Whangapoua Estuary: Changes Over the Long-Term

JOHN OGDEN  (Killara, Oruawharo, Aotea)

Whakapapa or genealogy, is understood in the context of family, but the landscape also has a ‘time before’ and a ‘time to come’, with each change influencing the next. Due to our short human lifespan, it is often difficult to see the slow changes occurring as an estuary slowly infills with silt and sand, or to notice the changes in a forest as one kind of tree or shrub disappears and is replaced by another species. In the water of any lake or estuary, there is always a fine ‘dust’ of particles derived mainly from erosion on the surrounding land. These tiny particles gradually settle out as ‘mud’ at the base of the water body. Consequently, each year’s sediment overlies earlier years. Young muddy sediments, reflecting recent erosion or other events are near the surface,  while the deeper, older, sediments compressed and solidified by the weight of material above, tell of things that happened in the past. This layering  provides a built-in story; with each passing year a record of the place is written and retained in the accumulating sediments. Bits and pieces of plants are frequently incorporated in the sediments – indeed in some cases (peat) the sediments are composed almost entirely of semi-decomposed plants. Plants are composed of carbon, and can be dated by the radiocarbon dating method. It is possible to remove the plant material from a layer of sediment at a certain depth and by radiocarbon dating it, to say how long ago the sediments at that depth were deposited. Radiocarbon dating provides the ‘clock’. 

The ‘story’ is provided by the sediments and the items trapped within the sediments – in particular by pollen grains. Any asthma sufferer knows that the atmosphere is full of drifting pollen grains, derived from many different flowers throughout the year. Some species produce vast quantities of pollen drifting in invisible clouds in the air, while others produce fewer but bigger pollen grains to attract insects to the flower. All these grains can be incorporated into the sediment, but likely there will be a lot more of the wind pollinated plants than of the insect pollinated plants (which do not blow about). 

Palynology is the study of pollen grains, especially their dispersal in the atmosphere and their incorporation into sediments. There are two important points about pollen grains: first, they are remarkably diverse – different plant genera or species can be unequivocally recognised under the microscope; second, the diagnostic outer layer of the pollen grain is amazingly tough, so that the grains can be extracted from the sediment by a procedure which removes everything else in the sediment sample using concentrated acids and alkalis, but leaves the pollen intact. The residual liquid containing these grains is then spread on a microscope slide and the numbers of each different pollen type counted. This process is carried out for samples from different depths, so that the changing proportions (percentages) of the different plant species can be plotted against depth – that is, the vegetation changes through time can be envisaged (refer to the diagram on the following page).

In the early 2000s, Yanbin Deng, myself and others from Auckland University carried out a detailed study of Whangapoua Estuary and Okiwi Spit. We extracted sediment cores from eleven sites around the estuary and obtained sediment samples from different depths for pollen analysis and other studies. The results were published in a series of papers with a final synopsis in 2006 (see references). Having replication allowed us to see what bits of the ‘story’ were common to all eleven sites, and what bits related to events at particular core locations. In estuaries, tidal currents and turbulence can blur the settlement process and sometimes wash away surface deposits. Moreover, the carbon in a layer doesn’t always represent the age of that layer (e.g., the plant material may be from old trees or derived from older sediments). Consequently, ‘interpreting’ the radiocarbon and pollen evidence can present a challenge, but our replication provides confidence in the general story from Whangapoua. 

Over the last 400,000 years there have been four major fluctuations in the climate of the world, referred to as glacial (cold!) and interglacial (warm) phases. The last glacial-interglacial cycle is represented by a few brown ‘clay’ dunes on Okiwi Spit. The present estuary was formed as climate warmed and sea-level rose at the end of the last glacial period. Although we have no direct evidence from Whangapoua, from other core sites on Aotea (Awana, Kaitoke wetland etc.), we know that c. 7500 years ago the sea spilled through the coastal dunes to create large estuarine lagoons behind enclosing sand spits. The sea level continued to rise until it was 2m above present levels, when it would have filled the Whangapoua estuary, probably up to the level of  Mabey  Road where it enters the Waikaro Block. A core near there (E8, Deng 2006) shows marine deposits (with shells) from c. 90 to c. 190 cm depth. Sea level began to fall again, commencing about 3500 years ago, and reaching present day levels about 600 years ago (1400 AD). Analysis of the E8 core shows clear evidence of Māori arrival about this time. A waka said to be buried in the sediments closeby  also implies that seawater reached this point at that time. Science and Maori tradition are well aligned: sometime between 600 to 800 years ago, it would have been possible to paddle from the estuary shore at Okiwi almost to the current level of Mabey road where it enters Mabey’s property. 

