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Ocean Limiting Nutrients – Examination from Space

Periodic Reporting for period 1 - OceanLiNES (Ocean Limiting Nutrients – Examination from Space)

Reporting period: 2015-05-01 to 2017-04-30

Oceanic phytoplankton are the base of the marine food web and contribute significantly to removal of carbon from the atmosphere. Monitoring how their distribution and activity are changing into the future is therefore a major scientific goal. It is widely recognised that nutrient availability is a critical factor regulating phytoplankton distribution in the global ocean, but exactly which nutrients are limiting phytoplankton in different regions is less well resolved. A potential route for synoptic monitoring of phytoplankton nutrient limitation could lie in observations of phytoplankton fluorescence made by sensors on earth orbiting satellites. Prior work—both in the field and laboratory—has repeatedly shown that phytoplankton living under conditions with insufficient availability of the ‘micronutrient’ iron emit significantly more fluorescence per unit biomass than those living under growth limitation by ‘macronutrients’, such as nitrate or phosphate. Therefore, synoptic observations of phytoplankton fluorescence made by sensors on earth orbiting satellites represents a potential approach for distinguishing regions of iron-limited phytoplankton growth from those limited by nitrate or phosphate. This would allow for robust descriptions of where, and how much, of the ocean is currently limited by iron or nitrate/phosphate. Looking to the future, this would allow for a low cost means for monitoring how these distributions are changing with time. A major factor confounding direct application of available satellite fluorescence data lies in correcting the fluorescence signal received by satellite sensors for additional, poorly resolved physiological mechanisms employed by phytoplankton. In particular, down-regulation of fluorescence emission from phytoplankton under different levels of incident irradiance at the instant the fluorescence is emitted/detected by the satellite sensor occurs, and is not the same between phytoplankton communities. The main aim of this project was to perform measurements and experiments throughout the global ocean to determine (i) if there were large regional variations in down-regulation of phytoplankton fluorescence with increasing incident irradiance, and (ii) if the environmental or biological factors that might be driving this. Such understanding might then be used to correct satellite fluorescence signals to reveal patterns of iron-stressed phytoplankton growth at a global scale.
To achieve the goals set out above, three oceanographic research cruises were conducted to collect samples and perform experiments to evaluate down regulation of phytoplankton fluorescence under variable incident irradiance and elucidate potential controlling factors. These were located in (i) the tropical North Atlantic, (ii) the equatorial Pacific and the Peruvian upwelling zone, and (iii) the South Atlantic (see blue dots in attached image). Collectively these regions cover multiple oceanographic regimes, with (i) amounts of phytoplankton in seawater varying by three orders of magnitude, (ii) containing different types phytoplankton, and (iii) host to phytoplankton under growth limitation by different nutrients. Using a state-of-the-art fluorometer, experiments were conducted to evaluate the magnitude of fluorescence reductions phytoplankton exhibit upon exposure to progressively increasing irradiance (or ‘non-photochemical quenching’; NPQ). Alongside these experiments, samples were collected to determine how much phytoplankton biomass was present, what types of phytoplankton were present, the concentrations of nutrients (e.g. nitrate and phosphate) and trace elements (e.g. iron), alongside measurements of ocean physics (e.g. how deep phytoplankton in surface waters were being mixed down to). A suite of longer duration experiments directly tested which nutrients were limiting phytoplankton growth. Collectively, thousands of samples were collected on research cruises and were subsequently analysed in GEOMAR Centre for Ocean Research, Kiel (Germany) and Dalhousie University (Canada). Although the experimental work in this project has suggested that an important initial aim of the proposal, to develop a robust NPQ correction to apply to satellite fluorescence data, has considerably greater complexity than previously envisaged, the new data have led to significant advances in understanding processes of nutrient limitation and the complexities of using phytoplankton fluorescence to infer nutrient limitation. The advances have been presented at international conferences and published in international peer reviewed journals (or are under review/in preparation for publication).
This project has furthered knowledge of phytoplankton nutrient limitation in several ways. Firstly, work conducted in the tropical North Atlantic has revealed how iron availability can control microbial access to the major nutrient phosphorus, through its requirement in alkaline phosphatase enzymes (enzymes that are used to hydrolyse organic phosphorus molecules). This new finding is significant, as it has elucidated a direct biochemical linkage between cycles of iron and phosphorus in the ocean, which could be important for predicting phytoplankton responses to the projected changing input of these nutrients in the future. Secondly, the work conducted in the South Atlantic has demonstrated how phytoplankton communities in broad regions of the surface ocean can be living under conditions of simultaneous nutrient co-limitation (where the availabilities of multiple nutrients are restricting phytoplankton growth at the same time). This finding is significant as it demonstrates that co-limitation is prevalent in the ocean, which has not been shown previously, and that this needs including in biogeochemical models that are used for projecting the impacts of climate change. Thirdly, the suite of experiments conducted has demonstrated how poorly studied nutrients such as cobalt and vitamin B12 can play a role limiting phytoplankton growth in the ocean. Forth, the experiments have demonstrated how reductions in phytoplankton fluorescence with increasing irradiance appear to be moderated significantly by iron availability. This last finding is important as it indicates significant additional complexity in developing a robust correction for phytoplankton NPQ (i.e. without firstly knowing the iron stress status of the phytoplankton community). This could represent a difficult obstacle to overcome in evaluating detailed patterns of iron stress from existing space-based passive sunlight induced fluorescence measurements (e.g. fluorescence line height products available from MODIS and MERIS). However, planned future sensors on satellites that will have the capacity for measuring phytoplankton fluorescence at multiple wavelengths, alongside enhanced sensitivity for fluorescence detection from low chlorophyll waters, could provide a route for overcoming this obstacle (e.g. the planned NASA PACE mission; see: When such data become available (estimated year 2023), the field-based observations collected in this study will provide crucial ground-level phytoplankton physiological information needed evaluate the signals carried in such data.
Cruise locations for the OceaLiNES project (blue dots are sampling stations)