Investigating how environmental change affects benthic biogeochemistry

Lead Research Organisation: University of Aberdeen
Department Name: Inst of Biological and Environmental Sci


Microscopic plants, or phytoplankton, use the sun's energy to combine atmospheric carbon dioxide (CO2) and water to produce particulate organic matter (POM). A proportion of this sinks to the seabed, where bacteria and animals use it for energy production, maintenance of biomass and growth. These organisms maintain a strict balance of carbon (C), nitrogen (N) and polyunsaturated fatty acids (PUFAs) in their tissues. Their growth can thus be limited by any of these substrates, depending on the quantity and biochemical composition, or 'quality', of the POM available. Limitation by a single substrate necessitates that all others are in excess and must therefore be released to the environment. This can be achieved by adjusting the efficiencies with which C and N are assimilated or by liberating them as CO2 or dissolved inorganic N (DIN). It follows that the quantity and quality of POM arriving at the seabed, and the dietary demands of its inhabitants, influence the fate of C and N in marine sediments. Any POM that escapes ingestion, or that which is released as faecal pellets, may persist in the sediments for thousands of years. Geological storage of POM in marine sediments represents a means by which the ever-increasing atmospheric concentration of CO2 can be reduced. However, it also removes N from the biosphere. This reduces the potential for further phytoplankton growth, which is limited by the supply of DIN. If DIN was only supplied via the recycling of POM in marine sediments, it follows that a sustained net biological drawdown of atmospheric CO2 into the oceans can only occur when the C:N ratio of the POM arriving at the seabed is greater than the ratio of CO2:DIN released. Essentially nothing is known about how POM quantity and quality affect the offset between these ratios, or how it is liable to change in the future. Manmade nutrient enrichment and climate change are already changing the quantity and quality of POM arriving at the seabed, but we do not understand, or have the capacity to predict, how this influences the roles that marine sediments play in C storage and DIN release. In turn, this greatly restricts our ability to accurately represent the cycles of C and N in global models that are designed to make meaningful forecasts about future climate change. I will grow species of phytoplankton with different ratios of C, N and PUFAs, which therefore differ in terms of their quality. The C and N in the phytoplankton will be replaced with 13C and 15N, stable isotopes (SIs) of these elements that behave in an identical manner, but differ subtly in their mass. They are scarce and hence easy to follow in the natural environment. My research will, for the first time, introduce increasing quantities of the different, dual SI-labelled algae onto the seabed and follow the fate of 13C and 15N into 13CO2, DI15N, bacterial and animal biomass and the sediments, thereby providing a detailed insight into the ways in which the quantity and quality of POM influence the burial of C and the release of DIN. The experiments will be conducted in coastal and deep-sea (> 1 km deep) habitats as these are considered to be the most important areas for global seabed C turnover. My research will also provide information on the relative roles that bacteria and animals play in elemental cycling in shallow and deep-water habitats, a topic that currently remains hotly debated. The ultimate goal of this project is to generate a mechanistic understanding of the ways in which POM quantity and quality affect the fate of C and N in marine sediments. This will be used to produce a mathematical model that is capable of predicting the quantities of C stored, and DIN released by the seabed, given a known quantity and quality of POM. This will represent a significant step towards being able to accurately represent the role of marine sediments in global climate models.


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Giering SLC; Sanders R; Lampitt RS; Marsay C; Cook K; Mayor DJ (2011) Mesozooplankton diel migration balances twilight zone carbon budgets

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Giering SLC; Sanders R; Lampitt RS; Marsay C; Mayor DJ (2011) Mesozooplankton demands exceed carbon flux in the twilight zone

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Giering SLC; Sanders R; Lampitt RS; Poulton AJ; Mayor DJ (2011) Iron recycling by mesozooplankton supports phytoplankton growth in the Irminger Basin

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Gontikaki E (2011) Processing of 13C-labelled diatoms by a bathyal community at sub-zero temperatures in Marine Ecology Progress Series

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Main C (2015) Hydrocarbon contamination affects deep-sea benthic oxygen uptake and microbial community composition in Deep Sea Research Part I: Oceanographic Research Papers

