Lake Pupuke Research Articles

A Holocene palaeomagnetic secular variation record from Lake Pupuke, New Zealand

ABSTRACT Palaeomagnetic records from three cores of lake sediment have been merged, producing a new Holocene geomagnetic secular variation record for northern New Zealand. The cores are from Lake Pupuke, a maar lake in Auckland Volcanic Field (AVF). They contain tephra from Rangitoto Volcano (530 ± 10 yr BP), Taupō (1718 ± 10 yr BP), Tūhua/Mayor Island (7637 ± 100 yr BP) and Rotomā tephra from Ōkataina Volcanic Centre (9423 ± 120 yr BP). These tephra dates are supplemented by selected radiocarbon age estimates. The cores were correlated using tephra and fine-scale variations in magnetic susceptibility. The natural remanent magnetisation is strong, stable and carried by fine-grained titanomagnetite from AVF’s basaltic volcanoes. The resulting palaeosecular variation (PSV) record spans from 10,000 to 1500 yr BP. It shows well-defined, millennial-scale swings in direction between 10,000 and 8000 yr BP and between 4000 and 1500 yr BP, while between 8000 and 4000 yr BP variations are of higher frequency and lower amplitude and centre on the geocentric axial dipole field direction. This is in excellent agreement with published PSV records from Mavora Lakes, 1000 km further south, indicating the regional nature of PSV – resulting from broad-scale changes in the circulation of conducting iron-rich fluid of Earth’s outer core.

Density separation in pollen preparation: How low can you go?

In palynology and other laboratory-based disciplines, methodological literature often lags the development and adoption of new practices. Here we highlight the lack of literature on the application of heavy liquid density separation for pollen preparations, a technique that has become common practice in recent years. In a study of Holocene-age sediments from Lake Pupuke, northern New Zealand, we found that the density of the heavy liquid used to separate pollen from the minerogenic fraction, within the range of reported practice, affected pollen counts. When a relatively low density was used (2.0 g/cm3), buoyant pollen grains such as Prumnopitys taxifolia and Dacrydium cupressinum were overrepresented, whereas small, compact pollen grains such as Libocedrus and Metrosideros were underrepresented. This result raises wider concerns, as heavy liquid densities reported in the literature range from 1.88 to 2.40 g/cm3. We draw attention to this problem and recommend steps that palynologists can take to ensure that their enumerated pollen assemblages are representative and do not lead to spurious interpretations.

Geophysical constraints on the structure and formation of Onepoto, Orakei, Pupuke and Tank Farm maar volcanoes, Auckland Volcanic Field

ABSTRACTWe report on a geophysical study of maar volcanoes in the Auckland Volcanic Field which was performed in 1975, whose results remain relevant. Modelled gravity and magnetic anomalies indicate that substantial funnel-shaped magnetised basaltic bodies underlie the mud-filled craters at Onepoto and Orakei Basin. These bodies may represent magma that solidified within the craters after initial explosive eruptive phases that involved direct interaction between magma and surface or near-surface water. The estimated volume of residual magnetised basalt beneath the crater is roughly equal to the ejected quantity of basalt at Onepoto and about 3 times larger at Orakei. Magnetic anomaly data show that the Lake Pupuke crater also partially filled with basaltic material after an explosive phase than in turn followed an initial effusive eruptive phase. In contrast, there is no magnetic indication of a substantial magnetised basaltic body beneath Tank Farm crater, so it appears that magma supply ceased at the end of the explosive phase.

Holocene temperature, humidity and seasonality in northern New Zealand linked to Southern Hemisphere summer insolation

The Holocene thermal maximum (HTM) is a spatio-temporally variable period of generally warmer conditions during the early and middle Holocene that is often used as an analogue for future climate change. Global scale climate reconstructions and models tend to smooth out the variations and complexity of the HTM and inconsistencies between reconstructions from different locations and proxies are often attributed to bias arising from different locations or proxies. We use these differences as a source of information about seasonality and precipitation during the Holocene in a multi-proxy investigation of the sediments of Lake Pupuke, northern New Zealand. The sediments, spanning the last 16 kyr, were analysed for pollen, from which mean annual air temperatures (MAAT) and effective precipitation were estimated, and chironomids, from which summer air temperature (SmT) was estimated. We found no evidence for an HTM in the MAAT reconstruction, questioning the validity of treating the early-to-mid Holocene as an analogue for future climate change in northern New Zealand. SmT increases between 10 and 3 cal kyr BP, correlating strongly with integrated local summer insolation. Early-Holocene low seasonality (from 12 to 9.3 cal kyr BP) was likely driven by low local summer insolation intensity. An early-to-mid-Holocene wet period (9.6–7.5 cal kyr BP) corresponds to relatively high southern westerly wind (SWW) strength. Mid-to-late-Holocene summers following the wet period were hot and dry, especially 4.0–2.4 cal kyr BP, allowing the tall conifer, kauri (Agathis australis) to expand throughout northern New Zealand. Low effective precipitation at this time is consistent with increased evapotranspiration due to higher SmT, although reduced precipitation due to southward displaced SWW or increased El Niño frequency may also have contributed.

