Extent of coastal habitats

Monitoring sites

We have mapped the extent of mangrove habitat in four estuaries: the southern Firth of Thames, Whāingaroa (Raglan) Harbour, Tairua Harbour, and Coromandel Harbour. We also monitor other aspects of ecosystem health, such as sediment-dwelling animal communities, sedimentation rates, and water quality, in these estuaries.

Monitoring frequency and history

Mangrove extent has been mapped using aerial photographs from 2002, 2012 and 2017 (i.e., every 5 to 10 years).

Monitoring technique

The extent of mangrove habitat was digitised in GIS using Waikato Regional Aerial Photographs (WRAPS). The identification of mangrove habitat in 2002 and 2012 imagery was helped by using information from field surveys undertaken between 1998 and 2012 that described estuarine vegetation in the monitored estuaries. Aerial oblique imagery collected between 2016 and 2018 was used to help identify mangrove habitat in 2017 WRAPS imagery.

Mangrove extent was digitised for each estuary and then the total area calculated in GIS for each WRAPS year. The percent change from the extent measured in 2002 (i.e., the baseline year) compared to the most recent imagery (2017) was also calculated for each estuary. Specific areas of mangrove expansion and contraction (e.g., due to consented or unconsented clearance) were identified in GIS maps.

Available information on factors that could cause changes in mangrove extent (such as consented clearance and sedimentation rates) was used to help explain the changes identified in this indicator.

Spatial resolution and accuracy of WRAPS imagery has increased over the monitored period, from 1m resolution (and ±3.0 m at 95% confidence level) in 2002, to 30cm resolution (and ±0.5 m at 95% confidence level) in 2017. In older imagery it was difficult to discriminate small amounts of mangrove habitat, such as individual trees, so these may not have been included in digitised mangrove extent.

In 2017, the Firth of Thames mangrove forest covered 1476 ha in area, and there was 83 ha of mangroves in Coromandel Harbour, 36 ha in Whāingaroa (Raglan) Harbour and 30 ha in Tairua Harbour. Between 2002 and 2017, the total extent of mangrove habitat increased in three out of the four monitored estuaries. In Coromandel Harbour and Whāingaroa Harbour, mangrove extent increased by nearly 40%. Mangrove extent also increased by 12% in the Firth of Thames. In Tairua Harbour, mangrove extent increased between 2002 and 2012, but the consented clearance of c. 9 ha of mangroves between 2012 and 2017 resulted in an overall decrease in mangrove habitat of 11% between 2002 and 2017.

The expansion of mangrove habitat in Coromandel Harbour, Whāingaroa Harbour, and the Firth of Thames (and in Tairua Harbour before mangrove clearance) is likely a result of increased sedimentation. Sediment is deposited on the intertidal flats in estuaries, and this provides habitat for mangroves to grow. This indicator shows that mangrove expansion is mostly occurring around river mouths in our estuaries, which could indicate that this sediment is coming from rivers and the catchment. Once established, the mangrove forest traps more sediment, preventing the sediment from moving elsewhere in the estuary where it could negatively affect intertidal flora and fauna, such as seagrass and shellfish. Sedimentation rates measured in mangrove forest are often much higher than those measured on unvegetated intertidal flats. For example, sedimentation rates in the Firth of Thames mangrove forest have been measured to be as much as c. 60 mm/year [1], whereas sedimentation rates on intertidal flats outside of mangroves in Whāingaroa Harbour and the Firth of Thames are only up to c. 5 to 7 mm/year, and at some sites we have found that sediment is not building up at all [2]. 

[1] Green and Zeldis 2015. Firth of Thames water quality and ecosystem health. Waikato Regional Council Technical Report 2015/23

[2] Hunt 2019. Regional Estuary Monitoring Programme (REMP) intertidal sedimentation measurements, results and review of methodologies. Waikato Regional Council Technical Report 2019/04.

Mangrove expansion around the Piako and Waihou River mouths in the Firth of Thames

Mangrove expansion south of the Waiau River mouth in Coromandel Harbour

Mangrove expansion around the Waingaro River mouth in Whāingaroa (Raglan) Harbour

Decrease in mangrove habitat in Pepe Inlet in Tairua Harbour

Area of mangrove habitat (hectares) in the monitored estuaries in 2002, 2012 and 2017