How Does Circulation Affect Stratification?

What is stratification, and why must we circulate water? 

Before we get into how circulation affects stratification, we’re going to briefly and simply explain what stratification is.

Stratification in a pond or lake is when water forms distinct layers that do not mix. 

Stratification may occur for a number of reasons, including salinity, density, and temperature. But for our purposes, we’re going to discuss only two, and only at a very simple level: Stratification is often about water temperature, and the distribution of dissolved oxygen. This is where stratification leads to problems for people who manage lakes or ponds. 

Everything, even plants and animals that live in water, needs oxygen to survive. Deep water loses its oxygen because it is quickly consumed by decomposing organics on the bottom and has no light or contact with the atmosphere to absorb new oxygen. Likewise, fertile, green, productive water has a lot of organic material present and loses oxygen relatively fast because sunlight is inhibited from reaching deeper in the water. 

Where does circulation come in?

Without circulation, a relatively deep pond can stratify into layers, resulting in a narrow band of oxygen-rich water at the top, and progressively less oxygen in the bottom layer. Stratified layers can be aerobic, anoxic, or anaerobic. 

a visual of a stratified, deoxygenated waterway; brown sides of a river and layer of of water getting progressively darker. Water has no circulation to prevent stratification
Stratified water can lead to problems.

Anoxic and anaerobic conditions can suffocate life-forms in that zone, and greatly reduce potential for biodiversity and water health. Especially in the presence of heavy nutrient loads, circulation is required to expand the top/aerobic zone into the lower, oxygen-poor layers.

So, how does circulation affect stratification?

Circulation in a pond or lake can prevent stratification; circulation helps mitigate HABs, and circulation helps prevent fish kills. 

Suffice it to say that water needs circulation and aeration to provide a healthy oxygen-rich environment to support the aquatic food web. The other necessary element is surface area, to ensure that algae faces competition from healthy wetland microbes.

Without circulation, an excessive anoxic or anaerobic condition can quickly develop…in other words, a potential opportunity for a fish kill. Circulation can be natural, caused by the flow of water through the pond from inflow to outflow, or mechanical, provided by pumps and aerators that move water, or by nanobubblers and another form of natural circulation called “Brownian Motion.” Circulation almost always improves aeration and the cycling of nutrients, both of which contribute to water health.

The Evolution of BioHaven Circulation

Beds of aquatic vegetation, rock, cobble, deadheads and other structures are natural forms of surface area. BioHaven Floating Island matrix is a “concentrated” form of engineered surface area. All types of surface area are important for providing a structure on which biofilm can grow. Biofilm—which is essential for cleaning water—is composed of microorganisms that need nutrients and oxygen for their survival and growth. Circulation is how oxygen and nutrients are delivered to the microorganisms. 

Early on in our research efforts, some twelve years ago, a study being run at the Montana Center for Biofilm Engineering in Bozeman found that nutrient uptake could be increased by circulation and aeration. Nutrients will cycle through a waterway’s food web at least four times faster within the aerobic zone than within anoxic or anaerobic zones, which really shows that circulation affects stratification and argues for added aeration/circulation, especially in nutrient rich water.

Reduction of nutrients is key to water quality. 

Based on these findings, here at Shepherd we asked the question: How do you maximize for surface area and circulation? Conventional BioHaven Floating Islands with gravity-based circulation and wind/wave action were developing a history of excellent performance. But we continued to notice that the very best results corresponded with the greatest amount of circulation. At times we would see results that were ten-fold higher than our modeling projections. So we explored small-bubble, high-pressure vertical aeration, horizontal aeration via an impeller pump, and finally, air lift pump systems that use low pressure to move water by displacing it with air.

Adding in Circulation

This work led to our Floating streambed system (formerly known as Leviathan), run by an air lift pump. It is approximately 600% more efficient than a pump that uses compressed air to generate bubbles. At Shepherd, we have two of these streambed units, powered by three-horsepower air blowers, on either end of Fish Fry Lake. These streambeds are the reason Fish Fry lake’s water clarity has gone from 14 inches to as much as 19 feet, and why nitrogen that enters Fish Fry in unpredictable volumes, intermittently, is taken to levels of non-detect in the lake’s outflow, and ultimately why Fish Fry Lake, at 28 pounds of wild fish harvest per acre foot, with no feeding, is currently the most productive wild fishery of over 50 acre-feet in Montana.

What does it mean to be such a productive fishery? It means that kids catch fish—about one every two minutes! Many who have never fished before. It means that we keep two drum scalers on hand and operational so folks can process fish quickly and easily, and go home with meals of delicious fish fillets. It means that the water of Fish Fry Lake is inviting, and a pleasure to snorkel or dive…certainly when compared to its initial status. Just years prior, swimming was impossible due to the mats of algae.

It means that dragonflies and damselflies are almost a constant. It means that other life forms, like freshwater sponge, are now occurring in the lake, and expanding biodiversity. Freshwater sponge is also a filter feeder, so it’s also helping the lake transition to even higher water quality. It means that life forms that only occur in healthy, well-oxygenated water can now happen in the lake, where before they were disallowed.

The reverse is also true. Life forms that vector with impaired water, including mosquitoes and the midge that spreads Blue Tongue, exist here in minuscule numbers, if at all.

