Otago Shelf Research Articles

A new genus and five new species of Phoxocephalidae (Crustacea: Amphipoda) from the Eastern Tasman Sea, the Otago Shelf, and The Snares, New Zealand

A new genus and five new species of Phoxocephalidae (Crustacea: Amphipoda) from the Eastern Tasman Sea, the Otago Shelf, and The Snares, New Zealand

Episodic Summer Chlorophyll‐a Blooms Driven by Along‐Front Winds at Aotearoa's Southeast Shelf Break Front

AbstractThis study investigates the influence of along‐front wind forcing on chlorophyll‐a (Chl‐a) at the Otago Shelf Break (OSB) in southeast Aotearoa/New Zealand using remotely‐sensed and in situ data. Summer wind stress over the OSB was shelf‐aligned, oscillating between upfront and downfront. Surface Chl‐a concentrations along the OSB were shown to increase episodically (≤10 days duration) following upfront wind stress. This response occurred over most of the 350 km long shelf break, and was most intense north of the Otago Peninsula. Peak Chl‐a enhancement at the shelf break occurred following periods of upfront winds, exhibiting a lagged response of approximately 5 days. Moored thermistor data indicated that upfront wind events were followed by increased thermal stratification over the mid‐shelf, whereas downfront wind events were followed by a well‐mixed water column. In situ temperature, salinity, and nutrient measurements suggested an offshore movement of the surface expression of the front following upfront winds, and a reduction in nitrate over the shelf break. From these observations a model of Ekman restratification driven by upfront winds is proposed for this system, wherein off‐shelf Ekman transport converts strong horizontal isopycnal gradients at the shelf break front into vertical stratification. This stratification holds phytoplankton in the upper water column, allowing increased access to light, which is marked by the increased drawdown of nitrate at the shelf break. Downfront winds break down this stratification, move the shelf break front onshore, and mix phytoplankton through the water column, reducing the surface expression of chlorophyll.

Voluntary fishing restrictions alone do not promote growth of bryozoan-dominated biogenic habitat on the Otago shelf, southeastern New Zealand

AbstractOn the continental shelf, New Zealand bryozoans dominate ecologically-important three-dimensional benthic habitat providing structural complexity which hosts a wide variety of fauna, including economically valuable species (e.g. oysters and blue cod). The association between these species and bryozoan-dominated biogenic habitat commonly results in trawling damage to the benthos; eliminating pressure from destructive fishing practices could support bryozoan regrowth. In 2002, a voluntary fishing restriction was designated over part of one such bryozoan-dominated biogenic habitat, but the efficacy of this ban has not been assessed. Statutory marine protected areas have been proposed in the same area; they aim to achieve bryozoan protection and recovery. A comparison of the bryozoan fauna using benthic images from 2003 and 2019 from the Otago shelf, South Island, New Zealand/Te Waipounamu, Aotearoa shows that a 17-year voluntary fishing ban has been ineffective at increasing the cover and richness of bryozoan-dominated biogenic habitat on the Otago shelf. This study provides a baseline for future monitoring and develops useful metrics for recovery, while calling for further research on the biology and ecology of habitat-forming bryozoans.

Larval quality and fecundity trade-offs are linked to the maternal environment in sea perch (Helicolenus percoides, Sebastidae)

