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Submarine canyons can facilitate the transfer of land‐derived organic matter to the deep sea. Here, we investigated links between variability in organic matter availability from land and marine sources and infauna communities in two contrasting canyon systems off New Zealand and used stable isotope analyses to identify potential food sources of benthic invertebrates. Kaikōura Canyon, a steep, short canyon close to the shore, was characterized by high food availability and varying proportions of marine‐ and land‐derived organic matter, whereas Hokitika Canyon, a narrow and lower‐gradient canyon that extends further from the coast, was characterized by low food availability and mostly land‐derived sediment organic matter throughout. Both macrofaunal and meiofaunal abundance and biomass were greater in Kaikōura Canyon than in Hokitika Canyon. Correlation analyses suggested that land‐derived organic matter may contribute to increased meiofaunal abundance in Kaikōura Canyon. Carbon and nitrogen isotopic analyses, however, did not provide unequivocal support for the uptake of land‐derived material by large benthic invertebrates in Kaikōura Canyon. Infaunal abundance and biomass were low throughout Hokitika Canyon despite similar concentrations of land‐derived organic matter in sediments of both canyons, which suggests that variations in marine‐derived organic matter inputs is the main driver of community differences among canyons. Refractory vascular plant material by itself may not provide an adequate food supply to infaunal organisms, but may represent a complementary food resource when more labile marine phytodetritus is also readily available.

Field measurements were used to validate predictions for the initial dilution of negatively buoyant, cold-water inflows to Lake Taupo, as part of a study to quantify mixing processes associated with the two largest inflows to the lake. The predictions were made using a formulation originally derived for positively buoyant, warm-water inflows to cooling ponds. The formulation predicts the total dilution of an inflow during its inertia-dominated phase between its entrance to the lake and the point where buoyancy forces are great enough to cause the inflow to plunge and form a submerged density current. In one of the measured inflows, the inflowing jet was free to entrain from both sides; in the other, entrainment was restricted on one side by attachment of the inflowing jet to the shoreline of a bay just upstream of the plunge point. In the former example, the unmodified coefficients from the cooling pond formulation provided an excellent prediction of initial dilution. In the latter example, entrainment was reduced and different coefficients were derived. In both examples the inflows remained attached to the lake bed throughout their course until their liftoff at depths of 45-55 m to form interflows. The difference between coefficients for the two inflows indicates that the coefficient values should be considered site-specific. The formulation is not valid for inflows that separate from the bottom of the inflow channel before plunging. The entrance mixing formulation was incorporated in a more general model of lake stratification, DYRESM, which already includes a well-documented routine for routing underflows down submerged channels on the bed of a lake. Application of the model to the inflows measured in Lake Taupo gave good results for two model outputs that were not involved in the calibration of the entrance mixing formulation, but that are affected by the result of the initial dilution calculation—the temperatures in the river plume after it has plunged, and the insertion depth.

Phytoplankton are recognised as the primary source of food for cultured mussels. Small phytoplankton (<5 μm), however, are often not efficiently retained as food and therefore phytoplankton size can affect bivalve growth and condition. In Beatrix Bay, New Zealand, small picophytoplankton (phytoplankton <2 μm in diam.) sized cells contributed on average 29% of the phytoplankton biomass. They dominated the biomass (52%) in June (winter), when light was limiting, and (40%) during September (spring), when light levels were increasing but nutrients were becoming depleted. This substantial contribution of picophytoplankton to total phytoplankton biomass may affect the growth and condition of mussels. Our results highlight the need for a better understanding of prey selection by mussels and raise questions about the relevance of total chlorophyll a concentration as an accurate measure of phytoplankton food supply for mussels.

The influence of density stratification on sedimentation, suspension, and resuspension in Tasman Bay and Beatrix Bay, New Zealand, two contrasting coastal environments, was studied with specific reference to the implications for modelling aquaculture sustainability. Tasman Bay, an enhanced scallop (Pecten novaezelandiae Reeve) fishery, is a very large coastal indentation gently shelving to c. 20 m deep c. 10 km from shore and open to the Tasman Sea, whereas Beatrix Bay, an area of intense Greenshell™ mussel (Perna canaliculus Gmelin) aquaculture, is a small (22 km2 × 30-40 m deep) enclosed embayment off the side of the nearby Pelorus Sound. Sediment trap arrays were used to determine the vertical fluxes of suspended solids and the associated chlorophyll component. Benthic chambers were used to investigate sediment nutrient regeneration. In summer, salinity gradients in both bays are minimal or non-existent because of low inputs of fresh water and density stratification is mainly controlled by water temperature. The data from mid summer exhibited different spatial distribution patterns for detritus and phytoplankton biomass (as indicated by chlorophyll) in these two very different bays, although they had similar turbulent environments. The density discontinuity at the thermocline had a strong influence on settling of phytoplankton. There was evidence of upwards entrainment of suspended paniculate matter into the upper water column from the thermocline in Beatrix Bay. Benthic resuspension was estimated to contribute up to 90% of the suspended solids caught in sediment traps near the sea floor in both bays. The trapping rate of phytoplankton was thought to be dependent on species dominance. Possible mechanisms of resuspension included turbulence in the benthic boundary layer, and high velocities below the thermocline associated with internal seiches. The presence of a mid water column chlorophyll maximum in Beatrix Bay is discussed in terms of nutrient and light regimes at the thermocline, and species composition. In Tasman Bay, the chlorophyll maximum was thought to be caused by resuspension of benthic microphytes and their subsequent confinement in a thin layer (2-4 m thick) of high turbulence between the thermocline and the seabed.

