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THE STUDY AREA: TAURANGA HARBOUR

CHAPTER 2 MANGROVES AND THE RESEARCH STUDY AREA

2.6 THE STUDY AREA: TAURANGA HARBOUR

2.6.1 The physical setting

The study sites (Welcome Bay, Waikaraka and Waikareao estuaries) are small embayments situated within Tauranga Harbour. The harbour is a large (over 200 km2) barrier enclosed estuarine lagoon (Healy and Kirk, 1992) that extends roughly 40 km along the Bay of Plenty coast (Davies-Colley and Healy, 1978).

The lagoon is impounded by Matakana Island, a sandy barrier spit that has developed between two tombolos, Bowentown to the north and Mount Maunganui to the south. Similar to other barrier enclosed estuaries of New Zealand, extensive tidal flats are exposed at high tide (Healy et al., 1996).

The Geology

The Kaimai Ranges separate the Waikato and the Tauranga basins. The ranges are made up of Miocene – Pliocene basalt to rhyolitic rocks that were uplifted during a period of activity along the Hauraki Fault around 1-2 Ma (Briggs et al., 1996). The Tauranga Basin formed over the last 2 to 4 million years through the process of subsidence (Whitbread-Edwards, 1994) associated with activity of the Taupo Volcanic Zone (Davis and Healy, 1993). Tectonic controls of uplift and subsidence vary from site to site in New Zealand (Berryman and Hull, 2003) and to date opposing views have been presented as to whether the Tauranga Basin is still subsiding or currently stable (Shepherd et al., 1997).

Thick ignimbrite deposits are the prominent geological features of the Tauranga Basin. Toward the harbour margins, the ignimbrites are overlain with Holocene and Late Pleistocene alluvium and tephras (Harmsworth, 1983). A number of Miocene rhyolite domes protrude through the plateaus (Briggs et al., 1996), one of which (Minden Peak) creates a watershed for the Waikaraka Estuary catchment.

Some ignimbrites, particularly in the central basin area, are non-welded and as such, are prone to erosion. The terraces along the north-west margin of Waikareao Estuary, the largest field area of this study, are made up of Te Ranga Ignimbrites, a non-welded deposit that is structurally weak and prone to gully erosion (Briggs et al., 1996), as shown in Figure 2.6.

The Harbour system

Tauranga Harbour possesses two tidal entrances, Katikati Inlet to the north, and the Tauranga Entrance at the south-eastern end of the harbour, which is the access route to the Port of Tauranga (Davies-Colley and Healy, 1978). A channel has been dredged across the ebb delta to improve ship navigation (Davies-Colley and Healy, 1978). Tidal velocities can peak at 2 m s-1 on spring-ebb tides at the Katikati inlet throat (Hume and Herdendorf, 1992), and peak spring-ebb tides of 1.2 -1.3 m s-1 have been recorded either side of the Tauranga inlet gorge (Davies-Colley and Healy, 1978).

Seventy percent of Tauranga Harbour is exposed at low tide, therefore a combination of climate (inducing wind-waves) and tidal stage (providing inundation or exposure) will influence the entrainment of sediment over the greater part of the harbour floor (de Lange and Healy, 1990). The dominant orientation of sediment transport in small embayments within Tauranga Harbour have been reported as flood-dominated (White, 1979) and interchangeable depending on the season (Hope, 2002), with reduced current speeds across the tidal flats compared to adjacent tidal channels (Perano, 2000).

Bottom sediments of both harbour entrances and ebb deltas consist mostly of medium and coarse sand with some shelly gravel (Davies-Colley and Healy, 1978; Kruger, 1999). Fine sands and muds accumulate near the head of the many sub-estuaries of the harbour (White, 1979; Hope, 2002; Park, 2003).

2.6.2 The Climatic setting

The mean summer monthly maximums experienced in Tauranga township range from 22 to 24 °C over the months December to March. Mean winter maximums range from 14 to 15 °C and minimums from 5 to 6 °C. Mean rainfall is around 1,200 mm per year 1. The dominant wind directions measured at Tauranga Aerodrome tend to be north to north-east and west to south-west with the strongest (10.5 – 22.5 m s-1) mostly coming from the west and south-west (Hope, 2002).

