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4.7.1 Historical sedimentation

The geomorphic evolution of Waikaraka Estuary over the last century can be linked to increased sedimentation triggered by deforestation of the catchment.

Quarrying of the local rhyolite in this area has also increased sediment deposits in the estuary. Analysis of 210Pb dated sediments collected toward the seaward end of the neck of the estuary identified a sharp transition from slow deposition of silts to more rapid sedimentation associated with poorly-sorted sands. This change was found 1 m below the surface on the west side of the channel and approximately 0.5 m below the surface east of the channel (Stokes et al., 2009), and may correlate with the onset of land-clearance for agriculture. A bed of angular coarse sands positioned 42 cm below the surface was analysed using 210Pb and dated as being deposited within the 1920s. This deposit may be representative of the impacts from rhyolite quarrying, which commenced production at that time and continued through to the 1960’s. Sediment accumulation rates of 35 mm yr-1 occurred in the early years of quarry operation and fell to 10 mm yr-1 between 1925 and 1950, with a further reduction of sedimentation (2.3 mm yr-1) recorded over the last half century.

The average SAR of < 0.1 mm yr-1 from ~ 7000 BP to approximately 1920, inferred from 14C results, is likely to be an underestimate. The use of carbon dates


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accumulation and does not account for any sediment compaction which is likely to occur under load. Pre-European sedimentation rates determined for other North Island estuaries range from < 1 mm yr-1 (Sheffield et al., 1995; Swales et al., 2002a) to < 1.5 mm yr-1 (Hume and McGlone, 1986).

The lag between the increased sedimentation associated with European land-use and the onset of rapid mangrove colonisation in the 1950s/1960s can be explained, at least partially, by the critical tidal limits required for the survival of Avicennia seedlings (Swales et al., 2007). The lower elevation limit (LEL) for Avicennia is typically between Mean Sea Level (MSL) (Clarke and Myerscough, 1993) and 0.3 m above MSL (Swales et al., 2007). The sediment accumulation rate of 10 mm yr-1 between approximately 1925 and 1950, as discussed above, would afford sufficient vertical growth of the tidal flats to bring the surface topography close to mean sea level and therefore provide suitable conditions for seedling survival. Historical SAR’s toward the head of the estuary would be relatively higher due to the proximity of the sediment supply and minimal tidal influence, and therefore likely to have reached the critical elevation for mangrove survival earlier. This gradient of SAR is common (Swales et al., 2002a; Ellis et al., 2004;), and further evidenced by aerial photo analysis of sites in Tauranga Harbour which identifies narrow mangrove stands isolated to the upper estuary (Site 1 and 2 as identified in Figure 4.1) in the early 1940s. Presently, the mangrove stands in Waikaraka Estuary are positioned mostly at or above 0.3 m above MSL (Moturiki datum) and the unvegetated sandflats are mostly at MSL (Park, 2004).

4.7.2 Mangrove expansion and contemporary sedimentation

The purpose of this study was to report on the mangrove expansion at Waikaraka Estuary and investigate the physical changes that have occurred as a result of mangrove removal. Photogrammetry documented a 23 % increase in mangrove coverage over the total estuary area between the years 1943 to 2003, with the greatest rate of expansion occurring between 1982 and 1996. The expansion rate has subsequently slowed, possibly as a result of human intervention via physical removal of propagules from the estuary. The main driver for mangrove expansion at this site may be sedimentation. The Waikaraka Estuary catchment area has experienced considerable land clearance since European settlement (approximately 150 - 200 years), and during this time sediment loads entering the estuary appear to have been greater than the present-day. Prior to the 1980s, stock

New Zealand estuaries, which may have truncated any estuarine vegetation establishment during that time. Recent prohibition of these activities may play some role in the success of mangrove expansion. Other possible factors include increases in nutrient run-off as a result of agricultural and horticultural activities, or a reduction in the occurrence of chilling temperatures during the establishment phase of mangrove propagules (Beard, 2006).

Mangrove shrubs in Waikaraka Estuary display a mean plant height of <1.5 m, in contrast to other New Zealand sites where tree heights range between 2 and 6 m in similar physical conditions (Young and Harvey, 1996; Osunkoya and Creese, 1997; May, 1999; Morrisey et al., 2003; Ellis et al., 2004; Alfaro, 2005). The study site is located toward the southern limit of mangrove distribution in New Zealand, and the limited plant growth can be attributed to climatic stress (Beard, 2006). Spatial variation in plant height is commonly found in mangrove habitat (e.g. Burns and Ogden, 1985; Ellis et al., 2004) and in this study could not be attributed to age. Other possible causes such as salinity (Crisp et al., 1990) and nutrient availability (Fry et al., 2000; Naidoo, 2006) were not measured.