Two-step human–environmental impact history for northern New Zealand linked to late-Holocene climate change

Following resolution of a long-standing debate over the timing of the initial settlement of New Zealand from Polynesia (late 13th century), a prevailing paradigm has developed that invokes rapid transformation of the landscape, principally by fire, within a few decades of the first arrivals. This model has been constructed from evidence mostly from southern and eastern regions of New Zealand, but a more complicated pattern may apply in the more humid western and northern regions where forests are more resilient to burning. We present a new pollen record from Lake Pupuke, Auckland, northern New Zealand, that charts the changing vegetation cover over the last 1000 years, before and after the arrival of people. Previous results from this site concurred with the rapid transformation model, although sampling resolution, chronology and sediment disturbance make that interpretation equivocal. Our new record is dated principally by tephrochronology together with radiocarbon dating and includes a cryptotephra deposit identified as Kaharoa tephra, a key marker for first settlement in northern New Zealand. Its discovery and stratigraphic position below two Rangitoto-derived tephras enables a clearer picture of environmental change to be drawn. The new pollen record shows an early phase (step 1) of minor, localised forest clearance around the time of Kaharoa tephra ( c. 1314 AD) followed by a later, more extensive deforestation phase (step 2) commencing at around the time of deposition of the Rangitoto tephras ( c. 1400–1450 AD). This pattern, which needs to be corroborated from other well-resolved records from northern New Zealand, concurs with an emerging hypothesis that the ‘Little Ice Age’ had a significant impact on pre-European Māori with the onset of harsher conditions causing a consolidation of populations and later environmental impact in northern New Zealand.

Extracting a primary Holocene crytoptephra record from Pupuke maar sediments, Auckland, New Zealand

ABSTRACTPost‐depositional reworking of volcanic glass shards is a common problem in tephrochronological studies. Distinguishing between primary and reworked tephra is challenging, particularly in cryptotephra investigations that depend on identifying peaks of glass shard concentrations in sediment sequences rather than discrete macroscopic tephra layers. Here, we investigated macro‐ and cryptotephra in sediments of Lake Pupuke, New Zealand, to identify primary from reworked tephra and thus refine the record of tephra‐fall events in the region over the last ca. 9 cal ka. Geochemical fingerprinting, stratigraphic correlation, 14C ages and X‐ray fluorescence and magnetic susceptibility core scanning were employed to filter out reworked tephra and thus establish a new Holocene tephrochronological framework for the region. Five primary macrotephra layers and 18 primary cryptotephra were identified. Confidence levels are attached to the newly identified cryptotephra, and geochemical characterization of well‐dated rhyolite tephra was used to establish an associated age model. Our results suggest that Holocene tephra fall from both local and distal volcanoes affected Auckland more frequently than previously documented, with a possible average frequency of at least once every 424 years. Furthermore, the study demonstrates the importance of identifying the extent of post‐depositional reworking to establish the veracity of the cryptotephra record.

Middle and Late Holocene paleotemperatures reconstructed from oxygen isotopes and GDGTs of sediments from Lake Pupuke, New Zealand

Quantitative Holocene temperature reconstructions for Southern Hemisphere terrestrial environments are still restricted to few records, and multi-proxy methods are rarely applied to the same system. Here, we test the applicability and comparability of temperature reconstructions inferred from different methodological approaches. We combined sedimentary cellulose and diatom oxygen isotope data from Lake Pupuke (North Island New Zealand) to derive a record of lake water temperatures. This record is complemented with mean annual air temperatures derived from the branched GDGT index MBT measured on the same sedimentary record. The datasets of both reconstructions show little temperature variation for the period 1320–7110 cal. BP. The exceptions are a warmer period culminating between 1700 and 1600 cal. BP reflected in both proxies and centennial-scale intervals with cooler water temperatures at 3380 and 2750 cal. BP recorded in the oxygen-isotope-based temperature reconstruction but not in the MBT-inferred air temperature estimates. We suggest that variations in lake circulation are the likely reason for deviations between temperature reconstructed from different proxies.

