Effects of Mangrove Removal - Whangamata Harbour

The results of the study indicated that surface sediments in the 2005 cleared mown site are similar in 2008 to those in the existing mangrove habitat, with over 50% mud present at both sites. Silty (mud-dominated) sediments occur to depths greater than 5 cm in the cleared site WMA, suggesting that the mud is either continuing to accumulate and/or is not being redistributed after mangrove removal. Core stratigraphy revealed silty sands to depths below 12 cm at all cleared sites, suggesting that it would be some time, if at all, before these sites evolve to a purely sandy environment under the present-day conditions. Further core sampling will provide valuable information on erosion of fine sediments over time at these sites. Surface macrofauna were generally only present in low numbers at all sites. Analyses of temporal trends in the data indicate that benthic invertebrate abundance and diversity are increasing with time and that the disparity in population structure between the mangrove site and the cleared site may decrease as time goes on. However, given that manual (and consented) removal of mangrove seedlings is now a regular occurrence, and that the below-ground environment has been significantly altered in places, the cleared sites are unlikely to revert back to mangrove habitat. The results indicate that the benthic communities at the cleared sites have not recovered within the observation period (2005 – 2008). Purpose: In September 2005, unauthorised clearance of mangroves took place in the Moanaanuanu Estuary, Whangamata Harbour. Two hectares of mangrove vegetation were removed on this occasion; a subset of which was also mown with a tractor to remove seedlings and pneumatophores (above-ground breathing roots). Three studies were subsequently undertaken by Environment Waikato to investigate the environmental impacts of this vegetation clearance. The first study focused on the effects of incineration of mangrove detritus (Riddell, 2005), the second looked at impacts of mangrove removal on vegetation (Wildland Consultants, 2005), and the third assessed the effects of vegetation removal on benthic fauna. Environment Waikato collected a series of benthic samples in November 2005, and again in November 2006. Analysis of the 2005 samples was reported by Felsing (2006) and a comparison of the 2005 and 2006 results was discussed in a report by Stokes (2008).

Sediment cores and epifaunal counts in impacted and non-impacted areas. Grain size analysis of samples. The results from this study provide some insight into the physical characteristics of the mangrove habitat at Moanaanuanu Estuary, and the intertidal areas that were cleared of mangroves. The high mud content and below ground biomass documented at one of the cleared sites highlights the importance of considering the intertidal position and hydrodynamics of a site before planning the removal of any coastal vegetation.

Moanaanuanu Estuary, Whangamata Harbour

2005 - 2008

Five habitats in the cleared and undisturbed areas were sampled: 1. WMA: cleared and mown habitat 2. WMB: cleared but not mown habitat 3. WMC: recently cleared habitat 4. WMD: mangrove habitat (undisturbed) 5. WME: bare (undisturbed) intertidal flat habitat Within each of the areas 10 samples were taken. The following samples and information were collected: • 30 cm sediment core (diameter 5 cm). • Three 25 cm x 25 cm quadrat counts of epifauna; seedlings; % macroalgae cover; % microalgae cover; pneumatophores. • Five replicate bulked sediment samples were collected from the top 20 mm of surface sediment within each site for the following analysis: - LaserSizer grain size analysis - Total organic content - Chlorophyll A analysis Sediment texture and colour changes along the core were recorded. On completion of stratigraphic logging, sediment was seived (0.5 mm) and any mangrove root material collected. Root material was weighed wet and again after drying to constant weight at 60°C. Frequency of collection: Data collected in 2005 and 2008

Data quality: Good.

Sensitivity/Confidentiality: n/a

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