Robert Strzepek

University of Tasmania · Institute for Marine and Antarctic Studies (IMAS)

The necessity to understand the influence of global ocean change on biota has exposed wide-ranging gaps in our knowledge of the fundamental principles that underpin marine life. Concurrently, physiological research has stagnated, in part driven by the advent and rapid evolution of molecular biological techniques, such that they now influence all li...

The influence of global change on Southern Ocean productivity will have major ramifications for the future management of polar living resources. A prior laboratory study investigated the response of a batch-cultured subantarctic diatom to global ocean change (concurrent increases in temperature/CO2/irradiance/iron; decreased macronutrients), reveal...

Carbon dioxide removal (CDR) and emissions reduction are essential to alleviate climate change. Ocean macroalgal afforestation (OMA) is a CDR method already undergoing field trials where nearshore kelps, on rafts, are purposefully grown offshore at scale. Dissolved iron (dFe) supply often limits oceanic phytoplankton growth, however this potentiall...

Manganese (Mn) is an essential element involved in photosynthesis, yet concentrations in Southern Ocean open waters are very low, arising from biological uptake and limited external inputs. At southern latitudes, waters overlying the Antarctic shelf are expected to have much higher Mn concentrations due to their proximity to external sources. In th...

Phytoplankton indirectly influence climate through their role in the ocean biological carbon pump. In the Southern Ocean, the subantarctic zone represents an important carbon sink, yet variables limiting phytoplankton growth are not fully constrained. Using three shipboard bioassay experiments on three separate voyages, we evaluated the seasonality...

Ocean alkalinity enhancement (OAE) aims to increase atmospheric CO2 sequestration in the oceans through the acceleration of chemical rock weathering. This could be achieved by grinding rocks containing alkaline minerals and adding the rock powder to the surface ocean where it dissolves and chemically locks CO2 in seawater as bicarbonate. However, C...

Deep Chlorophyll Maxima (DCMs) are ubiquitous in low-latitude oceans, and of recognized biogeochemical and ecological importance. DCMs have been observed in the Southern Ocean, initially from ships and recently from profiling robotic floats, but with less understanding of their onset, duration, underlying drivers, or whether they are associated wit...

Ocean Iron Fertilization (OIF) aims to remove carbon dioxide (CO2) from the atmosphere by stimulating phytoplankton carbon-fixation and subsequent deep ocean carbon sequestration in iron-limited oceanic regions. Transdisciplinary assessments of OIF have revealed overwhelming challenges around the detection and verification of carbon sequestration a...

Viruses can alter the abundance, evolution, and metabolism of microorganisms in the ocean, playing a key role in water column biogeochemistry and global carbon cycles. Large efforts to measure the contribution of eukaryotic microorganisms (e.g., protists) to the marine food web have been made, yet the in situ activities of the ecologically relevant...

In the Subantarctic sector of the Southern Ocean, vertical entrainment of iron (Fe) triggers the seasonal productivity cycle but diminishing physical supply during the spring to summer transition forces microbial assemblages to rapidly acclimate. Here, we tested how phytoplankton and bacteria within an isolated eddy respond to different dissolved F...

Ocean alkalinity enhancement (OAE) is a proposed method for removing carbon dioxide (CO2) from the atmosphere by the accelerated weathering of (ultra-)basic minerals to increase alkalinity – the chemical capacity of seawater to store CO2. During the weathering of OAE-relevant minerals relatively large amounts of trace metals will be released and ma...

Abstract We present annual cycles of chlorophyll a, phytoplankton carbon, nitrate and oxygen for Subtropical (STW), Subantarctic (SAW), and Subantarctic Mode (SAMW) waters near Aotearoa New Zealand from data collected by two Biogeochemical (BGC) Argo floats. We develop two simple models of depth-integrated net primary production (NPP), tuned agains...

The trace metal iron (Fe) controls the diversity and activity of phytoplankton across the surface oceans, a paradigm established through decades of in situ and mesocosm experimental studies. Despite widespread Fe-limitation within high-nutrient, low chlorophyll (HNLC) waters, significant contributions of the cyanobacterium Synechococcus to the phyt...

