From: Quarterdeck Volume 5, Number 1, Spring 1997
Visibility in the ocean and the effects of mixing
Wilford D. Gardner
How far can you see under water? Travel agents would have you believe you can see forever in the warm waters of the Caribbean. Alas, it isn't true. We can't see as far under water as in the air. Why not?
Light is attenuated much more by water than by air, limiting the distance we can see. Attenuation is caused by a combination of light being scattered and absorbed by water, particles, and dissolved organic matter.
Although dissolved organic matter is abundant in the ocean, most of it is colorless and causes little attenuation of light. The amount of attenuation by water is the same whether the water is fresh or salty. Therefore, it is the type and abundance of particles that control visibility through water in one location versus another.
Light attenuation in the ocean is measured with transmissometers and a-c meters. (See "Tools of the Trade" for information about instruments discussed in this article.) The research we are doing with colleagues and graduate students at Texas A&M University and several other institutions explains the causes of attenuation and investigates its effects on visibility in the ocean.
So who cares about how far you can see under water other than scuba divers, snorkelers, travel agents, and treasure hunters? Remember the Gulf War in 1991? The United States Navy had ships ready to launch an attack from the Persian Gulf against the Iraqis in Kuwait, but it required that personnel carriers traverse coastal waters and the beach, and these areas were known to be littered with explosive mines. The only way to detect some mines is to see them. Therefore, it is vitally important to know how far you can see underwater and what sort of atmospheric and oceanographic processes and conditions can change the visibility in seawater. For this purpose, the Navy has funded projects to study coastal mixing and underwater visibility.
To answer these questions we first need to consider where the particles in sea-water come from and how fast they settle to the ocean floor. Are all the particles pollutants? By no means. In coastal areas, rivers dump tons of sediment and dissolved nutrients into the sea. As the rivers enter the ocean, coarser materials quickly settle out near the mouth since there is a rapid decrease in the turbulent mixing that keeps particles in suspension. These sediments fill up estuaries and create deltas suchas the huge delta of the Mississippi River.
Finer materials drift farther from the river mouth until they are eaten and incorporated into fecal pellets, or react with salt water and organic matter to form large aggregates (clumps of particles), which settle faster than tiny, individual particles. Without aggregation, small particles could take a century to settle to the bottom of the ocean far from land, whereas large aggregates or fecal pellets could travel 5000 meters to the seafloor in a month or two.
See "As clear as mud: Particles where you least expect them" to see particles from different water sources.
The huge sediment input from the Mississippi and other rivers that empty into the Gulf of Mexico is one reason that the water along the Texas coast is much murkier than in the Caribbean. However, you only have to go a few miles from the coast to see clearer, greener surface water.
A quick look through a microscope at a sample of the greenwater would reveal very little riverine sediment, but you would see thousandsof tiny marine plants (phytoplankton) and animals (zooplankton). These organisms thrive in water that has been fertilized by nutrients brought in from riversor mixed up from deeper waters. The green color comes from the chlorophyllin the phytoplankton, as these phytoplankton are busy converting carbondioxide, nutrients, and sunlight to plant material and oxygen through the process of photosynthesis. The abundance of phytoplankton can be estimated by the amount they fluoresce when a flash of light hits them using a fluorometer.
When phytoplankton die or, more likely, are eaten, theyare usually incorporated into a fecal pellet or an aggregation of sticky particles and slowly sink. They take with them the nutrients that they used to make plant parts. As the phytoplankton settle on the sea-floor, they decompose and turn back into nutrients, carbon dioxide, and dissolved organic matter. On the continental shelf, where water ranges from zero to about 100 meters in depth, storms or winter cooling and mixing can return the nutrients to surface waters, and phytoplankton and the rest of the foodchain can thrive again.
If we are concerned about seeing through seawater, we are concerned not only with the number of particles but also the type of particles present. Clay and sand absorb and scatter light differently than phytoplankton.Clay particles are tiny jagged-edged plates a few thousandths of a millimeter across, while phytoplankton are somewhat larger spheres, tubes, and other shapes that are designed to absorb light for the purpose of photosynthesis.
Last fall our group joined researchers from several other institutions around the country in the first phase of an intensive studyof a patch of shelf water south of Cape Cod, Massachusetts. The area was far enough from any rivers that most of the particles in the upper water column were biological in origin.
The purpose of our study was to measure all the physical conditions that cause mixing of the ocean-including wind, waves, currents, tides, storms, cooling, and heating-and simultaneously measure the visibility through seawater at different wavelengths. We also collected and analyzed the particles in the water that affect visibility.
