John Zeldis

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.

Particle transformations and export flux during an in situ iron-stimulated algal bloom in the Southern Ocean

During the first Southern Ocean Iron RElease Experiment (SOIREE), a suite of biogeochemical measurements (water column 234Th and δ13Corg inventories, particle fluxes from sediment traps, phytoplankton sinking rates) were undertaken to test the hypothesis that the vertical export of particulate organic carbon (POC) is enhanced due to iron-induced increases in phytoplankton production. During the 13-days that the SOIREE bloom was monitored, export fluxes within the iron-fertilised patch were not substantially different to those in waters outside the bloom. On days 11–13, iron enrichment may have caused particle transformations that could lead to elevated future export via particle aggregation and/or diatom chain formation. The unknown time-lag between increased production and export, the longevity of the SOIREE bloom, and the absence of nutrient limitation over days 1–13, however, prohibit prediction of any iron-induced export. This conclusion highlights the difficulties of fully testing the “Iron Hypothesis” and for evaluating the implications for global climate change.

Validation of SeaWiFS data from around New Zealand

Abstract The near-surface concentration of chlorophyll- a ( C a ) was measured during 11 research voyages around New Zealand between 1998 and 2000. Over 400 measurements of spectral normalised water-leaving radiance were made using a high precision profiling spectroradiometer. The data were used to assess the quality of remotely sensed measurements of C a by the NASA ocean colour sensor, SeaWiFS, in New Zealand waters. The results in the open-ocean regions are consistent with a combination of negative biases in the C a algorithm of ca. 20% (some regional dependency), and positive biases due to under-estimation of water reflectance towards the blue end of the spectrum by SeaWiFS.

Snapper (Chrysophrys auratus): a review of life history and key vulnerabilities in New Zealand

Snapper (Chrysophrys auratus) is an important coastal fish species in New Zealand for a variety of reasons, but the large amount of research conducted on snapper has not been reviewed. Here, we review life history information and potential threats for snapper in New Zealand. We present information on snapper life history, defining stages (eggs and larvae, juvenile and adult), and assess potential threats and knowledge gaps. Overall we identify six key points: 1. post-settlement snapper are highly associated with certain estuarine habitats that are under threat from land-based stressors. This may serve as a bottleneck for snapper populations; 2. the largest knowledge gaps relate to the eggs and larvae. Additional knowledge may help to anticipate the effects of climate change, which will likely have the greatest influence on these early life stages; 3. ocean acidification, from land-based sources and from climate change, may be an important threat to larval snapper; 4. a greater understanding of population connectivity would improve certainty around the sustainability of fishery exploitation; 5. the collateral effects of fishing are likely to be relevant to fishery productivity, ecosystem integrity and enduser value; 6. our understanding of the interrelationships between snapper and other ecosystem components is still deficient.

Sediment–seawater solute flux in a polluted New Zealand estuary

We investigated the sediment-seawater solute flux at five sites in the polluted Avon-Heathcote Estuary, New Zealand, to provide a point of comparison for future studies of the effects of the closure of a major wastewater outfall into the estuary. Sediments collected in winters 2007 and 2008, and summer 2008, ranked consistently in organic matter content. Microelectrode profiling and sediment-core incubations revealed (1) a dominant role of microphytes in regulating solute flux causing significant differences in the dark and light sediment O₂ consumption (R(d), R(l)), total sediment O₂ utilisation (TOU(d), TOU(l)), and inorganic nutrient flux, (2) consistent ranking of sites in solute flux, and (3) a clear solute-flux signature of the wastewater effluent. Sediment near the wastewater outfall exhibited the highest absolute R and TOU(,) the lowest ratio R(l)/R(d,) the highest dark efflux of dissolved reactive phosphorus and ammonium, and the highest dark and light uptake of nitrate+nitrite.

Microbial assemblage during the spring‐summer transition on the northeast continental shelf of New Zealand

Abstract The northeast shelf of the North Island of New Zealand was sampled on four occasions from early spring to late summer in 1996–97. During this period, tight linkages between the physical and chemical forcings and the response from the microbial food‐web were observed. During bloom conditions in early spring the phytoplankton population was dominated by large diatoms with the picophytoplankton contributing an average of 8% of the phytoplankton biomass. The picophytoplankton fraction increased to contribute a mean of 57% of the phytoplankton biomass in late summer when upwelling had ceased, the water column was strongly stratified and dissolved inorganic nitrogen concentrations were negligible. The changes in the phytoplankton assemblage were accompanied by changes in the microzooplankton population. In early spring the microzooplankton population was dominated by larger species, including copepod nauplii and >20 μm aloricate ciliates, with copepod nauplii contributing up to 50% of the microzooplankton biomass in early and late spring. Peak microzooplankton biomasses of 880–2093 μg C m‐2 in the mixed layer occurred in early and late spring, depending on the station. The large ciliate Laboea strobila was an important component of the microzooplankton biomass, and increased substantially at three out of the five stations between early and late spring. Strong upwelling in early summer resulted in substantial decreases in picophytoplankton and microzooplankton abundance. In late summer the composition of microzooplankton population had changed substantially, with the <20 μm aloricate ciliates increasing from 35 to 90% of the total biomass. The overall increase in the importance of the microbial components of the food‐web between early spring and late summer was closely linked with changes in the physical and chemical drivers of the system.

Embryology and early larval development of orange roughy (Hoplostethus atlanticus Collett)

Abstract Orange roughy (Hoplostethus atlanticus Collett) egg development is described from fertilisation in a series of 29 stages based on morphological criteria. Two early larval stages are also described. Observations were made of eggs obtained from strip‐fertilisation using trawl‐caught adults and from plankton net samples of eggs caught off the east coast of New Zealand, and then cultured on‐board ship. Live eggs of each stage are described and illustrated with line drawings and photographs and age‐at‐stage data are presented. The mean development times from fertilisation to hatching were 278 h at 8°C, 235 h at 10°C, and 146 h at 12°C. Eggs cultured at 6°C did not hatch. The mean length of the larvae at hatching was 5.33 mm notochord length. Most criteria used in staging are readily observable with a standard dissecting microscope at x25 magnification using transmitted light. Damaged eggs are common in egg production surveys for orange roughy. The staging of damaged eggs is described. Orange roughy eg...

A national-scale GIS-based system for modelling impacts of land use on water quality

Management of freshwater quality requires modelling tools for rapid evaluation of land use and management scenarios. This paper presents the CLUES (Catchment Land Use and Environmental Sustainability) model to address this need. CLUES provides steady state, spatially distributed, integrated catchment models tightly coupled to GIS software to predict mean annual loads of total nitrogen, total phosphorus, sediments and E.coli, and concentration of nutrients throughout New Zealand (268,000km2) with a subcatchment resolution of 0.5km2. CLUES also estimates potential nutrient concentrations for estuaries and provides key farm socio-economic indicators. The model includes a user interface for study area selection, scenario creation, data geo-visualisation, and export of results. It is pre-populated with spatial data and parameter values for New Zealand. Evaluation of the model and a summary of applications demonstrate the tractability and utility of national-scale rapid scenario assessment tools within a GIS framework. Display Omitted Stream water quality prediction at mean annual time-scale throughout New Zealand at kilometre-scale resolution.Rapid scenario generation and evaluation of land use change and management measures (mitigation factors, intensification).Includes links to an estuary nutrient component and simple socio-economic indicators.Has a growing number of applications for catchment, regional and national scenario assessment.

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.