The arrival of Māori ancestors is clearly marked by an increase in bracken/aruhe spores and microscopic charcoal, with an associated decline in the pollen of forest tree species. These changes resulted from burning of forest in the surrounding catchment. It is difficult to date this point in our samples because the charcoal coming from burning ancient trees could already be several hundred years old before it was incorporated into the sediments. However, dates from elsewhere on Aotea, imply arrival of Māori soon after 1314±12 AD, when the Kaharoa tephra (ash) was deposited following an eruption near Taupo (Lowe et al. 1998.  Hogg et al. 2003). Carried by strong winds from the south, this eruption deposited a thin (c. 2 cm) layer of fine white ash in the wetlands of Aotea, which can now be identified in sediment cores from Awana and Kaitoke. The fact that this ash is not present at Whangapoua strongly suggests that the cored sites were all marine/tidal at the time, so that a rain of fine ash would not be easily preserved as a distinct layer in that turbulent environment. Radiocarbon dates on Māori midden shells on the Okiwi spit span the period 1390-1670 AD (Nichol et al. 2003), again indicating arrival before, possibly well before, 1390 AD.        

Burning of forest in the fourteenth century set in train a process of soil erosion and infilling of the estuary, such as can be seen following similar events, especially after forest destruction by cyclonic storms, today. From our cores the sedimentation rate before Māori arrival was between c. 2 and 4 mm per decade in the estuary and 5 -10 mm per decade in the surrounding fresh-water swamps. Following forest burning these rates increased to c.  9- 11 mm and 24 to 29 mm respectively. The greatly increased sedimentation rates also indicate a reduction in the area of the estuary as it became infilled from the shallowing margins, both by stream-carried inorganic matter and by the growth and death of plants, with the latter accounting for about half the amount of material deposited at the core sites. 

The estuarine vegetation change, or successional sequence, is particularly clear at Whangapoua. As sediments accumulate, tidal sand-flats are colonised by marine sea-grass (Zostera) and mangroves, which reduce water turbulence and increase the deposition of fine suspended matter as mud. This is colonised by rush-like plants, including oioi (Leptocarpus/Apodasmnia similis) and twig rush (Baumea/Machaerina juncea), forming a salt-meadow behind the mangroves. The twig rush is very resistant to decay, so its underground parts (roots / rhizomes) accumulate as peat, lifting the ground surface. Consequently, tidal inundation gradually declines. Eventually, when sea-water flooding is absent or rare, other plants, especially mānuka and raupō can become established. This sequence is moving through time and through space, so that the three visible zones (mangroves – salt-meadow – mānuka) represent a sequence of decreasing water-depth and frequency of tidal inundation, as the estuary is converted from water to land.

This successional process commenced in the inner estuary soon after Māori arrival and has progressed from tidal sand-flats through mangroves to mānuka in less than 700 years. On average over that time 78 cm of sediment has accumulated. More significantly, from the northern end of the estuary (near site E8) the linear extension of vegetation has been about 1.6 km, equivalent to the creation of about 3 square kilometers of semi-terrestrial surface, or 77% of the old estuary area. It is important to recognise that this infilling is not only due to Māori burning many centuries ago. A second pulse of erosion and infilling occurred as Europeans commenced kauri exploitation and forest clearance for farms. The silt input during this second pulse, after 1800 AD (12-13 mm/decade), was at a slightly greater rate than the first, perhaps because old sediments trapped in the surrounding freshwater swamps were remobilised and  transported to the estuary. This process is probably still on-going during big rain events on Aotea.   

It is speculative to extrapolate further from these results, which arise from a project that dealt with vegetation change rather than sedimentation rates. Moreover, the rate of estuary infilling depends as much on its (mostly unknown) depth profile as it does on the amount of sediment input. However, a couple of estimates suggest that, if processes continue as they have in the last 700 years, it will take another 200 – 400 years to infill the wide expanse of the Whangapoua estuary. But, although with climate warming, big erosional storm events dumping sediments into the estuary, are likely to be more frequent, the inexorable rise of  sea-level, on average by 35 mm/decade will refill seawater into more of the estuary and build up the Okiwi Spit as a sand barrier. It has happened before. Current sea-level rise is greater than any of our estimates of sedimentation, implying that past trends will be reversed. On this basis, rather than infilling, It is probably more likely that sea-water will flood the estuary more frequently and for longer in each tidal cycle, pushing the succession back and creating new sediment layers over the next century. In the shorter term, if we wish to maintain the estuary, the spit, and the biota currently using those ecosystems (including ourselves), some engineering work, catchment management  and good luck may be required. 

