Mark G. Hadfield

A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilization

Changes in iron supply to oceanic plankton are thought to have a significant effect on concentrations of atmospheric carbon dioxide by altering rates of carbon sequestration, a theory known as the ‘iron hypothesis’. For this reason, it is important to understand the response of pelagic biota to increased iron supply. Here we report the results of a mesoscale iron fertilization experiment in the polar Southern Ocean, where the potential to sequester iron-elevated algal carbon is probably greatest. Increased iron supply led to elevated phytoplankton biomass and rates of photosynthesis in surface waters, causing a large drawdown of carbon dioxide and macronutrients, and elevated dimethyl sulphide levels after 13 days. This drawdown was mostly due to the proliferation of diatom stocks. But downward export of biogenic carbon was not increased. Moreover, satellite observations of this massive bloom 30 days later, suggest that a sufficient proportion of the added iron was retained in surface waters. Our findings demonstrate that iron supply controls phytoplankton growth and community composition during summer in these polar Southern Ocean waters, but the fate of algal carbon remains unknown and depends on the interplay between the processes controlling export, remineralisation and timescales of water mass subduction.

Pilot trophic model for subantarctic water over the Southern Plateau, New Zealand: a low biomass, high transfer efficiency system

Abstract The Southern Plateau subantarctic region, southeast of New Zealand, is an important feeding area for birds, seals and fish, and a fishing ground for commercially significant species. The Southern Plateau is a major morphometric feature, covering approximately 433,620 km2 with average depth of 615 m. The region is noted for its relatively low levels of phytoplankton biomass and primary production that is iron-limited. In order to evaluate the implications of these attributes for the functioning of this ecosystem a steady-state, 19-compartment model was constructed using Ecopath with Ecosim software of Christensen et al. [ www.ecopath.org ]. The system is driven by primary production that is primarily governed by the supply of iron and light. The total system biomass of 6.28 g C m−2 is very low compared with systems so far modelled with a total system throughput of 1136 g C m−2 year−1. In the model, the Southern Plateau retains 69% of the biomass in the pelagic system and 99% of total production. Although fish are caught demersally, most of their food is part of production in the pelagic system. Top predators represent about 0.3% of total biomass and account for about 0.24 g C m−2 year−1 of food consumed made up of birds 0.058 g C m−2 year−1, seals 0.041 g C m−2 year−1, and toothed 0.094 g C m−2 year−1 and baleen whales 0.051 g C m−2 year−1. This amounts to 105,803 tonnes carbon over the whole of the Southern Plateau and is about 17% of the total amount of food eaten by non-mesopelagic fish. Mean transfer efficiencies between trophic levels II and IV of 23% are at the high end of the range reported in the literature. In the model, adult fish production is almost completely accounted for by the fisheries take (32%), consumption by seals (7%), toothed whales (21%), other adult fish (13%), and squid (20%). Fish and squid catches are at the trophic levels of 4.8 and 5.0, respectively. The gross efficiency of the fishery is 0.018% (catch/primary production). Although not all data come from direct knowledge of this system, the model reflects its general characteristics, namely a low primary production system dominated by the microbial loop, low sedimentation to the seafloor, high transfer efficiencies, a long food web and supporting high-level predators.

Aspects of the hydrodynamics of Beatrix Bay and Pelorus Sound, New Zealand

Abstract Hydrographic and current meter measurements were performed to determine the important physical processes in Beatrix Bay, an enclosed bay in Pelorus Sound, South Island, New Zealand over a 9‐month period. The water within the bay was highly variable in temperature, salinity, and density, but strongly stratified on all occasions, with the stratification approximating a two‐layer system. The stratification was usually dominated by salinity, but on one occasion, following a period of low rainfall, the stratification was dominated by temperature. A crude estimate of the residence time for the water in Pelorus Sound is calculated from the relaxation towards oceanic salinities during the dry period. The currents measured were largely tidal, and showed evidence of a strong internal tide as a result of the quasi‐two‐layer stratification. The tidal flows were larger than those predicted by a simple model.

A hydrodynamic model of Chatham Rise, New Zealand

Abstract A three‐dimensional hydrodynamic ocean model, the Regional Ocean Modelling System (ROMS), was applied to a region encompassing the Chatham Rise, New Zealand and forced by surface fluxes from an atmospheric reanalysis data set. The model outputs fields were validated against a number of observation‐based data sets, including the CSIRO Atlas of Regional Seas 2000 (CARS 2000) climatology of sub‐surface temperature and salinity, a sea surface temperature (SST) climatology from the NIWA SST Archive (NSA), and sea surface height (SSH) from the Archiving, Validation and Interpretation of Satellite data in Oceanography (AVISO) Mapped Sea Level Anomalies (MSLA) data set. The model reproduced the flow around Chatham Rise well and had a realistic seasonal cycle in the upper ocean. Its biggest deficiency was that the Wairarapa Eddy was too steady and pushed warm water too far south towards the head of Hikurangi Trough. This exercise confirms the need to validate a model against multiple data sets and shows the value of SST data for revealing underlying oceanographic features.