Visibility is quantified by using sensitive instruments that measure the optical properties of seawater-especially scattering, absorption, and attenuation of light-at several wavelengths across the visible and even the infrared parts of the spectrum. Physical and optical conditions were monitored by deploying highly instrumented moorings to measure meteorological conditions like air temperature, wind speed, humidity, and sunlight; and water conditions such as currents, temperature, salinity, wave height, turbulence, absorption and attenuation at one to nine wavelengths, chlorophyll abundance, and particle-size distributions. The moorings were also fitted with traps to collect samples of settling particles.
During a three-week period, one ship, the R/V Endeavor , steamed in small and large box-shaped paths around the site. She towed an instrument that descended and ascended between the surface and the seafloor every five minutes to characterize the spatial variability of temperature, salinity, and optical properties in the region.
Our ship, R/V Seward Johnson , generally stayed near the moorings and made careful measurements through the whole water column of the same parameters mentioned above.
Our Particle and Optics Profiling System (POPS) measured the abundance and size of particles and aggregates in the water from one micron to several millimeters long. In addition, we collected water samples to analyze the size and composition of particles that affect the optical properties of the water. This information will help to calibrate and interpret the optical signals from the moored, towed, and profiling instruments. Satellites passing overhead measured the surface temperature and wave characteristics, giving us instantaneous "big picture" views of the region.
The intent of the project was to study the region first in the late summer when solar heating had warmed the surface water and it was hard to mix the water column. The sun had warmed the surface leaving the cold, dense, nutrient rich water below. Then in the spring we would study the region after cold winter storms had cooled the surface and vertically mixed most of the water column.
All went well during the first two weeks at sea. The water was well stratified with a warm surface layer that gradually got colder with depth, just as a house heated by the sun will be warm in the attic and cold in the basement.
Chlorophyll fluorescence profiles through the water column and water samples showed that phytoplankton were growing happily, especially in a layer at about 25 meters below the surface. At the seafloor, sediment was being resuspended about five to ten meters into the water column by waves and currents. Everything looked as expected. We even had some subsurface internal waves pass by that affected most of the water column. These waves cause no change in sea-surface elevation, but can cause ten- to forty-meter vertical displacement of layers in the water in just a few minutes! Undoubtedly these are important mix-masters on the shelf.
Just when we began to tire of our daily routine at the same spot, word came of a hurricane heading ourway. We watched the weather reports with anticipation, and it soon became clear the storm was headed right for us.
For two days the swells grew larger. The phytoplankton either slowed their growth or were getting mixed as the chlorophyll peak at 20 meters dissipated. Resuspension of bottom sediments increased. When the swells reached 12 feet and the winds intensified, the captain headed for the safe harbor of Newport, Rhode Island, as we tied everything down and cleared the deck.
The next day, sustained winds of 50 miles per hour at the dock made us glad we weren't at sea. Hurricane Edouard passed just east of our site, but the moorings recorded the whole event, showing massive resuspension of bottom sediments.
After the hurricane passed we raced back to the site to see how conditions had changed. The surface-water temperature dropped from19°C to 15°C as the hurricane sucked heat from the water and mixed the colder water upward.
What had been a water column that gradually grew colder and denser with depth was now a two-layer system. A cool, upper layer was fairly homogeneous in temperature and optical properties. A lower, cold layer was well mixed in temperature but had b optical gradients with increasing turbidity toward the seafloor, the source of the resuspended sediment.
Turbidity at the bottom decreased rapidly over the next few days as sediment settled out or cleaner water moved into the area. A week later Hurricane Hortense passed further to the east, but we were safe lyin port by then. Mooring data revealed that Hortense caused a moderate episodeof resuspension. Data from all instruments indicate that the fine-grained sediment was most easily resuspended by the hurricanes and that the large aggregates measuring greater than 0.5 millimeters were torn up during the intense storms.
If you were looking for something in the water, you wouldn't do so during a storm, but it was surprising how rapidly the resuspended sediment settled out and visibility increased after the hurricane passed. As our study continues we will learn how visibility in this region changes with the seasons.
The area of our study was covered with a fine-grained mud, which presented a challenge for visibility. Visibility would be much greater in a region where the bottom is covered with sand rather than mud for three reasons:
1) Sand is more difficult to resuspend than mud.
2) Once resuspended, sand settles much more rapidly than fine-grained particles.
3) Billions of fine-grained particles make the water much murkier than a much smaller number of sand grains, even if the total weight of both types of particles is the same.
Normally oceanographers want to avoid hurricanes that landlubbers bill as going "harmlessly out to sea," but for this project, the event couldn't have been timed better. As this issue goes to press, we will be back at our site on a different ship trying to understand the forces that mix the sea and the particles that affect underwater visibility. We won't have to worry about hurricanes in April, but we'll be on the lookout for nor'easters!