Pearse James Buchanan

CSIRO Marine And Atmospheric Research

Microorganisms in marine oxygen minimum zones (OMZs) drive globally impactful biogeochemical processes. One such process is the multi-step denitrification, the dominant pathway for bioavailable nitrogen (N) loss and nitrous oxide (N2O) production. Denitrification-derived N loss is typically measured and modeled as a single step, but observations re...

Climate change could irreversibly modify Southern Ocean ecosystems. Marine ecosystem model (MEM) ensembles can assist policy making by projecting future changes and allowing the evaluation and assessment of alternative management approaches. However, projected changes in total consumer biomass from the Fisheries and Marine Ecosystem Model Intercomp...

Diatoms are prominent eukaryotic photoautotrophs in today’s oceans. While dominant in nitrate-rich conditions, they face competitive exclusion by other phytoplankton when ammonium forms the bulk of bioavailable nitrogen. The extent to which this competitive exclusion defines diatom abundance worldwide and the consequences of potential future ammoni...

ABSTRACT: Humans are exposed to toxic methylmercury mainly by consuming marine fish. While reducing mercury emissions and releases aims to protect human health, it is unclear how this affects methylmercury concentrations in seawater and marine biota. We compiled existing and newly acquired mercury concentrations in tropical tunas from the global oc...

Climate change is one of the top three global threats to seabirds, particularly species that visit polar regions. Arctic terns migrate between both polar regions annually and rely on productive marine areas to forage, on sea ice for rest and foraging, and prevailing winds during flight. Here, we report 21st-century trends in environmental variables...

Anaerobic metabolisms are thought to dominate nitrogen cycling in anoxic marine zones (AMZs). However, thriving populations of aerobic nitrite-oxidizing bacteria (NOB) in AMZs challenge this assumption and remain unexplained. Using theory and modelling, we show how periodic oxygen intrusions sustain aerobic NOB in AMZs alongside more competitive ae...

Multiple environmental forcings, such as warming and changes in ocean circulation and nutrient supply, are affecting the base of Arctic marine ecosystems, with cascading effects on the entire food web through bottom‐up control. Stable nitrogen isotopes (δ15N) can be used to detect and unravel the impact of these forcings on this unique ecosystem, i...

Significance Humans are exposed to toxic methylmercury mainly by consuming marine fish. New environmental policies under the Minamata Convention rely on a yet-poorly-known understanding of how mercury emissions translate into fish methylmercury levels. Here, we provide the first detailed map of mercury concentrations from skipjack tuna across the P...

The open ocean nitrogen cycle is being altered by increases in anthropogenic atmospheric nitrogen deposition and climate change. How the nitrogen cycle responds will determine long-term trends in net primary production (NPP) in the nitrogen-limited low latitude ocean, but is poorly constrained by uncertainty in how the source-sink balance will evol...

Plain Language Summary Measurements of dissolved oxygen (O2) in the ocean since the 1960s have revealed a multidecadal decrease in ocean O2 levels that is a major concern. It represents a fingerprint of human‐induced climate change and is also an ecological hazard in the highly productive upwelling systems of the lower latitudes that host present‐d...

Nitrogen stable isotopes (δ15N) are used to study food web and foraging dynamics due to the step-wise enrichment of tissues with increasing trophic level, but they rely on the isoscape baseline that varies markedly in the Arctic due to the interplay between Atlantic- and Pacific-origin waters. Using a hierarchy of simulations with a state-of-the-ar...

The hydrography of the Arctic Seas is being altered by ongoing climate change, with knock‐on effects to nutrient dynamics and primary production. As the major pathway of exchange between the Arctic and the Atlantic, the Fram Strait hosts two distinct water masses in the upper water column, northward flowing warm and saline Atlantic Waters in the ea...

The ocean seems the same everywhere, whether at your local beach or in the cold Arctic. But ocean properties vary greatly. Like different types of soil on land, some seawater is rich in nutrients, while others are nutrient-poor. If more nutrients are available, then more life can grow. With more life, the ocean can remove more carbon dioxide from t...

Roughly a third (~30 ppm) of the carbon dioxide (CO2) that entered the ocean during ice ages is attributed to biological mechanisms. A leading hypothesis for the biological drawdown of CO2 is iron (Fe) fertilisation of the high latitudes, but modelling efforts attribute at most 10 ppm to this mechanism, leaving ~20 ppm unexplained. We show that an...

The isotopes of carbon (δ13C) and nitrogen (δ15N) are commonly used proxies for understanding the ocean. When used in tandem, they provide powerful insight into physical and biogeochemical processes. Here, we detail the implementation of δ13C and δ15N in the ocean component of an Earth system model. We evaluate our simulated δ13C and δ15N against c...

The isotopes of carbon (δ¹³C) and nitrogen (δ¹⁵N) are commonly used proxies for understanding the ocean. When used in tandem, they provide powerful insight into physical and biogeochemical processes. Here, we detail the implementation of δ¹³C and δ¹⁵N in the ocean component of an Earth system model. We evaluate our simulated δ¹³C and δ¹⁵N against c...

We use a global ocean model to investigate the role of dinitrogen (N2) fixation within the global carbon (C) cycle. We find that: 1. N 2 fixation is ~25% (800 Pg C) of the biological C store under preindustrial conditions. 2. The biological C pump is severely limited without N 2 fixers in its capacity to respond to increases in aeolian iron (Fe)...

The biogeochemistry of the ocean exerts a strong influence on the climate by modulating atmospheric greenhouse gases. In turn, ocean biogeochemistry depends on numerous physical and biological processes that change over space and time. Accurately simulating these processes is fundamental for accurately simulating the ocean's role within the climate...

Zooplankton are the intermediate trophic level between phytoplankton and fish, and are an important component of carbon and nutrient cycles, accounting for a large proportion of the energy transfer to pelagic fishes and the deep ocean. Given zooplankton's importance, models need to adequately represent zooplankton dynamics. A major obstacle, though...

The ocean's ability to store large quantities of carbon, combined with the millennial longevity over which this reservoir is overturned, has implicated the ocean as a key driver of glacial–interglacial climates. However, the combination of processes that cause an accumulation of carbon within the ocean during glacial periods is still under debate....

This poster describes experiments using the CSIRO Mk3L climate system model that investigates the contribution of physical and biogeochemical changes that occurred in the glacial ocean to the net carbon storage of the ocean. We find that in order for the ocean to store a greater amount of carbon during glacial conditions, which explains the drawdow...

3 Minute Thesis presentation at the University of Tasmania (July 2016) for the Science, Engineering and Technology session.

The ocean's ability to store large quantities of carbon, combined with the millennial longevity over which this reservoir is overturned, has implicated the ocean as a key driver of glacial-interglacial climates. However, the combination of processes that cause an accumulation of carbon within the ocean during glacial periods is still under debate....

Chaetognaths are ubiquitous marine zooplankton strongly influenced by oceanographic conditions. Epi-pelagic plankton samples taken across the Leeuwin Current system, the poleward flowing eastern boundary current of the Indian Ocean, were examined for chaetognath diversity and distribution. Sampling occurred at shelf, shelf break, and oceanic enviro...

Southern Tasmanian shelf waters are host to the seasonal interplay of Australia’s two poleward boundary currents; the East Australian Current (EAC) and the Leeuwin Current (LC). While the behaviour and properties of the LC remain underexplored, strong research focus has allowed insight into how an intensifying EAC has created greater subtropical in...