Light is a ubiquitous physical phenomenon that we experience everyday on this planet. Undoubtedly, vast majority of the light we receive is from the Sun. For centuries, natural scientists and physicists were fascinated by the “birth” of light as well as its path to Earth; but it was not until recent decades, when the scientific community began to systematically study the “death” of light and recognize its significance. Light does not simply “die,” its energy is usually redirected or conserved in another form. The redirection of light in the atmosphere gives the sky its blue color, while the absorption of red light by the ocean allows blue light to penetrate relatively deeper in the water column.
Light in a marine environment is usually diminished in two ways: absorption and scattering. This concept can be easily demonstrated with an old-fashioned projector that uses transparencies to project images. Now imagine that I placed two clear glass dishes containing mystery liquids on the projector. As an audience, you would see the projected image were two shadows of the dishes’ shape; these shadows indicate that the two mystery liquids completely and equally diminish light from the projector. However, if I were to invite you to look directly at the two dishes, you would see one is filled with milk, and the other is filled with fountain-pen ink. Even though these two kinds of liquids give light the same fate, the kind of particles in each liquid is evidently very different. Milk scatters more light than it absorbs which is why it appears white; while ink absorbs more light than it scatters, so it appears to have a dark color. While this simple demonstration presents the unique optical properties of two kinds of homogeneous solutions, optical phenomena in the ocean are far more complex.
Absorption and scattering in the ocean depend heavily on water and the small particles residing in it. These two processes are collectively termed as “inherent optical properties” (IOPs) – which indicates that the ability for water and small particles to interact with light is “inherent” and is not dictated by ambient light. This is why many IOP instruments have their own light source and can be deployed in the dark. Light is typically absorbed by the following five components in the ocean: water, phytoplankton, non-algal particles (NAP), and colored dissolved organic matter (CDOM). Phytoplankton are unicellular plant organisms living in the ocean. They function in a marine ecosystem like their terrestrial counterparts on land. Their unique pigments for conducting photosynthesis have a significant absorption of light; the effect is more noticeable during a period of intense phytoplankton growth (aka. “bloom”). Non-algal particles are materials that are not alive and are not dissolved. NAPs often include cell walls of phytoplankton, detritus waste from organisms, as well as suspended minerals and sediments. Isolated NAPs often appear translucent or even transparent; however they can aggregate and become large particles after the end of a bloom when a massive amount of waste materials is readily available; therefore they can be significant sources for light absorption. Colored dissolved organic matter (CDOM) is the most mysterious among the four components that absorb light in the ocean. CDOM is in a dissolved phase, and so it is very hard to be captured with conventional filtration methods and without contamination. No one has a comprehensive understanding of CDOM even in the present day and, for decades, optical oceanographers simply referred to it as “gelbstoff” or “yellow matter.” This is because measuring this material is extremely difficult, and in addition the main components of CDOM differ significantly by region. The effect of CDOM is mostly pronounced in cases of “dead” lakes or ponds after a toxic algal bloom; in these scenarios, the aquatic environment is largely lifeless and the water has a hint of dark hue, which indicates light absorption. Light in the ocean is not only diminished by absorption; it can also be redirected by scattering. Size, concentration, and the intrinsic “texture” of particles can significantly contribute and influence scattering. These IOPs are elemental components that dictate the amount and intensity of light leaving the ocean-air interface. The outgoing light signal is usually computed as a ratio among the IOPs, and these ratios are commonly referred to as “apparent optical properties” (AOPs). AOPs are relatively easier to measure in comparison to IOPs; it gives rise to optical signals that can be detected by remote radiometers (a fancy light meter in the sky).
At this point, if it was a conversation, most people would begin to ask: ‘why is this important?’ And ‘why should we measure optics in Antarctica?” The answers to these questions would require an explanation of two orders – the first order concerns with biology and ecology, while the second order is operational and procedural. First, though only accounting for < 1% of the plant/algal biomass on Earth, phytoplankton produce approximately 50%—70% of the oxygen content. In order to grow and produce oxygen, phytoplankton needs light and carbon dioxide for photosynthesis. A careful examination of this bio-optical process can reveal how photosynthesis functions in the marine environment. Because phytoplankton uptake carbon when they conduct photosynthesis, they can also act as a biological pump and sink atmospheric carbon to great depths. The Southern Ocean, surrounding the continent of Antarctica, is the largest carbon sink in the world’s ocean both during the last ice age and in present-day. The western Antarctica Peninsula (wAP), where FjordEco field campaign took place, is one the most productive region among the Southern Ocean. The dark winters in the polar region makes light availability one of the limiting factors controlling phytoplankton growth. In addition, phytoplankton is a primary producer and it is the first link in the marine food web that plays a key role in oceanic ecology and ecosystem.
Furthermore, optical measurements are often very dynamic, and they would usually yield more information than what the researchers are seeking. This would allow a great opportunity to develop new proxies for studying specific oceanographic regions. For an example, using absorption data at 676nm of a scanned spectrum, we could derive chlorophyll concentration and refer pigment-based biomass; another example would be the relatively robust relationship between particle backscattering and particulate organic concentration in the water column. While the interpretation of many optical measurements is difficult and still has room for improvement, it undoubtedly provides valuable information for a holistic investigation of natural phenomena and ecosystems.
Moreover, the National Aeronautics and Space Administration (NASA) has been launching a suite of Earth observing satellites in the last two decades. Many of these missions are aimed to understanding oceanic processes. Passive sensors, such as the Moderate-Resolution Imaging Spectroradiometer (MODIS) onboard of the Aqua satellite, are capable of detecting ocean color signals (which are computed from water-leaving AOPs); since these AOPs rely heavily on IOPs and ambient light condition, they can be utilized to measure surface chlorophyll concentrations. This realization is perhaps one of the greatest technological advancements in the history of ocean science. For centuries, prior to remote measurements of ocean color, marine scientists only covered less than 10 percent of the entire world’s surface ocean. Since the launch of the first ocean color sensor, the Coastal Zone Color Scanner (CZCS), surface chlorophyll concentration data coverage around the globe has been extended to twice per day. This kind of data acquisition covering a large spatiotemporal scale was unprecedented, and it was only achieved through the understanding of fundamental optical components. However, it has been known that these satellite sensors are not well calibrated in the high latitudes; this is due to both the challenges of fieldwork and light availability/angles in these extreme environments. A great effort in ocean color algorithm validation and correction is urgently needed in the polar regions. NASA proposed to launch a next generation of ocean color sensor by 2020. The sensor is called “Pre-Aerosol, Clouds, and Ecosystem” (PACE) and it is “hyperspectral” – this means the detected signals will only have 5 to 10 nm intervals, and allow the data to have a finer spectral structure and shape. These information are crucial to understanding ocean biology and ecosystems.
Figure 1. Phytoplankton from Andvord Bay during the FjordEco Research Expedition. Photo by Maria Stenzel.
Figure 3. NASA’s Aqua satellite, which carries the MODIS sensor on board. It can detect phytoplankton pigment concentration from space.
Figure 2. Jack deploys HOBI HS-6, an optical instrument that measures backscattering of the water column.