Will rocks covering the bottom of my pond help with algae or make it worse?

This is a common question asked by new pond owners and people planning to install a pond.  Nobody wants their new pond to look like a bowl of slime, and there are different schools of thought regarding the best way to keep a pond looking nice.  We have been building ponds and water features for 15 years, and we go with a rock bottom every time.  Here's why:

• The foremost reason is that rocks will help stabilize the ecosystem.  They act as a biological buffer for the water chemistry.  Rock is a better surface for the beneficial bacteria colonies that convert fish waste and plant debris into neutral forms.  As the bacteria reproduce, they tie up phosphorus and nitrogen in their cells so it's no longer available as food for unwanted algae.  Most of the bacteria will populate on the rocks lining the pond.  Some of the bacteria remain in the water, where they process dead phytoplankton and soluble nutrients.  This action serves to clean the water and keep algae in check. 

• It is true that rocks will trap sediment and will likely need to be washed out once a year.  The good part, however, is that by trapping the sediment it keeps your water from clouding up when the fish get frisky.  Without the rocks to trap organic debris, the fish will stir it up as they swim around.  It's sort of like putting grates on the floor of a beach shower so people don't have to stand on piles of sand rinsed off the feet of previous guests.  

• A bare bottom pond is much more susceptible to minor changes in water chemistry.  I've seen a bare-liner pond go from crystal clear to solid green in an afternoon.  This is because the pond lacks the bacterial biofilm that would typically neutralize the more extreme effects of a sudden change, whether it is caused by a lot of rain, a heat wave, binge feedings, or any other event that impacts the water.  This isn't to say that a rock-bottom pond will never experience algae problems, but when the conditions do favor an algae bloom it will be much less severe when the pond has a thriving bacteria population.  

 

• The larger the pond, the more stable it will become after the elements of the ecosystem have been established and harmonized.  It's like the difference between adding a pinch of pepper to a large pot of soup, or adding that same pinch to your spoonful.  Larger is more forgiving, and the advantages with a rock-bottom pond become more prominent in a larger pond.  We have seen large ponds that haven't been cleaned in years and still look gorgeous; that is less common in smaller ponds, where there is less water volume and bacteria to absorb the impact of any changes to the water chemistry.  

 

• Another benefit to a rock-bottom pond is the safety factor.  This is often overlooked when choosing a pond, but is very important to consider.  It is not a fun surprise when someone slips into a pond and can't get their footing to climb out.  If you haven't experienced it for yourself, take my word for it: a wet, algae-covered rubber liner is one of the slickest surfaces known to man.  I've heard of guys needing to lasso the nearest tree with their hose to escape a pond because they can't walk out on their own.  It can be comical to watch, but it can also be dangerous.  Without a rock bottom, any kids or animals that unexpectedly find themself in a slick bowl of water will have difficulty climbing out on their own without requiring a rescue.  ​Rocks can still get slippery when they're coated in algae, but a slippery escape sure beats no escape.  

 

• Another benefit of a rock-bottom pond is extending the longevity of the liner.  One of the biggest factors that degrades an EPDM liner is UV damage from the sun.  Covering the liner with rocks will keep the sun from hitting it and prevent this UV damage.  Our liners do come with a 20-year warranty, but that doesn't cover the cost of taking the old liner out and replacing it, so why chance it?  

These are the reasons we do full-rock-coverage on our water features.  It is a proven technique that works for pond builders around the world.  It is a key element of the aquatic ecosystem and allows for greater harmony in the pond, ultimately leading to a cleaner, nicer, safer pond with less regular maintenance.