 21.624 31st July

Table 1. Location of Central Datum Markers for Zostera survey sites

2.03 Method At each site a central line was lowered to the sea floor to act as a datum from which all bearings and distances were measured. The position of this line was chosen by eye to be approximately central in the bed to be surveyed by means of a surface swim over the bed to determine its extent. Care was also taken to ensure that the datum marker was located in a sand patch within the bed to avoid disturbing any plants. Once the central line was positioned, pairs of divers were then sent to take samples from points at randomly selected distances, between 0 and 30m, and bearings, between 0° and 359°, from the central line. At each of the sites, a 0.25m 2 quadrat, constructed of a 0.5m by 0.5m square constructed from copper tubing was positioned so that the bottom left hand corner of the quadrat lay against the right hand edge of the tape at the indicated distance.

Figure 1 A Quadrat

9 The bearing was measured using a standard divers compass and the distance from the central line was measured using a marked tape. Once at the indicated position, all the plants within the bottom left hand quarter were then cut above the rhizomes and stored in a marked plastic bag for later analysis. Care was taken to ensure that only the plants with their rhizomes directly under the quarter square were taken by ensuring that all leaves were traced to the base.

2.04 Data Analysis For each sample bag, the following variables were recorded:

- Bearing and distance at which sample was taken - Number of plants per bag - Maximum plants length - Amount of Labrynthula infection and percentage number of infected leaves - Amount of Epiphytic cover - Number of flowering plants

Amount of Percentage of Score Diagrammatic Representation of Infection Infection Infection

0 Uninfected 0%

Some infection 1 <10% apparent As much as half the leaf 2 10 - 50% infected Over half the leaf 3 50 - 80% infected

Almost all the leaf 4 >80% infected

Table 2 Scoring system for Leaf Infection and Epiphyte cover

The data were initially recorded on a preformatted sheet and then loaded into an Excel spreadsheet for further analysis where the following was calculated:

- The density of plants per 0.0625m 2 - One-way Analysis of Variance (ANOV) on maximum leaf length and percentage of infected leaves between the different sites surveyed to examine if any significant difference (with a 95% confidence level) exists between the five sites. - Correlation matrix of the variable listed above to investigate the existence of any relationship between these variables. - Frequency analysis of longest leaf length to investigate the population structure at the five sites.

10 2.1 Data Analysis for 2002 The sampling of the seven sites took place during the period 27 July to 3 August 2002 from the boat EVA. A summary of the results from these surveys appears in table 3.

West Old Higher Broad Little Grimsby Town Broad Ledge, Arthur, Bar Rushy Harbour, Bay, St. Ledge, St. Eastern Point, St. Bay, Tresco Martin's Tresco Martin's Isles Mary's Bryher Total Number of Quadrats 24 24 25 25 25 25 25 Number of Empty Quadrats 8 14 1 14 0 11 17 Total Number of Plants 116 173 277 49 290 151 79 Total Number of Leaves 509 745 1072 194 1305 656 413 Percentage Leaves Infected 67.5 52.8 70.9 74.2 62.8 71.4 55.6 Mean Infection Score 1.06 0.65 1.07 1.00 0.82 1.03 0.68 Mean Epiphyte Score 1.05 0.83 0.82 1.51 0.59 0.52 1.63 Mean Plant Length (cm) 43.8 34.8 34.0 55.8 85.6 57.0 55.9 Standard Deviation of plant length 16.33 11.64 12.62 17.71 26.07 18.83 20.60 Mean Plant Density / 0.0625m 2 1 4.83 7.21 11.08 1.96 11.60 6.04 3.16 Mean Biomass Index 211.45 250.79 376.56 194.00 993.14 344.24 65.11 Number of Flowering Plants 1 3 1 0 4 3 4 Number of Plants with Eggs 50 60 155 11 87 34 13

Table 3 Summary of Zostera analysis, 2002 survey

Included this year is reference to the number of null bags recorded at each of the sites surveyed. It is important to include these bags in the density calculation as it reflects the patchy nature of most of the Zostera beds and can highlight the changing condition of the bed such as at Higher Town Bay, St. Martin’s. These figures however also mask the density of the plants where growth does occur so a second mean plant density figure is included, calculated by ignoring the null bags, for comparison.