Spatial differences in demographics and nutrition can result in variations in reproductive output and larval quality among marine subpopulations. We investigated how local differences in maternal provisioning influenced the quality and quantity of larvae in sea perch (Helicolenus percoides, Sebastidae), a long-lived, viviparous species, widely distributed around New Zealand, in order to determine how local differences in maternal provisioning influenced the quality and quantity of larvae. Subpopulations from Deep Cove within Fiordland, on the west coast, and the Otago shelf on the east coast of New Zealand's South Island were compared. We used fatty acid analysis of adult liver and gonad tissues to investigate potential spatial differences in maternal nutrition. Fatty acid analysis of larvae on the day of parturition, larval notochord length and oil globule volume on the day of parturition, change in length of cohorts of larvae and change in average size of the oil globule were used as proxies to establish differences in larval quality between the Deep Cove and Otago shelf subpopulations. Our results demonstrated a trade-off between fecundity and oil globule size, where sea perch from the Otago shelf were more fecund but provisioned their larvae with smaller oil globules than females from Deep Cove. We observed faster change in length and slower rates of change in size of the oil globule among cohorts of larvae produced by females from Deep Cove. The maternally provisioned concentrations of the key fatty acids docosahexaenoic acid (DHA) and arachidonic acid (ARA) were higher, and the eicosapentaenoic acid (EPA)/ARA ratio was lower among larvae produced by females from Deep Cove. The results indicated that potential visual and osmoregulatory advantages linked to these fatty acids are passed on to the offspring, necessary to survive in the low-light, low productivity environment of the inner fjords, characterised by high freshwater inputs. The results of the present study highlight the important link between the maternal environment and the quality and fitness of fish larvae.

Late Quaternary (marine isotope stages 6-1) seismic sequence stratigraphic evolution of the Otago continental shelf, New Zealand

A proposed chronostratigraphic model for the Late Quaternary evolution of the Otago continental shelf, New Zealand, includes fluvially incised sequence boundaries eroded during marine isotope stages 2, 4 and 6, bounding three sequences subdivided into lowstand, transgressive and highstand/regressive systems tracts. Datable material is limited to the uppermost 1 m of the sequence, and consequently the model is based on correlation of new high-resolution seismic reflection data (Geopulse™) to a compilation of independent global sea-level curves. A second chronostratigraphic model that lacks a stage 4 sequence boundary and distinct stage 3 highstand/forced regressive deposit is discounted because its subsidence-corrected paleoshoreline depths do not match lowstands in the eustatic curves or temporally constrained sequences at other far-field locations. In the preferred chronostratigraphic model, sea-level lowstands during stages 2, 4 and 6 reached maximum depths of 126–131 m, 101–111 m and 116–135 m, respectively, producing sequence boundaries, landward-pinching deltaic wedges on the outer Otago shelf, and submarine canyon incision. Lowstand paleoshoreline depths have been corrected for subsidence from thermal cooling, sediment compaction and loading, but may be erroneously deep owing to erosion of paleoshoreline indicators. Clastic, shallow marine wedges and back-barrier valley-fill deposits accumulated during the stage 6–5, 4–3 and 2–1 marine transgressions. Contrary to the traditional sequence stratigraphic model, deltaic and strandline units deposited during highstand through falling sea level (stages 5–4, 3–2, 1) volumetrically dominate Otago Late Quaternary sequences. Discrete units and minor marine erosion surfaces within the transgressive and regressive systems tract deposits are interpreted as strong evidence for seventh- (∼ 20 ka) or higher-order sea-level fluctuations influencing sedimentation on the Otago shelf.

Carbonate mineralogy of free-living bryozoans (Bryozoa: Otionellidae), Otago shelf, southern New Zealand

On the open temperate shelf off Otago Peninsula, eastern South Island, New Zealand, in water depths ranging from 90 to 140 m, free-living motile bryozoans of the genus Otionellina survive in shifting quartzofeldspathic sands. Unlike most bryozoans, these free-living colonies characteristically precipitate skeletons of aragonite. Here, we report on skeletal carbonate mineralogy of 104 specimens of Otionellina from four different species, across the shelf, which are also in different stages of development. All were dominantly composed of aragonite, with only five specimens containing 0–15 wt.% calcite (but with an overall mean of <1 wt.%), with low to intermediate MgCO 3 content in the calcite (<5 wt.% MgCO 3). Mineralogy in Otionellina varies, but is not related to species, colony size, or environmental factors such as water temperature, salinity, water depth, and substratum. Otionellinids may lack the environmentally influenced biomineralisation pathways found in other carbonate-producing taxa. It appears that most free-living (or “vagrant”) bryozoans are aragonitic, although some rare examples contain up to 5% calcite; there may be no free-living genera which are always entirely aragonite. While preservation of aragonitic skeletons is by no means certain, the presence of free-living bryozoans indicates a sandy outer-shelf environment and a diagenetic history that allows for preservation of aragonite.