Late Holocene hydrologic changes in northern New Zealand inferred from stable isotope values of aquatic cellulose in sediments from Lake Pupuke

Isotopic records of aquatic cellulose are becoming increasingly important for palaeohydrolog- ical reconstructions, but widespread application of this climate proxy is hampered by minerogenic contami- nation that affects oxygen isotope measures in cellu- lose. Few records of isotopes in aquatic cellulose are available from palaeoclimate archives in the Southern Hemisphere. In this study, we used a new bulk cellulose extraction method and determined the oxy- gen (d 18 O) and carbon (d 13 C) isotope values in

Late Holocene climate variability from Lake Pupuke maar, Auckland, New Zealand

Spectral analyses of quasi-annual organo-diatomaceous laminae couplets in an Auckland maar lake indicate brief (sub-decadal scale) episodes with strong spectral power and long periods of weak to no spectral power between c. 1700 to c. 550 cal. yr BP. Laminae couplet thickness appears to be a function of changes in wind flow over the basin, with enhanced wind flow deepening the mixing zone and providing additional nutrients for laminae formation. Aeolian dust from Australia amplifies the wind signal. Spectral signals in the high power episodes are focused in <4 years and 6–8 years windows. These are consistent with El Niño–Southern Oscillation (ENSO) periodicity. This climate system is known to play a major role in the modern Auckland climate whereby strongly negative (positive) ENSO are associated with enhanced (diminished) SW airflow over Auckland. ENSO events interact in the modern climate and the spectral results indicate that this is the case when spectral power is strong in the laminae. These results highlight strong but intermittent ENSO activity between 600 and 1400 cal. yr BP.

A late glacial Antarctic climate teleconnection and variable Holocene seasonality at Lake Pupuke, Auckland, New Zealand

We present the first continuous paleolimnological reconstruction from the North Island of New Zealand (37°S) that spans the last 48.2 cal kyr. A tephra- and radiocarbon-based chronology was developed to infer the timing of marked paleolimnological changes in Lake Pupuke, Auckland, New Zealand, identified using sedimentology, magnetic susceptibility, grain size and geochemistry (carbon, nitrogen and sulphur concentrations and fluxes, carbon and nitrogen stable isotopes). Variable erosional influx, biomass and benthic REDOX conditions are linked to changing effective precipitation and seasonality within three inferred broad intervals of climatic change: (1) the Last Glacial Coldest Phase (LGCP) of reduced effective precipitation and cooler temperatures, from 28.8 to 18.0 cal kyr BP, (2) the Last Glacial Interglacial Transition (LGIT) of increasing effective precipitation and warmer conditions, from 18.0 to 10.2 cal kyr BP, and (3) a Holocene interval of high effective precipitation, beginning with a warm period of limited seasonality from 10.2 cal kyr BP and followed by increasing seasonality from 7.6 cal kyr BP. The LGCP and LGIT also contain millennial-scale climate events, including the coldest inferred glacial conditions during the LGCP from 27.8 to 26.0 and 22.0–19.0 cal kyr BP, and a climate reversal in the LGIT associated with lower lake level, from 14.5 to 13.8 cal kyr BP, coeval with the Antarctic Cold Reversal. The onset of seasonal thermal stratification occurred at 5.7 cal kyr BP and was linked to natural eutrophication of Lake Pupuke, which produced enhanced organic sedimentation.

A diatom-inferred record of reduced effective precipitation during the Last Glacial Coldest Phase (28.8–18.0 cal kyr BP) and increasing Holocene seasonality at Lake Pupuke, Auckland, New Zealand