Other Key Features

Other key features of the BioHaven StreamBed include:

  1. Maximal nutrient cycling
  2. Potential spawning habitat for riverine species like trout (we successfully hatched Yellowstone Cutthroat in an early embodiment of streambed)
  3. Maximal water filtration
  4. Maximal water aeration, as cooler water which re-oxygenates more readily than warm water is perched at the surface for an extended period
  5. Maximizes for circulation and corresponding performance enhancement for conventional BioHavens
  6. Very quiet, inexpensive year round operation

Putting such a system into a static waterway expands potential for biodiversity. There are many life forms that occur in riverine habitat, but which don’t occur in static waterways. And the more biodiverse your waterway is, the more resilient it is to the nutrient and other contaminant issues freshwater faces today.

So: BioHaven Floating Islands passively remove the nutrients that cause HABs.  

With our floating streambed, we showed that adding standard aeration, such as an air-blower, improves their performance fourfold. 

A study by Mark Reinsel of Apex Engineering published in May 2012 issue of Environmental Science & Engineering Magazine titled, “Floating treatment wetlands mitigate lake eutrophication” noted that the product now known as the BioHaven StreamBed extrapolates the passive functioning of BioHaven islands by maximizing surface area and circulation, key components of wetland effectiveness. 

That same study provided positive results of this system (StreamBed) which is designed to provide the complete “wetland effect,” including aerobic, anaerobic and anoxic microbial nutrient conversion. This allows it to treat large, nutrient-rich stratified bodies of water, including “dead zones,” in both freshwater and marine settings. Removal of ammonia, nitrate, phosphate and soluble organic carbon has been demonstrated.

The system can move nutrients from any depth into and through the islands’ biologically active substrate. In the process, these nutrients are digested by beneficial microbes and form periphyton (attached plant and animal organisms embedded in a polysaccharide matrix, similar to biofilm), which is the base of the freshwater food chain. As these excess nutrients transition into the food chain via biofilm/periphyton, both water quality and fish growth rates can be dramatically improved.

BioHaven StreamBed can de-stratify water bodies, resulting in greatly expanded habitable zones for targeted fish species. As part of this process, high DO levels can be achieved and maintained, and water temperatures homogenized.

The new aeration scheme in the lake improves water quality by incorporating dissolved phosphorus and nitrogen into the aquatic food web, in the form of periphyton, while limiting the growth of deleterious algae. Total phosphate concentrations are reduced from about 0.04 mg/L to 0.02 mg/L, while nitrate-nitrogen concentrations decrease from about 0.6 mg/L to 0.01 mg/L.

BioHaven Streambed (BSB) / Forced-Flow StreamBed (FFSB) BSBs integrate an efficient, low-energy airlift system into the BioHaven matrix. There are two standard models:

  • The BSB has a stream channel on top that allows deep water to be drawn up into it and exposed to the atmosphere. Its main purpose is aeration.
  • The Forced-Flow BSB sits mostly below the waterline to promote optimal biofilm growth. The stream channel is submerged, allowing water to circulate through the matrix under pressure. Its main purpose is forced circulation particularly in a wastewater environment.

Both BSBs can be anchored or tethered. The blower assembly in both systems is housed onshore.

How is Aeration different from Circulation? 

Aeration or oxygenation injects oxygen into the water. In the content above, you may have observed that we used the terms aeration and circulation almost synonymously. That is because they work together to prevent fish kills and stratification of water bodies. As long as water can be exposed to the air, then an ample supply of oxygen can enter the pond, but deeper layers of water often don’t get the oxygen they need to sustain life through circulation alone. 

Aeration, like circulation, delivers important benefits to your lake or pond by mitigating problems caused by water stratification. To reiterate what we’ve said above, if your water body runs out of dissolved oxygen at the bottom (or any level!) then nothing will survive. Not the good microbes that consume the decomposing plants and fish at the bottom of the pond, and not fish. It becomes a zone of death and reduces the usable habitat for fish, depletes the food chain in the lake (the ecosystem) and allows harmful, disease causing bacteria to accumulate and emit foul odors. In fact, when the lake-bottom runs out of oxygen, chemical reactions occur that release the inorganic Phosphorus previously locked up in the sediment into the water column above. Algae love the fact that all those decomposing things are at the bottom releasing nutrients they need to thrive.

Welcoming New Nanobubbler Technologies 

Now, in 2021, innovation has allowed for the modeling of our latest product that incorporates nanobubble technology. Nanobubbles are just what they sound like, tiny bubbles. In fact, according to some sources, a nanobubble is 2500 times smaller than a grain of salt, and 64 million nanobubbles fit in one microbubble. These extremely small bubbles have properties that larger bubbles don’t have, and travel through the layers of a body of water in what’s called a “Brownian motion.” They don’t rise and burst at the top, instead, they travel all around and release dissolved oxygen randomly. They do this for a very long time. 

This is a breakthrough development, as nano bubbles are neutral buoyant. They take fourteen days to transmit their oxygen, completely, into water. And they disperse through the entire water column, including that intractable benthic zone of dense, stratified water that otherwise slips into anoxic status.

Nanobubblers thoroughly aerate an entire water column from top to bottom. Nanobubbles deliver far superior oxygen saturation than standard aerators: we expect by an order of magnitude.

Nanobubblers benefit from the surface-area provided by the BioHaven matrix, which augments the lake biology and brings microbial processes to the oxygen source.

This winter we are developing an off-grid nanobubbler system, called the NanoHaven. If the NanoHaven is successful, it means that even deep lakes can be productive from top to bottom.  It will also bump up water clarity, meaning that sunlight, the second “engine of life,” will be brought to bear on most, if not all, of the water column.

The NanoHaven can be powered by solar panels to clean impaired water off-the-grid.

Clear water, healthy, life-filled well-circulated layers of water, are all part of a healthy, balanced ecosystem, with a little help from the BioHaven matrix and its partners.