Karl A. Safi

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 of phytoplankton in Manukau Harbour, New Zealand

Abstract Grazing by microzooplankton on phytoplankton in Manukau Harbour was measured by size‐fractionated dilution experiments at monthly intervals from October 1994 to October 1995. Grazing rates were always highest on the 22 μm phytoplankton was measurable only during the February bloom of the large diatom Odontella sinensis. The grazing rate was low, being a small percentage of phytoplankton growth rate in that size fr...

Distribution and biomass of benthic microalgae in Manukau Harbour, New Zealand

Abstract Benthic microalgal biomass (as sediment chlorophyll a (Chl. a), by spectrophotometry) and taxonomic composition (by HPLC pigments analysis) were investigated at intertidal locations throughout Manukau Harbour, North Island, New Zealand. Benthic microalgal biomass averaged 97.5 mg Chl. a m‐2 for all sediment samples. Benthic microalgal biomass was higher in sediments containing at least some sand than in muddy sediments, in contrast to previous findings from Manukau Harbour. Loading of fine sediments from erosion within the harbours basin may, therefore, affect the amount and distribution of benthic microalgal biomass in the harbour. Average benthic microalgal biomass for the entire area of the harbour was at least 62.5 mg Chl. a m‐2. The latter value is at least 4 times higher than mean annual, spatially integrated phytoplankton biomass in Manukau Harbour, suggesting that benthic microalgae are a more important food source for estuarine consumers. High fucoxanthin:chlorophyll a ratios indicated that benthic microalgae were primarily diatoms. The physical characteristics of Manukau Harbour and similarity in taxonomic composition of the phytoplankton and benthic microalgae suggest that resuspended benthic microalgae are an important component of the harbours phytoplankton biomass.

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.

Importance of different size classes of phytoplankton in Beatrix Bay, Marlborough Sounds, New Zealand, and the potential implications for the aquaculture of the mussel, Perna canaliculus

Abstract 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.

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.

Summer―winter transitions in Antarctic ponds I: The physical environment

Abstract Meltwater ponds are one of the most widespread aquatic habitats in ice-free areas of continental Antarctica. While most studies of such systems occur during the Antarctic summer, here we report on ice formation and water column attributes in four meltwater ponds on the McMurdo Ice Shelf during autumn, when they went from ice-free to > 80 cm thickness of ice. Ice thickness grew at an average rate of 1.5 cm d-1 in all ponds and as ice formed, salts and gases were excluded. This resulted in conductivity rising from 3–5 to > 60 mS cm-1 and contributed to the ebullition of gases. Incorporation of gas bubbles in the ice resulted in a high albedo and under-ice irradiance declined faster than incident, the former falling below 1 W m-2 (daily average) by early April. After two months of ice formation, only 0–15% of the volume of each pond was still liquid, although this represented 5–35% of the pond sediment area, where much of the biological activity was concentrated. We suggest that the stresses that the freezing process imposes may be as important to structuring the biotic communities as those during the more benign summer growth period.