Other Ambient-noise Sources
Ambient noise is also produced by intermittent and
local effects such as earthquakes, biologics,
precipitation, ice, and breakage of waves.
PRECIPITATION.– Rain and hail will increase
ambient-noise levels at some frequencies (usually
between 500 Hz and 15 kHz). Large storms can
generate noise at frequencies as low as 100 Hz and can
substantial y affect sonar conditions at a considerable
distance from the storm center.
ICE.– Sea ice affects ambient-noise levels in polar
regions. Provided that no mechanical or thermal
pressure is being exerted upon the ice, the noise level
generally is relatively low during the growth of ice.
According to investigations carried out in the Bering
Sea, the noise level should not exceed that for a sea state
2, even for winds over 35 knots. The exception to this
rule is extremely noisy conditions due to entrapped air.
BIOLOGICS.– Biological noise may contribute
significantly to ambient noise in many areas of the
ocean. The effect of biological activity on overall noise
levels is more pronounced in shallow coastal waters
than in the open sea. It is more pronounced in the tropics
and temperate zones than in colder waters. By far the
most intense and widespread noises from animal
sources in shallow water observed to this time are those
produced by croakers and snapping shrimp. Fish, more
than crustaceans (crabs, lobsters, shrimp), are the source
of biological noise in most of the open ocean.
Marine Mammals
Mammal sounds include a much greater range of
frequencies than do the sounds of either crustaceans or
fish. They have been recorded as low as 19 Hz (whale
sounds) and as high as 196 kHz (porpoise sounds).
EVALUATING THE IMPACT OF
BIOLUMINESCENCE
LEARNING OBJECTIVES: Identify the
primary sources of bioluminescence in the
oceans. Recognize distinguishing features of
sheet, spark-type, glowing ball, and exotic light
display luminescence.
Plankton organisms are chiefly responsible for
bioluminescence in the sea. The smallest forms are
luminescent bacteria that usually feed on decaying
matter or live in various marine animals. However, with
a supply of the proper nutrients, luminescent bacteria
can develop in great masses in the sea, causing a general
bluish-green glow in the water. The glow is usually
diffused and barely detectable, although exceptionally
bright displays caused by luminous bacteria
occasionally are observed in coastal regions near the
outflow of large rivers. The light given off frequently
outlines the current front where the river and ocean
meet.
TYPES OF BIOLUMINESCENT DISPLAYS
Bioluminescent displays may be classified
according to their appearance. They are sheet,
spark-type, glowing ball, and exotic light.
Sheet Bioluminescence
Most bioluminescence in the oceans is of a
sheet-type display and is produced by one-celled
organisms. This type is most commonly observed in
coastal waters. The color is usually green or blue and
many displays appear white when the organisms are
present in great numbers.
Spark-type Bioluminescence
Spark-type displays are created by a large number
of crustaceans. Most of these displays occur in colder,
disturbed waters and only rarely in tropical waters. The
light emitted gives the ocean surface a “twinkling”
appearance.
Glowing-ball-type Bioluminescence
Glowing ball or globe-type displays are seen most
frequently in the warmer waters of the world. The ocean
may seem to be full of balls or discs of light, some
flashing brightly as they are disturbed, and others
dimming after the initial disruption has ceased. The
light given off is usually blue or green; displays of white,
yellow, orange, or red have occasionally been reported.
Luminescent jellyfish also cause many
glowing-ball displays.
Large shining round or oval
spots of light may appear in the water.
Exotic Light Displays
Exotic light formations like wheels, undulating
waves of light, and bubbles of light appear to be separate
and distinct from the displays previously discussed. The
cause of such phenomena are still unknown.
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