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4.5.1 Mangrove Physiognomy

Plant height, stem girth at 0.05 m above stratum, and pneumatophore density were measured at four sites along the estuary (Figure 4.1). At each of these sites, three 4 x 4 m plots were randomly selected, marked out, and all trees measured for the above-mentioned parameters. Pneumatophores were counted in three separate, randomly selected 1 m2 quadrats within each plot. Mean values reported in Table 4.2 represent results of the three plots combined for each site.

4.5.2 Sediment characteristics

In July 2006 (southern-hemisphere winter) triplicate sediment samples were collected along transects at Sites 2, 3 and 4. In February 2007 (summer), sites were resampled, with the inclusion of Site 1, to provide baseline grain size data in light of potential mangrove removal in the future. Two sampling stations were located inside mangrove habitat, and three stations on the bare flats (Figure 4.3f).

Samples were also collected at three locations within cleared plots 1, 2 and 3, in May 2005, and again in summer 2006 and 2007.

Sediment samples were treated with 10 % hydrogen peroxide (H2O2) to remove organic material. Calgon was then added for deflocculation, and samples analysed for grain size distribution using the Malvern Mastersizer S. Version 2.19.

Three sediment cores, 1.5 m – 3 m in length, were collected in 70 mm diameter aluminium tubes using a vibracorer (Figure 4.3). Cores were returned to the lab for stratigraphic logging, and sub-samples were removed for grain size analysis and color notations, using Maunsell color charts.

It was only possible to collect cores in proximity to the main access point, which is mid-way along the estuary, roughly 25 m south of Site 4 (see Figure 4.1 for site location). Core (a) represents the sediment profile beneath a recently cleared

zone, and Core (c) was collected toward the middle of the intertidal flats, approximately 15 m east of the main tidal channel. A short core (35 cm deep) was collected within the mangroves in the vicinity of Site 2, in the middle (longitudinally) of the mangrove zone.

An additional series of cores were collected in 2007, 100 m west of Site 4 on intertidal flats and within mangroves on the adjacent side of the tidal channel (see Figure 4.1). Of three long cores, one yielded a contiguous 160 cm sediment profile of the unvegetated tidal flats. From this core, a cockle (Austrovenus stuchburyi) shell was collected at 155 cm depth and submitted to the Waikato Radiocarbon Dating Laboratory where Accelerated Mass Spectrometry (AMS) was used to calculate a 14C date. Wood samples collected at various depths were sent to SCION Laboratory (New Zealand) for identification, however due to their advanced state of decomposition, only very broad taxonomic classifications could be provided.

Sediment texture and colour, and broad descriptions of mineralogy (under petrological microscope) were also described.

A smaller core (7 cm diameter x 70 cm) collected in the mangroves opposite Site 4 was analysed for 210Pb to provide a sedimentation history. On return to the laboratory, the core was split and sub-sampled at 1 cm intervals then oven-dried to a constant weight. Approximately 10 grams of each sub-sample were retained for grain size analysis using a Malvern Mastersizer S Version 2.19, after 48 hours in 10 % hydrogen peroxide (H2O2) and 24 hours in Calgon. Selected samples were then analysed at the State Key Laboratory of Nanjing University for 210Pb.

Sediment Accumulation Rates (SAR) were determined from regression analysis of log-transformed data (Swales et al., 2002).

4.5.3 Surface elevation changes from erosion pins and RSET

Surface elevation changes on the mudflat surface were measured with a series of Rod Surface Elevation Tables (RSET), as described in Cahoon et al. (2002) (Figure 4.3). Benchmark poles were driven 3 m into the substrate with around 50 cm protruding from the estuary floor, then further stabilised with cement. A detachable arm with nine measuring pins attaches to the benchmark pole via a rod-collar coupling device, and for this study was rotated 180°, giving a total of 18 readings per RSET, which were then averaged after each visit to give a single

al., 2002). Each RSET benchmark was manually surveyed one month after installation, and again 14 months later to ensure the poles had maintained their original position.

Three transects of four RSETs were positioned in the upper estuary in the vicinity of Sites 2 and 3 (see Figure 4.1 for site location and Figure 4.3f for transect lay-out). RSETs are a permanent fixture in the environment and because of the potential for injury or interference, only three transects were installed. The intertidal RSETs along Transect 1 were positioned in Cleared Plot 3 (10 m from mangrove fringe) and Cleared Plot 1 (20 m from mangrove fringe) while RSET Transects 2 and 3 were positioned within mangroves and on bare tidal flat to assess variation in surface elevation changes in the absence of mangrove removal.