Plain Language Summary Phytoplankton fluorescence is a relatively easy and consistent measurement that can be made in the ocean. The measured fluorescence stems directly from the chlorophyll contained in phytoplankton cells and is therefore very specific to the tiny plants. Great oceanic coverage of fluorescence measurements is achieved because flu...

Ocean alkalinity enhancement (OAE) is a proposed method for removing carbon dioxide (CO2) from the atmosphere by the accelerated weathering of (ultra-) basic minerals to increase alkalinity – the chemical capacity of seawater to store CO2. During the weathering of OAE-relevant minerals relatively large amounts of trace metals will be released and m...

The Subantarctic Southern Ocean is far away from terrestrial iron inputs. Low dissolved iron (DFe) supply strongly limits the growth of phytoplankton in subpolar surface waters. However, phytoplankton benefit from vertical Fe supply from a subsurface reservoir (termed new Fe) which triggers the beginning of the phytoplankton growth season. However,...

Submission to Journal of Plankton Research - Horizons Article (topical and provocative reviews). In the article we ask the question: how do we translate the wealth of information on physiological potential from omics-based studies to quantifiable physiological rates and, ultimately, to biogeochemical fluxes? Based on the trajectory of the state-...

It has recently been shown that Southern Ocean phytoplankton species have evolved to optimize their light‐harvesting potential without increasing the high iron‐requiring proteins used for photosynthesis. We measured molecular and physiological responses of phytoplankton cultures under a combination of iron and light conditions. While iron‐replete c...

Manganese (Mn) is an abundant element in the Earth's crust. However, its concentrations in open ocean seawater are low, where external inputs are scarce. In this study, we report the dissolved Mn and particulate Mn distributions in the Southern Ocean, measured along the GEOTRACES—SR3 transect, from Tasmania (Australia) to Antarctica in the Southern...

Model projections for the Southern Ocean indicate that light, iron (Fe) availability, temperature and carbon dioxide (CO2) will change concurrently in the future. We investigated the physiological responses of Southern Ocean phytoplankton to multiple variables by culturing the haptophyte Phaeocystis antarctica and the diatom Chaetoceros flexuosus u...

Evaluation of photosynthetic competency in time and space is critical for better estimates and models of oceanic primary productivity. This is especially true for areas where the lack of iron (Fe) limits phytoplankton productivity, such as the Southern Ocean. Assessment of photosynthetic competency on large scales remains challenging, but phytoplan...

Mesoscale eddies are ubiquitous in the iron-limited Southern Ocean, controlling ocean-atmosphere exchange processes, however their influence on phytoplankton productivity remains unknown. Here we probed the biogeochemical cycling of iron (Fe) in a cold-core eddy. In-eddy surface dissolved Fe (dFe) concentrations and phytoplankton productivity were...

The Southern Ocean is responsible for approximately 40% of oceanic carbon uptake through biological and physical processes. In the Southern Ocean, phytoplankton growth is limited by low iron (Fe) and light supply. Climate model projections for the Southern Ocean indicate that temperature, underwater irradiance and Fe supply are likely to change sim...

Evaluation of photosynthetic competency in time and space is critical for better estimates and models of oceanic primary productivity. This is especially true for areas where the lack of iron limits phytoplankton productivity, such as the Southern Ocean. Assessment of photosynthetic competency on large scales remains challenging, but phytoplankton...

Abstract Observations from two research cruises made in 2008 and 2012 to east of New Zealand are put into context with satellite data to contrast and compare surface chlorophyll a evolution in the two years in order to explore mechanisms of phytoplankton bloom development in the southwest Pacific Ocean. In 2008, surface chlorophyll a largely follow...

A mesocosm experiment was used to investigate the fractionation of silicon (Si)isotopes in subtropical surface waters east of New Zealand. Surface waters in this region were characterised by relatively low concentrations of silicic acid (Si(OH) 4 )(~2 μmol L ⁻¹ )and higher nitrate (~5 μmol L ⁻¹ )and dissolved iron (Fe)concentrations (~0.4 nmol L ⁻¹...