Table 3a Supplementary data from Zostera data analysis, 2002 survey

11 2.2 Analysis of Time Series Data

2.21 Leaf Infection

West Old Higher Broad Little Grimsby Town Broad Ledge, Arthur, Bar Rushy Harbour, Bay, St. Ledge, St. Eastern Point, St. Bay, Tresco Martin's Tresco Martin's Isles Mary's Bryher August '02 68 53 71 74 63 57 56 August '01 51 65 76 66 86 69 68 August '00 47 56 65 60 58 62 47 August '99 60 67 71 69 52 55 69 August '98 57 67 76 61 65 n/a n/a August '97 43 37 44 49 n/a n/a n/a July '96 69 63 61 n/a n/a n/a n/a June '95 63 42 n/a n/a n/a n/a n/a August '94 56 79 n/a n/a n/a n/a n/a July '93 59 65 n/a n/a n/a n/a n/a

Table 4 Mean percentage number of leaves infected at survey sites

Comparison of 2002 percentage leaf infection data for the seven beds with data from pervious years show a marked increase in the amount of Labrynthula infection from the previous year in the beds at Old Grimsby Harbour, Tresco and West Broad Ledge, St. Martin’s. The amount of infection recorded at West Broad Ledge, St. Martin’s is the highest of any year since recording began in August 1997 and that recorded at Old Grimsby Harbour is the highest since July 1996. However, the amount of infection in the plants at Higher Town Bay, St. Martin’s, Little Arthur, Eastern Isles, Bar Point, St. Mary’s and Rushy Bay, Bryher is markedly reduced.

90 80 70 60 50 40 Mean Percent Infection 30 July '93 August '94 June '95 July '96 August '97 August '98 August '99 August '00 August '01 August '02

Old Grimsby Harbour, Tresco Higher Town Bay, St. Martin's Broad Ledge, Tresco West Broad Ledge, St. Martin's Little Arthur, Eastern Isles

Figure 2 Mean percentage number of leaves infected at survey sites

12 Seagrass Bed Count Sum Average Variance Old Grimsby Harbour, Tresco 120 80.98 0.67 0.05 Higher Town Bay, St. Martin's 173 91.37 0.53 0.08 Broad Ledge, Tresco 277 196.44 0.71 0.05 West Broad Ledge, St. Martins 49 37.09 0.74 0.03 Little Arthur, Eastern Isles 291 182.74 0.63 0.04 Bar Point, St. Mary's 151 107.85 0.71 0.04 Rushy Bay, Bryher 79 43.93 0.56 0.07

One way Analysis of variance Source of variation SS df MS F P-value F crit Between Groups 5.49 6 0.92 18.51 1.3E-20 2.11 Within Groups 56.09 1134 0.05 Total 61.58 1140

Table 5 One-way Analysis of variance of Percentage Leaf Infection

The one-way analysis of variance for the null hypothesis that ‘there is no significant difference in the amount of leaf infection between sites’ shows that this null hypothesis must be rejected, as the calculated F value of 18.51 is greater than the F-Critical value of 2.11. This indicates that there is a significant difference in the amount of infection between beds.

13 2.22 Plant Size Distribution and Plant Density The plant size distribution, measured as the length of the longest leaf, is illustrated in figure 4 below. This shows that, as seen in previous years, the distribution is highly different between each site. The distributions for Higher Town Bay, St. Martin’s and Broad Ledge Tresco have a near normal distribution while those for Old Grimsby Harbour, Tresco and West Broad Ledge, St. Martins are more even over the range of leaf lengths. The distribution Little Arthur, Eastern Isles continues to be slewed towards the longer leaf length.

West Old Higher Broad Little Grimsby Town Broad Ledge, Arthur, Bar Rushy Harbour, Bay, St. Ledge, St. Eastern Point, St. Bay, Tresco Martin's Tresco Martin's Isles Mary's Bryher August '02 43.8 34.8 34 55.8 85.6 67.0 55.9 August '01 33.8 41.0 47.4 45.3 69.5 60.6 61.2 August '00 38.9 39.9 32.7 53.3 89.2 46.5 53.0 August '99 37.5 45.8 40.4 64.5 79.0 58.2 66.5 August '98 29.0 50.0 47.0 62.0 86.0 n/a n/a August '97 49.0 48.0 41.0 53.0 n/a n/a n/a July '96 59.0 54.0 40.0 n/a n/a n/a n/a June '95 n/a n/a n/a n/a n/a n/a n/a August '94 56.8 61.0 n/a n/a n/a n/a n/a July '93 69.4 48.8 n/a n/a n/a n/a n/a

Table 6 Maximum leaf lengths at survey sites

100 90 80 70 60 50 40 30 20 Mean Max. Plant Length (cm) Length Plant Max. Mean 10 0 July '96 August '97 August '98 August '99 August '00 August '01 August '02

