Limnology of Lake Ngahewa,North Island,New Zealand

The water chemistry, flora, and fauna of Lake Ngahewa are described. Its waters are coloured by humic material from a flax swamp and the mean carbon: nitrogen ratio of the sediments lies in the range for dystrophic lakes. Thermal stratification is very weak or absent during summer, although deoxygenation occurs at depths below 3 m in late summer. The lake is highly productive and has a diverse phytoplankton and zooplankton population, which is different in species composition from other nearby eutrophic lakes; some algal species common in nearby eutrophic lakes are absent, and there is a high ratio of rotifers to cladocerans and copepods.

The high levels of dissolved phosphorus (42.2 mg.m^‐3) and nitrogen (up to 36.1 mg.m^‐3 NO3‐N) in the lake waters, and the differences in the flora and fauna compared with neighbouring lakes give only weak support to its tenuous classification as a dystrophic lake on the basis of water colour and sediment carbon values. The concept of dystrophy is discussed.     

Sedimentary processes operating around the entrance to a river mouth port,Westport, New Zealand

The paper presents investigations of wave climate, tidal inlet hydraulics, and sand sediment bypassing at the entrance to Westport Harbour, South Island, New Zealand. The results complement and extend those of studies of bar morphologies and sediment characteristics already published. Longshore transport of about 1 × 10⁶ m³/year is directed in a net eastward fashion across the inlet because of an in‐built misalignment of the harbour training walls. Approximately 90% of the drift is bypassed, and has been since 1921, by deflection and splitting of the main sediment streams through the inner and outer bars and a transverse channel aoross the entrance. The outer bar appears to be the submarine, downdrift extension of Carters Breach and river load appears to contribute an order of magnitude less sediment to the complex than annual littoral drift. River sediments and littoral drift are mixed off the harbour and a declining proportion over time is recirculated to cause progradation of North Beach. The tidal compartment contributes little to scour of the entrance because of the predominance of bar bypassing. Contrary to the recommendations of several past studies, it is argued that improvements in navigation depths at West‐port are more likely to be obtained through modification of the littoral drift system than they are from tidal compartment enlargement.     

Turbulent mixing in a stratified estuarine tidal channel: Hikapu Reach,Pelorus Sound,New Zealand

Channel constrictions within an estuary can influence overall estuary-sea exchange of salt or suspended/dissolved material. The exchange is modulated by turbulent mixing through its effect on density stratification. Here we quantify turbulent mixing in Hikapu Reach, an estuarine channel in the Marlborough Sounds, New Zealand. The focus is on a period of relatively low freshwater input but where density stratification still persists throughout the tidal cycle, although the strength of stratification and its vertical structure vary substantially. The density stratification increases through the ebb tide, and decreases through the flood tide. During the spring tides observed here, ebb tidal flow speeds reached 0.7?m?s^?1 and the buoyancy frequency squared was in the range 10^?5 to 10^?3?s^?2. Turbulence parameters were estimated using both shear microstructure and velocimeter-derived inertial dissipation which compared favourably. The rate of dissipation of turbulent kinetic energy reached 1?×?10^?6?m²?s^?3 late in the ebb tide, and estimates of the gradient Richardson number (the ratio of stability to shear) fell as low as 0.1 (i.e. unstable) although the results show that bottom-boundary driven turbulence can dominate for periods. The implication, based on scaling, is that the mixing within the channel does not homogenise the water column within a tidal cycle. Scaling, developed to characterise the tidal advection relative to the channel length, shows how riverine-driven buoyancy fluxes can pass through the tidal channel section and the stratification can remain partially intact.     

