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The answer to this question about surface area to volume led me to an interesting article:

LOW OXYGEN AND POND AERATION William
A. Wurts, Senior State Specialist for Aquaculture
Kentucky State University Cooperative Extension Program
(PDF)

From which I quote:

Oxygen Depletion

Sudden death of phytoplankton or algal bloom, "bloom crash", may result from insufficient light (e.g. cloud cover) for photosynthesis, inadequate pond nutrients (a bloom too dense to be supported by available nutrients and oxygen) and/or bloom senescence (the plant cell line becomes too old to continue reproduction). Oxygen is consumed or depleted when dead phytoplankton/algae decay. During the nighttime hours, a dense phytoplankton bloom can remove all oxygen from the water for respiration (to breathe) alone. When a bloom crash occurs, the water appears to have become "black" or clear overnight.

It would seem that certain algae levels are helpful for oxygenating ponds.

I maintain several pet fish ponds. What role does algae play in establishing a healthy pond environment for my fish and other aquatic or semi-aquatic species I might introduce?

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  • just an update to establish the pet relationship of the pond, tweak as desired. – John Cavan Dec 6 '13 at 3:23
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    This reminds me of how dead spots in the oceans and bays occur. – jmort253 Dec 6 '13 at 7:55
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I could write a book on this; in fact, there are many books on this. The short version is -

  • Algae is beneficial to fish the same way that plants are - they convert nitrate to a usable nitrogen source for consumption with the byproduct of oxygen. This oxygenates the water and removes harmful (to fish) nitrogen compounds. This is assuming alkaline water, it work differently in acidic water. Take a look at this simplified article on the nitrogen cycle. Disclaimer: it's my article on my site where I also may or may not advertise products.
  • Algae is beneficial to snails, fish and other organisms that may eat it. It can be a great food source, depending on the type of algae and the type of organism.
  • It is an indicator of adequate (or excess) nutrients in the ecosystem. Because it is a plant, it indicates that there are sufficient nutrients and light for it to grow. It also indicates that carbon (the most common limiting factor) is adequate.
  • It's a detriment to other plant species. Because it is so efficient at utilizing nutrients and expanding to choke out other organisms competing for them (and light), it can and will damage other plants. It does this to assure its own survival. It can also perform types of allelopathy which can be detrimental to other plant species.
  • By choking out other plants, it may choke out food sources, oxygenating sources or camouflage/cover for other organisms. This is, once again, detrimental to the balance of the ecosystem and may cause it to crash.

In summary, algae, in and of itself, is a good indicator of a healthy ecosystem, but (like anything) in excess, it can be harmful. It's great to see some algae and know things are going well, but it's also very important to keep it under control to ensure it doesn't disrupt the balance.

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Algae for ponds, fountains, wall waterfalls or any other water feature:

Algae really performs an essential and imperative part, much the same as some other plant in your pond - that is to assimilate nitrates, which is what's left in the water after your pond's useful microorganisms are done corrupting fish and plant waste. Nitrates are incredible plant manures, however left unchecked, can be hurtful to amphibian life. Along these lines, a tiny bit of algae is okay and it is likewise a wellspring of nourishment for goldfish and koi.

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