Old Grimsby Harbour, Tresco Higher Town Bay, St. Martin's Broad Ledge, Tresco West Broad Ledge, St. Martin's Little Arthur, Eastern Isles

Figure 3 Maximum leaf lengths at survey sites

Seagrass Bed Count Sum Average Variance Old Grimsby Harbour, Tresco 119 5206 43.75 268.95 Higher Town Bay, St. Martin's 173 6019 34.79 135.60 Broad Ledge, Tresco 277 9414 33.99 159.38 West Broad Ledge, St. Martins 49 2732 55.76 313.69 Little Arthur, Eastern Isles 290 24829 85.62 679.46 Bar Point, St. Mary's 151 8606 56.99 354.54 Rushy Bay, Bryher 79 4416 55.90 424.53

One way Analysis of variance Source of Variation SS df MS F P-value F crit Between Groups 478905 6 79817.5 227.53 2E-190 2.11 Within Groups 396762 1131 350.81 Total 875667 1137

Table 7 One-way Analysis of variance of Maximum Leaf Length

From table 7 it can be seen that the Null Hypothesis that ‘no significant difference existed between the leaf length from plants at the five sites’ is not supported with the F value of 227.53 being larger than the F-Critical value of 2.11 supporting the evidence above that the distributions are markedly different. The plant densities recorded at the four sites surveyed in the 1997 expedition showed a similar mean plant density. Higher Town Bay, St. Martin’s again had the greatest plant density though less than last year. The site with the lowest density was again Old Grimsby Harbour, Tresco but was slightly higher than the previous year.

15 50

25 Number

0 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 Maximum Leaf Length

Old Grimsby Harbour, Tresco Higher Town Bay, St. Martin's Broad Ledge, Tresco West Broad Ledge, St. Martins Little Arthur, Eastern Isles

Figure 4 Distribution of maximum leaf lengths at survey sites.

16 2.23 Mean Plant Density

West Old Higher Broad Little Grimsby Town Broad Ledge, Arthur, Bar Rushy Harbour, Bay, St. Ledge, St. Eastern Point, St. Bay, Tresco Martin's Tresco Martin's Isles Mary's Bryher August '02 4.83 7.21 11.1 1.96 11.6 6.04 3.16 August '01 5.44 12.24 7.80 4.72 8.76 5.76 5.04 August '00 2.48 11.44 10.40 4.92 9.50 4.00 2.32 August '99 3.62 10.20 9.17 5.12 9.84 7.39 4.59 August '98 7.32 14.08 4.20 7.36 8.28 n/a n/a August '97 6.78 11.79 4.83 9.52 n/a n/a n/a July '96 7.17 15.96 7.50 n/a n/a n/a n/a June '95 n/a n/a n/a n/a n/a n/a n/a August '94 8.83 9.64 n/a n/a n/a n/a n/a July '93 4.05 5.45 n/a n/a n/a n/a n/a

Table 8 Mean Plant Density per 0.0625m 2 at survey sites

18 16 2 14 12 10 8 6 4

2 Mean Plant Demsity / 0.0625m 0 July '96 August '97 August '98 August '99 August '00 August '01 August '02

Old Grimsby Harbour, Tresco Higher Town Bay, St. Martin's Broad Ledge, Tresco West Broad Ledge, St. Martin's Little Arthur, Eastern Isles

Figure 5 Mean Plant Density per 0.0625m 2 at survey sites

The bed with the highest plant density (11.6 plants per 0.0625m 2) was that at Little Arthur in the Eastern Isles while the bed at West Broad Ledge, St. Martin’s continued the downward trend seen over the past 5 years to 1.96 plants per 0.0625m 2. The bed at Higher Town Bay, St. Martin’s also had a sharp fall in density down from 12.24 to 7.21 plants per 0.0625m 2.

17 2.24 Mean Biomass Index

West Old Higher Broad Little Grimsby Town Broad Ledge, Arthur, Bar Rushy Harbour, Bay, St. Ledge, St. Eastern Point, St. Bay, Tresco Martin's Tresco Martin's Isles Mary's Bryher August '02 211.6 250.9 377.4 109.4 993.0 404.7 176.6 August '01 183.7 501.3 369.8 214.0 608.9 349.2 308.5 August '00 96.5 456.5 340.1 262.1 847.4 186.0 123.0 August '99 135.6 467.2 371.0 329.9 777.4 430.0 305.1 August '98 212.3 704.0 197.4 456.3 712.1 n/a n/a August '97 332.2 565.9 198.0 504.6 n/a n/a n/a July '96 423.0 861.8 300.0 n/a n/a n/a n/a June '95 n/a n/a n/a n/a n/a n/a n/a August '94 501.5 588.0 n/a n/a n/a n/a n/a July '93 281.1 266.0 n/a n/a n/a n/a n/a