Stainless steel erosion pins were installed at 15 locations within the cleared areas as well as the mangrove zones at Sites 1 and 4, the locations of which are displayed in Figure 4.7. Erosion pins (0.7 m long, 5 mm diameter) were deployed in clusters of seven pins (Figure 4.3) and driven into the substrate with 0.2 m remaining above the sediment surface. The height above substrate of the seven pins was averaged to provide a single measurement of elevation change. Erosion pins have been used in other mangrove environments (e.g. Spenceley, 1977), and although the accuracy has not been specified in published surveys, it can be estimated to the nearest millimetre (Thomas and Ridd, 2004).

Site 4 was partially cleared of mangroves in mid-March 2006, roughly one year after sections in the vicinity of Site 2 and 3. Cleared Plot 1 (CP1) was cleared on 21 May 2005; CP2 on 13 August 2005 and CP3 on 30 August 2005 (Figure 4.2).

In this study, recorded measurements from RSETs and erosion pins are referred to as ‘surface elevation change’. These devices measure the rise or fall in the substrate, therefore any sediment compaction, shallow subsidence, root decomposition, or root growth are incorporated in the result of elevation change (Cahoon et al., 2000). It is important to note that the RSETs have a base datum 3 m below the surface, which is much deeper than the pins at 0.5 m, although the datum of both instruments is positioned at depths below the mangrove root zone.

Both techniques will therefore measure the processes of root development and root decomposition that are likely to influence surface accretion at these sites.

Any sediment compaction between 0.5 m and 3 m will be picked up by the RSET but excluded by the pins.

Most studies that use RSETs also put down marker horizons as a method of differentiating between rates of surface elevation change and depths of sediment

in proximity to each RSET. Unfortunately the success of the marker horizons in this study was limited. This was due to a) removal of some of the posts that marked each location (therefore making it difficult to find); b) difficulty in extracting small cores; and c) some trampling of the surface. Because of the limited time-frame of the study, rather than re-establish a second series of marker horizons, sediment traps were deployed as an alternative. The difference between the two methods, however, is that traps measure the weight of sediments collected over an area rather than a vertical depth of sediment accumulation.

Figure 4.2 Aerial vertical image of Waikaraka Estuary, 2003. Mangroves have expanded

4.5.4 Sediment traps

Sediment accumulation, or gross sediment deposition, was measured using cylindrical PVC sediment traps with an internal diameter of 70 mm and an aspect ratio of 1:8 (Figure 4.3). Sediment traps have been used in mangrove environments to investigate temporal and spatial variability in sediment loads that move across the intertidal surface (Ellis et al., 2004; Wolanski et al., 2005; Victor et al., 2006). They provide an inexpensive and robust method to gain some understanding of not only the load of sediment but also the characteristics of the sediment that is settling out of the water column.

Measuring pins (a)




Substrate surface (c)

70 mm

100 mm

Buried 490 mm 100mm

200 mm 500 mm


-20 m - 10 m 0 m 10 m 20 m 40 m mangrove habitat bare flats

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(f) mangrove


Figure 4.3 Images of instruments used in this field study. (a) Tripod component of the motorized vibracorer used to collect sediment cores; (b) schematic diagram of erosion pin cluster, (c) sediment traps installed on bare intertidal flats and within mangrove zones; (d) the permanent benchmark of the RSET device; (e) conceptual diagram of the portable RSET arm with adjustable measuring pins

The limitations of this technique however lie in their inability to reflect the on-going natural process of periodic settlement and resuspension that is likely to occur before longer term deposition occurs, and therefore are likely to over-estimate sediment flux (Kozerski, 1994).

Traps were deployed for approximately one month in May and June 2006 (winter) and January and February 2007 (summer). Transects of sediment traps were installed at the four monitoring sites, with two traps inside the mangroves and one on the bare flats (Figure 4.3f). Sediment accumulation rates of dry sediment are expressed in g m2 mo-1. A combination of tampering, mishandling and growth of filamentous algae over traps, has reduced the final analyses however.