1 Climate data for the observation period 1969-1998, accessed via MetService.com. Data on MetService.com supplied by

The Bay of Plenty region experiences occasional tropical cyclone systems that tend to travel south-east, bringing strong winds and heavy rains (Quayle, 1984), such as the event that caused numerous slope failures along the terraces of Waikareao Estuary after 309 mm of rain fell in a 24 hour period1 (see Figure 2.6).

The La Nina phase of the El Nino Southern Oscillation (ENSO) tends to provide more opportunity for cyclonic weather (de Lange, 2000). An analysis of storm surges and associated wind events suggests a period of higher frequency and greater magnitude of storm surge occurred between 1960 and 1976, a cycle which may be prevailing presently (de Lange, 2000).

Figure 2.6 Slope failures on the margins of Waikareao Estuary after heavy rain in May 2005.

2.6.3 Land Use Changes and Sedimentation

Significant changes in land use have occurred within and around Tauranga Harbour since European settlement (150 – 200 years). Forested areas of the Tauranga basin have been cleared for agricultural and horticultural purposes, and a growing human population is creating growth in the building sector and an increase in the amount of earthworks being undertaken. Construction of causeways, bridges and the port facility has altered the physical and hydrodynamic environment of the harbour. One example is Waikareao Estuary (one of the field sites of this study), where the tidal entrance has been narrowed from 400 m to 200 m as a result of land reclamation and the construction of road and rail causeways (White, 1979).

Sedimentation within Tauranga Harbour has been cited as a leading public concern (Lawrie, 2005), however contemporary and historic rates of infilling have

(14C) and radio isotope techniques (in this instance, 137Cs) to infer sedimentation rates in Waikaraka Estuary (Hope, 2002). Cesium (137Cs) is a by-product of nuclear weapons testing and peaks found in sediment cores correspond to nuclear test dates undertaken in 1953, 1955-1956 and 1963-64 (Swales et al., 2002a). The results of 14C dating suggested sedimentation rates within Waikaraka estuary of 0.05 mm yr-1, a lower result than commonly reported for other New Zealand estuaries (listed in Table 2.2). No peak of 137Cs was detected, however. In the absence of a cesium peak, the sedimentation rate calculated from the single carbon dating sample can provide only an average rate of deposition, and any temporal variation of sedimentation rates which are commonly recorded in the stratigraphy of other New Zealand estuaries (see Table 2.2), are not identified in this instance.

Estuarine sediments are often sourced regionally, at relatively short distances, and so land-use practices in the surrounding catchment will influence the volumes of sediment entering an estuary. Although rates of infilling in Tauranga Harbour are yet to be investigated (beyond this study at least), it has been suggested that fine-grained, catchment-derived sediments are accreting in the upper reaches of many of the quieter embayments of Tauranga Harbour, particularly along the western harbour from Katikati to Te Puna (Hope, 2002; Park, 2003). However, temporal changes in the quantity of terrestrial sediment entering the tidal system are largely unknown. A one-off, extensive study estimated sediment yields entering Tauranga Harbour over the monitoring period of July 1990 to June 1991 (Surman, 1999). The study addressed the erosional state of freshwater streams entering the harbour, and reported monthly measurements of suspended sediment concentration in freshwater inflow from the larger streams and rivers.

Interestingly, the highest suspended sediment concentrations and the highest sediment yields were not arriving from the largest inflow at Wairoa River (7 g m3), but from the Kopurereroa catchment (49 g m3) which drains into the Waikareao Estuary (Surman, 1999). The lower yields from the Wairoa River may be due to the damming upstream for hydro-electricity (Perano, 2000). Also, the Waikareao catchment is a large one, and one that has experienced considerable earthworks over the last 30 years. Park (2003) suggests that water quality may have improved since Surman’s study in 1990-1991, following monitoring of Kopurererua Stream (Waikato estuary) in 2001 that yielded a mean suspended

2.6.4 Mangroves in Tauranga Harbour

Mangroves are found in most of the low energy embayments of Tauranga Harbour (Figure 2.7). An almost exponential increase in mangrove coverage was reported for seven sub-estuaries of the harbour between 1943 and 2003 (Park, 2004; Figure 2.8). A small decline in mangrove habitat was recorded after 1999, presumably due to unauthorised vegetation clearance by local residents.

Canopy cover identified on aerial photos of the 1940s and 1950s was usually less than one hectare, increasing to between 5 ha at Waimapu Estuary and 35 ha at Te Puna Estuary by 2000 (Park, 2004).