Table 9 Mean Biomass index at main survey sites

1200.0

1000.0

800.0

600.0

400.0 Index Mean Biomass

200.0

0.0 July '96 August '97 August '98 August '99 August '00 August '01 August '02

Old Grimsby Harbour, Tresco Higher Town Bay, St. Martin's Broad Ledge, Tresco West Broad Ledge, St. Martin's Little Arthur, Eastern Isles

Figure 6 Mean Biomass index at main survey sites

The bed at Little Arthur again had the highest mean biomass index, 993.0 the highest figure recorded at any bed since the surveys began. The fall in mean plant density at West Broad Ledge, St. Martin’s is reflected in the lowest mean biomass figure for this bed, 109.4, and also of all the beds in 2002. The sharp fall in density at Higher Town Bay, St. Martin’s is also reflected in the mean biomass index for this bed, 250.9.

18 2.3 Individual Bed Reports

2.31 Old Grimsby Harbour, Tresco Following the 2000 survey of the Zostera bed, Cook et al (2001) highlighted the bed at Old Grimsby Harbour as being a source of great concern. This was due to the decline in plant density from 7.32/0.0625m 2 plants in 1998 to 2.48 plants per 0.0625m 2 in 2000 and it was suggested that the bed might be in danger of extinction. In the 2001 survey of the bed, it was found that the average plant density was had improved to 5.44 plants per 0.0625m 2 though the authors suggested that due to the position of the bed and the activity there the bed was still under threat. The average plant density recorded during the 2002 survey was 4.83 plants per 0.0625m 2, slightly down on the previous year but still much improved on the low described above.

The bed lies along the southern edge of the natural harbour formed by the small bay on the eastern side of Tresco that forms one of the main access points to the island from the sea. Although this access is dependant on the state of the tide, a large number of boats use the stone quay situated in the centre of the western side of the bay. The bay is found on the eastern side of the island and it provides shelter for both the visiting boats that anchor on the edge of the bay and local boats that use the permanent mooring buoys in the bay, from the prevailing south westerly winds. Though as a result of the large tidal range experienced throughout the whole of the Scillies, large areas of the bay are exposed at low water. This includes the southern edge of the Zostera bed which lies close to one of the beaches used by visiting tourists. Here, the water depth along the southern edge of the Zostera bed is shallow enough to allow trampling of the plants. Cook et al. (2001) and Cook (2002) described the highly patchy nature of the bed at this location. It was suggested that the movement of the mooring chains might be one factor. These moorings are anchored to base weight by means of a heavy sinker chain with a large buoy on the surface. The chains have to be long enough to allow for the rise and fall of the tide, which means that at low water there is a large amount of chain lying on the sea floor and over the Zostera plants. As the direction of the wind changes the moorings move round causing the chains to be dragged over the plants. This can cause plants to be dislodged and even for the rhizomes to be damaged. The presence of exposed and dislodged rhizomes within the arc of the chains movement help to confirm this theory. Although Cook et al . (2001) did not find any direct correlation between the position of the buoys and the patches it was suggested that this was possibly due to the extreme patchiness of the bed and the overall low plant density during the 2000 survey. Surface swims during both the 2001 survey (Cook, 2002) and again this year, when plant densities were higher, confirmed the continued patchy nature of the growth. The correlation of the patches with the positions of the mooring chains described by Cook (2002) was again noted during the 2002 survey with the rhizomes forming the bed in the swinging range of the chain having been removed.

The seabed in Old Grimsby Harbour is comprised of mainly medium sand. As described by Cook (2002) the Zostera was intermixed with some over lying loose macro algae and again, Sargassum was noted on any of the survey swims though this is probably due the

19 lack of any suitable holdfasts being present as Sargassum is present in several other locations around Tresco.

