How are the weather patterns at the lake? If no toxic chemicals or contaminations are present, then could it be weather or climate? Phytoplankton are a main source of oxygen in ponds and lakes, however, it may be the water chemistry in a specific climate or weather change. The warmer the water, the less dissolved oxygen it can hold. Even if there is a bloom of phytoplankton that normally produce plenty of oxygen, the phytoplankton may get trapped in the middle layers of the water where it cannot be mixed in order to reach the surface for sunlight to produce oxygen. Also, in warmer weather, oxygen will not stay in the water, it will escape into the atmosphere, decreasing the water's oxygen levels. Additionally, this results in more oxygen being taken out by blooms at night than what it makes during the day, further depleting oxygen levels. Eventually, this can lead to the fish suffocating.
Also, a more indirect approach to suggest is that any prolonged minimal exposure to oxygen can lower a fishes immune system, allowing them to be more prone to bacterial infections and diseases, resulting in their death.
Hope this helps!
In the Amazon I have frequently observed mass fish deaths in floodplain lakes. It normally occurs after heavy rain. The cold rain water causes water turnover and brings warmer de-oxygenated water up into the water column. As sensitive aquatic life dies from oxygen starvation warm tropical conditions quickly produce a cascade effect as bacterial decomposition removes more oxygen and the waters become almost anoxic. Only fish able to air breath or take water from the surface film may survive. Although some fish seem to go into an inactive state that allows survival. In some lakes strong winds can also cause the same water column turnover effect.
Sudden die-offs of algae can occur in lakes.Because of the bloom collapse oxygen level was dramatically dropped due decrease of photosynthesis.Oxygen depletion results in massive fish deaths.
If it is not toxin it must be the weather. If it is winter season may be oxygen deficiency and if summer eutrophication.
I have observed mass fish death in Zanzibar during the hot seasons like January to March. Its due to the heat but its not to all fish only certain types of fish are affected. What about in your lake? How was it
Oxygen deletions are rather common occurrences depending on weather patterns, eutrophication, lake bed depth and morphology. From your description stating that the lake has a phytoplankton bloom oxygen depletion is a strong candidate for the cause. But don't forget that lightning strikes have been known to cause fish kills in proximity to a strike, especially in small embayments. Also you are located in a somewhat seismically active region of the world where subtle shocks could disturb anaerobic regions or layers in the lake and also cause an oxygen depletion occurrence.
If the fish are observed dead in the early morning,, and there is phytoplankton in the lake, and it was a hot still night, then the cause is most likely lack of oxygen.
Phytoplankton produce oxygen when there is sunlight, but they use oxygen at night. Oxygen does not diffuse well in water, most water bodies require water movement to oxygenate the surface layers and turn the surface water over to expose deeper water to the surface. Wind works well. Add the effects of temperature (warm water holds less dissolved O2), and you have a recipe for disaster on hot still nights. The oxygen level will be low at about 3am, but the moment the sun rises, the phytoplankton begin to produce O2 and by the time people see the dead fish the oxygen levels are almost normal. What I am saying is easily checked with a continuous data logger. If you aerate a pond with paddlewheels, then certainly do so at night, its more important than at daytime.
Could be due to:
1. The phenomenon of upwelling / downwelling
2. explosion of toxic algae
3. acid rain
4. pathogen infection
In Mexico recurrently there mass death of fish in some lakes and lagoons and this occurs in the spring, when ambient temperatures reach their highest values (> 35 ° C) throughout the year, causing anoxic conditions accompanied by Exponential blooms of phytoplankton .
When the day is cloudy and rainy, light penetration to the bottom of the lake is minimum. It will dramatically decrease the photosynthesis(then algae start to intake oxygen available in water). At the same time rain will turbid the lake too. Again it decreases the light penetration. Low dissolved oxygen level and turbidity will suffocate the fish and finally they may die. In shallow lakes this is very common due to the high turbidity.
I’M AGREE WITH Peter Alan Henderson, Prama Hartami AND Sachithra Amarin Hettiarachchi.
Here are some possibilities
1. A bloom collapse can result in a depletion of oxygen.
2. A prolonged stratification can lead to oxygen depletion in the hypolimnion and bodies following their demise may float to the surface
3. Ichthyotoxic algae inducing mortality
all the best
Dan
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