National Oceanic and Atmospheric Administration 

U.S. Department of Commerce 

NOAA’s Oil Spill Response 

Guide to Distinguishing
 

Oil From Algal Blooms
 

When observed from a boat or aircraft, many kinds 

of natural phenomena can be mistaken for oil on 

the water surface. Kelp beds, accumulations of jellyfish, 

and cloud shadows on the water are sometimes mistaken 

for oil. Oil spill response experts call such phenomena 

“false positives.” Algal blooms are one kind of false 

positive. They are sometimes reported as oil on the water,

even by trained and experienced observers. This 

document describes algal blooms and provides tips to 

help you avoid mistaking algal blooms for oil. 

Noctiluca blooms 

Algal blooms happen naturally when some kinds of 

microscopic algae multiply quickly to form visible 

patches on the water surface. Noctiluca is a common, 

non-toxic alga that forms red, tomato soup-like blooms in

the Gulf of Mexico. A Noctiluca bloom can look similar 

to the reddish 

emulsified oil 

originating from the 

Deepwater 

Horizon wellhead. 

Noctiluca bloom in Hood Canal, Washington 

State. (Credit: University of Washington) 

Algal blooms can develop 

when nutrients become plentiful 

and other growing requirements (light, 

temperature, and salinity) are met. The Mississippi and 

Atchafalaya Rivers bring nutrient-rich water to the Gulf 

of Mexico, providing especially favorable conditions for 

blooms of Noctiluca and other species of algae from 

spring until the hurricane season. 

Distinguishing Noctiluca from oil 

If You’re Observing From a Vessel, Aircraft, or 

the Shore 

If you see a reddish patch on the water surface, look for 

these clues: 

1. 	Pattern: A patch of oil 

tends to appear streaky, or 

oil may appear as rows of 

streamers. Algae and water 

weeds tend not to appear 

streaky, but instead appear 

as an amorphous mass. Oil 

that has emulsified (mixed 

with water to form an 

emulsion) typically either 

has no texture, or 

appears streaky. 

continued on back 

Emulsified oil from the Deepwater 

Horizon appears as reddish streaks 

on the water. (Credit: USCG) 
2. 	Sheen: If you observe a dark or reddish patch on the 

water surface from an aircraft or vessel, look for sheen 

around it. Sheen is a very thin film of oil on the water 

that may be brown, silvery, 

rainbow-colored, or transparent. 

Sheen is usually present around 

patches of thicker oil on the 

water. You may see small 

quantities of colorless, sheen-like 

biofilm around algal blooms, but 

oil usually produces stronger 

sheen. Any colored sheen 

indicates oil (though not all sheen 

associated with oil is colored). 

3. 	Nighttime luminescence: Noctiluca is bioluminescent: 

when gently shaken, it produces light from chemical 

reactions within its cells. Boat crews operating at night 

may observe glowing stripes or patches on waves, 

which indicate the presence of Noctiluca. Oil does not 

produce light. 

It’s possible that in some cases in the Gulf of Mexico, 

spilled oil may be comingled with algal blooms. 

If You Have a Water Sample 

Avoid direct contact with substances that could be 

crude oil, because oil on bare skin can cause mild skin 

irritation. However, if you have obtained a fresh water 

sample that you think could contain oil, here are two ways 

to tell whether the sample contains Noctiluca instead, 

without using a microscope: 

1. 	Pour some of the water into a shallow dish, and inspect 

it closely. Look for tiny transparent balls somewhat like 

fish eggs. These balls are very small, less than a tenth of 

an inch in diameter. If you see them, your sample 

probably contains Noctiluca, and what you’re seeing are 

the mature cells 

of this tiny algae. 

2. 	Take your sample

into a completely 

dark room. Let 

your eyes adapt 

for several 

minutes, and then

swirl your sample

Watch for small 

flashes of light. 

If you see them, 

your sample contains Noctiluca. 

Learn more about NOAA’s response to the BP oil spill 

at http://response.restoration.noaa.gov/deepwaterhorizon. 

To learn more about NOAA, visit http://www.noaa.gov. 

A streamer of oil is surrounded by 

rainbow-colored sheen, observed 

during a May 6 overflight in the 

Gulf of Mexico. (Credit: USCG) 

.

Magnified view of Noctiluca cells. (Credit: 

Instituto de Oceanografia, Faculdade Ciências da 

Universidade de Lisboa) 

June 8, 2010