Unveiling secrets of the Atlantic Ocean through ‘spectral fingerprints’ of phytoplankton

A new study reveals the optical properties, or ‘spectral fingerprints’, of phytoplankton in the ocean, providing a valuable dataset for remote-sensing scientists, marine ecologists, biogeochemical modelers, and climate scientists alike.?

PML’s Dr Tom Jordan , Earth Observation Scientist in Ocean Optics, has led a new study resulting in a unique and valuable dataset that combines the optical properties of Atlantic seawater and phytoplankton pigment concentrations.

The decadal dataset provides a new resource for scientists to ground-truth data from existing ocean color satellite sensors that detect markers such as the chlorophyll pigment found in phytoplankton. The dataset can also be used to develop new phytoplankton algorithms for next-generation satellites.???In turn, increased confidence in satellite observations of phytoplankton enables improved monitoring of ocean health and the global carbon cycle, which feeds into climate science.?

Phytoplankton are a crucial part of the marine ecosystem as they drive photosynthesis and play a significant role in absorbing carbon dioxide in seawater that is drawn down from the atmosphere and sequestering it to the deep ocean. In so doing, phytoplankton produces about 50% of the oxygen on Earth. Phytoplankton also form the base of the marine food web, providing food for life forms higher up the chain and its productivity are indicative of the wider health of our Ocean.?

Ocean colour data obtained by satellites helps monitor phytoplankton?by indicating their abundance through the presence of chlorophyll, a pigment that gives them a greenish hue and an identifiable ‘spectral fingerprint’. The greener the ocean appears, the higher the concentration of phytoplankton, allowing scientists to track plankton blooms and overall ocean health.?

The surface water data was collected over nine Atlantic Meridional Transect (AMT) research cruises between 2009 and 2019 on RRS Discovery, James Cook and James Clark Ross. Each cruise spanned about 40 days and collected data from 50°N to 50°S, crossing eight distinct ocean regions between the UK and the southern tip of South America, including the remote and rarely sampled Atlantic gyres.??

As it happens, Dr Jordan has just returned from the 31st Atlantic Meridional Transect cruise (AMT 31) on RRS James Cook, which travelled from Southampton to Montevideo (Uruguay)?on a five-week voyage. He describes the dataset – unique in its coverage across space and time – and the work involved:??

“The new dataset focuses on the inherent optical properties (IOPs) of seawater which describe how water and its constituents, such as phytoplankton, absorbs, scatters, and transmits light, enabling scientists to estimate phytoplankton pigment concentrations with precision. You could think of it as giving phytoplankton ‘spectral fingerprints’.”? ?

“The IOP data were collected using a semi-autonomous ‘flow-through’ system which was developed by former PML scientist Dr. Giorgio Dall’Olmo who now works at OGS, Italy. ?The systems’ continuous sampling enables rapid variability in phytoplankton abundance to be observed, such as sharp transitions across ocean fronts.”?

“We also compiled phytoplankton pigment concentrations, such as chlorophyll that is measured using High Performance Liquid Chromatography (HPLC), which reveal the types and abundance of these tiny oceanic plants.”?

The pigment analysis was carried out by expert laboratories at NASA Goddard (USA) and DHI (Denmark) and Dr Ruth Airs at PML, who are co-authors on the paper.?

On AMT 31, Dr Jordan was joined by PML colleague Dr Gavin Tilstone who was Principal Scientist on the cruise and a co-author of the paper. Dr Tilstone describes the science on-board in relation to this latest dataset:??

“AMT 31 has particular relevance for the optical data we collect, since one of the objectives of this cruise was to validate the recently launched NASA’s PACE (Plankton, Aerosol, Cloud, ocean Ecosystem) mission. The cruise crosses the Atlantic gyres, the ocean deserts that are dominated by pico-phytoplankton, some of the smallest plants on the planet, and the unique, hyperspectral optical data being collected on this AMT will be used to sea-truth this latest NASA mission. In addition, the past data that we have released will be used to develop algorithms for estimating the biomass of key phytoplankton groups, that will be transferable to PACE satellite data.”??

?“Different phytoplankton groups absorb CO2 at different rates. On AMT 31, both the seawater and atmospheric concentrations of CO2 were measured. So, the algorithms being developed by PML and the measured CO2 concentrations will potentially provide us with a better understanding of the global carbon cycle.”??

Tom added:??

“AMT 31 was my first time crossing the Atlantic on a research cruise, and seeing the data collection in real-time gave me a much deeper appreciation for the biodiversity and optical signatures of the ocean provinces. Having been run for 30 years, AMT is a truly unique scientific programme. It is only via the efforts of multiple scientists on multiple cruises that we can produce decadal-scale datasets such as in this paper.”??

Access the full paper: ‘A compilation of surface inherent optical properties and phytoplankton pigment concentrations from the Atlantic Meridional Transect’ >>??

The dataset is open-access, for more information please see: https://seabass.gsfc.nasa.gov/archive/PML/AMT/??

Other co-authors on the paper include colleagues from the National Institute of Oceanography and Applied Geophysics, the University of Exeter and CLS Group (Collecte Localisation Satellites) .??