Pond algae blooms can deplete pond water of oxygen and kill all fish in the pond
Because pond algae are plants they both exhale and inhale oxygen in the photosynthesis process. Under dense conditions algae can remove all oxygen from water... so algae blooms (dense dark green pea soup like pond water) are potentially disastrous for pond life
The Danger Of Algae BloomsThis section that follows is important not just as far as alga blooms are concerned but also for a general understanding of what happens within any pond.
Talking of a Pond That Breathes.
Think of your pond as breathing in oxygen during the day and breathing out carbon dioxide at night
Carbon dioxide in pond water results from a number of sources including:
1. Waste products decaying at the bottom of the pond
2. Respiration by pond inhabitants.... fish, insects, plants, algae
Of course all of us think first and foremost of oxygen in pond water as being essential and of course this is very true. Carbon dioxide is also of critical importance. Oxygen and carbon dioxide also work in concert with each other.
In simple terms as carbon dioxide levels increase in a pond then oxygen levels tend to decrease. This follows a natural pattern as can be seen in the sketch.
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Here you can see that from dawn to dusk (ie daytime) oxygen levels in a planted pond increase then decrease once evening sets in.
Carbon dioxide concentrations act in reverse fashion. They fall during the daytime and increase throughout the night. Oxygen levels are at their highest at dusk and carbon dioxide levels are at their highest at dawn.
Let's equate the above to what living organisms do in the pond. Think of the flora and fauna as exhaling carbon dioxide while inhaling oxygen during the night.
From this you will agree that dawn is the critical time in a pond. Often people wake up to find dead fish and wondered what happened. Such deaths could be associated with very low oxygen levels coinciding with high carbon dioxide levels. When ponds are full of suspended algae such problems can arise.
If there is a very small amount of algae bloom in the pond then you will find oxygen and carbon dioxide levels will not change significantly between early morning and late afternoon. On the other hand dense pea soup type water will show very significant variations.
Seasonal Impacts upon carbon dioxide problems
The first thing to realise is that oxygen concentrations are highest in winter because water is cooler. Because oxygen concentrations are high the oxygen reserve is not depleted so quickly during the night. Plant (algae) and animal life has also slowed down significantly.
*****In summer water can hold much less oxygen and the animal and plant life (algae) is also thriving due to higher temperatures along with more nutrients in the water associated with feeding fish. The living organisms are therefore emitting more carbon dioxide in a situation of potentially disastrously low oxygen levels. Fish can die from suffocation under these conditions.
Measuring carbon dioxide is possible using a test kit but it is not normally required to do this.
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