Ocean optics illuminates aquatic algae

Traditionally, microalgal particles are collected in the field and then analyzed under a microscope to characterize their size, shape, morphology, and taxa or species.³ Field collection techniques have been extended to flow cytometry, in which pictures of microalgal cells suspended in a flow tube are captured on a camera and analyzed by either human analysts or computer programs. Recently, DNA barcoding has also been used to identify microalgal taxa and their living environment. Although the techniques are relatively accurate, they rely on physically handling the samples. Optical instruments, on the other hand, measure the microalgal particles through nontouch techniques.

The history of optical measurements of the ocean dates back to the initial efforts in the late 19th and early 20th centuries to understand the color of the ocean and how sunlight penetrates it. The light penetration, or water transparency, is determined by the depth at which a submerged Secchi disk—divided into quadrants and colored white and black—is no longer visible to the human eye. That is called the Secchi depth.⁴ While relatively accurate, the handheld instruments measure only one property of the ocean water. To describe other optical properties, researchers needed to develop digital instruments that can accurately measure them.

Such requirements drove advances in ocean optics: the study of how light interacts with water and its constituents and how such interactions can be measured and interpreted. It employs radiative transfer theory, based on physical principles, to describe the optical properties of the ocean and its constituents and how light interacts with the atmospheric gases and particles (that is, aerosols).^5,6 A combination of physics and oceanography, ocean optics made its formal debut as an interdisciplinary science between the 1950s and the 1980s, when its pioneers systematically studied how natural light is absorbed, scattered, and fluoresced by marine particles and dissolved matter^7,8 (see figure 2).

Advances in ocean optics have led to new methods for measuring various microalgal particles. Early on, the optical properties (absorption and scattering) of microalgae and non-algae particles were measured in both the field and the laboratory using shining tubes, filter pads, and other instrumentation.⁹ For example, the concentrations of chlorophyll a—the primary pigment used in photosynthesis—and other pigments were determined using fluorometers because once the microalgae absorb light at one wavelength, they can emit light at a longer wavelength.

Today, more sophisticated instruments and methods are available to measure all those properties. One such way is through high-performance liquid chromatography (HPLC). Different types of microalgae have different pigment compositions,¹⁰ and each pigment has its own absorption spectrum, which is known a priori. The total pigment absorption spectrum can thus be decomposed into individual pigment absorption spectra from which the type and size of the microalgae can be estimated. For example, the freshwater cyanobacterium Microcystis aeruginosa (responsible for most freshwater HABs) can often be identified through its phycocyanin pigment, which contributes to its blue-green color, and the saltwater cyanobacterium Trichodesmium sp. can be identified through its phycourobilin, phycoerythrobilin, and phycocyanin pigments.

Alternatively, the ratio of microalgal backscattering to absorption can be used as an index to separate otherwise similar microalgae. That technique was employed to differentiate the toxic dinoflagellate Karenia brevis (responsible for most HABs producing red tides in the Gulf of Mexico) from diatoms because the former has a lower backscattering efficiency.¹¹ More importantly, surface spectral reflectance data from satellites have become a critical tool because inversion algorithms allow for the determination of absorption and scattering spectra.

Optical techniques are also used to measure macroalgae. For the case of the Atlantic pelagic Sargassum, most field and laboratory efforts rely on eye sighting, sample collection using a net,¹² and measuring the sample’s pigment composition and chemical composition—carbon, nitrogen, phosphorus, arsenic, and more—using HPLC and other techniques. Optical measurements of Sargassum did not start until the 2010s.¹³ 

Because Sargassum on the surface forms individual clumps (see figures 1a and 1b) and multiclump mats, its reflectance is measured in the field in order to establish a relationship with its surface biomass density, or weight per area. The area of a Sargassum mat is determined by comparing its full size against a reference quadrat (see figure 3a) before collecting the marked-off Sargassum in a large net. It is weighed both wet, after removing the ambient water, and dry, after drying the sample in an oven. Around 80% of the wet weight is from the water alone.

