While it's relatively common to spot unidentified dark or reddish patches on the surface of the ocean in coastal areas around the U.S., it's not always easy to discern by sight what the substance is that's creating the disturbance. Often, offshore patches of discolored water are the result of algal blooms or oil slicks.

Algal blooms occur when colonies of algae—simple ocean plants that live in the sea—grow out of control. While algal blooms come in many colors (and some have no color at all), they are popularly known as 'red tides' because some are deep red in color.

Oil slicks, on the other hand, are simply films of oil floating on top of the water. While some slicks may be a few inches thick, most are thinner than a human hair. They may form naturally, but they are often introduced by man in incidents ranging from refined fuels or crude oil spilled from a ship to larger events such as last year's Deepwater Horizon oil spill. Oil sometimes emulsifies under certain conditions. Emulsified oil is a mixture of oil and water that often resembles chocolate mousse or pudding.

How do you tell the difference? It can be difficult. Even the experts can be fooled, especially when looking at the ocean from an aircraft.

Color. Oil can vary greatly in color, from the commonly expected black or dark brown, to red, orange, yellow, and even some more exotic colors. When oil becomes emulsified, its color becomes much lighter. For example, South Louisiana crude oil is generally a dark red, but appears bright red to orange when emulsified—as it did during the Deepwater Horizon spill. If the oil has a reddish hue, it may be mistaken for so-called "red tide." Once the emulsion breaks, the color reverts back to its normal dark color. Once an oil slick spreads out and becomes very thin, the color varies from grey to silver. It's easier to observe from a boat or a plane than it is from on the beach, but if you see a thin film that is rainbow-colored, you're looking at oil. However, other than rainbow sheen, both oil and algal blooms can have a large range in colors that are similar, and algal blooms often create sheens.

Odor. The most reliable difference is odor. Oil slicks nearly always have a characteristic petroleum smell. Algal blooms may have a strong smell as well, but the smell is distinctly different from that of the more-familiar smell of petroleum.

In the water or just on top? Algal blooms are mostly in the water column although often in the upper layers, but they may have a floating layer. Oil slicks are generally only floating on the surface. But certain oils may be naturally or chemically dispersed into the water column.

Nighttime Bioluminescence.: Oil isn't bioluminescent—it doesn't produce light—but some of the algae that form surface blooms do. The light comes from chemical reactions in the algal cells. So when it's dark out, the water may glow—especially when waves break. During the day, you can take some water into a very dark room, let your eyes adapt to the darkness for several minutes, and then swirl the container. A blue glow or individual flashes of light indicate bioluminescent algae. Keep in mind, though, that some red tide algae do not produce light.

Unsure? Sometimes it's best to leave it to the experts. If you think you see oil on the water, report it to the National Response Center at 1-800-424-8802.

For more information
What is a 'red tide?' (Ocean Fact)
Why do harmful algal blooms occur? (Ocean Fact)
What is Natural Resource Damage Assessment? (Ocean Fact)
How does oil impact marine life? (Ocean Fact)
Oil and Chemical Spill Incident News, NOAA's Office of Response & Restoration
Oil & Chemical Spills
Harmful Algal Blooms
Responding to Oil Spills (podcast)
Harmful Algal Blooms (podcast)
Oil in the Ocean (video)
Predicting Harmful Algal Blooms (video)

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The Earth’s outer crust is made up of a series of tectonic plates that move over the surface of the planet. In areas where the plates come together, sometimes volcanoes will form. Volcanoes can also form in the middle of a plate, where magma rises upward until it erupts on the sea floor, at what is called a “hot spot.”


The Hawaiian Islands where formed by such a hot spot occurring in the middle of the Pacific Plate. While the hot spot itself is fixed, the plate is moving. So, as the plate moved over the hot spot, the string of islands that make up the Hawaiian Island chain were formed.


The Hawaiian Islands form an archipelago that extends over a vast area of the North Pacific Ocean. The archipelago is made up of 132 islands, atolls, reefs, shallow banks, shoals, and seamounts stretching over 2,400 kilometers (1,500 miles) from the island of Hawaii in the southeast to Kure Atoll in the northwest.



