Shedding Light on PACE
Secton IV: How Does the Ocean Interact With Light
Light Attenuation Through the Ocean
As we learned in the last section , light traveling through space – sunlight, starlight, light reflected off the moon's surface – contains a broader spectrum of wavelengths than the light that ultimately reaches Earth's surface. This reduction of certain wavelengths of light through the atmosphere is referred to as attenuation. Light attenuation also happens as light penetrates the ocean.
Similar to the atmosphere, light attenuation in the ocean is dependent on the concentration of "stuff" in the water. For example, light can shine all the water to the bottom of clear Caribbean waters, whereas you can barely see your hand beneath the surface in muddy river waters. Additionally, even in clear waters some wavelengths can penetrate further than others. For example, the red portion of sunlight is absorbed near the surface so only the blues are able to penetrate into deeper waters. But those can only go so far. Light is able to move much more readily through the atmosphere than the ocean. Under most conditions, only the faintest light reaches beyond the top 200 meters (650 feet) of the ocean, with waters deeper than 1 km (3300 feet) completely dark. To put this in perspective the light traveling from the top of the atmosphere to the Earth’s surface traveled 10,000 km. Read more about light in the ocean in this National Ocean Service article.
What's That Stuff in the Water?
If you spend time around an aquatic body, be it a pond, stream, lake, or ocean shoreline, you’ll observe it isn't the same color or clarity all the time: it changes over seasons, or perhaps after a heavy rain. A usually clear stream turns muddy after a heavy rain from all the sediment and debris washed downstream. A pond can grow a nice "crop" of algae in the hot summer months, giving it green tint. The scientific term for "stuff" that interacts with light is optically active constituents (OACs). OACs can interact with incoming light to give color to our watery worlds. They span a wide size range from molecules (e.g., H2O) all the way up to meters–long kelp plants anchored along shallow shorelines. PACE's Ocean Color Instrument (OCI) is designed to collect light reflected from water bodies, known as reflectance. OCI data will help scientists to answer the important question, "What's that stuff in the water?" based on calculations of color, size, and concentration of OACs. To focus on ocean color, we also need data from the two polarimeter sensors on PACE (SPEXone and HARP2) working in tandem with the OCI, to help subtract the light contributed by the atmosphere a calculation called an atmospheric correction. Read more about atmospheric correction in the Ocean Optics Book.
There are many ways to categorize OACs, but ocean scientists can group them in the following ways:
- Water contains H2O molecules plus inorganic dissolved materials such as salt and CO2, each of which interacts in various ways with sunlight. In clear ocean waters, the eﬀect of water on ocean color through both absorption and scattering of light in the visible range must be considered.
- Colored dissolved organic material (CDOM) are small organic matter molecules with light-reacting properties in the visible range. CDOM is often created from decaying detritus, or plant matter. Need a better visual? Scroll down to What Makes Your Tea Brown?
- Phytoplankton are microscopic organisms that live in the water column. Phytoplankton are mostly single-celled plants though some are also bacteria or protists. All phytoplankton photosynthesize, that is they have chlorophyll pigments that allow them to use the Sun's energy to fix carbon dioxide into organic matter.
- Non-algal particles (NAP) are those that do not contain phytoplankton pigments. NAP includes living and dead and decaying matter, bacteria, and viruses. Inorganic particles are another type of NAP, including mineral particles made by organisms such as shells and those derived from non-living sources such as clay, silt, and sand. Sadly, given the current state of the global ocean, we must add tiny particles of plastic to the list of NAPs.
- Bubbles are mainly atmospheric gases mixed into the upper surface waters by wave and wind action, or in much smaller quantities, produced by or exuded into the water by living organisms. Reach more about sea foam in this National Ocean Service article.
Want to know more? Check out the Ocean Optics Book: Optical Constituents
Measuring What's in the Ocean
Just as bananas have spectral signatures, so do OACs in the ocean. The spectral signatures of each of these constituents – for example, chlorophyll, CDOM, sediment – tells scientists about their Inherent Optical Properties, information that is critical to deciphering which elements are being observed by PACE.
Click on the boxes below to explore OACs in the ocean and their spectra.
Advanced Topic – Harmful Algal Blooms
Bloom or Bust? Phytoplankton make up the base of most ocean food webs, using the Sun's energy to power photosynthesis. When phytoplankton have access to enough nutrients and light, there can be exponential growth resulting in "blooms". Blooms of many algae fuel marine ecosystems, eventually supporting large fish populations and fisheries. However, the helpfulness versus harmfulness of blooms depends on the type of phytoplankton and where the bloom occurs. Instead of positively supporting to the natural food web, blooms may become harmful to other organisms in their immediate environment. Harmful Algal Blooms (HABs) occur when high concentrations of cells produce toxins or have other harmful effects such as depleting the oxygen needed by fish, shellfish, or marine mammals. Monitoring HABs is important as they may lead to human and marine wildlife sickness and mortality, contaminated aquaculture populations, and beach closures.
Friend or Foe? Many HAB species contain distinct color pigments which dominate water color, and have earned them colloquial colorful names: Green Slime, Red tides, Brown tides. But there are other optical indicators, as well. PACE's OCI data will help scientists differentiate among different types of phytoplankton, including helpful versus harmful species. HABs are hard to predict, so early detection is our next best tool. PACE's hyperspectral imaging capability combined with optical algorithms developed by the PACE community will take HAB detection to the next level by providing wide spatial coverage and quicker response times to mitigate HAB impacts.
What Makes Your Tea Brown?
The process of making a "cuppa" tea is familiar to many of us. Dried tea leaves are steeped in hot water to release aromatic smells and flavors, often turning the water a brown-ish color. As the tea leaves steep, they release "tannins", a type of CDOM with a brown color. Like steeped tea, organic matter can release CDOM into water bodies as it decomposes. PACE's OCI is designed to measure the light-altering capacity of CDOM from space, allowing ocean scientists to calculate how much is in water and where it travels on its watery journey.
Coastal Tea: The picture on the right shows variations in water color after a large rain storm in the coastal Chesapeake Bay (Eastern US). Look for clear blue water offshore (top of photo), and observe the contrast with shallower waters where there is a mix of light and dark brown waters. The lighter brown colors are suspended sediments churned up during the storm. The darker brown colors reveal waters full of CDOM that flowed into the Bay from a nearby marsh during the rain storm. Marsh tea, anyone?