Significance Controls on phytoplankton productivity in the extreme environment of the Southern Ocean has received widespread attention. This polar region presents three challenges to primary producers: how to subsist under the joint constraints of low temperatures, low light, and growth-limiting concentrations of the essential trace element iron. T...

In this study we investigated the distribution of dissolved and particulate zinc (dZn and pZn respectively) and its isotopes in the Subantarctic Zone as part of a Geotraces Process voyage. dZn and pZn depth profiles contrasted each other, with dZn showing depletion within the euphotic zone while pZn profiles showed enrichment. Fitting a power law e...

Zinc (Zn) isotope fractionation by the coccolithophore Emiliania huxleyi (a Tasman Sea isolate) was investigated by culturing it across a range of free Zn2+ ion concentrations (1.2 pmol L−1 to 3.2 nmol L−1), which span the natural range observed in the global oceans. Across the range of free Zn2+ ion concentrations tested, the amount of bio-availab...

Phaeocystis antarctica is an abundant phytoplankton species in the Southern Ocean, where growth is frequently limited by iron and light. Being able to grow under low iron conditions is essential to the species’ success, but there have been hints that this ability differs among clones. Here, we compare the growth, cell size and chlorophyll a concent...

Average cell volume of the P. antarctica clones grown under different iron- and light-conditions. This table provides the average cell volume (fL cell-1) measured for each of the four P. antarctica clones grown under different iron- and light-conditions. Each biological replicate was measured three times, and the standard error of the mean cell vol...

Percentage of solitary cells of the four P. antarctica clones grown under different iron- and light-conditions. This table provides the percentage of solitary cells (out of the total solitary + colonial cells) of the four P. antarctica clones grown under different iron- and light-conditions. Each biological replicate was measured three times, and t...

Intracellular chlorophyll a concentrations of the four P. antarctica clones grown under different iron- and light-conditions. This table provides the intracellular cell volume-normalized chlorophyll a concentrations (mmol chlorophyll a LCV-1) of the four P. antarctica clones grown under different iron- and light-conditions. Each biological replicat...

Criteria used to determine whether clones exhibit a particular response to the iron limiting treatments. This table shows the statistical significance (p-value) and fold-change (FC) thresholds which were used to identify clones that exhibit particular responses, such as a change in cell size or growth limitation, to the iron limiting treatments. Th...

Cell surface area to volume ratios (μm-1) of the P. antarctica clones grown under different iron- and light-conditions. This table provides the cell surface area to volume ratios (μm-1) of the four P. antarctica clones grown under different iron- and light-conditions. Each biological replicate was measured three times, and the standard error of the...

The fractionation of silicon (Si) isotopes was measured in two Southern Ocean diatoms (Proboscia inermis and Eucampia Antarctica) and a coastal diatom (Thalassiosira pseudonana) that were grown under varying iron (Fe) concentrations. Varying Fe concentrations had no effect on the Si isotope enrichment factor (ε) in T. pseudonana, whilst E. Antarcti...

We investigated the effects of iron (Fe) limitation on the elemental stoichiometry, silicic acid (Si(OH)4) uptake kinetics and cell morphology in two Southern Ocean diatoms Eucampia antarctica and Proboscia inermis and the temperate diatom Thalassiosira pseudonana. An increase in Fe-stress resulted in reductions in specific growth rate and decrease...

Relief of iron (Fe) limitation in the surface Southern Ocean has been suggested as one driver of the regular glacial-interglacial cycles in atmospheric carbon dioxide (CO2). The proposed cause is enhanced deposition of Fe-bearing atmospheric dust to the oceans during glacial intervals, with consequent effects on export production and the carbon cyc...

Apparatus for sublimation of ice-core material for dry extraction of large masses of ice core dust (based on [24]). (TIF)

Supporting Data. (XLSX)

Calculating Eucampia antarctica cell volume. (DOCX)

2016 Conway et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.Relief of iron (Fe) limitation in the surface Southern Ocean has been suggested as one driver of...

