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Assessment of Physical Changes after Mangrove Removal: Whangamata Harbour 2008

TR 2009/13
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Report: TR 2009/13

Author: Debra Stokes

Abstract

Sediment cores were taken and epifaunal counts carried out in the following areas:
1. cleared and mown mangrove habitat (site WMA)
2. cleared but not mown mangrove habitat (site WMB)
3. recently cleared mangrove habitat (site WMC)
4. intact mangrove habitat (site WMD)
5. bare (undisturbed) intertidal flat habitat (site WME)

Sample analyses confirmed that surface sediments in the 2005 cleared mown site (WMA) 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 hydrodynamic regime.  Surface sediments in cleared sites WMB and WMC were dominated by fine and medium sands (with some mud), indicating that some of the mud particles have been winnowed out over time, but that any further significant coarsening of sediments may either not occur, or will occur very slowly.  Further core sampling will provide valuable information on erosion of fine sediments over time at these sites.

The rate of biomass decomposition influences how surface sediment characteristics change over time, as the mass of fine roots remaining in the sediment still functions to trap fine sediment particles until such time that the roots decompose and break down completely.  Sizes and ages of (former) trees at sites WMA, WMB and WMC are unknown because they were cleared before any such measurements could be taken.  Interestingly however, the below-ground biomass average of 3.47 kg m2 in the highly impacted cleared site (WMA) is higher than that which has been measured in healthy (intact) mangrove habitat in an estuary in Tauranga Harbour (Stokes and Healy submitted).  The other cleared sites also still have relatively high levels of root biomass with over 1 kg (dry weight) of root material per square metre. 

There are two possible explanations for the significantly greater mass of root material found at the highly impacted cleared site (WMA) compared with all other sampling sites; firstly that the highly anoxic and hydrodynamically quiet environment is inhibiting decomposition and breakdown processes, and secondly that these processes have been further inhibited by sediment compaction resulting from vehicle use in the area.  Compaction reduces the erosion potential of surface sediments and slows down decomposition processes by creating a highly anoxic zone (as demonstrated by the black colour of the sediments). 

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 changes observed in the sediment composition and the structure of benthic communities in the cleared areas, along with a significant decline in benthic faunal abundance and species richness indicate that the cleared sites have not recovered within the observation period (2005 – 2008).

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.

Assessment of Physical Changes after Mangrove Removal: Whangamata Harbour 2008 [PDF, 252 KB]