To investigate the extent of spatial and temporal variation in groundwater quality along the eastern shores of Lake Taupo, New Zealand, data were collected from May 1975 to March 1976 from 14 boreholes sited to represent a variety of land use and human population density. Physical parameters investigated were water level, temperature, and electrical conductivity; the chemical constituents monitored were soluble reactive phosphorus, total dissolved phosphorus, nitrate and nitrite nitrogen (combined), ammonium nitrogen, total soluble iron, major cations and anions, and pH. The results show that groundwater quality can vary greatly over time and space. For example, spatial variation in mean soluble reactive phosphorus levels was 1-340 mg.m-3, in mean nitrate and nitrite nitrogen (combined) 3-13 930 mg.m-3, and in ammonium nitrogen from 5-633 mg.m-3. The most extreme variation with time concerned nitrate nitrogen at a site in Taupo township, where the concentration dropped from 16 430 mg.m-3 in early June 1975 to 1170 mg.m-3 in March 1976. Sometimes temporal variation was relatively small: at Motutere nitrate nitrogen stayed within the range 131-202 mg.m-3 during the study period. Generally, a few infrequently monitored boreholes should not be relied on to describe variations in groundwater quality.

Lake Rotoiti (North Island, New Zealand) is a deep mesotrophic lake that has declined in water quality over the last 30 years. The main river entering the lake was identified as the primary enrichment source, but its interaction with the surrounding lakewater varied with season and time of day. During winter the river was colder than the lake and penetrated 6–8 km into the main basin as an underflow. In summer the river often entered as a plunging inflow during the early morning, but it warmed during the day, and in the afternoon entered the lake as a buoyant jet that flowed directly to a nearby outlet river. From continuous temperature measurements in the inflow and lake surface it was estimated that the river plunged and penetrated the lake as an interflow or underflow for 60.2% of the year. This translated into 31% of the N and 64% of the P loading on the lake. The river also injected phytoplankton into the main basin of Lake Rotoiti, including populations of bloom-forming cyanobacteria. The underflow was the dominant term in calculating the hydraulic flushing time of the main basin, and also made a large contribution of dissolved oxygen to the subsurface waters. These complex interactions between Lake Rotoiti and its inflowing river were controlled by small temperature differences ( < 3 °C, and had wide-ranging implications in the eutrophication process.

The removal of ammonium—nitrogen (NH4‐N) from groundwater flowing through a grey willow (Salix cinerea) and native raupo (Typha orientalis) lake edge wetland was investigated over 10 mo. Monthly groundwater sampling showed that more than 95% of incoming NH2‐N was removed by the wetland. Laboratory assays found that the potential for NH4‐N removal by nitrification and subsequent denitrification was significantly higher in the organic surface sediments than in deeper, sandy, root zone sediments. In a laboratory 15N‐tracer experiment, Salix cinereaand Typha orientalisreadily assimilated NH2‐N from the root zone and were found to enhance the natural diffusion of NH4‐N up through the organic sediment layer and into the overlying water. Estimated rates of removal of NH2‐N by coupled nitrification‐denitrification and plant assimilation were 8 to 9 mg of N m−2d−1and 9 to 46 mg of N m−2d−1, respectively. Assimilated N, returned to the overlying water‐sediment surface via litter fall may be subject to sequential mineralization, nitrification, and denitrification, resulting in gaseous N loss. Sixty‐five percent of the 15N in Salixleaf litter was transformed in such a way. The results of this study indicate that plants play a central role in the NH4‐N processing capacity of wetlands. The relevance of our findings to constructed wetland design is discussed.

Two sampling techniques have been developed which enable ground water samples to be obtained from various depths within shallow aquifers and littoral sediments. Both techniques involve the insertion of a percussion‐driven spike to the desired depth. One method employs a sampling probe consisting of a perforated tip and a mechanism for the prevention of blockage by grit during the driving process, with samples being withdrawn through an internal plastic tube. The other technique involves permanent installation of a perforated plastic capsule connected to the surface by plastic tubing. Some examples of chemical data obtained during extensive testing of these techniques are presented. It is concluded that the techniques could have considerable application in the investigation of shallow ground water and littoral sediment chemistry.