Physical and chemical forcing of diazotrophy in the (sub)-tropical Atlantic Ocean
Lead Research Organisation:
University of East Anglia
Department Name: Environmental Sciences
Abstract
The oceans play a central role in the global carbon cycle, and have taken up ca. 30-40% of the anthropogenically produced CO2. It has long been known that ocean biota play a major role in sequestering CO2 on very long time scales (>1000 y). Recent evidence also suggests that the ocean biota play an important role on shorter time scales (10-100 y). The balance between phytoplankton photosynthesis and community respiration determines the ability of the oceans to take up CO2. Nitrogen (N) is generally considered to be the nutrient that limits phytoplankton photosynthesis. However, it is unclear what controls the amount of N in the ocean. Unlike most phytoplankton, which are N-limited, N2 fixing cyanobacteria (diazotrophs) have an unlimited supply of N in the form of N2 gas. N2-fixers play a significant role in ocean nutrient and biogeochemical cycles as they are a major source of N, providing N for up to 50% of primary productivity in nutrient poor oceanic regions. N2 fixation is a key process that modulates the ability of the oceans to sequester CO2 on time scales of 10 to 10,000 y. Limitation of N2 fixation results in lowered N availability for other primary producers reducing the potential of oceans to sequester carbon. Whilst the colony forming Trichodesmium is considered the most important oceanic diazotroph, recently a range of new diazotrophs have been discovered in the ocean. This brings us to the questions of 'what constrains the amount of N2 fixation in the ocean?', and 'what determines the species distribution of diazotrophs in the ocean?' Iron appears to be the key environmental factors constraining N2 fixation based on a recently observed direct link between Fe and N2 fixation in the Atlantic, with Fe determining surface ocean P cycling. The goal of this project is to investigate quantitatively the link between iron supply and N2 fixation in the Atlantic, and for this it is essential to understand the importance and strengths of various iron sources. The iron sources are considered to be atmospheric dust deposition and low oxygen shelf sediments in the NW African upwelling region. The strengths of these sources are expected to change in future with changes in dust deposition and expansion of the oxygen minimum zones in the oceans. Identification and quantification of the sources is hence key to undertake model estimates of N2 fixation under future climate scenarios. This proposal will relate the supply and biogeochemical cycling of Fe and P to N2 fixation and the community structure of diazotrophs in the (sub)-tropical Atlantic Ocean. We will undertake this research using a combination of ship-board observations and radiotracer uptake experiments, and modeling activities involving nutrients and Fe. We will quantify sources of these elements for the diazotroph community from the atmosphere and ocean circulation, and by use of chemical source tracers. We will link the supply of nutrients and Fe to the activity and species distribution of diazotrophs. Molecular techniques will be used to determine the different diazotrophs in the study region. We will undertake the work on a dedicated cruise in the (sub)-tropical Atlantic which involves east-west transect from the African shelf to characterise the trace element gradient in the oxygen minimum zone and thus the potential for lateral advection from the shelf. The cruise will also traverse the dust/redox plumes in the study region and characterize the horizontal trace element gradients along the edges of the dust/redox plumes. We will sample the common diazotroph Trichodesmium and study its uptake of Fe and P using radiotracers. We will use a circulation model to provide a large scale context for the programme, with sources and cycling of nutrients and Fe adapted according to the observational studies. This research will ultimately assist with oceanographic studies on nutrient cycling and modeling with a view on the future importance of the oceans as C sink.
Organisations
Publications
Jickells TD
(2016)
Atmospheric transport of trace elements and nutrients to the oceans.
in Philosophical transactions. Series A, Mathematical, physical, and engineering sciences
Baker A
(2010)
Estimation of atmospheric nutrient inputs to the Atlantic Ocean from 50°N to 50°S based on large-scale field sampling: Fixed nitrogen and dry deposition of phosphorus
in Global Biogeochemical Cycles
Powell C
(2015)
Estimation of the Atmospheric Flux of Nutrients and Trace Metals to the Eastern Tropical North Atlantic Ocean*
in Journal of the Atmospheric Sciences
Ussher S
(2013)
Impact of atmospheric deposition on the contrasting iron biogeochemistry of the North and South Atlantic Ocean
in Global Biogeochemical Cycles
Milne A
(2017)
Particulate phases are key in controlling dissolved iron concentrations in the (sub)tropical North Atlantic
in Geophysical Research Letters
Bridgestock L
(2016)
Return of naturally sourced Pb to Atlantic surface waters.
in Nature communications
Ducklow HW
(2018)
Spring-summer net community production, new production, particle export and related water column biogeochemical processes in the marginal sea ice zone of the Western Antarctic Peninsula 2012-2014.
in Philosophical transactions. Series A, Mathematical, physical, and engineering sciences
Bridgestock L
(2017)
The Cd isotope composition of atmospheric aerosols from the Tropical Atlantic Ocean
in Geophysical Research Letters
Packman H
(2022)
Tracing natural and anthropogenic sources of aerosols to the Atlantic Ocean using Zn and Cu isotopes
in Chemical Geology
Description | atmospheric deposition important to ocean biogeochemistry |
Exploitation Route | continued research particularly on potential effects of climate change on the atmospheric deposition |
Sectors | Environment |
Description | To help understand the controls on ocean primary production and carbon uptake |
First Year Of Impact | 2012 |
Sector | Environment |
Impact Types | Policy & public services |