Dissolved iron supply is pivotal in setting global phytoplankton productivity and pelagic ecosystem structure. However, most studies of the role of iron have focussed on carbon biogeochemistry within pelagic ecosystems, with less effort to quantify the iron biogeochemical cycle. Here we compare mixed‐layer biotic iron inventories from a low‐iron (~...

Significance The supply and bioavailability of dissolved iron sets the magnitude of surface productivity for approximately 40% of the global ocean; however, our knowledge of how it is transferred between chemical states and pools is poorly constrained. Here we utilize the isotopic composition of dissolved and particulate iron to fingerprint its tra...

Phytoplankton growth rates are limited by the supply of iron (Fe) in approximately one third of the open ocean, with major implications for carbon dioxide sequestration and carbon (C) biogeochemistry. To date, understanding how alteration of Fe supply changes phytoplankton physiology has focused on traditional metrics such as growth rate, elemental...

“ It takes a village to finish (marine) science these days ” Paraphrased from Curtis Huttenhower (the Human Microbiome project) The rapidity and complexity of climate change and its potential effects on ocean biota are challenging how ocean scientists conduct research. One way in which we can begin to better tackle these challenges is to conduct...

The individual reactions norms (specific growth rates per day) of all measured strains and cultures obtained by fitting a thermal tolerance function (see Methods) to these data. (TIF)

Intraspecific variation in thermal reaction norms for species in which only one strain was available. (TIF)

A summary of temperature optima (°C) obtained here (red boxes) and from the literature (blue boxes), expressed as box and whiskers plots, for each of the species used in our study (red). The thick black line in each box represents the median temperature. (TIF)

(DOCX)

A summary of temperature optima, maximum growth rates and niche width – expressed as box and whiskers plots - for each of the species used in our study. The black bands denote the median value, the bottom and top of the box represent the 1st and 3rd quartile of the data, respectively. The ‘whiskers’ extending from the boxes indicate the positions o...

A summary of niche width (°C), expressed as box and whiskers plots - for each of the species used in our study. (TIF)

A summary of maximum growth rate expressed as box and whiskers plots - for each of the species used in our study. (TIF)

A plot of niche widths versus annual temperature range. Data points are coloured by species. This plot omits the niche widths that are poorly resolved (i.e. the 6 T. pseudonana +1 A. sanguinea strain). Niche widths increase as the annual temperature range increases, in accordance with the climate variability hypothesis. (TIF)

The rate of carbon fixation by phytoplank- ton in marine surface waters is often tied to the supply of growth-limiting nutrients such as iron (Fe). While average cellular requirements and ratios for various elements are well known in the literature, especially through laboratory culture work, the plasticity of these relationships in natural plankto...

1] Diatom blooms play a central role in supporting food-webs and sequestering biogenic carbon to depth. Oceanic conditions set bloom initiation, whereas both environmental and ecological factors determine bloom magnitude and lon-gevity. Our study reveals another fundamental determinant of bloom dynamics. A diatom spring bloom in offshore New Zealan...

Subantarctic water (SA) close to New Zealand belongs to high nutrient low chlorophyll (HNLC) region. HNLC water is Fe limited. Low Fe concentration in water limits primary production. Oceanographic studies in the last 10 years have shown that Fe in subnanomolar concentration plays a significant role in primary production. However Fe is not only the...

In laboratory experiments we examined the interplay of light and iron availability on the intracellular iron concentrations, specific growth rates, and photosynthetic physiology of Southern (S.) Ocean diatoms (Eucampia antarctica and Proboscia inermis) and the haptophyte Phaeocystis antarctica. Intracellular iron concentrations and iron : carbon (F...

1] The emerging field of ocean iron biogeochemistry has prompted interest in the identification and quantification of Fe supply mechanisms. However, less attention has been given to estimating biological Fe utilization, and using the magnitude of Fe utilization to enhance our understanding of modes of supply. Here, we combine regionally validated d...