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42 ROGER L. HERMAN AND FRED P. MEYER
<br />Winter oxygen depletion occurs when ice and
<br />snow cover prevent photosynthesis or other aera-
<br />tion. Mortalities may occur at any time of the day.
<br />Other environmental indicators are the same as
<br />those listed above.
<br />Care must be exercised to avoid confusing an
<br />oxygen depletion due to a natural cause with a deple-
<br />tion caused by an herbicide, which can result in a
<br />kill that begins at any time and continues unabated
<br />throughout the day and night.
<br />A fish kill that results from natural causes, such
<br />as oxygen depletion, is usually preceded by indi-
<br />cators that should alert an investigator. Before
<br />lethal oxygen depletion occurs, heavy growths of
<br />aquatic vegetation or thick blooms of blue-green
<br />algae may be present for several days or weeks.
<br />Dissolved oxygen may exceed saturation between
<br />noon and 2:00 p.m. and approach the critical lower
<br />limit for fish survival just before daybreak. Accom-
<br />panying this phenomenon is a wide shift in pH with
<br />readings of 10 or above at midday and 6.9 or below
<br />at daybreak. These signs are readily apparent to a
<br />trained observer and provide advance warning. In
<br />contrast, fish kills due to toxic substances are
<br />abrupt, large-scale, catastrophic events that occur
<br />without warning.
<br />Toxic Algal Blooms
<br />In certain unique situations, a single species of
<br />toxic alga may become dominant in the flora. Some
<br />blue-green algae and certain dinoflagellates release
<br />toxins that kill or inhibit other algae. When competi-
<br />tion for nutrients becomes intense, the level of toxin
<br />released climbs. Susceptible species of algae gradual-
<br />ly disappear until only the single dominant species
<br />remains, usually in high abundance. As the alga uses
<br />up the available nutrients, the species competes with
<br />itself and the level of toxin released continues to rise.
<br />Eventually, the water may become toxic to zooplank-
<br />ton, insects, fish, and sometimes even to animals
<br />that drink the water. Red tides, which occur in
<br />marine waters because of blooms of the dinoflagel-
<br />late Gymnodinium brevis, are a common example.
<br />Mortalities due to toxic algal blooms are unique
<br />in that production of the toxin is strongly related
<br />to photosynthetic activity. Kills begin at about 9:00
<br />a.m., continue through the day until 4:00 p.m., and
<br />then subside, only to be repeated the following day.
<br />Unless some factor intervenes, the phenomenon con-
<br />tinues until the algal bloom ends or an oxygen deple-
<br />tion occurs. Often there is a large-scale die-off of the
<br />problem alga, sometimes followed by signs of a
<br />classical oxygen depletion (e.g., low 02, low pH,
<br />high CO2, dark water color, sour-cabbage odor).
<br />Unless the observer has information about the early
<br />phases of the fish kill, the role of the toxic alga may
<br />be overlooked.
<br />In toxic algal blooms, pH is very high (9.5 to 11.0)
<br />at midday, dissolved oxygen is near saturation or
<br />above, and water temperatures are above 27° C. A
<br />single species of alga is present in large numbers.
<br />Species of Anabaena, Aphanizomenon, Dinobryon,
<br />Glenodinium, Gleotrichia, Gymnodinium, and
<br />Microcystis are some that have been reported to
<br />cause toxic blooms.
<br />Turnovers
<br />Occasionally, weather-related disturbances trigger
<br />fish kills. In shallow lakes, high-velocity winds can
<br />break the thermal stratification and cause a turn-
<br />over. Cold, heavy rainfall following prolonged hot
<br />weather or a severe hailstorm can also cause a sum-
<br />mer turnover that brings anoxic water and decay-
<br />ing organic materials into the total water column
<br />and greatly increases the total oxygen demand. Oxy-
<br />gen depletion can result, in spite of the aeration by
<br />wave action. Typical signs are low dissolved oxygen,
<br />decaying organic matter, foul odor, color change,
<br />and others, as normally seen during an oxygen
<br />depletion.
<br />Hydrogen Sulfide Poisoning
<br />Severe weather can also cause different kinds of
<br />fish kills. Disturbance of thermal stratification often
<br />releases large quantities of hydrogen sulfide (H2S).
<br />High dissolved H2S, even in the presence of ade-
<br />quate dissolved oxygen, can cause a "brown blood"
<br />condition and mortalities in fish. The brown color
<br />of the blood is caused by the formation of sulfhemo-
<br />globin, which drastically reduces the ability of the
<br />blood to carry oxygen. Some fish usually survive and
<br />ultimately recover. The largest fish are most severe-
<br />ly affected. Environmental signs include (1) an odor
<br />of H2S in the water-especially downwind from the
<br />site, (2) black, decaying organic matter on the wind-
<br />ward shore, (3) disoriented, dying fish, and (4) fish
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