• No results found

CHAPTER 6 IMPLICATIONS FOR THE FUTURE: WAIKARAKA ESTUARY

6.8 DISCUSSION

Figure 6.5 Conceptual model: mangrove expansion and surface topography, in ten year increments, at Site 4.

m of the mangrove fringe. These sluggish tidal currents promote sediment deposition, with re-suspension unlikely. Any erosion events within the mangrove habitat would therefore only occur during high energy events (Woodroffe, 1983) and/or high rainfall events (Tolhurst et al., 2005).

Interestingly, no strong tidal asymmetry in suspended sediment or tidal current velocity was identified at Waikaraka Estuary over the 4 day deployment of OBS and current meters. Indeed, the strength of the tidal flows, both inside and outside the mangroves, was well below the 0.3 m sec-1 required for re-suspension of sediment fines (Wolanski et al., 1995). SSC in mangroves at Waikaraka is not influenced by tidal stage (i.e., no flood peaks), instead showing a gradual decline over the inundation period.

Tidal current velocities of <0.02 m s-1 measured on the mudflats seaward of mangroves at Site 4 are considerably lower than that reported in studies of tidal dynamics within mangrove systems that fringe tidal creeks. For example, Wolanski (1992) found current velocities leaving tidal channels regularly exceeded 1 m s-1, whereas current speeds remained >0.07 m s-1 in mangrove forest 50 m from the creek edge. Van Santen et al. (2006) reported a dampening of flood tide velocities across mudflats fronting a mangrove forest in Vietnam which ranged from 0.15 to 0.5 m s-1. Tidal currents through fringing mangroves, measured by Van Santen et al. (2006), generally did not exceed 0.03 m s-1 which is still higher than the 0.01 m s-1 measured in mangroves at Waikaraka Estuary.

One factor that will act to attenuate tidal velocities in the fringing mangroves at Waikaraka is the fronting ‘cleared zone’ which extends approximately 30 m toward the channel. The mudflat surface of this cleared area consists of watery muds with numerous protruding tree and pneumatophore stumps which will be producing some frictional force against the tidal currents. Figure 6.6 illustrates an area recently cleared of mangroves and highlights the roughness of the bed surface.

Figure 6.6 Cleared area (Plot 1) two days after mangroves were removed in May 2005.

Distance from left to right (middle of photo) approximately 40 m.

Sediment loads, as measured by sediment traps, were highest on the bare mudflats in Waikaraka. Despite the relatively higher sediment loads and SSC observed over the mudflats the mudflat surface is subsiding at >10 mm yr-1, suggesting either a scouring surface or a collapsing substrate induced by root collapse after mangrove removal, or more likely a combination of the two. Couple this with sluggish tidal flows and it can be expected that much of the re-suspended sediment is delivered into the neighbouring mangrove habitat on an incoming tide.

Suspended sediment entering mangroves at Site 4 appear to settle over relatively short distances, with between 30% and 60% of the initial SSC deposited within 10 m of the seaward mangrove fringe. This is typical of sedimentation processes in mangroves, however the volume and gradient of deposition will be site-specific and reliant on inundation height and sediment supply (Furukawa and Wolanski, 1996). For example, Furukawa and Wolanski (1996) measured a 50% decrease in SSC within 35 m of the tidal creek/mangrove edge, whereas Victor et al. (2006) suggested much of the incoming suspended sediment in a microtidal site may be deposited within the seaward 25 m of mangrove forest. Similarly, Van Santen measured high sediment trap accumulation rates in sparse, pioneering mangrove vegetation of 20 – 40 g cm2 yr-1, compared with 0.5 – 2.5 g cm -1 yr-1 in dense

accumulation rates on the mudflats fronting mangroves in Waikaraka were between 1 and 7 g cm2 yr-1 which reflects the smaller sediment yields delivered into Waikaraka Estuary.

6.8.2 Mangrove expansion

The predicted future geomorphology in the presence of mangrove expansion at Waikaraka Estuary was based on empirical data of substrate accretion, combined with an assumption of a constant expansion of mangroves of 10 m per decade.