The reflectance of the Sargassum mat is measured using a spectrometer with a fiber-optic probe, which collects the sunlight reflected in the quadrat. Figure 3b presents several sample reflectance spectra of Sargassum mats. The visible wavelength range 400–700 nm has a local reflectance maximum around 600–640 nm, which is why Sargassum appears yellowish or brownish. The local minimum at about 620 nm is caused by light absorption by chlorophyll c pigments, and the local minimum at around 670 nm is caused by light absorption by chlorophyll a pigments. In the near-IR wavelengths of 700–900 nm, the sharp increase is often called red-edge reflectance, which is typical for both terrestrial and large marine plants.

In addition to the reflectance and biomass density measurements in the field, Sargassum samples collected from the quadrat are also analyzed for their pigment composition and elemental concentrations of carbon, nitrogen, and phosphorus, among others.¹³ Because the makeup of the samples is measured with the Sargassum reflectance, which can also be measured from satellites, field data are essential for validating satellite observations. Using data based on field and laboratory measurements, researchers develop algorithms that they apply to space-based measurements in order to estimate Sargassum biomass density and other properties.

With new ocean color remote sensing techniques, along with advances in theory, instrumentation, and algorithm development, scientists can not only measure and characterize the underwater light field but also observe global oceans and lakes from space. The satellite-detected signals are used to estimate surface spectral reflectance of the ocean through a process called atmospheric correction⁶ (see figure 2). Reflectance carries information on pigment concentration, absorption, scattering, fluorescence, type, and size distribution of microalgal particles and on macroalgal biomass density and other properties. To derive those secondary properties, researchers rely on well-designed inversion algorithms developed from field and laboratory measurements. Because the whole process depends on interpreting the ocean’s spectral reflectance, or color, it is also called ocean color remote sensing. (The same terminology is also used for freshwater lakes.)

The history of ocean color remote sensing goes back to the pioneering works of many individuals.¹⁴ Since 1978, when the proof-of-concept Coastal Zone Color Scanner was one of eight instruments aboard the Nimbus-7 spacecraft, modern satellite sensors specifically designed to measure reflected sunlight have been equipped with more spectral bands, better signal-to-noise ratios, and finer spatial resolutions. Recent missions also allow for more frequent revisits to the same area. The satellite-derived surface reflectance measurements produce unprecedented data to study not only light penetration in the ocean but also, through the use of inversion algorithms, properties of microalgal particles and macroalgal mats.

Indeed, since the 1980s the number of publications that use the phrases “ocean optics” and “ocean color” has grown exponentially. Global distribution maps of microalgal particles that show concentration, absorption, scattering, type, size, and other variables have been made available by various space agencies (including NASA, at https://oceancolor.gsfc.nasa.gov). Those data products have been used by the research community to not only quantify chlorophyll a and primary production but also differentiate different phytoplankton functional groups (see figure 4), monitor HABs, and understand their physiological response to the changing ocean environment. For example, US federal and state agencies have applied tailored algorithms to multisensor ocean color data to generate and distribute HAB bulletins for HABs in both oceans and lakes.

Ocean color remote sensing is now a crucial tool for studying not only microalgae but also macroalgae. Among the many types of macroalgae, the Atlantic pelagic Sargassum represents a special case because the recurrent great Atlantic Sargassum belt (GASB)¹⁵ represents an emerging phenomenon that has significant implications for research, the environment, tourism, and the economy.

The relationship between Sargassum’s reflectance and biomass density—established from field measurements—can be applied to satellite-measured reflectance in each valid image pixel to map its biomass density from space. In that process, contributions from the atmosphere and other perturbation factors are removed (see figure 2), and radiative transfer equations estimate how such removals may modulate the functional relationship. Cloud cover and other factors frequently lead to data gaps in images. But gaps in a given location during a certain period can be filled by calculating an average Sargassum biomass density from all valid pixels in the images of preceding and subsequent days.

The distribution map of Sargassum density for the month of June 2021 is shown in figure 5, where a continuous Sargassum-heavy “belt,” the GASB, is observed from western Africa to the Gulf of Mexico. The water area in the belt is about 5 × 10⁶ km², and the Sargassum mats scattered around would occupy a surface area of about 6000 km² if aggregated together, with an estimated total wet biomass of 18 million metric tons.

The discovery of the GASB, which has been recurring annually since 2011, resulted from advances in ocean optics and ocean color remote sensing. Traditionally, Sargassum was known to be abundant in the Sargasso Sea—a subocean in the North Atlantic named after the macroalgae—but satellite data show much higher abundances of the macroalgae in the tropical Atlantic in recent years. Researchers are hard at work studying the GASB, including how it was formed, whether it indicates a regime shift resulting from a changing climate, how it affects local biology and ecology, how it may change carbon cycling, and whether it can be used to sink carbon to the ocean floor.