N 30

0 1.000 2.000 3.000 20 4.000 5.000 6.000 7.000 8.000 9.000 10 10.00 11.00 12.00 13.00 14.00 0 15.00 W E 16.00 -30 -20 -10 0 10 20 30 17.00 18.00 19.00 20.00 -10 21.00 22.00 23.00 24.00 25.00 26.00 -20 27.00 28.00 29.00 30.00 31.00 32.00 -30 33.00 34.00 S 35.00

Figure 7 Location of sample points and individual plant densities, Old Grimsby Harbour

2.32 Higher Town Bay, St. Martin’s The bay is situated on the southern edge of St. Martin’s and is bounded by Cruther’s Point to the west and English Point to the east. At the western end of the bay is a small stone harbour which acts as one of the main access points to the island from the sea. The bay is also used as a anchorage for a number of small vessels and the fringing beach and dune system are a popular destination for tourists. The Zostera bed lies at the eastern end of the bay and runs from English Island along the edge of the bay. Strong tidal streams flow across the bay and the bed is also exposed to the prevailing south-westerly winds. The sea floor here is comprised of medium sands which given the strong tidal streams is liable to erosion. This sediment movement and erosion is prevented in some places however by the Zostera rhizomes that help bind the sand and also promote accretion to the extent that the Zostera forms prominent platforms that stand up to 10cm above the surrounding sea floor. The strong tidal streams bring large fronds of loose macro algae from the rocky ground of the eastern isles and although there are very few other species growing here, there are large fronds of transported material that overlie the Zostera .

20 N 30

0 1.000 2.000 20 3.000 4.000 5.000 6.000 7.000 8.000 10 9.000 10.00 11.00 12.00 13.00 W 0 14.00

E 15.00 -30 -20 -10 0 10 20 30 16.00 17.00 18.00 19.00 -10 20.00 21.00 22.00 23.00 24.00 -20 25.00 26.00 27.00 28.00 29.00 30.00 -30 31.00 32.00 S 33.00 34.00 35.00

Figure 8 Location of sample points and individual plant density, Higher Town Bay

This has, in previous years, been one of the most productive sites in terms of plant numbers with densities being above 10 plants per 0.0625m 2 since 1996. This year, however, despite a mean percentage leaf infection of 53%, is lowest level since 1997 and the lowest of all seven beds surveyed in 2002, the density was only 7.21. This was due to the increased patchiness of the bed as reflected in the large number of null bags (14) recorded. If the null bags are ignored the mean plant density was 17.30 plants per 0.0625m 2, the highest density of any of the seven beds surveyed. Examination of the edge of the Zostera platforms showed exposed rhizomes and some obvious signs of erosion. The edges of the platforms are open to a variety of forces which might lead their reduction and although the reasons for this erosion and hence reduction in the area of the bed are not clear though there are several possible reasons. • The strong tidal streams that flow across the bay may erode the base of the platform and cause the collapse of the overlying rhizome system. • The bed is the shallowest of all those found in the Scillies, it drys at low water springs and as it is exposed to the south-westerly winds, wave action will erode the edges of the bed. • The proximity of the bed to the beach and the shallowness of the bed mean that it is liable to trampling, however this is not thought to be a major problem.

21 • As mentioned above, the small harbour at the western end of the bay is used as an access point to the island and is used by several boats, including those transporting divers to and from their dive sites. During the survey of the bed during the 2002 expedition, one of these boats was observed attempting to gain access to the harbour across the western edge of the bed. As this was close to low water, the water over the bed was too shallow to allow the stern of the boat to travel across the bed and the boat ground on the bed. To overcome this, the passengers on the boat climbed onto the bow of the boat and then, with the stern partially lifted, the boat, at full power, ploughed through the bed into deeper water. Zostera plants were observed ripped from the bed.

2.33 Broad Ledge, Tresco Broad Ledge lies on the southern edge of Tresco and together with Crab Ledge, Tabaccoman’s Ledge and Green Island to the east, forms part of the large inter tidal area that fringes the southern coast of Tresco. There is a small jetty that allows access to the island from the sea and is used by tourist boats when the tides allows. The bay is used on an occasional basis as and anchoring point for smaller yachts. The area is open to the prevailing south-westerly winds and weak tidal streams. The seabed here is comprised of coarse sand, mixed with small gravel, pebbles and some cobbles and some Sargassum muticum plants and small macro algae can be founds attached to the small material. The Zostera bed here is extensive and competes well with the other species. N 30 0 1.000 2.000 3.000 4.000 20 5.000 6.000 7.000 8.000 9.000 10.00 11.00 10 12.00 13.00 14.00 15.00 16.00 0 17.00 W 18.00

E -30 -20 -10 0 10 20 30 19.00 20.00 21.00 22.00 -10 23.00 24.00 25.00 26.00 27.00 28.00 -20 29.00 30.00 31.00 32.00 33.00 34.00 -30 35.00 S Figure 9 Location of sample points from and individual plant density, Broad Ledge

22 This bed had the second highest mean plant density, 11.1 plants per 0.0625m 2, of any of those surveyed during the 2002 expedition. This was also the highest mean plant density recorded at this site since recording began at this site in 1996. The mean percentage leaf score of 71% was slightly lower than that recorded the previous year, 76%, but in line with the percentages recorded during the previous 4 surveys.