For more information:
Hawaiian Archipelago, Coral Reef Information System
Seamounts and Hot Spots, New Millennium Observatory

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Water levels in the Great Lakes have long-term, annual, and short-term variations. Long-term variations depend on precipitation and water storage over many years. Annual variations occur with the changing seasons. There is an annual high in the late spring and low in the winter. These changes occur at a rate that can be measured in feet per month.

True tides, changes in water level caused by the gravitational forces of the sun and moon, do occur in a semi-diurnal (twice daily) pattern on the Great Lakes. Studies indicate that the Great Lakes spring tide, the largest tides caused by the combined forces of the sun and moon, is less than five centimeters (two inches) in height. These minor variations are masked by the greater fluctuations in lake levels produced by wind and barometric pressure changes.

Consequently, the Great Lakes are considered to be essentially non-tidal.

For more information:
Center for Operational Oceanographic Products and Services
Great Lakes Water Level Data
Great Lakes Online
Tide Predictions and Data Frequently Asked Questions
Tides Tutorial, NOS Education
What are Tides? - Diving Deeper audio podcast

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The ocean is the lifeblood of Earth, covering more than 70 percent of the planet's surface, driving weather, regulating temperature, and ultimately supporting all living organisms. Throughout history, the ocean has been a vital source of sustenance, transport, commerce, growth, and inspiration.

Yet for all of our reliance on the ocean, 95 percent of this realm remains unexplored, unseen by human eyes.

NOAA’s Office of Ocean Exploration and Research is leading efforts to explore the ocean by supporting expeditions to investigate and document unknown and poorly known areas of the ocean. These expeditions represent a bold and innovative approach by infusing teams of scientist-explorers with a "Lewis and Clark" spirit of discovery and equipping them with the latest exploration tools.

From mapping and describing the physical, biological, geological, chemical, and archaeological aspects of the ocean to understanding ocean dynamics, developing new technologies, and helping us all unlock the secrets of the ocean, NOAA is working to increase our understanding of the ocean realm.

For more information:
NOAA Ocean Explorer


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Lightering is the process of removing oil or other hazardous chemicals from a compromised vessel to another vessel to prevent oil from spilling into the surrounding waters.

Lightering is not possible in all oil spill scenarios. It depends on many factors including the type of oil that is spilled. As time passes, the oil can become more viscous, or thicker, and therefore more difficult to pump. This can, in turn, make lightering difficult, if not impossible. While there are benefits to removing oil in this way, there can also be accidents and spills that result from lightering.


Lightering is also used to transfer cargo between vessels of different sizes like a barge and a bulker or oil tanker to reduce the vessel's draft in order to enter port facilities.

For more information:

Office of Response and Restoration

Office of Coast Survey

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Losses from catastrophic events such as hurricanes can be extensive. The economic losses from the 2005 hurricane season, which included Hurricanes Katrina and Rita, were $200 billion, the costliest season ever.

For some threats, such as sea level rise, only the projected economic losses are indicated. For example, the vast majority of our nation’s commercial and recreational fisheries are dependent on coastal marshes. Approximately two-thirds of those fisheries spend some stage of their lives in tidal marshes. As sea levels rise, the built-up areas behind these marshes will provide no opportunities for wetlands to migrate. The net result will be billions of dollars in economic impacts affecting the livelihoods and sustainability of many coastal communities.

Coastal threats are different throughout the United States. For example, in the Pacific Islands, there are more potentially catastrophic coastal hazards such as tsunamis, flooding, and even droughts. In the North Atlantic, there are more severe storms, population and development pressures, and regional-scale impacts such as climate change.

It is critical to develop hazard-resilient communities to prepare for these threats and enhance the ability of these communities to absorb impacts and bounce back. This preparation will reduce the lives lost in disasters, secure the economic stability of these communities, and support the health of our coastal ecosystems, including wetlands which are essential for reducing storm impacts on our coastal communities.

For more information:
NOAA Coastal Services Center
Coastal Hazards, NOAA's Office of Ocean and Coastal Resource Management
Local Strategies for Addressing Climate Change (pdf, 1.12 mb)
Diving Deeper Podcast, Episode 10 (June 3, 2009) - What is resilience?
Explore: Natural Hazards Assessment

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