Sargassum is a critical habitat for ocean life, but excessive amounts of it in coastal waters and on beaches have caused tremendous problems because dead Sargassum can sink to the bottom and smother corals and seagrasses. Rotten Sargassum on beaches can destroy turtle nests, attract insects, grow and spread bacteria, and cause respiratory problems in humans. All of those issues adversely affect local tourism and the economy, turning Sargassum from a critical habitat to a beach nuisance.

Countering Sargassum’s negatives will require more research efforts in understanding its biology and forecasting its blooms and in developing new methods to mitigate its effects and make it into useful products, such as fertilizers. Ocean optics and ocean color remote sensing are expected to continue playing an essential role in all those aspects. Improved understanding of the Sargassum biomass, growth rate, and changing patterns will provide more accurate data for forecasting models and for improved monitoring and tracking capacity through the application of artificial intelligence.¹⁶ 

Globally, Sargassum is not the only macroalgae that can be detected, characterized, and quantified using optical means. Other types, either floating on the water surface or growing on the shallow bottom, have also been reported worldwide. They include Ulva prolifera, a green macroalgae, in the Yellow Sea; Sargassum horneri, a brown macroalgae that grows on the shallow ocean floor but can be detached to float on the surface, in the East China Sea¹⁷ (see figure 4); giant kelp in California (see the opening image); and seagrass around the world’s coastal oceans. In each case, the macroalgae can be studied both in the field and from space, based on the same optical principles applied to the Atlantic Sargassum. With more research dedicated to macroalgae, their roles in carbon science and other Earth-science disciplines will be better recognized and quantified.

After a century of development, ocean optics continues to greatly affect technology and science. The theory of underwater visibility has been revised from relying on assumptions to enabling direct estimates of water clarity. New sensors have been invented to measure variable fluorescence. New flow cytometers have been created to analyze thousands of microalgal cells per second. Those and other developments allow for more accurate assessments of phytoplankton physiology, and some sensors have been customized to be mounted on autonomous platforms, such as ocean gliders and the floats used by the international Argo program to collect ocean data.

The sensors include direct and derivative measurements of light scattering, absorption, fluorescence, attenuation, and reflectance—thus making it possible to characterize the underwater light environment and to automatically estimate microalgae and other ocean constituents. For example, as it drifts with ocean currents, each Argo float cycles between the surface and depths of 2000 m every 10 days, collecting a depth-dependent profile of ocean properties through its mounted sensors. There are 4000-plus Argo floats in the world’s oceans, so such bio-optical data will provide an unprecedented capability for mapping microalgae and other ocean properties in three dimensions, developing remote-sensing algorithms, and validating remote-sensing data products.

Those developments also support new polar-orbiting and geostationary satellite missions that use detectors that combine spectroscopy with imaging, polarimetry, and active techniques, such as lidar. Planned efforts include NASA’s forthcoming Plankton, Aerosol, Cloud, Ocean Ecosystem (PACE) mission; NASA’s Geostationary Littoral Imaging and Monitoring Radiometer (GLIMR), currently being developed; and NOAA’s future Geostationary Extended Observations (GeoXO) satellite system.

Of particular importance are the emerging constellations of easy-to-produce small satellites known as CubeSats that can measure land and coastal waters at a spatial resolution of just a few meters with near-daily revisits around the globe. Such a capacity has never been possible until recently because traditional satellites could not address the simultaneous requirements of high spatial and high temporal resolutions needed for studying the fast-changing microalgae and macroalgae with fine details. Indeed, the PlanetScope constellation of more than 200 CubeSats already has shown a much-improved capacity to track macroalgae near beaches.¹⁸ 

Computer artificial intelligence has found increased applications in field and laboratory measurements and optical remote sensing, often with more robust algorithms and data products than what’s offered by traditional methods. All the advances will continue to enhance our capability to characterize, quantify, and understand both microalgae and macroalgae in the field and from space.

Chuanmin Hu is a professor of oceanography at the University of South Florida in St Petersburg. He has over 30 years of experience in ocean optics and ocean color remote sensing.