Previously, I explained how a research project carried out by staff and students at Auckland University in the early 2000s clarified the timing of the formation of Whangapoua Estuary. The effects of early Māori forest fires, and European logging and clearance for farming, on the infilling process were outlined; in less than 700 years, about three square kilometres of former tidal estuary have been turned into relatively dry scrubby land, with freshwater wetlands. These apparently rather slow processes have created the visible zonation from the sandy estuary, filled at every high tide, through bright green mangroves, a zone of brownish salt-meadow, and finally a mosaic of mānuka scrub and raupō swamps. While parts of the area have been drained and most of the surrounding forest is modified by fire and logging, it remains in my experience one of the best natural areas of estuary, wetland, dunes and beach anywhere in the North Island, deserving better recognition. The outstanding wildlife, and the need for protection of Whangapoua estuary, was stressed by the (then) New Zealand Wildlife Service as early as 1980 (Ogle 1980, 1981). 

In the 1970’s Max Burrill drained some of the wet mānuka covered flat land near the head of the estuary by digging a wide drain (Burrill’s Drain) with many tangential smaller drains feeding it. In doing so, he may have unwittingly created excellent habit for pāteke (brown teal Anas chlorotis) both in the drain and surrounding cleared and drained paddocks. However, the nearer the drain got to the tidal boundary, the more likely sea-water could enter and penetrate further inland. The consequence was that incoming saline water allowed mangroves to establish along the seaward drains and salt-meadow to spread in areas formerly occupied by mānuka, thus reversing the natural successional process in places. 

Annual-ring counts from polished cross-sections of dead mānuka stems, both in Whangapoua estuary and Oruawharo wetlands, indicate that pre-existing stands of mānuka have died synchronously several times in the last hundred years. The cause appears to have been exceptionally high tides coupled with on-shore gales, driving sea-water into the mānuka zone. The last such event was in June 2021, when seawater breached the spit, but earlier events can be recognised in 1968 (Wahine storm?) and 1997. Thus, vegetation spread, estuary infill, and the dynamics of the spit are inter-related, and changes may be sudden and episodic rather than the gradual shifts implied by the coarser long-term studies recounted previously.   

For their resident and visiting birds, such as New Zealand dotterel/tūturiwhatu and variable oystercatcher/tōrea pango,  the estuary, Okiwi Spit and Whangapoua beach ecosystems are linked; the estuary provides the main feeding ground and the spit provides resting or nesting sites. Long-distance migrants, such as bar-tailed godwit/kuaka and Pacific golden plover/kuriri also use both estuary and spit, although they nest elsewhere. The estuary is also linked to the surrounding successional vegetation, especially the fresh-water wetlands, by another suite of birds, exemplified by Australasian bittern/matuku-hūrepo, white-faced heron/matuku moana, brown teal/pāteke and pied stilt/poaka. Avian activity varies with the tides and also with the seasons. Waders and ducks follow the declining tide as it deposits their food on the mud surface, and before the buried filter-feeding molluscs withdraw their siphon tubes.

In March-April dotterels and oystercatchers gather up after breeding and the international migrants fatten up on the estuary in preparation for long flights across the Pacific Ocean. At this time of year there are also gatherings of gulls/tarāpunga, flocks of white-fronted terns/tara and a few Caspian terns/taranui on the ocean beach. Both species of tern occasionally risk nesting on the spit, but usually fail.  A second peak in bird abundance is usually in Nov/Dec, when bitterns/matuku-hūrepo boom in the surrounding wetlands and the godwits return from Siberia and golden plovers from elsewhere in the Pacific. These intersecting daily and seasonal cycles, driven by the moon’s orbit around the earth, are truly remarkable, and deserving of our observance and protection. 