A continuous, 1,420-cm sediment record from Lake Pupuke, Auckland, New Zealand (37°S) was analysed for diatom taxonomy, concentration and flux. A New Zealand freshwater diatom transfer function was applied to infer past pH, electrical conductivity, dissolved reactive phosphorus and chlorophyll a. A precise, mixed-effect regression model of age versus depth was constructed from 11 tephra and 13 radiocarbon dates, with a basal age of 48.2 cal kyr BP. Diatom-inferred changes in paleolimnology and climate corroborate earlier inferences from geochemical analyses (Stephens et al. 2012), with respect to the timing of marked climate changes in the Last Glacial Coldest Phase (LGCP; 28.8–18.0 cal kyr BP), the Last Glacial Interglacial Transition (LGIT; 18.0 to ca. 12–10 cal kyr BP) and the Holocene, the onset of which is difficult to discern from LGIT amelioration, but which includes an early climatic optimum (10.2–8.0 cal kyr BP). The LGCP is readily defined by a reduction in lake level and effective precipitation, whereas the LGIT represents a period of rising lake level, with greater biomass during the Holocene. There was limited change in diatom assemblage structure, influx or inferred water quality during a Late Glacial Reversal (LGR; 14.5–13.8 cal kyr BP), associated with heightened erosional influx. In contrast, an LGIT peak in paleoproductivity is recorded by increased diatom influx from 13.8 to 12.8 cal kyr BP. Changes in sediment influx and biomass record complex millennial-scale events attuned to the Antarctic Cold Reversal (ACR; 14.5–12.8 cal kyr BP). Additional millennial-scale environmental change is apparent in the Holocene, with marked changes in lake circulation beginning at 7.6 cal kyr BP, including the onset of seasonal thermal stratification and rapid species turnover at 5.7 cal kyr BP. The most rapid diatom community turnover accompanied widely varying nutrient availability and greater seasonality during the last 3.3 cal kyr. Rising seasonality appears to have been linked to strengthened Southern Westerlies at their northern margins during the middle and late Holocene.

Environmental changes in northern New Zealand since the Middle Holocene inferred from stable isotope records (δ15N, δ13C) of Lake Pupuke

Maar lakes in the Auckland Volcanic Field are important high-resolution archives of Holocene environmental change in the Southern Hemisphere mid-latitudes. Stable carbon and nitrogen isotope analyses were applied on bulk organic matter and the green alga Botryococcus from a sediment core from Lake Pupuke (Auckland, North Island, New Zealand) spanning the period since 7,165 cal. year BP. The origin of organic matter was established using total-organic–carbon-to-nitrogen ratios (TOC/TN) as well as organic carbon (δ13COM) and nitrogen (δ15N) isotope composition of potential modern sources. This approach demonstrated that the contribution of allochthonous organic matter to the lake sediment was negligible for most of the record. The sedimentary TOC/TN ratios that are higher than Redfield ratio (i.e. >7) are attributed to N-limiting conditions throughout the record. Variations of nitrogen and carbon isotopes during the last 7,165 years are interpreted as changes in the dominant processes in the lake. While epilimnetic primary productivity controlled isotope composition before 6,600 cal. year BP, microbial processes, especially denitrification and methane oxidation, caused overall shifts of the δ15N and δ13C values since the Mid-Holocene. Comparisons with climate reconstructions from the Northern Island suggest that changes in the wind-induced lake overturn and a shift to more pronounced seasonality were the most likely causes for lake-internal changes since 6,600 cal. year BP.

Multi-proxy identification of the Laschamp geomagnetic field excursion in Lake Pupuke, New Zealand

We present palaeomagnetic and cosmogenic radionuclide records of the Laschamp geomagnetic excursion in Lake Pupuke, a maar lake in Auckland, New Zealand. Laschamp was identified by a combination of relative palaeointensity, 10Be and 14C data from the lake sediments and represents the first such record from the Southern Hemisphere. Despite the high organic carbon content, which causes relatively weak natural remanent magnetisations, the geomagnetic intensity minimum associated with the Laschamp excursion is identifiable as a relative palaeointensity minimum that is synchronous with (i) a peak in 10Be concentration and (ii) an anomaly in Δ 14C. The Lake Pupuke time scale, provided by 14C data calibrated with INTCAL09, places the 10Be maximum at the same time as a 10Be maximum in Greenland ice cores when secured to the GICC05 time scale. The central age of the Laschamp geomagnetic excursion in Lake Pupuke as defined by the 10Be prediction peak is c. 41 kyr, which confirms its global application as a palaeomagnetic isochron. Anomalous palaeomagnetic directional data at c. 32 kyr in the Lake Pupuke sediments may represent the Mono Lake geomagnetic excursion, but tephra layers caused by frequent eruptions in the Auckland volcanic field during this excursion probably disrupted the palaeointensity signal. The study highlights the value of combining traditional palaeomagnetic methods with measurements of cosmogenic radionuclides in the quest for accurate and precise geochronologies during MIS3, a time of rapid global climate change.

