CRC REEF RESEARCH CENTRE TECHNICAL REPORT No. 14

Distribution Of Seagrasses In The Lizard Island Group - A Reconnaissance Survey, October 1995.

L.J. McKenzie, Department of Primary Industries & Fisheries
W.J. Lee Long, Department of Primary Industries & Fisheries
E.J. Bradshaw, Department of Primary Industries & Fisheries

(THIS REPORT IS NOT AVAILABLE FOR DISTRIBUTION AND WAS PUBLISHED FOR GOVERNMENT AGENCIES ONLY)

EXECUTIVE SUMMARY

Key Results

1. ± 78 ha of seagrass habitat was mapped in the survey of intertidal and sub-tidal areas surrounding the Lizard Island Group in October 1995.

2. Seven seagrass species (1 previously undescribed) from two Families, were recorded in the survey area. Thalassia hemprichii was the most commonly encountered species (38% of seagrass sites), although it was restricted to the shallow reef-tops. Halophila ovalis was the second most commonly encountered species (36% of seagrass sites) and was found in both intertidal and subtidal areas. Other species found in the survey area included Halodule uninervis, Halophila spinulosa, Syringodium isoetifolium, Cymodocea serrulata and Halophila sp.

3. Halophila ovalis had the widest depth distribution and occurred at both the shallowest (0.4 m below MSL) and the deepest (4.4 m before MSL) sites where seagrass was found. Mean depths of occurrence for individual species were mostly 15m below mean sea level.

4. A previously undescribed seagrass species, Halophila sp., was found in the survey area near Watson's Bay in depths >10m below MSL. This species was similar to Halophila capricorni although several differences were noted. Advice is being sought to help clarify its identification.

5. Seagrass communities included Halophila ovalis and Thalassia meadows in the shallow sandy areas of the reef flat and Halophila species dominated meadows in deep water. Shallow (3m) parts of Watson's Bay contained medium dense meadows of Halodule uninervis and Halophila ovalis.

Key Issues

1. The inventory of seagrass species from the Lizard Island Group, including the present survey, now totals 8, which is high for a Great Barrier Reef locality.

2. Anchor damage (scars) was observed in the seagrass meadows of Watson's Bay. Shallow water (10m) scars showed evidence of recolonisation/recovery, however deep water (>10m) scars appeared recent, as there was little evidence of seagrass recovery. It is unknown whether recovery occurs at a faster rate in shallow waters compared to deep water meadows and further research is necessary.

3. We recommend that permanent moorings be considered for large passenger vessels using the Watson's Bay National Park Zone, if use of the site continues to increase and if the deep water seagrass habitats are to be protected.

4. Indications from this seagrass survey are that the Watson Bay seagrasses are already affected by vessel anchoring. Seasonally and year to year changes in seagrass distribution, abundance and species composition make assessment of such impacts difficult. Anthropogenic impacts may not manifest in the seagrass until 6 to 12 months after the impacting event. Large scale changes may only be detected over a period of years as loss can be gradual. Investigations over long terms therefore need to be designed to detect gradual change.

THIS PUBLICATION IS CITED AS:
McKenzie, L.J., Lee Long, W.J. & Bradshaw, E.J.
Distribution of seagrasses in the Lizard Island Group - a reconnaissance survey, October 1995.
Technical Report No. 14
Townsville; CRC Reef Research Centre Ltd, 26 pp.

ISBN 1 876054 14 X.