Figure 2.7 Location of mangrove habitat within Tauranga Harbour (Park, 2004). Study sites are outlined. Image from Google Earth, 2006.

0 5 10 15 20 25 30 35

1960 1970 1980 1990 2000

YEAR

HECTARES

Bluegum Hunter Tanner N Taupiro Te Puna Waikaraka Waimapu Welcome Bay LOCATION

Table 2.2 Sedimentation rates measured in New Zealand estuaries, including dating techniques used.

AUTHORS LOCATION METHOD SEDIMENTATION RATES (mm yr-1) LAND USE GRAIN SIZE Hume & McGlone,

1986

Waitemata Harbour 14C Pollen

2 mm Present day 3 mm 1840 - 1985

<1.5mm Pre-Polynesian settlement

Rural and urban

Upstream mostly mud;

decreasing but always more than 50%

Sheffield, 1995 Whangamata Harbour

14C

210Pb

Pollen

20 mm from 1940s;

18 mm from 1920s-1940s

< 1 mm pre European settlement

Mining;

Forestry;

Steep land

Predominantly fine sand over intertidal flats

Swales et al., 1997 Mahurangi Estuary Cores Probes modelling

2-21 mm since 1850

Double the sediment loads of many other catchments in Auckland

Pasture and bush steepland

muddy sands and alternative mud/sand beds.

Swales et al., 2002a

Pakuranga Estuary (Auckland)

Pollen

137Cs

3-33 mm Urbanisation

1-1.6 mm European settlement 0.2-0.6 mm Polynesian settlement

3 fold increase in soil erosion over pasture because of urbanization

Urban

development Intertidal mud and fine sand

Swales et al., 2002b

Auckland estuaries Pollen

210Pb

137Cs

1.5 - 34.5 mm Post 1950 Mangrove sites to 30 mm

Urban

development mostly muddy fine-sands.

Ellis et al., 2004 Whitford Embayment, Auckland

Traps – mthly for 7 months

0 to 23 mm

Urban development

55 – 99% mud

Swales et al., 2007 Firth of Thames 210Pb 100 mm mangroves since 1950 20 mm 1850-1920 deforestation

Pasture Forestry, mining

Muds and fine sands

Figure 2.9 Mangrove seedlings prograding across bare intertidal flats seaward of established dense mangrove habitat (roughly 50 m width) bordered by saltmarsh habitat (in the foreground), Waikareao Estuary.

2.6.5 Ecology of Tauranga Harbour

The benthic ecology of mangrove habitat in Tauranga Harbour is mostly unidentified. Ecological studies undertaken within the Harbour have to date focused on the population structure and function of mollusc species of subtidal and (unvegetated) intertidal habitat (e.g. Cole et al., 2000; Gouk, 2001). The flood tidal delta of the Tauranga entrance appears to support a wide diversity of bivalves, with 31 taxa identified by Cole et al. (2000). The most common species identified in the area were Paphies australis, Tawera spissa and Ruditapes largillerti (Hull, 1996). Bivalve diversity and abundance has been found to decline in the upper reaches of other North Island locations, where the substrate contains mud and/or experiences increased turbidity (Thrush et al., 2004; Norkko et al., 2006). Paphies australis (pipi) appear to be particularly susceptible to increased turbidity (Teaioro, 1999).

2.6.6 Summary

The mangrove systems in New Zealand may not be as diverse or productive as their tropical counterparts, however ecosystem services are still provided via the contribution of organic matter to the detrital food web and habitat for some benthic, pelagic and terrestrial species. Through the use of aerial photographs,

within their natural range, and in Tauranga Harbour canopy cover has increased significantly since the 1960s. Increased sediment loads are a suggested leading causal factor in the changes to the vegetation mosaic, however other potential influences such as climate (e.g. less chilling temperatures) and greater nutrient loads have not been widely investigated.

The complex above-ground root structures of mangrove plants generate a substantial influence on the substrate morphology, however the extent to which mangrove expansion in New Zealand has amplified sediment retention is largely unknown. Because of the variability in forest structure and catchment characteristics, continued site-specific studies are required to address this question. Reduced biodiversity and increased coastal erosion are often highlighted as deleterious consequences of the large-scale removal of mangroves in tropical regions, however detailed studies of the physical or ecological changes that occur after mangrove removal are few. Temperate mangrove habitats are vastly different ecosystems which, to date, have received little attention in relation to their evolution, ecology and the impacts of their removal.