Description The composition of phytoplankton communities are changing in response to environmental pollution and climate change. These changes are affecting the quantity and biochemical composition ('quality') of the particulate organic matter (POM) that ultimately sinks down to the seabed. This project examined how changes in the quantity and quality of POM arriving at the seafloor influence the fate of carbon (C) and nitrogen (N) in a range of contrasting environments. A key driver of this research was to better understand the processes that favour the storage of C in marine sediments, and hence the removal of atmospheric CO2. It explored the following hypotheses: 1) high quality resources stimulate rates of biological activity and the uptake of C and N by benthic organisms, thereby reducing the capacity for burial of these elements; 2) increased availability (quantity) of resources enhances the efficiency with which they are used and hence decreases the proportional amounts of C and N that are respired and incorporated into organismal biomass, thereby increasing the potential for burial of these elements.

Conceptually identical, isotope tracer experiments were conducted on deep-sea, Arctic shelf and intertidal estuarine habitats. Different quantities and qualities of isotopically labelled (13C/15N) marine phytoplankton were added to the resident sediment communities and the subsequent fate of the constituent 13C and 15N were followed into the sediments, the biota and into the overlying seawater. My dual-isotope experiments were the first to concurrently describe the consumption of dissolved and particulate organic carbon, the production of inorganic carbon and nitrogen species and the uptake of C and N into macrofaunal and bacterial biomass. This represents a significant development in our capability to study the sources and sinks of >1 element in marine sediment ecosystems and allows us to examine how the availability of one element influences the fate of others. This is important because in reality multiple elements occur in biological compounds and hence their fate is interrelated.

The data arising from my experiments are broadly consistent with the original hypotheses, and demonstrate that the rates at which C and N cycle in marine sediments are dependent upon the quantity AND the quality of the resource being consumed i.e. there are interactions between the quantity and quality of available substrates. The observed interactive effects were not consistent across all of the studied ecosystems and apparently differed by temperature regime: Resource quality effects predominated in the cold-water, deep-sea (-0.7 deg. C) and Arctic (1.8 deg. C) environments i.e. C and N turnover rates were as follows: high-quantity + high quality > low-quantity + high-quality > high-quantity + low quality > low-quantity + low-quality. In contrast, resource quantity effects predominated in the milder (5.5 deg. C), estuarine sediments i.e. C and N turnover rates were as follows: high-quantity + high quality > high-quantity + low quality > low-quantity + high-quality > low-quantity + low-quality.

These findings indicate that any climate-driven changes in phytoplankton community composition, and hence the amount and biochemistry of organic matter reaching the seafloor, will influence the potential for burial of elements in the underlying sediments. They also demonstrate that elemental cycling in marine sediments cannot be predicted by the availability of resources alone, as currently assumed in many global biogeochemical models.
Exploitation Route This research will be useful to politicians and environmental campaigners alike to help justify measures to mitigate the effects of climate change. It will also be of use to policy makers and enviornmental managers for better understanding and managing how the functioning of marine sediment ecosystems are likely to change in the future. This research will be used to inform other academics, environmental managers, NGOs and the public about how carbon and nitrogen cycling in marine sediment ecosystems may change in the future. The methods developed and employed for this work will be helpful for other individuals that are interested in understanding if, how and why climate change affects the environmental fate of carbon and nitrogen.
Sectors Environment

Description Bart Veuger 
Organisation William Harvey Research Foundation
Country United Kingdom 
Sector Charity/Non Profit 
PI Contribution I conducted a series of expensive experiments to examine C and N cycling in contrasting marine sediment ecosystems.
Collaborator Contribution My collaborator analysed samples to determine the uptake of carbon and nitrogen into bacterial biomass.
Impact Many conference presentations. I am currently writing up the data for publication.
Start Year 2010
Description Bradley Eyre 
Organisation Southern Cross University
Country Australia 
Sector Academic/University 
PI Contribution I conducted a series of complex experiments to examine rates and pathways of carbon and nitrogen cycling in contrasting marine sediment ecosystems.
Collaborator Contribution The collaborator determined the production and release of dissolved organic carbon in my experiments.
Impact Numerous conference presentations. I am currently writing up the data for publication in a peer-reviewed journal.
Start Year 2010
Description Ellon Academy vist to Oceanlab (University of Aberdeen) 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Schools
Results and Impact An afternoon of talks for students from Ellon Academy, a local school.

Many of the visiting students have subsequently requested work experience at our research station.
Year(s) Of Engagement Activity 2012