Growth rates of five species of surf clams on a southern North Island beach,New Zealand

Abstract

Growth rates of five species of surf clam were estimated on the Kapiti Coast, southern North Island, New Zealand. Length‐frequencies of sequential population samples were analysed by the computer program, MULTIFAN. Incremental growth of marked individuals was analysed by the computer program, GROTAG. The von Bertalanffy growth parameters k and L ₈ estimated by MULTIFAN for Spisula aequilatera (Deshayes in Reeve, 1854) were 0.80 yr^?1 and 52.1 mm; for Mactra murchisoni Deshayes in Reeve, 1854 were 0.60 yr^?1 and 72.3 mm; and for M. discors Gray, 1837 were 0.35 yr^?1 and 60.1 mm. MULTIFAN could not model growth of Paphies donacina (Spengler, 1793) and Dosinia anus (Philippi, 1848) from the population samples. The growth parameters estimated by GROTAG for D. anus were 0.53 yr^?1 and 53.0 mm and for M. murchisoni 1.84 yr^?1 and 72.4 mm. The growth rates of P. donacina, S. aequilatera, and M. discors were estimated for the size range of their incremental growth data and the estimates are usable for this limited size range only. Growth rates of each species varied with depth in the surf zone in the same way in the North and the South Island. The growth rate of two species was faster in the South Island and asymptotic size of all species was greater in the South Island.     

Contribution of photosynthetic gains during tidal emersion to production of Zostera capricorni in a North Island,New Zealand estuary

Photosynthetic characteristics of intertidal Zostera capricorni were measured under different tidal conditions in Whangapoua Harbour on the eastern Coromandel Peninsula, New Zealand, and compared with permanently submerged seagrass beds. Photosynthetic characteristics were measured using pulse amplitude modulated (PAM) fluorom‐etry and oxygen (O2) electrode techniques. Gross light saturated photosynthesis measured as oxygen exchange averaged 5.74 and 5.36 mg O₂ g^–1 dry weight (DW) h^–1 and leaf respiration rates averaged 1.22 and 1.38 mg O₂ g^–1 DW h^–1, for intertidal and subtidal plants respectively. Photosynthesis of both intertidal and shallow subtidal plants was light saturated at between 195 and 242 μmol photons m ^–2 s^–1, suggestive of acclimation to a high light environment. Despite the period of exposure at low tide clearly being an important time for photosynthetic gains for intertidal plants, when water clarity was sufficiently high, maximum rates of photosynthesis were also possible when the beds were submerged. If average water clarity was at the clearer end of a range measured for this site (K_d = 0.85 m^–1) then it was calculated that for intertidal seagrass beds growing at mean sea level in Whangapoua, c. 50% of above‐ground production could occur while plants were submerged.     

Spatial and temporal variation in the abundance and composition of ichthyoplankton in a large South Island,New Zealand river estuary

Larval and juvenile fish drifting on ebb tides were collected from early summer to mid autumn at various locations in the Taieri River and Waipori River estuary, South Island, New Zealand. A total of seven species of fish were recorded, however only three species were regularly collected. These were the larval common bully (Gobiomorphus cotidianus McDowall), larval inanga (Galaxias maculatus Jenyns), and postlarval cockabully (Tripterygion nigripenne Valenciennes). Common bully densities were highest in mid January, with the highest densities being recorded drifting out of Lake Waihola. Inanga densities were highest in March and April, with the highest densities being recorded in the channels draining wetland areas where extensive tidal inundation of riparian vegetation occurred. Cockabully were recorded from February to April, mostly at more downstream sites. Inland penetration by marine species was considerable, with two marine species, clingfish (Trachelochismus pinnulatus Forster) and smooth leatherjacket (Parika scaber Forster), being recorded c. 12 km upstream in late summer.     

Age and growth of bluenose,Hyperoglyphe antarctica (Pisces: Stromateoidei) from the lower east coast,North Island,New Zealand

Growth of the bluenose, Hyperoglyphe antarctica (Carmichael, 1818) from the lower east coast, North Island, was determined by counting growth check rings in otoliths. One growth check ring appeared to be laid down each year, so this technique is probably valid for aging bluenose. Female fish had a significantly higher growth rate than males. Comparisons of samples from three sites indicated no significant regional differences in growth. The von Bertalanffy parameters L8,K, and t0 fitted to back‐calculated fish lengths were, respectively, 81.1 cm, 0.308, ‐0.627 for males, and 86.1 cm, 0.308, ‐0.384 for females.     

