Lake Superior Zoo, Duluth, Minnesota

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Thursday, May 20, 2010

Blog written by - Animal Curator Peter Pruett

As I move through the zoo guests sometime ask why the pool is ‘green’. Most people’s idea of a pool tends to be the one we and 100,000 other people swim in during the summer. Big, crystal clear water, smells of chlorine and has that shiny white or light blue bottom. All great for vacationing humans but not so good for animals that spend all or the majority of life in water.

Why then do we swim in such pools? Well for instance accidents happen and diseases could possibly be transmitted to many people. That’s where the chlorine comes in, it’s a great sanitizer but has side affects. Everybody who’s ever spent time in a pool knows that their eyes turn red, itch and become painful. Hair ends up dry and feels like straw, mine fell out. Imagine a harbor seal spending over 20 hours a day in that pool! The white or blue bottom allows us to see how clear the pool is, reflects light and makes us much more likely to go swimming.

The pools for the animals are ‘green’ because our swimming pools are not healthy for them and their quality of life will suffer. We want some algae growing in the water it’s a sign of a healthy aquatic system. When a pool is solid green and you can’t see the animals that would be no good. Too much green is BAD. To maintain that delicate balance between good and bad we do use chlorine. The levels are very low and at times are less than the levels in our drinking water. Everyday keepers test the chlorine and adjust when needed. Once a week water samples are taken to an environmental lab for testing. The tests tell us if there are any bad things growing in the water. If there are the pool is drained and cleaned. Cleaning a pool is not fun so keepers work very hard at maintaining happy healthy water.

Let’s go back to human pools and look at why they aren’t good for your harbor seals. Remember chlorine makes our eyes red and painful and the seals spend all day and night in the water. The seals eyes do become red and painful but more importantly lengthy exposure to all that chlorine can cause cataracts. A cataract is the clouding of the lense in an eye which leads to blindness. The shiny pool bottom can also cause cataracts. When light hits the shiny bottom it is reflected back everywhere. It would be similar to me sitting in a small white room with 100’s of light bulbs on. I would eventually go crazy and blind due to cataracts. Not fun at all. A darker pool with algae growing on the sides means more light is being absorbed not reflected and we’ll have happy healthy seals!

Ok, it is safe and fun for us to go swimming. We just don’t want to live our entire life in a swimming pool. The appropriate levels of algae growing in a zoo pool is good, it makes the animals happy. Next time your at the zoo and you hear someone comment on the ‘green’ pool tell them why!

posted by Keely Johnson at


Blogger david said...

Great content, thanks for sharing.
Always wondered why the pools are green. Would like to share this to my followers over at בריכות שחיה ביתיות

July 1, 2020 at 12:18 AM  

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