The conceptual model highlights the potential for seaward colonisation of mangroves to limit vertical growth of the seafloor on the landward side of the colony. Sediment trap results and surface elevation changes measured at Site 4 suggest that the majority of the incoming sediment is deposited within 20 – 30 m of the mangrove fringe. The progression of the mangrove fringe results in the older landward mangroves being subject to less tidal inundation, and therefore less sediment supply, as they move back relative to the tidal frame. The resultant cross-shore profile becomes convex, with development of a relative depression toward the landward side of the mangrove forest which may become hyper-saline and compacted. The landward margins could therefore see a progression in plant communities dominated by either stunted mangrove plants or a shift to saltmarsh species such as Juncus.

Progression of mangrove habitat can occur only where intertidal areas are positioned above mean sea level. At Waikaraka, over 90% of the estuary is above mean sea level (Park, 2004), indicating the potential for continued mangrove expansion. Following the evolutionary progression subscribed by Thom (1975) and Woodroffe (1992), for example, mangrove could continue to prograde across the tidal flats with the eventual morphology being a contiguous mangrove/saltmarsh wetland dissected by tidal channels.

Surface elevation of mangrove habitat in the Waikaraka Estuary averaged 3 mm yr-1, similar to the average rate of sea-level rise (Hannah, 2004). Numerous studies have identified a feedback mechanism at play in wetlands whereby surface elevation keeps pace with rising sea-level (Lynch et al., 1989; Cahoon and Lynch,

suspended sediment has been identified (e.g. Temmerman et al., 2003), substrate accretion is a function of a complex set of physical and biological processes (Cahoon and Lynch, 1997). It has been suggested elsewhere that below-ground biomass can contribute to surface accretion (McKee and Faulkner, 2000; Cahoon et al., 2006; McKee et al., 2007), and this becomes particularly important in regions of low terrestrial sediment input such as coral islands (Krauss et al., 2003;

Day et al., 2008).

The impact of sea-level rise in Tauranga Harbour becomes a more serious issue upon consideration of the effects of mangrove removal.

6.8.3 Mangrove removal

Between 2003 and 2009 approximately 10% of the mangrove habitat in Waikaraka Estuary was cleared. The associated decline in surface elevation of between 14 mm yr-1 and 17 mm yr-1 would amplify the effects of sea-level rise by effectively increasing the depth of tidal inundation. The double effect could effectively lower relative sea-level more than 20 mm yr-1. This may have wider implications if all mangroves are cleared because no buffer would exist to protect the remaining saltmarsh from potentially higher tidal inundation. In Waikaraka, similar to the other sites of this study, there is little to no room for any landward progression of saltmarsh to accommodate significant increases in RSLR.

Mangrove removal in Waikaraka Estuary is altering both the forest dynamics and the estuarine geomorphology. In the 12 months after mangroves were felled approximately 7.4 – 8.9 kg of sediment was released for each square metre that was cleared. These ‘released’ loads will include mangrove roots, micro-organics (i.e. algae) and mineral sediments.

By converting vertical measurements of substrate change to sediment mass, as mentioned above, an assumption is made that the seafloor changes are due to surface erosion. What is not taken into account is the effect of root collapse, which has been found to occur as fine roots die and compact, effectively lowering the relative surface level. This process was identified after Hurricane Mitch

mangrove peats of 11 mm yr-1 (Cahoon et al., 2003). Mangrove sediments in Waikaraka Estuary are less biogenic than the peat soils identified by Cahoon et al.

(2003), making any attempt to estimate the effect of root mortality on seafloor collapse difficult in this instance.

The impacts of any sediment redistribution as a response to vegetation removal in Waikaraka Estuary can only be speculated. Considering the quiet nature of the tidal regime in the regions where mangroves are being cleared, it is probable that surface sediments will be entrained and re-deposited within the estuary, particularly within the remaining mangroves where tidal velocities are extremely low. A remnant mangrove fringe may therefore be an important consideration for coastal management purposes, particularly in its potential to buffer the detrimental effects that high turbidity and sedimentation can impose on estuarine ecology, such as bivalve productivity (Thrush et al., 2003; Norkko et al., 2006).