2.34 West Broad Ledge, St Martin’s West Broad Ledge lies on the south-western edge of St. Martin’s and on the southern edge of the channel between St. Martin’s and Tean. This channel is used by pleasure boats navigating between the islands but not often as an anchoring point, boast generally choosing to anchor further to the north off the access jetty. The seabed is comprised of medium and coarse sand with small gravel and pebbles on which some fronds of Sargassum and other species of small macro algae were noted. The Zostera bed covers a wide area but is highly patchy in nature.

N 30 0 1.000 2.000 3.000 4.000 20 5.000 6.000

7.000 8.000 9.000 10.00 10 11.00 12.00 13.00 14.00 15.00

16.00 W 0 E 17.00 18.00 -30 -20 -10 0 10 20 30 19.00 20.00 21.00 -10 22.00 23.00 24.00

25.00 26.00 27.00 -20 28.00 29.00 30.00 31.00 32.00 33.00 -30 34.00 S 35.00

Figure 10 Location of sample points from and individual plant density, West Broad Ledge

The mean plant density has fallen every year since recording began at this site in 1997, from 9.52 plants per 0.0625m 2 to 4.72 plants per 0.0625m 2 recorded during the 2001 expedition. The mean plant density recorded during the 2002 expedition was 1.96 plants per 0.0625m 2. The reason for this crash in density is not clear and although the mean

23 percentage leaf infection, at 74%, was the highest recorded at this site, this is not thought to be a factor in the decline. As mentioned above, this bed is highly patchy and it is possible that the nature of the seabed in this area is a contribution factor. Where the bed is predominantly comprised of sand, the Zostera can become established but where there is more of the larger material, it will be more difficult for the Zostera rhizomes to survive, especially if this material is mobile. The presence of larger material will also allow some of the larger macro algae to become established and compete for light. No data is available on the composition of the seabed.

2.35 Little Arthur, Eastern Isles This bed lies in the eastern isles and to the east of Little Arthur where it is sheltered from the prevailing south-westerly winds and strong currents that flow round the islands. The eastern isles are also home to a colony of grey seals which attract several boats of tourists which come to view them though few boats anchor here. The majority of the substrate within the islands is comprised of bedrock and large boulders which are covered by dense growths of macro algae. The Zostera patch, however, lies in a small patch of medium sand. Despite the large amount of surrounding macro algae, the Zostera bed is relatively free from any covering plants. This is one of the deepest beds surveyed in the islands and although small in area, exists as a complete single bed with few patches.

N 0 30 1.000 2.000 3.000 4.000 5.000 20 6.000 7.000

8.000 9.000 10.00 11.00 10 12.00 13.00 14.00 15.00 16.00 0 17.00 W E 18.00

-30 -20 -10 0 10 20 30 19.00 20.00 21.00 22.00 -10 23.00 24.00 25.00 26.00 27.00 -20 28.00 29.00 30.00 31.00 32.00 33.00 -30 34.00 S 35.00

Figure 11 Location of sample points from and individual plant density, Little Arthur

24 The mean plant density recorded at this site during the 2002 survey was 11.6 plants per 0.0625m 2, the highest density recorded at any of the seven sites and also the highest recorded at this bed since surveys of this bed began in 1998. The mean percentage number of leaves infected was 63%, a big reduction on the 86% percent of leaves infected in 2001.

2.36 Bar Point, St. Mary’s The bed is situated in the shallow water that lies in the small bay to the west of Bar Point. It is highly patchy in nature and intermingled with large fronds of macro algae some of which grows in situ and some that are brought there by the strong tidal currents that flow across the bed. The presence of the Zostera here is thought to be important in stabilising the sand and preventing the erosion of Bar Point.

N 30 0 1.000 2.000 3.000 4.000 20 5.000 6.000 7.000 8.000 9.000 10.00 10 11.00 12.00 13.00 14.00 15.00 16.00 0 17.00 W E 18.00

-30 -20 -10 0 10 20 30 19.00 20.00 21.00 22.00 -10 23.00 24.00 25.00 26.00 27.00 28.00 -20 29.00 30.00 31.00 32.00 33.00 34.00 -30 35.00 S Figure 12 Location of sample points from and individual plant density, Bar Point

The mean plant density at recorded at Bar Point during the 2002 survey was 6.04 plants per 0.0625m 2. This included 11 empty bags, which, if ignored, give a plant density of 10.79 plants per 0.0625m 2. The bed is relatively small and rather patchy in nature and the range of the surveys at up to 30m from the central datum marker may, at some of the survey points, be outside the limits of the bed. The mean percentage number of infected leaves was 57%, down from 69% in the 2001 survey.