Stable isotope values in modern bryozoan carbonate from New Zealand and implications for paleoenvironmental interpretation

Bryozoan carbonate contains useful geochemical evidence of temperate shelf paleoenvironments. Stable isotope values were determined for 103 modern marine bryozoan skeletons representing 30 species from New Zealand. δ18O values range from‐1.4 to 2.8‰VPDB, while δ13C range from ‐4.5 to 2.8‰ VPDB (values uncorrected for mineralogical variation). These values are distinct from those of both tropical marine skeletons and New Zealand Tertiary fossils. Most bryozoans secrete carbonate in or near isotopic equilibrium with sea water, except for Celleporina and Steginoporella. The complex and variable mineralogies of the bryozoans reported here make correction for mineralogical effects problematic. Nevertheless, mainly aragonitic forms display higher isotope values, as anticipated. Both temperature and salinity constrain δ18O and δ13C values, and vary with latitude and water depth. Ten samples from a single branch of Cinctipora elegans from the Otago shelf cover a narrow range, although the striking difference in carbon isotope values between the endozone and exozone probably reflects different mineralisation histories. Our stable isotope results from three different laboratories on a single population from a single location are encouragingly consistent. Monomineralic bryozoans, when carefully chosen to avoid species suspected of vital fractionation, have considerable potential as geochemical paleoenvironmental indicators, particularly in temperate marine environments where bryozoans are dominant sediment producers.

Mexicope sushara sp. nov., the first New Zealand record of the isopod crustacean family Acanthaspidiidae (Asellota)

Mexicope sushara sp. nov. is recorded from southeastern New Zealand coastal waters, the first record of the genus from the Pacific and the first record of the Acanthaspidiidae from New Zealand. The species is from the continental shelf, taken in association with a bryozoan colony at a depth of 80 metres on the Otago Shelf, southeastern South Island. The distinguishing characters are a rostral spine and prominent and acute pre-ocular lobes; these characters in conjunction with stalked eyes separate the species from all others in the family.

Optical Properties of Otago Shelf Waters: South Island New Zealand

The optical properties and concentrations of optically active water components were measured at 31 stations on the Otago coast. For the broad-band (PAR), diffuse light attenuation decreased 15-fold ( K d=0·76 to 0·05 m −1) and reflectance 7-fold ( R=11·95 to 1·45%) from the most turbid harbour water to the clearest oceanic sites 30 km offshore. The K d values therefore suggested, that Otago shelf waters were appreciably clearer than at first thought. The absorption and scattering coefficients ( a and b, respectively) for PAR were calculated using published nomograms. Multiple linear regression showed the dependence of a and b on changes in the concentration of gilvin, g 440, chlorophyll a, [ C], and inorganic particles, [ I]: a (PAR)=0·019+0·221 g 440 +0·037[ C]+0·018[ I] b (PAR)=0·239[ C]+0·342[ I] Substituting the mean concentrations of gilvin (0·078 m −1), chlorophyll a (1·58 mg m −3) and inorganic particles (3·23 g m −3) into the above equations suggested that gilvin absorbed 11%, phytoplankton 39%, inorganic particles 38% and water itself 12% of PAR and on average. Phytoplankton cells caused 25% and inorganic sediments 75% of the scattering. Spectral irradiance measurements showed, that absorption by gilvin and suspended sediments could dominate absorption in the blue-green waveband for the most turbid harbour waters. Gilvin and phytoplankton levels were lowest in winter and inorganic sediment concentration were generally low offshore (<1 g m −3). Thus, oceanic waters were clearer and brighter in winter when compared to spring (July: K d=0·05 m −1; R=3·1%; November: K d=0·11 m −1; R=2·1% on average). However, spectral irradiance measurements showed anomalous features in R (λ) and K d(λ) spectra above ∼590 nm which suggested light emission resulting from Raman scattering and chlorophyll fluorescence.