The estuary and enclosing spit are the main post-breeding feeding grounds for New Zealand dotterels and variable oystercatchers. Most individuals of both these species nesting on the eastern beaches of Aotea move north to Whangapoua after breeding, although a few remain on their breeding beaches. This northern accumulation in the Autumn and early winter is presumably because the wide expanse of sand, silt and mud at low tide in the estuary provides them with a wealth of invertebrate food. The spit on the other hand provides a place to hang-out when the tide is high and, from my point of view, is a convenient place to count them.  But other places are utilised too; when New Zealand dotterel numbers are lower on the spit, they tend to be higher on Kaitoke Beach, because birds can fly! Also mostly moving to Whangapoua in autumn, variable oystercatcher pairs seem to remain together, but single and juvenile birds form a more distinct flock. These birds can move about too, utilising the Okiwi Airfield and the paddocks behind Kaitoke beach when worms are brought to the surface during periods of prolonged rain. The data for both species suggest a pattern of several years of increase, followed by sudden declines (figures on following page). However, overall, they appear to have gradually increased in abundance between 2000 and 2020, probably due to local conservation efforts to protect nests and keep dogs away from beaches when the young birds are most vulnerable (Lord et al. 2001; Ogden & Dowding 2013).  Interestingly, following 2020 – 2021, when the estuary outlet moved further north, breaching the spit, counts of both species started to decline, and this was particularly noticeable for variable oystercatchers in the count immediately after Cyclone Gabrielle (February 2023). It is quite likely that some of the oystercatchers and dotterels had gone elsewhere in March (the count month) of that year, but it is also probable that many birds were killed; there was nowhere to hide from that storm. 

Variable Oystercatcher Trends - Maximum post-breeding counts, Okiwi Spit 1999-2024

For New Zealand’s internal migrants, such as banded dotterels, wrybills and South Island pied oystercatchers, Whangapoua estuary is close to their northern limit. Banded dotterel numbers vary from c. 30 to c. 60 from year to year, generally peak in March, and showed no significant changes between 2000 and the present. A few pairs remain through the summer, nesting at Whangapoua and Kaitoke.  In contrast, migratory wrybills have declined from 10 -20 birds each winter to local extinction over the same time period (Ogden 2015). 

Wrybills and Poaka

Wrybills are tiny waders, and one of very few bird species with a sideways-curved bill. The species is a highly specialized and unusual member of the plover family; its sideways bill allowing it to poke under river pebbles for hidden insects, worms and crustaceans. 

They breed on the braided rivers of the eastern South Island, and migrate to North Island estuaries to feed in the winter.  Wrybills are found only in New Zealand, and are just as iconic as kiwi or kaka. Their un-noticed disappearance from Aotea beaches indicates vulnerability to mammalian predation, and human exploitation of water resources, on their nesting sites elsewhere in New Zealand (Ogden 2015).  

The pied stilt/poaka is a long-legged black and white wader, delicate in appearance but raucous by nature. It arrived on Aotea sometime last century, but is now restricted to Whangapoua, where it has also declined since 2000 (see: Birds of Aotea 2022).

Whether this is due to a failure of migration from the south, or unsuccessful breeding by resident pairs is unknown.  This year my best count was five birds.  

The estuary and surroundings are home to the largest population of pāteke on the island. Despite intensive management by DOC at Okiwi, that population remains at risk. Again, years of increase seem to be followed by inexplicable declines. The status of other rare wetland birds in the upper estuary, surrounding freshwater and saline wetlands is largely unknown. Although Bittern have declined on Aotea since Ogle’s (1980) survey, there seems to be a healthy population currently around the estuary. A 2023 survey indicated four pairs, with probably at least two of these breeding (Ogden, Moore & McIntyre (2023)). Spotless crake have been recently recorded in audio surveys (Stewart 2021). Fernbirds are certainly present, possibly marsh crake; many commoner bird species (white-faced heron, harrier, banded rail, paradise shelduck and pūkeko) are present, and the anticipation of occasional visitors such as black swans, spoonbills, whimbrels and eastern curlews on the open estuary or spit, create ornithological excitement!  

Insofar as the tidal flats provide food for the various birds (and humans) using the area it is important to know how that resource has changed and continues to change. The marine area provides molluscs, such as mussels, cockles and pipi, crustacea such as crabs and shrimps, and various ‘worms’. In the long term, as the area inundated daily by the sea has declined, and mangroves encroached, the area and quality of these resources has declined. Even within living memory some marked changes have occurred; for example, the disappearance of beds of harvested mussels from the creek margins. Monitoring these changes, and reporting the results, is key to understanding their ramifications through the whole ecosystem. 

