Estuarine Vegetation - Response to Sea Level Rise

This pilot study aimed to help the Waikato Regional Council to identify the potential impact of projected sea level rise on the extent and distribution of estuarine vegetation within Waikato estuaries. In particular, the report examines the potential influence of surrounding land uses on the response of estuarine vegetation to sea level rise. Coromandel Harbour was chosen as the pilot study site as it has a range of different land uses around the harbour margin typical of the increasing human pressure on Waikato estuaries – including agricultural and urban land uses. The study investigated the influence of sea level rise and surrounding land use on estuarine vegetation in sites with different wave exposure and an area where estuarine wetland has already been significantly modified by human activities. The objectives of the pilot study were to: - Identify the factors influencing the boundaries of vegetation communities with existing sea level, particularly bed level and the effect of different wave exposure. - Survey the harbour and low-lying harbour edge to identify areas where wetland vegetation might expand to with predicted sea level rise. - Examine implications of various sea level rise scenarios for wetland loss/expansion. - Identify any relevant recommendations for the management of harbour margin land uses to ensure the protection of estuarine wetland vegetation in the event of projected sea level rise. The results of the study showed a strong correlation between the type of estuarine vegetation and bed levels, with certain communities only occuring at given bed levels. Another influence found to be particularly important was wave exposure. In particular, the study has highlighted the potential for peripheral estuarine vegetation communities (i.e. rush and sedgeland, sea meadow and riparian species) to be significantly reduced in area. In areas where landward topography rises steeply there will inevitably be a significant loss of rushland/sea meadow with projected sea level rise. There is potential for this loss to be balanced by increased areas of rushland/sea meadow where adjacent topography is low-lying - provided landward expansion is not constrained by development or other human activities. The area of mangrove communities is likely to be less significantly impacted by sea level rise, though some reduction may occur. Seagrass communities are unlikely to be reduced in area.

Table of Contents: Introduction 2 Objectives 3 Methods 3.1 Estuarine Vegetation Communities 3.2 Surveying and Study Sites 3.3 Analysis 4 Results 4.1 Survey Benchmarks and Tidal Parameters 4.2 Species Occurrence 4.3 Response to Most Likely Sea Level Rise 4.3.1 Exposed Site 4.3.2 Moderately Exposed Site 4.3.3 Modified Area - Long Bay Road Wetland 5 Conclusions and Management Recommendations 5.1 Summary of Conclusions 5.2 Recommendations References Appendices Appendix 1: Estuarine vegetation communities Appendix 2: Benchmarks Appendix 3: Additional cross-sections

Coromandel Harbour

For the purpose of this investigation, estuarine plant species are split into six groups: seagrass, mangrove, rush and sedgeland, sea meadow, saltmarsh ribbonwood, and riparian shrub communities. Benchmarks were levelled to mean level of sea (MLOS) using high tide levels predicted by the NIWA tide forecaster. At least two different predicted high tides were used to establish the elevation of each datum, with a third predicted tide surveyed if there was a significant difference between the first two measurements. Each datum was established with an accuracy of +0.01 m. Levelling of spot elevations using a laser level with position fixed by GPS. Spot elevations were taken to define bed levels within and along the boundaries of different vegetation communities and to establish the elevation of low-lying land around the harbour margin (the latter measurements to examine the potential for landward expansion of wetland vegetation with sea level rise). The surveyed bed level data for each of the different vegetation communities was graphed to identify community boundaries and to examine the effect of exposure and, for the Long Bay area, human alteration. Bed levels were also mapped and a typical cross-section chosen for each of the three surveyed areas. These cross-sections and the established vegetation community boundaries were used to predict the likely wetland vegetation change for different sea level rise scenarios. One-off study.

Two sites were chosen, one moderately exposed and one exposed, so the data does not cover the entire area of the Coromandel Harbour.

Results are presented in a written report. Report available in digital format from Environment Waikato website. www.ew.govt.nz/publications

Planning for future climate change and sea level rise - management of estuarine margins and vegetation.

Freely available. No confidentiality.

Complete.