An integrated approach to assess the ecological and chemical status of lakes with HOD/AHOD: a case study of two lakes

The thermal strati?cation of the lakes impedes the transfer of atmospheric oxygen into the lower layers of the lake. In lakes which are af fected by diverse anthropogenic in?uences, the increasing organic matter amounts lead to a sharp decrease in hypolimnetic oxygen amounts, aided by thermal strati?cation,and anaerobic conditions arise. The determination of hypolimnetic oxygen demand(HOD) and areal hypolimnetic oxygen demand(AHOD) and their monitoring represent an integrated approach to investigate the oxygenation of lakes, the nutrient conditions, and the physicochemical dynamics. In this study, two lakes dif fering by size and af fected by dif ferent anthropogenic sources, are investigated in this respect. At?rst, bathymetric studies were conducted to determine the depth, surface area, and volume relationships.Then, based on monitoring studies conducted in 2013 and 2014, the thermal strati?cation dynamics and layer properties were established using the relative thermal resistance to mixing(RTRM) index based on temperature and density pro?les. Following this, the oxygen depletion rates were determined by oxygen and temperature pro?ling in the hypolimnion. For the years of 2013 and 2014, the AHOD values for the Borabey Pond which is far from anthropogenic in?uences, were found to be 0.848 and 0.569 g O 2/(m·d), respectively.The AHOD values for the Porsuk Reservoir which was overburdened for years by industrial and domestic pollution were found to be 4.263 and 5.099 g O 2/(m·d), larger than its counterpart by almost sevenfold. The HOD and AHOD monitoring can be considered to a valuable tool for assessing the ecological and chemical status of lakes within Annex 5 of the Water Framework Directive and as an integrated approach to assess and monitor the status of lakes.     

Phyllosoma larvae of Jasus edwardsii (Hutton) (Crustacea: Decapoda: Palinuridae) and their distribution off the east coast of the North Island,New Zealand

The complete series of phyllosoma larval stages of spiny lobster lasus edwardsii are described. Eleven stages are recognised from specimens captured in plankton samples collected along a transect extending 185 km east of Castlepoint, North Island, New Zealand between July 1969 and December 1971. A table of distinguishing characteristics and a key for identification of stages are presented. First stage larvae occur between August and October and last stage larvae about 15 months later. Mortality through the phyllosoma stages is estimated to be 98%. Early stage larvae occur mainly inshore in Southland Current water in the upper 5 m, and later stage larvae occur more frequently offshore in East Cape Current water at 40–60 m depth.     

Biomarkers of Middle to Late Jurassic marine sediments from a canonical section: New records from the Yanshiping area,northern Tibet

The Yanshiping section, which includes the Quemo Co, Buqu, Xiali, Suowa and Xueshan Formations (Yanshiping Group) exposes organic-rich Middle to Late Jurassic deposits in the Qiangtang Basin of northern Tibet. The biostratigraphic data, from bivalves, brachiopods as well as dinoflagellate cysts, define a Bajocian to Tithonian age. This study focuses on the biomarkers present in these mudstones and limestones to determine the sources, thermal maturity and depositional environment of the organic matter. Most samples show a clear dominance of short-chain (C₁₅–C₂₀) n-alkanes with a maximum at C₁₉ or C₁₉ with a secondary maximum at C₂₃ except for the sample BP01(22)S1 where the predominant range is C₂₂ to C₂₆ with a maximum at C₂₄, significant CPI and odd-to-even predominance. The hopanoids and steroids suggest that the sources of organic matter were dominated by phytoplankton, especially algae, as the primary source. Furthermore, the Pr/Ph, Pr/nC₁₇ and Ph/nC₁₈, with relatively low values plus high abundance of 17α(H)-hopanes, support deposition in dysoxic to reducing, relatively shallow-water depositional settings, and the presence of gammacerane indicates normal marine salinity and/or water-column stratification. All samples are fairly mature with respect to petroleum generation, a conclusion supported by maturity parameters such as C₃₁ 22S/(22S + 22R) hopanes and C₂₉ ααα20S/(20S + 20R) steranes.