The Community Shellfish Monitoring Programme was an initiative of the Hauraki Gulf Forum, Auckland Council, assisted by various agencies including the Ministry of Fisheries and the Department of Conservation. A grid of  49 sample points (map on following pages)  was established in the estuary and measured for the first time in 2007. All point grid reference data are loaded onto two GARMIN GPS units held by DOC, so that samples are taken very close to the same points every year. An excellent set of Guidelines, and a Teacher’s Resource Kit are available as hard-copy and on CD. Most of the actual counting and measuring has been done under supervision by school children from Okiwi and Kaitoke schools. 

During the years in which I was involved (2007 – 2014), cockle and pipi numbers declined by 70 – 80%, and average cockle shell size diminished every year (figures above).  The largest cockles were greater than 40 mm in 2007, but only 30-35mm by 2014, and the median size of all cockles dropped from 20-25mm to 15-20mm.

These results - decline in number and size - are typical for an over-harvested shellfish population. However, not all the trends can be attributed to direct human action, because some other unharvested shell types, such as horn shells, also showed a decline. It is well documented that removal, or a large reduction, of a key component of an ecosystem, in this case the most abundant filter-feeding molluscs, can lead to unanticipated changes elsewhere in the food chain.

In the case of Whangapoua estuary, the numerical changes in populations of dotterels, oystercatchers, stilts or wrybills, might have nothing to do with mollusc numbers, but until a detailed study of the estuarine food-web is undertaken, we cannot be certain. So far as I know, the total Whangapoua data set collected by the schools has never been fully analysed or reported on. That would be a start. 

References

1. Deng Y., Ogden J., Horrocks M., Anderson S (2006). Application of palynology to describe vegetation succession in estuarine wetlands on Great Barrier Island, northern New Zealand. Journal of Vegetation Science. 17:765-782.

2. Hogg A, Higham C, Lowe D, Palmer J, Reimer P, Newnham R (2003). A wiggle-match date for Polynesian settlement of New Zealand. Antiquity. 77:116-125.

3. Lowe DJ., Mcfadgen BG., Higham TFG., Hogg AG., Froggatt PC., Nairn JA (1998). Radiocarbon age of the Kaharoa tephra, a key marker for Late Holocene stratigraphy and archaeology in New Zealand. The Holocene. 8(4). 487-495.

4. Nichol SL, Lian OB, Carter CH (2003). Sheet-gravel evidence for a late Holocene tsunami run-up on beach dunes, Great barrier Island, New Zealand. Sedimentary Geology 155: 129-145.

5. Ogden J., Deng Y., Horrocks M., Nichol S., Anderson S (2006). Sequential impacts of Polynesian and European settlement on vegetation and environmental processes recorded in sediments at Whangapoua Estuary, Great Barrier Island, New Zealand. Regional Environmental Change. 6:25- 40. 

6. Birds of Aotea. The status of the birds of Aotea Great Barrier Island. Full Report. 2022. Pp 176. Great Barrier Island Environmental Trust. https://www.gbiet.org 

7. Lord, A., Waas, JR., Innes, J., Whittingham, MJ. 2001. Effects of human approaches to nests of New Zealand dotterels. Biological Conservation 98: 233-240.  

8. Ogden, J. 2014. Great Barrier Island Schools Whangapoua shellfish sample 2014. Report and data for Schools. Unpublished report. 

9. Ogden, J. 2015. Wrybills disappearing from Great Barrier. Environmental News. 34: 11-12. Aotea/Great Barrier Environmental Trust. https://www.gbiet.org/en34-wrybills

10. Ogden, J., Dowding, JE. 2013. Population estimates and conservation of the New Zealand dotterel (Charadrius obscurus) on Great Barrier Island, New Zealand. Notornis 60:210-223.

11. Ogden, J. Moore, S. McIntyre, L. 2023. Great Barrier Island Bittern Survey. 18th October 2023. Unpublished Report to Oruawharo Medlands Ecovision and Department of Conservation. 

12. Ogle, CC. 1980. Wildlife and wildlife habitat of Great Barrier Island. Pp 55. New Zealand Wildlife Service.

13. Ogle, CC.1981. Great Barrier Island Wildlife Survey. Tane 27: 177-200.

14. Stewart, P. 2021. Autonomous acoustic bittern distribution survey in the southern Auckland region and on Waiheke and Great Barrier/Aotea islands 2020. Report to Auckland Council. www.soundcounts.com