To recap last week's blog post, the problem with retention ponds or simple earthen bottom ponds is that they're designed for one specific purpose.
It's important to reverse the effects of aging of these ponds, known as eutrophication, and in order to do so we need to think of them as aquatic ecosystems. Our goal is to manage pond nutrients since they're the building blocks of algae, higher plants, and the entire food web. By controlling nutrients we can control the water quality and the entire ecosystem.
There are two major forms for nutrients to enter and cycle within a pond. Allocthonous organic matter, is a generic term for any material that originated from outside of the pond, such as leaf debris, lawn clippings, wind blown dust etc. Versus, Autocthonous matter which describes any material that came from within the pond, like leaves and stalks of aquatic plants, algae, fish waste etc. Both are connected, but the source that has the greatest impact in most situations is the allocthonous material as it's responsible for adding large quantities of nutrients over the life of the pond.
Let's use the pond at McCannon Farm (advanced CAC build at Pondemonium 2014) as our example.
The most effective long term strategy for managing nutrients starts by creating a skimmer system designed to capture the allocthonous organic matter. The skimmer will house a series of pumps; these pumps are discharging the water from the skimmer to several strategic areas around the pond.
As the pumps evacuate water out of the skimmer area, water from the pond will refill the void left within the skimmer. The key is the weir structure, as this is where the skimmer connects to the pond. The goal is to have a shallow sheet of surface water flowing into the skimmer.
Think of a standard pond or swimming pool skimmer … only a lot larger!
On the McCannon Farm project this was achieved by creating a concrete weir structure which became our skimmer opening.
The desired effect was achieved by placing a series of boards in the weir zone to achieve the appropriate depth and water velocity. Fast water flowing into the skimmer will draw surface debris into the skimmer collection area for periodic removal.
Collection of floating organic debris (leaves, lawn clippings, algae) is an important part of the process; by keeping the organic stuff from accumulating on the bottom of the pond we limit the overall nutrients (food) found in the pond, which will have a direct relation to sediment accumulation (matter accumulating faster than it can decompose).
This then stabilizes the bacterial process of decomposition and overall dissolved oxygen levels. The bacteria will now be able to keep up with the decomposition of organic matter and pond sediments. When balanced properly, there will not be a drop in dissolved oxygen levels which eliminates the stress on the fish and other aquatic organisms. Less sediments require longer periods before needing to be dredged, thereby saving time and money!
Consistent water depth is also beneficial because it will keep the pond cooler, meaning higher dissolved oxygen. Less sunlight reaching the bottom of the pond will inhibit nuisance aquatic plants and algal growth.
I have a simple calculation to determine the size of the skimmer; the minimum size/volume of a custom built skimmer is 1 minute of water volume held within the Aquablox. For this project we have approximately 700 gpm, therefore I have 22 Aquablox minimum (1 Aquablock 32 gallons). I increased the number to 24 (24 x 32 gal = 768 gal) because it was a better configuration and the extra volume is a good safety factor.
In the McCannon pond, there are 5 submersible pumps in the skimmer. The large number of pumps versus one or two large pumps is simple; 5 pumps give us a very cost effective way to control the overall flow and function of the system.
We can run just one pump or any combination for ultimate control. Another benefit is the availability and redundancy; these pumps are all stock items so if one goes down for service or replacement, we still have an operational system.
One of the pumps in the skimmer will discharge the water into a large constructed wetland filter, which I will need to discuss next week due to the amount of information connected with it. The second pump is discharging the water through a series of underwater jets; the goal and position of the jets is on the opposite side of the pond from the skimmer. The underwater jets will create a push of water across the surface of the pond, which helps to move surface debris towards the skimmer for efficient collection while adding oxygen to the water.
The final three pumps are primarily aesthetic as they create a dramatic waterfall, however I do believe that the waterfall is a very important feature which is why I said primarily aesthetic (not completely). The waterfall will help to increase the overall circulation of the pond including the dissolved oxygen content which as we already discussed, is very important to the overall health and function of the system.
Nothing beats the sight and sounds of a well-designed waterfall as they have the ability to captivate pond viewers for hours. It all starts with a thorough understanding of aquatic systems and realistic goals for the water feature; using nature as our guide we can increase the function and aesthetics of any pond.
Remember next week we'll discuss constructed wetland filters. These filters work in conjunction with the skimmer, jets and waterfalls to deliver the desired water quality in a healthy ecosystem pond.