25 2.37 Rushy Bay, Bryher This is the smallest bed of all those surveyed in the Scillies and lies at the southern end of the island of Bryher and is possibly what Cook (2001) suggested was the remnant of what was once a much larger bed that extended south to Yellow rock and eastwards towards Tresco across the Tresco flats. Cook (2000) noted sparse growths of Zostera across this area and small outgrowths of plants were again observed, but not surveyed, during the 2002 expedition. The area is sheltered from the prevailing south-westerly winds but does experience strong tidal streams. The seabed is comprised of sand and patches of larger material though Zostera was recorded growing through both. The bed is boarded to the north by much larger material and outcrops of bedrock onto which are attached dense growths of macro algae including Sargassum muticum . 30 N 0 1.000 2.000 3.000 4.000 20 5.000 6.000 7.000 8.000 9.000 10.00 10 11.00 12.00 13.00 14.00 15.00 16.00 0 17.00 W E 18.00 -30 -20 -10 0 10 20 30 19.00 20.00 21.00 22.00 -10 23.00 24.00 25.00 26.00 27.00 28.00 -20 29.00 30.00 31.00 32.00 33.00 34.00 -30 35.00 S Figure 13 Location of sample points from and individual plant density, Rushy Bay

Despite its small size and competition from other algal species, this bed continues to thrive. During the 2002 survey, the mean plant density was 3.16 plants per 0.0625m 2, though included in this figure are 17 empty bags. This again is largely due to small size of the bed resulting in several of the survey locations being beyond the extent of the bed. The mean percentage number of infected leaves was 56%, down from 68% in the 2001 survey.

26 2.4 Correlation of variables recorded Old Grimsby Harbour % Leaves Infected Mean Infection Score Mean Epiphyte Score Total Plant Length % Leaves Infected 1.00 Mean Infection Score 0.77 1.00 Mean Epiphyte Score 0.05 0.07 1.00 Total Plant Length 0.19 0.17 -0.08 1.00

Higher Town Bay % Leaves Infected Mean Infection Score Mean Epiphyte Score Total Plant Length % Leaves Infected 1.00 Mean Infection Score 0.90 1.00 Mean Epiphyte Score 0.18 0.20 1.00 Total Plant Length 0.32 0.32 0.53 1.00

Broad Ledge, Tresco % Leaves Infected Mean Infection Score Mean Epiphyte Score Total Plant Length % Leaves Infected 1.00 Mean Infection Score 0.73 1.00 Mean Epiphyte Score 0.02 0.24 1.00 Total Plant Length 0.20 0.13 -0.22 1.00

West Broad Ledge, St. Martin's % Leaves Infected Mean Infection Score Mean Epiphyte Score Total Plant Length % Leaves Infected 1.00 Mean Infection Score 0.60 1.00 Mean Epiphyte Score 0.13 -0.08 1.00 Total Plant Length 0.37 0.00 0.37 1.00

Little Arthur, Eastern Isles % Leaves Infected Mean Infection Score Mean Epiphyte Score Total Plant Length % Leaves Infected 1.00 Mean Infection Score 0.84 1.00 Mean Epiphyte Score 0.15 0.27 1.00 Total Plant Length -0.08 -0.03 0.23 1.00

Bar Point, St. Mary's % Leaves Infected Mean Infection Score Mean Epiphyte Score Total Plant Length % Leaves Infected 1.00 Mean Infection Score 0.71 1.00 Mean Epiphyte Score 0.11 0.13 1.00 Total Plant Length 0.10 0.07 -0.22 1.00

Rushy Bay, Bryher % Leaves Infected Mean Infection Score Mean Epiphyte Score Total Plant Length % Leaves Infected 1.00 Mean Infection Score 0.89 1.00 Mean Epiphyte Score 0.38 0.34 1.00 Total Plant Length 0.39 0.28 0.20 1.00 Table 10 Correlation of variables, Strong Positive Correlation

27 As recorded in previous years the analysis of the 2002 data shows that the strongest correlation is seen between Percentage leaf infection and Mean infection score. This was highest at Higher Town Bay, 0.90 and lowest at West Broad Ledge, St. Martins, 0.60 where the correlation is best described as ‘modest correlation’ ‘strong correlation’ (Fowler and Cohen, 1994). The bed at Higher Town Bay, St. Martin’s also shows ‘modest correlation’ between Plant Length and Mean Epiphyte Cover though this is not reflected at the other sites.