Stable isotope composition of modern bryozoan skeletal carbonate from the Otago Shelf, New Zealand

The oxygen (δ18O) and carbon (δ13C) isotope ratios of 10 species of living Bryozoa collected from the Otago Shelf, New Zealand were analysed to assess the extent to which isotopic equilibrium (relative to inorganic equilibrium isotope fractionation) is attained during the precipitation of skeletal calcium carbonate. The data reveal that whereas eight species of Bryozoa synthesise skeletal carbonate in apparent oxygen isotope equilibrium with respect to environmental conditions, two species (Celleporina grandis and Hippomonavella flexuosa) yield δ18Ocalcite values which indicate significant disequilibrium oxygen isotope fractionation during calcification. Sufficient data are available from one species (C. grandis) to demonstrate that disequilibrium is probably related to kinetic factors associated with diffusion‐controlled transport of HCO3‐ to the site of calcite precipitation. Carbon isotope signatures indicate significant departures from inorganic isotope equilibrium in all but one bryozoan species (Hippomenella vellicata). Although greater uncertainties are associated with estimates of the isotopic composition of total dissolved inorganic carbon (δ13CSDIC), the data suggest that two factors—kinetic fractionation and incorporation of respiratory CO2—are important in controlling carbon isotope disequilibrium. Where bryozoan species exhibit evidence for disequilibrium in both oxygen and carbon isotope systems (C. grandis, H. flexuosa), it is likely that kinetic factors are primarily responsible for observed departures from carbon isotope equilibrium. In contrast, the probable explanation for those species which display evidence for carbon isotope disequilibrium only, is that skeletal carbonate is precipitated from a DIC pool modified by the incorporation of respiratory CO2. Differences between the carbon isotope composition of skeletal elements from the same species and co‐existing species living in the same community suggests that significant variations may occur in the extent to which marine DIC and respiratory CO2 are utilised during calcification. Additional studies of carbon pathways associated with calcification are required to assess the relative effects of kinetic, metabolic, and environmental factors on the carbon isotopic composition of bryozoan skeletal carbonate.

A New Species of Ascothoracid Parasite (Maxillopoda) From the Otago Shelf, New Zealand, and a New Host Record

A new species of the Ascothoracida (Maxillopoda), Dendrogaster otagoensis, was found infesting the sea star Asterodon miliaris off the Otago coast, New Zealand. The infestation of a second sea star, Allostichaster insignis by Dendrogaster argentinensis, provides a new host record for this species.

Reactive silicate in shelf waters near Otago Peninsula, New Zealand

Reactive Si data from 19 monthly occupations of two transects near Otago Peninsula in 1986-87 showed low surface values (≤1 μM) during spring and summer, and high values (&gt;4 μM) in winter. Concentrations were greatest after flooding in the Clutha River, 100 km south of the study area. In spring and early summer, concentrations were lowest inshore at the surface. At other times of the year, concentrations were highest in inshore surface waters, and usually coincided with the salinity minimum. Vertical gradients across the Southland Front, a near-horizontal thermal front outcropping near the shelf break, were calculated and compared with phosphorus gradients from the same samples. Nine of 21 Si gradients were negative (i.e. concentrations decreased with depth), whereas only 1 of 24 phosphorus gradients was negative. This implies that riverine Si flux is important in establishing Si gradients across the Southland Front. The results are a further indication of the importance of the Clutha River to the Otago shelf system.