Sequential eruption of alkaline and sub-alkaline magmas from a small monogenetic volcano in the Auckland Volcanic Field, New Zealand

Rangitoto Volcano is the youngest and largest eruptive centre in the monogenetic intraplate Auckland Volcanic Field (AVF). The stratigraphy of Rangitoto pyroclastic deposits that have been preserved in swamps on nearby Motutapu Island and in Lake Pupuke on the mainland reveals that the volcano erupted twice; radiocarbon dating of 10 samples from the two tephra units in the swamps indicates eruption ages of 553 ± 7 and 504 ± 5 Cal years BP, for the lower and upper tephra layers, respectively. Geochemistry of the lava field and various scoria cones on Rangitoto Island itself reveals two distinct compositional groups: an alkaline olivine basalt group (that correlates geochemically with the lower tephra layer) and a group that is sub-alkaline and transitional to tholeiite (that correlates geochemically with the upper tephra layer). Based on this data, we infer that, following a phreatomagmatic vent-clearing phase, the early magmatic eruption of Rangitoto Volcano was Strombolian in character and produced an alkaline olivine basalt scoria cone and an associated thick ash deposit on nearby Motutapu Island. This was followed by a time gap of up to several decades, after which a second eruptive phase built the current summit scoria cone together with an encircling lava field. We suggest this later, sub-alkaline eruptive period was associated with the deposition of the thin upper tephra layer on Motutapu Island. The two suites of Rangitoto samples are chemically quite distinct, and each is associated with a distinct parental composition. Trace element modelling indicates the alkaline and sub-alkaline parental melts could have been derived by ~ 1 and 6 wt.% partial melting of an anhydrous garnet peridotite source at ~ 80 and 65 km depth, respectively. The compositional range within each suite is similar, and can be explained by mainly olivine together with minor clinopyroxene fractionation within a relatively simple conduit system in which mixing and mingling were not important. Significant olivine fractionation (< 25%) suggests that the magma may have spent some time in the upper conduit during ascent. This contrasts with a recently published model for the Crater Hill centre in the AVF, in which deep-seated fractionation of clinopyroxene followed by relatively rapid ascent to the surface has been invoked to explain the compositions seen there. The polycyclic nature of Rangitoto and in particular the reuse of the conduit system after a period of quiescence have implications for the concept of monogenetic volcanism. There are also implications for hazard assessment, such that when a future eruption occurs in the AVF, it will be necessary to consider the conduit a possible pathway for another eruption for up to several decades afterwards.

Multi-proxy evidence of late Holocene human-induced environmental changes at Lake Pupuke, Auckland (New Zealand)

New Zealand was the last major landmass in the world to be colonised by people. Despite the short time-depth of human presence in the country there is ongoing debate about the date of earliest arrival of people that resulted in the emergence of two contrasting colonisation hypotheses: the Early hypothesis and the Short prehistory. To decide between both hypotheses we employed a range of multi-proxy investigations (geochemistry, stable isotopes and mineral magnetism) on a lacustrine sequence from Lake Pupuke, a maar lake in the city of Auckland and an area potentially among the first places in New Zealand to have been colonised by early people. The environmental history reconstructed from the multi-proxy evidence identified a clear lack of catchment disturbance from c. 2000 B.P. until several decades before the eruption of the Rangitoto volcano. The nature and abruptness of disturbance unambiguously point to anthropogenic forcings and are likely to mark the onset of prehistoric human colonisation at the site. Linear interpolation between independently dated tephrochronostratigraphic marker beds present in the sediments allowed to date this event to c. 610 cal yr B.P. This date is in compliance with the Short prehistory for this region.

Rapid change in early Holocene environments inferred from Lake Pupuke, Auckland City, New Zealand

AbstractSediment cores from Lake Pupuke in Auckland City, New Zealand, contain a high‐resolution millennial to centennial‐scale record of changing climate and catchment hydrology spanning the past ca. 10 000 years. Here, we focus on the period between 9500 ± 25 and 7000 ± 155 cal. yr BP during which grain size, diatom palaeoecology, biogenic silica concentrations, sediment elemental and carbon isotope geochemistry reflect changes in sediment sources and lake conditions, with a significant event commencing at ca. 8240 cal. yr BP, commensurate with a lowering of lake level, faster erosion rates and increased sediment influx with a duration of ca. 360 yrs. However, the changes in the lake are not reflected in the terrestrial vegetation, where the pollen record indicates that podocarp forest dominated the Auckland region, with apparent environmental stability during this part of the early Holocene. The synchronous change in most of the proxies between ca. 8240 and 7880 cal. yr BP at Lake Pupuke indicates the presence of a sustained episode of relatively low lake level and concomitant increased rate of erosion in the early Holocene that appears to be at least partly coeval with the 8200 cal. yr BP meltwater event proposed for the North Atlantic region. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Biological and Geochemical Record of Anthropogenic Impacts in Recent Sediments from Lake Pupuke, Auckland City, New Zealand