Pneumatophore densities measured in this study are higher than those reported in other New Zealand estuaries (Young and Harvey, 1996; Morrisey et al., 2003;

Ellis et al., 2004; Alfaro, 2005) which may be due to the high mud content of surface sediments (Ellis et al., 2004). The low pneumatophore density measured within the youngest stand of mangroves in Waikaraka Estuary is consistent with a reported correlation between increasing plant age and higher pneumatophore densities (Morrisey et al., 2003). Pneumatophore density has also been found to correlate with increased sediment trapping capability (Young and Harvey, 1996).

Sediment trapping occurs within the mangrove vegetation at the study site, evidenced by the recorded increase in surface elevation. Surface elevation change averaged 3 mm yr-1, which is less than that recorded in other New Zealand estuaries (Swales et al., 1997; Ellis et al., 2004; Swales et al., 2007), although this is similar to values recorded in Florida (Cahoon and Lynch, 1997), Vietnam (Van Santen et al., 2006) and temperate Australia (Rogers et al., 2005; Rogers et al., 2006).

Sedimentation rates are influenced by sediment supply into the estuary, and hydrodynamic processes (Furukawa et al., 1997). As Waikaraka Estuary receives a relatively low volume of freshwater inflow, it is likely that suspended sediment input will also be relatively low, particularly in light of the small catchment area (10 km2).

The establishment of mangrove vegetation on previously bare tidal flats initiates a substantial change in surface sediment characteristics. Interpretation of core stratigraphy and surface sediment analysis suggests that bed material of fine and medium sand representative of the bare intertidal flats, is replaced by silt-dominated sediment once mangroves become established. The depth of mud is likely to vary spatially within the estuary, and was found to extend to a depth of 8 cm in the vicinity of a well-established mangrove stand located roughly equi-distant between the mouth and head of the estuary. Interestingly, medium and coarse silts were also found at depths of around 55 cm below the surface, suggesting that the study site has experienced accumulation of finer-grained material in the past.

Rates of surface elevation change associated with mangrove vegetation at Waikaraka Estuary ranged from -5 mm to 14 mm yr-1. The rate of surface elevation change is spatially and temporally variable with no clear seasonal fluctuations discernible over the monitoring period. A relationship between sedimentation with distance from the head of the estuary has been reported in other studies (Young and Harvey, 1996), but was not evident at this site. Higher values of surface elevation change recorded mid-estuary coincide with lower values along the RSET transects either side, suggesting the existence of a narrow depositional zone within this section of the estuary. This could be the result of tidal currents pushing released sediment from neighbouring cleared zones into this mangrove zone (approximately 200 m downstream), or may simply be due to a topographical/hydrodynamic anomaly favouring deposition at this location.

Sediment availability (determined from sediment traps) is lower within mangrove habitat than on the adjacent bare flats, further demonstrating the trapping capabilities of mangroves at the study site, particularly as the higher sediment accumulation rates of the bare flats do not result in a net gain in surface elevation.

This trend of decreasing sediment load between the bare tidal flats and vegetation zone, coupled with increasing sedimentation into fringing mangrove habitat, has been discussed by other authors and is considered to be a function of both the trapping capability of high vegetation density (Furukawa and Wolanski, 1996), and erosional episodes of the less stable sediments on the bare tidal flats (Van Santen et al., 2006). The monitoring undertaken in this study coincided with mangrove clearing activities, therefore the sediment accumulation rates quoted may not reflect typical, or ambient, sediment availability but is likely to reflect the injection of released sediment from cleared zones. A positive correlation between rainfall and sediment accumulation has been reported in other studies (Saad et al.,

periods of trap deployment at Waikaraka Estuary, possibly due to this remobilisation of sediment.

Since May 2005 approximately 9,600 m2 of mangrove vegetation has been removed from Waikaraka Estuary, resulting in significant changes to surface topography. Surface elevation within cleared areas declined at rates of 9 to 38 mm yr-1 (average 14 mm yr-1). The decomposition of mangrove root material has been found to contribute significantly to surface subsidence, following a study of mass tree mortality (Cahoon et al., 2003). Unfortunately, marker horizons were unsuccessful in this study and as such it is not possible to separate the processes of sediment erosion and root-mass decomposition. An apparent increase in grain size between winter 2005 and summer 2007, mostly of no more than 30 µm, is skewed by a systematic and substantial increase in grain size documented for summer 2006, coupled with a considerable range of mean values. Possible explanations for this anomaly are a) a function of spatially variable root-mass decomposition resulting in zones of released sediments along with trapped, coarser sediments within areas where root mass is still significant, b) the temporary exposure of underlying coarser material, c) the response to a period of increased flow velocities; d) an artifact of sample collection. A fining of surface texture between winter 2006 and summer 2007 occurred on bare flats adjacent to cleared zones at two of three sampling locations, which could possibly be due to deposition of silt released from nearby cleared areas.