Seasonal changes in biochemical composition of body walls, gonads and pyloric caeca in two populations ofSclerasterias mollis (Echinodermata: Asteroidea) during the annual reproductive cycle

Seasonal changes in biochemical composition of tissues are compared in a shallow inshore and deeper offshore population of the seastarSclerasterias mollis (Hutton, 1872) on the Otago shelf, New Zealand, from 1985 to 1986. The biochemical composition of the body wall, gonads and pyloric caeca of the two populations did not differ greatly, even though reproductive output and nutrient storage of reserves were consistently higher in the offshore population. The biochemical composition of the ovaries and testes differed. High levels of carbohydrate and lipid were maintained in the ovaries from the commencement of vitellogenesis until spawning occurred. The testes had higher ash levels during spermatogenesis. During the annual reproductive cycle, an inverse relationship in the carbohydrate levels between the ovary and the pyloric caeca suggests nutrient transfer from the pyloric caeca to the gonads during gametogenesis. Annual changes in biochemical composition indicate that the body wall is also used for nutrient storage.

New and little-known genera of cheilostome Bryozoa from the New Zealand region

Six new or little-known genera of cheilostome Bryozoa are recorded from the New Zealand region. Villicharixa n. gen. (Electridae), established for a species from South Island and from southern Chile, closely resembles forms from the lower Cretaceous of Europe. Daisyella n. gen. (Calloporidae) includes three uniserial species (two New Zealand and one Antarctic). Rhabdozoum Hincks (Rhabdozoidae), a little-known southern Australian genus, is newly recorded from the Otago Shelf, South Island. Plagiopora MacGillivray (Catenicellidae), a hitherto monotypic? Miocene Australian genus of the little-known subfamily Ditaxiporinae, is represented in Recent seas by a new species from the Norfolk Ridge. Yrbozoon n. gen. (Celleporidae) and Harpago n. gen. (Lekythoporidae), both monotypic, are based on new species from submarine plateaux southeast of New Zealand.

Aggregation of avian predators and zooplankton prey in Otago shelf waters, New Zealand

In Otago shelf waters surface swarms of krill (Nyctiphanes australis), hyperiid amphipods (Parathemisto spp.) and galatheid crab larvae (Munida gregaria) provide an abundant summer food source for planktivores. We tested the hypothesis that aggregation of avian planktivores depends upon the spatial distribution of their prey. Gulls (Larus scopulinus, L.bulleri, L.dominicanus), sooty shearwaters (Puffinus griseus) and white-fronted terns (Sterna striaXa) showed significantly aggregated distributions. The distribution of birds sitting on the sea surface was correlated with the abundance pattern of krill but was not correlated with the distribution of smaller hyperiid amphipods. The distribution of flying red-billed gulls (L.scopulinus), black-billed gulls (L.bulleri) and sooty shearwaters was correlated with the krill distribution but black-backed gulls (L.domini- canus) were not. Stomach contents of black-billed gulls were dominated by krill, in contrast to sooty shearwaters, which ate a higher proportion of Minida, and black-backed gulls, which contained Munida and fish. No amphipods were found in bird stomachs. There was no significant correlation between bird distributions and the hydrographic regime, water depth or distance offshore. The distribution of prey rather than hydrographic regime was a more important determinant of bird distributions at this spatial scale (2.6-12 km) and location.

A budget for modern‐Holocene sediment on the South Otago continental shelf

Abstract A sediment budget for the South Otago continental shelf and coast, between Nugget Point and Otago Peninsula, reveals modern (post 6500 y) sediment input is dominated by the Clutha River (total 3.14 Mt y‐1; Mt = 106 tonnes). Contributions from the Taieri River (0.6 Mt y‐1), the adjacent Southland shelf (0.4 Mt y‐1), and the biogenic production of calcareous shell debris (0.25 Mt y‐1) account for only 28% of the input. About half of the bedload (sand and gravel) reaching the Otago shelf is stored within a large nearshore sand wedge in the protected waters of Molyneux Bay, off the Clutha River. Bedload that escapes storage (1.1 Mt y‐1) is transported north‐eastwards to be deposited on beach and inner shelf environments just north of Otago Peninsula. Suspended load (mud) accounts for over half of the sediment input (2.33 Mt y‐1) and is nearly all transported from the study area to accumulate in north‐easterly shelf and slope depocentres.