Frozen sediment cores from Lake Pupuke in Auckland City, New Zealand, contain a high resolution decadal to annual scale record of changing lake paleoenvironments and geochemistry that reflects changing landuse and hydrology in the catchment over the past c. 190 years. A reliable chronology is available from AMS 14C and 210Pb dating of the sediments, with the timing of the older part of the record supported by the first appearance of pollen of introduced Pinus and Plantago lanceolata associated with European settlement of Auckland in the early 1840s. Diatom stratigraphy, sediment elemental and carbon isotope geochemistry reflect changes in sediment sources and lake conditions commensurate with European development of the Lake Pupuke catchment, in particular enhanced algal productivity controlled by the influx of nutrients after c. 1920 AD. Attempts to prevent nuisance algal blooms in 1933, 1934 and 1939 using CuSO4 addition produced Cu spikes in the sediment that allowed confirmation of the accuracy of the 210Pb chronology. Hence, the elemental and isotopic composition of the Lake Pupuke sediments reflect the timing of significant anthropogenic activities, rather than climatic variations, that have occurred within the watershed over the past c. 190 years. The comparison of records of land use change in the catchment with the multi-proxy record of changes in the sediments usually allowed unambiguous identification of the signatures of change and their causes.

Holocene vegetation, environment, and tephra recorded from Lake Pupuke, Auckland, New Zealand

Lake Pupuke provides a near‐complete, high‐resolution environmental record of the Holocene from northern New Zealand. Tephra beds constrain the timing of a range of proxy indicators of environmental change, and demonstrate errors in a radiocarbon chronology. Agathis australis forest progressively increases from c. 7000 yr BP and, in conjunction with indicators of reduced biomass productivity, support a model of long‐term climate change to drier conditions over the Holocene. However, except for Agathis, conifer‐hardwood forest dominated mainly by Dacrydium cupressinum shows little change throughout the pre‐human Holocene, suggesting environmental stability. Dramatic vegetation change occurred only within the last millennium as a result of large‐scale Polynesian deforestation by fire. This happened a short time before the local eruption of c. 638 cal. yr BP Rangitoto Tephra. The identification of two eruptions of tephra from Rangitoto volcano has implications for future hazard planning in the Auckland region, because the volcanoes were previously considered single event centres. Changes in atmospheric circulation since the Late Glacial, possibly causing lower frequency of distal ashfall in Auckland during the Holocene, complicates the use of long‐term records in hazard frequency assessment.

Submerged macrophytes of Lake Pupuke, Takapuna, New Zealand

Abstract Lake Pupuke is unique in New Zealand in that all four obligate submerged members of the adventive Hydrocharitaceae, Vallis‐neria gigantea Graebn., and the “oxygen weeds” Lagarosiphon major (Ridl.) Moss ex Wagner, Elo‐dea canadensis Michx., and Egeria densa Planch, co‐exist there. Dense stands of Vallisneria, up to 5 m tall, dominated the mid‐littoral zone (2–7 m depth) through to 1985. Egeria was first recorded as present in Lake Pupuke in 1980. It occurred in the mid‐littoral zone and in 1982 it was estimated to occupy 2%, in 1983 10%, and by 1985, 25% of total macrophyte volume in Lake Pupuke. This trend suggests that Egeria is the potential dominant of this community.

The ultrastructure of Chlorella minutissima Fott et Nováková (Chlorophyceae, Chlorococcales)

Abstract Chlorella minutissima Fott et Novakova has been recorded for the first time in New Zealand from the stagnant inshore waters of Lake Pupuke, Auckland, New Zealand. Its ultrastructure has been investigated by transmission electron microscopy, from serial sections and freeze-fracture preparations. The extremely small cells, 1 · 8–3 · 0 μm in diameter, are bounded by a thin cell wall with an outer sporopollenin-containing layer. Each cell contains a single nucleus, chloroplast, pyrenoid, dictyosome and branched mitochondrion. The spatial relationships of the various organelles are discussed.