Executive summary

This study uses satellite imagery paired with machine learning to quantify the extent of saltmarsh, mangrove, and seagrass ecosystems in the Auckland region and carbon sequestration rates measured primarily from New Zealand to estimate associated carbon sequestration rates. In addition, the extent and carbon sequestration by unvegetated habitats was estimated and the hypothetic extent of kelp habitat and associated carbon sequestration rate was quantified based on two scenarios (kelp cover occupying 10 per cent and 50 per cent of rocky reefs). This is the first study to create a regional layer of blue carbon ecosystem extent that uses data acquired from one time period (from 12 June 2023 to 11 January 2024) and uses Australasia sourced carbon sequestration measures (including samples from the Auckland region) to estimate associated carbon sequestration rates.

Saltmarsh occupied 3587 ha, mangroves 8181 ha, and seagrass 10,995 ha, with the west coast of Auckland containing over twice the extent of these habitats than the east coast (16,180 ha vs 6582 ha). Adjusted for their habitat extents, saltmarsh sequestered 3192 tC yr^-1, mangroves 5236 tC yr^-1 and seagrass 3518 tC yr^-1 throughout the Auckland region with a combined carbon sequestration of 11,946 tC yr^-1. Carbon sequestration by kelp was estimated at 160 and 798 tC yr^-1, based on kelp occupying 10 per cent and 50 per cent of all rocky reef habitats throughout the Auckland region, respectively. Rates of water-column phytoplankton net primary production (depth-integrated) in New Zealand range from approximately 0.4 to 17.8 tC ha-1 yr^-1, however, estimates of sediment carbon sequestration of phytoplankton carbon were not obtained. Unvegetated habitats had the lowest carbon sequestration rate per unit area of all habitat types. However, unvegetated habitats occupied an estimated 993,806 ha within the Auckland Region Coastal Marine Area, with an associated area adjusted carbon sequestration rate of 258,389 tC yr^-1, approximately 21-fold the combined rate of sequestration by other vegetated blue carbon ecosystems throughout the region.

The regional scale estimates of blue carbon ecosystem extent and sequestration quantified in this study illustrate their potential contribution to regional scale carbon abatement (in addition to the other important services provided by marine ecosystems), and highlight the importance of protecting and enhancing these ecosystems.

Auckland Council technical report, TR2024/10