28 2.5 Discussion and Future Work

The results given here again indicate the plant populations at the seven sites have a different size structure despite being within the same general area. The environmental conditions at each of the beds must have an impact though without further in situ observations, the variation in such factors as nitrate, phosphate and light levels as well as the tidal flow over the bed, is unknown.

What is clear, however, is that anthropogenic inputs are having an impact on the long term viability of the Zostera beds. The effects of the swinging mooring chains on the bed at Old Grimsby Harbour, Tresco have been documented and despite the increase in plant density over the past two years, the continued success of the bed must be in question. In addition, the damage done to the bed at Higher Town Bay, St. Martin’s by the passing boats dredging their way trough the bed at low water is clear. If this continues, the breakdown of the platform together with the natural erosion from tidal currents will reduce this bed to the point where it will no longer protect the eastern end of the bay.

In order to better understand the relationship between these environmental factors and the Zostera growth, it is suggested that further monitoring of the environmental conditions is undertaken. In order to understand the development of Labrynthula infection and increasing epiphyte cover it is suggested that monthly observations are made to record the density, growth and infection levels over the length of the growing season.

29 2.6 References

Cleator, B. 1993. The Status of the genus Zostera in Scottish coastal waters . (Contractor: B. Cleator, Edinburgh). SNH Review No. 22. Edinburgh, Scottish Natural Heritage.

Cleator, B., Nunny, R. and Mackenzie, G ., 1996. Coral Cay Conservation Sub-Aqua Club Isles of Scilly 1993 Expedition Report . Coral Cay Conservation Sub-Aqua Club, London.

Cook, K., 2000. Report on the Coral Cay Conservation Sub-Aqua Club 1999 Expedition to the Isles of Scilly .

Cook, K., 2001. Isles of Scilly Zostera marina monitoring 2000 Expedition Report.

Cook, K.J., Enderson, A., Reid, T., 1999. Report on the Coral Cay Conservation Sub- Aqua Club 1998 Expedition to the Isles of Scilly.

Cook, K.J., Hinton, C., Reid, T.J., 2001. Report on the Coral Cay Conservation Sub- Aqua Club 2000 Expedition to the Isles of Scilly . den Hartog, C., 1987. ‘Wasting disease’ and other dynamic phenomena in eelgrass Zostera beds. Aquatic Botany , 27: 3-14.

den Hartog, C., 1989. Early records of ‘wasting disease’ like damage in eelgrass Zostera marina . Dis. Aquat. Org ., 7: 223-226.

Fowler, J. and Cohen, L., 1994. Practical Statistics for Field Biology . John Wiley and Sons, Chichester.

Fowler, S.L., 1992. Marine monitoring in the Isles of Scilly, 1991. Report to the Nature Conservancy Council. 31pp.

Grassle, J. F., and J. P. Grassle, 1978. Life histories and genetic variation in marine invertebrates, in Marine Organisms . Edited by B. Battaglia and J. Beardmore, pp. 347- 364, Plenum, New York.

IGC, 1996 [Online] Marine Protected Areas . Available from http://www.igc.apc.org/habitat/treaties.mar-prot.html.

Irving, R.A., Gibb, J. and Mackenzie, G. 1996 . Coral Cay Conservation Sub-Aqua Club Isles of Scilly 1995 Expedition Report . Coral Cay Conservation Sub-Aqua Club, London.

Irving, R.A. and Mackenzie, G. 1997 . Coral Cay Conservation Sub-Aqua Club Isles of Scilly 1996 Expedition Report . Coral Cay Conservation Sub-Aqua Club, London.

30 Irving, R.A., Mackenzie, G. and Reid, T., 1998 . Report of the Coral Cay Conservation Sub-Aqua Club Isles of Scilly 1997 Expedition to the Isles of Scilly, 1 August - 16 August 1997 . Coral Cay Conservation Sub-Aqua Club, London.

Mackenzie, G. and Rickards, K., 1996. Coral Cay Conservation Sub-Aqua Club Isles of Scilly 1994 Expedition Report . Coral Cay Conservation Sub-Aqua Club, London

Raines, P., Nunny, R. and Cleator, B., 1993. Coral Cay Conservation Sub-Aqua Club Isles of Scilly 1992 Expedition Report . Coral Cay Conservation Sub-Aqua Club, London