Development of a regional ocean model for New Zealand

Abstract A regional ocean model with a horizontal resolution of 1/6° encompassing the New Zealand Exclusive Economic Zone is described. The regional model successfully downscaled solutions from a high resolution, global, coupled model HadCEM. Transport estimates from the global and regional models were compared with observations, and both models supported largely consistent, climatological mean solutions. The regional model used monthly mean forcing at the surface. Nevertheless, the regional model eddy kinetic energy (EKE) spatial patterns compared favourably with long‐term mean satellite altimetric estimates, although the modelled background EKE amplitudes were much lower than observed. A series of permanent eddies associated with the western boundary current system around the top of the North Island of New Zealand were reproduced, and an eddy adjacent to Norfolk Ridge was identified in both the global and regional models. The western boundary current system around the North Island of New Zealand and the associated eddies were the most sensitive components of the model solutions, being influenced by initial conditions, wind forcing, and the model domain size.

Dispersal of Oyster (Ostrea chilensis) Larvae in Tasman Bay Inferred using a Verified Particle Tracking Model that Incorporates Larval Behavior

ABSTRACT We present a particle tracking model that simulates the dispersal of the oyster Ostrea chilensis Philippi in Tasman Bay, New Zealand. The model was calibrated to reproduce observed vertical distributions of larvae, and was verified against an independent data set of such distributions. The seabed in Tasman Bay contains little hard substrate other than shell hash. By comparing simulated and observed settlement, we infer that larvae must either be able to sense shell hash at a distance and orient onto it, such that settlement success is higher than indicated by the 1-km2 resolution relative density (square meter of exposed shell per square meter of seabed), or be settling on material other than shell hash. The modeling indicates that only a very small proportion of those larvae that do settle travel more than 1 km from their natal location. There are plans to endeavor to restore the oyster fishery by provision of new shell hash to the bed. Our simulations suggest that these new beds will be colonized rapidly only if the new beds are placed close to the remnant beds, or if they are artificially seeded with mature oysters. Conversely, if the new beds are artificially seeded, our simulations imply that a large fraction of the spawn from these adults may be retained within the new bed, rather than dispersing into the surrounding, unsuitable soft sediments.

Power stations, oxides of nitrogen emissions, and photochemical smog: a modelling approach to guide decision makers

Abstract Emissions of oxides of nitrogen from proposed industrial developments in urban areas have the potential to affect photochemical smog. This paper outlines an assessment method, using the Integrated Empirical Rate (IER) Model, which can guide decision makers on the likely impact of such emissions. The approach is illustrated with a case study for a proposed gas fired power station in Auckland, New Zealand. The modeling in this case predicted that total production of photochemical smog in the Auckland region would generally be unaffected by the power station, and that ozone concentrations would usually be reduced by the power station emissions. The IER model provides a relatively simple method of assessing the likely effect on air quality of changes in photochemical smog precursor emissions, especially when only limited emissions and meteorological data are available.

Links between climate and recruitment of New Zealand hoki (Macruronus novaezelandiae) now unclear

Abstract We investigated the relationship between climate variation and year‐class strengths for hoki (Macruronus novaezelandiae) in New Zealand waters. Our analyses extended those of a previous study, by using an additional 6 years’ data and considering some additional predictands (total year‐class strength and proportion migrating) relating to an alternative stock‐structure hypothesis, and also updated them, by using revised versions of some predictors and year‐class strengths. Predictors considered were based on the Southern Oscillation Index, weather patterns, sea‐surface temperatures, wind speeds, and modelled mixed‐layer depths and nutrient concentrations. In contrast to the earlier analyses we found little or no predictive power for either year‐class strength or proportion migrating. The substantial correlations found in the earlier study were greatly reduced. Such reversals are not uncommon in the climate‐recruitment literature.

Wind-driven upwelling and surface chlorophyll blooms in Greater Cook Strait

ABSTRACT We present the results of a combined observational and numerical study to investigate cool plumes of nutrient-rich upwelled water that emanate near the Kahurangi Shoals and extend into Greater Cook Strait. Surface temperature and chlorophyll are mapped using satellite observations to produce surface climatologies, to validate a numerical simulation and to show the utility of using spatial temperature differences as a measure of upwelling. We find upwelling near the Kahurangi Shoals is strongly wind-driven in the weather band. Upwelling occurs at all times of the year, but its surface signature is only visible in summer months. The upwelled nutrient-rich water supports increased primary production compared to surrounding waters, particularly in summer when the water column is more stratified and surrounding surface waters are presumably nutrient depleted.