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Subtidal seagrass surveys at Slipper and Great Mercury Islands

TR 2019/29

Report: TR 2019/29

Author: Dana Clark and Dan Crossett (Cawthron Institute)

About this report

Subtidal seagrass meadows are rare in New Zealand, with most seagrass now found only in intertidal areas of estuaries. While subtidal seagrass beds may have once been common in subtidal channels of sheltered estuaries, permanently submerged meadows in New Zealand are now primarily restricted to offshore islands (Turner & Schwarz 2006a). The Waikato region has three known areas of subtidal seagrass; Huruhi Harbour1 (Great Mercury Island), South Bay (Slipper Island) and Whangapoua Harbour. The Huruhi Harbour and South Bay beds were surveyed in the 1970s (Grace & Whitten 1974; Grace & Grace 1976) and in 2004 (Schwarz et al. 2006) and were recently checked by divers in 2017.

Waikato Regional Council (WRC) commissioned the Cawthron Institute to carry out a non-destructive field survey of the subtidal seagrass meadows at Huruhi Harbour and South Bay. The aim of the survey was to delineate the extent of the subtidal seagrass meadows and describe the ecological health and condition of the meadows.

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Subtidal seagrass surveys at Slipper and Great Mercury Islands [PDF, 4.2 MB]

Contents

Executive Summary
1. Introduction
2. Methods
2.1 Study areas
2.2 Mapping seagrass extent
2.3 Determining seagrass health and condition
2.3.1 Seagrass cover
2.3.2 Seagrass leaf length
2.3.3 Seagrass biomass
2.3.4 Macroalgae cover
2.3.5 Epiphyte/sediment cover
2.3.6 Fungal wasting disease
2.3.7 Additional information
2.3.8 Statistical analyses
3. Results
3.1 Seagrass extent
3.2 Seagrass health and condition
3.2.1 Indicators of seagrass condition
3.2.2 Indicators of seagrass stress
3.2.3 Visual biomass assessment
3.2.4 Other observations
4. Discussion
4.1 Seagrass extent
4.2 Seagrass health and condition
4.3 Visual biomass assessment
4.4 Recommendations for future monitoring
5. Acknowledgements
6. References
7. Appendices
List of figures
List of tables
List of appendices
Appendix 1. Transect locations
Appendix 2. Seagrass cover classes
Appendix 3. Reference scale for visual biomass estimates
Appendix 4. Semi-quantitive scale for estimating epiphyte cover and severity of fungal wasting disease
Appendix 5. Statistical results
Appendix 6. Raw data from the Slipper Island and Great Mercury Island seagrass surveys