Julie A. Hall

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.

Retention of dissolved iron and Fe II in an iron induced Southern Ocean phytoplankton bloom

During the 13 day Southern Ocean Iron RE-lease Experiment (SOIREE), dissolved iron concentrations decreased rapidly following each of three iron-enrichments, but remained high (>1 nM, up to 80% as FeII) after the fourth and final enrichment on day 8. The former trend was mainly due to dilution (spreading of iron-fertilized waters) and particle scavenging. The latter may only be explained by a joint production-maintenance mechanism; photoreduction is the only candidate process able to produce sufficiently high FeII, but as such levels persisted overnight (8 hr dark period) —ten times the half—life for this species—a maintenance mechanism (complexation of FeII) is required, and is supported by evidence of increased ligand concentrations on day 12. The source of these ligands and their affinity for FeII is not known. This retention of iron probably permitted the longevity of this bloom raising fundamental questions about iron cycling in HNLC (High Nitrate Low Chlorophyll) Polar waters.

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.

Microzooplankton grazing in different water masses associated with the subtropical convergence round the south island, New Zealand

Abstract Grazing rates by microzooplankton feeding on bacteria, picophytoplankton and total phytoplankton were measured in subtropical, Subtropical Convergence and subantarctic waters in winter and spring 1993. Water samples were collected simultaneously for determinations of nutrient concentrations and biological parameters. Bacteria, phytoplankton and microzooplankton abundance were generally higher in spring than winter. Picophytoplankton were a large proportion of the phytoplankton populations in subantarctic waters. Microzooplankton grazing impact on total chlorophyll a standing crop and phytoplankton production ranged from 10–92% to 71–194% in winter and 4–57% to 20–126% in spring, respectively. The only major difference between grazing impacts in winter and spring was in subtropical waters, where grazing impacts on both chlorophyll a and picophytoplankton were higher in winter. Grazing impact was generally greater in subantarctic water and on picophytoplankton than on total phytoplankton. Grazing balanced growth of phytoplankton in all water masses in winter, but in subtropical waters in spring, growth of total phytoplankton exceeded grazing and grazing exceeded growth in picophytoplankton populations. This decoupling of growth of the larger phytoplantion and grazing by microzooplankton contributed to the spring bloom and potential flux of carbon out of the surface waters in subtropical water masses. These findings are further discussed in relation to spatial and seasonal trends in other parts of the STC.

Impact of Phytoplankton on the Biogeochemical Cycling of Iron in Subantarctic Waters Southeast of New Zealand During FeCycle

[1] During austral summer 2003, we tracked a patch of surface water infused with the tracer sulfur hexafluoride, but without addition of Fe, through subantarctic waters over 10 days in order to characterize and quantify algal Fe pools and fluxes to construct a detailed biogeochemical budget. Nutrient profiles characterized this patch as a highnitrate, low-silicic acid, low-chlorophyll (HNLSiLC) water mass deficient in dissolved Fe. The low Fe condition was confirmed by several approaches: shipboard iron enrichment experiments and physiological indices of Fe deficiency (Fv/Fm 40% of total chlorophyll. Whereas the picophytoplankton accounted for � 50% of total primary production, they were responsible for the majority of community iron uptake in the mixed layer. Thus ratios of 55 Fe: 14 C uptake were highest for picophytoplankton (median: 17 mmol:mol) and declined to � 5 mmol:mol for the larger algal size fractions. A pelagic Fe budget revealed that picophytoplankton were the largest pool of algal Fe (>90%), which was consistent with the high (� 80%) phytoplankton Fe demand attributed to them. However, Fe regenerated by herbivory satisfied only � 20% of total algal Fe demand. This iron regeneration term increased to 40% of algal Fe demand when we include Fe recycled by bacterivory. As recycled, rather than new, iron dominated the pelagic iron budget (Boyd et al., 2005), it is highly unlikely that the supply of new Fe would redress the imbalance between algal Fe demand and supply. Reasons for this imbalance may include the overestimation of algal iron uptake from radiotracer techniques, or a lack of consideration of other iron regeneration processes. In conclusion, it seems that algal Fe uptake cannot be supported solely by the recycling of algal iron, and may require an Fe ‘‘subsidy’’ from that regenerated by heterotrophic pathways.

Factors influencing autotrophic and heterotrophic nanoflagellate abundance in five water masses surrounding New Zealand

Abstract The aim of this study was to measure nanoflagellate abundance in New Zealand waters, and identify the key factors which both influence, and are influenced by, nanoflagellate abundance. Nanoflagellate populations were sampled in winter and spring 1993 from a series of sites representing different water masses around the South Island of New Zealand. Both numbers and biomass of heterotrophic (HNF) and autotrophic nanoflagellate (ANF) populations were larger in spring by a factor of four. ANF were about three times as abundant as HNF in both seasons. The physiochemical variables, temperature, NH4‐N, and urea combined with bacteria and picophytoplankton numbers explained between 67 and 94% of the variation in nanoflagellate abundance. In addition, there was evidence that variation in abundances between seasons and water masses was influenced by food availability, predation, and changes in species composition represented by large differences in cell biovolume.

Role of microzooplankton grazers in the subtropical and subantarctic waters to the east of New Zealand

Abstract Subtropical and subantarctic surface waters to the east of New Zealand were sampled in four seasons over a period of 3 years to evaluate the importance of the microzooplankton as grazers of the phytoplankton community. Subtropical waters (STW) to the north were warm with high salinity and seasonal macronutrient limitation. Subantarctic waters (SAW) to the south were colder, with lower salinity, high macronutrient concentrations and chlorophyll a concentrations between 0.20 and 0.28 μg litre‐1. In the STW, phytoplankton showed a typical seasonal pattern for macronutrient‐limited waters, with biomass dominated by large phytoplankton and a chlorophyll a maximum of 1.4 μg litre–1 in spring. Microzooplankton biomass varied from 4.5 μg carbon (C) litre‐1 in the STW in winter, to 13.8 μg C litre‐1 in the SAW in winter with the hetero‐trophic flagellates contributing between 22% and 78% of the biomass. In spring and winter in the STW, only 73% of the primary production was grazed by the microzooplankton compared with over 100% in autumn and summer. In contrast, over 100% of the primary production was consumed in all seasons by the microzooplankton in SAW. In the SAW in all seasons and the STW in summer and autumn, microzooplankton grazing on phytoplankton dominated the organic matter fluxes from the phytoplankton population. During these periods the picophytoplankton contributed a significant proportion of the phytoplankton biomass.

A sensitive fluorometric technique for the measurement of phycobilin pigments and its application to the study of marine and freshwater picophytoplankton in oligotrophic environments

A sensitive and specific technique is described for the estimation of phycobiliprotein in freshwater and marine picophytoplankton. The method uses fluorescent properties to detect phycoerythrin concentrations as low as 40 ng L-1 from a 1 L water sample and is capable of distinguishing between R-phycoerythrin, C-phycocyanin and C-phycoerythrin. The application of the method to the study of natural picophytoplankton populations in marine and freshwater environments is described. Nitrate concentrations appear to influence picophytoplankton cellular C-phycoerythrin concentrations in surface waters and increasing cellular C-phycoerythrin fluorescence with water depth suggests that this pigment plays a role as a photosynthetic accessory pigment.

Comparing past and present trophic states of seven Central Volcanic Plateau lakes, New Zealand

Abstract Data on seven lakes in the Rotorua District were examined to determine trophic state changes in these lakes. The method of data analysis was specifically designed to find small changes in the trophic state of lakes. For each variable, the P‐value from 12 monthly paired sample Student t‐test comparisons was calculated and a change of trophic state value (CTS) was assigned, based on the P‐value. The CTS values for all the variables examined for a single lake were averaged (C) with a standard error (CV). The resulting Change/ Confidence Value index (C/CV) indicated the nature of the trophic change observed and the degree of confidence that could be placed in the stated change. This study finds that in recent years Lakes Rotorua and Rotoiti have become less eutrophic; Lakes Okareka and Rotoma may have become less eutrophic; Lakes Okataina and Rotokakahi have not changed in trophic state in the last 22 years, and Lake Tikitapu may have become more eutrophic.

Grazing by protozoa in marine coastal and oceanic ecosystems off New Zealand

Abstract Uptake rates for ciliates and flagellates grazing on bacteria and picophytoplankton were measured in different water masses around South Island, New Zealand, in April 1992. Fluorescent particles were used to established uptake rates for major ciliate taxa, phytoflagellates, and hetero‐trophic flagellates. Protozoan grazing had little impact on the bacterial population, removing < 5% of the population per day. Heterotrophic flagellates and ciliates selected picophytoplankton in preference to bacterial‐sized particles, both groups removing 6–32% of the picophytoplankton population per day. Highest removal rates for picophytoplankton were found in coastal waters and for bacterial populations in subantarctic waters. This difference was attributed to differences in community composition and taxa‐specific clearance rates.