What causes a formerly pristine, 'good', healthy lake or pond, to go 'bad'?
Years ago, when the lake was 'good', there weren't so many shallow areas, and the water was clear enough to see well down into the water column. However, as the lake has aged, the build up of silt and decaying plant matter has occurred and the bottom of the lake has becomes a mucky, smelly mess.
This process, known as eutophication (ageing), can takes years, spurred by natural production of plants utilising energy from sunlight mixing with air and water through a process called 'photosynthesis'. However, even though it's a natural process, the uncontrolled accumulation of organic matter on a lake bed is the primary cause for a lake to go bad, and bad water leads to trouble, such as fish kills.
Ironically, as organic matter builds up, the problem is compounded. More organic matter feeds more plants which creates more organic matter. It's a vicious, normal and natural cycle.
Another irony revolves around mankind's modern life style. More cars, more people and more food production, leads to the faster eutrophication of a pond or lake.
Sewage treatment plant outflows and runoff from agricultural land discharge into streams and rivers. Rainfall collected from roads, pavements and rooftops is discharged into storm water drain systems, picking up whatever it can on the way, all this being flushed downstream, which often ends up in our lakes and ponds.
Even where lakes and ponds have no obvious surface inflow, fertilisers, grass clippings, tree leaves and septic tank outflows will percolate through the soil, depositing nutrients into your lake or pond.
Nature has many rules, one of these being that, when any kind of nutrient enters a water course, it won't just sit there. It will grow something, somehow, some time.
An increase in nutrient levels in a body of water increase biological productivity, especially algae and aquatic weed growth.
As this buildup occurs, natural processes work to break down dead organic matter, sucking oxygen from the water in the process, placing increased demand on dissolved oxygen levels.
In addition, living plants also consume oxygen during the hours of darkness.
Oxygen problems occur when this consumption exceeds the amount of oxygen produced through photosynthesis and diffusion from the air.
To compound the issue, lake and pond water responds in the most natural of ways, as greenery grows, water stratifies. Thermal stratification adds to an already growing problem.
Dense algae blooms accumulate near the surface to absorb sunlight, with this dense living microscopic mass hording sunlight and shading the lower depths. The result is that most of the oxygen produced is now located near the surface, leaving a larger volume of water underneath the fertile surface layer.
Deeper water is deficient in oxygen. As the sun warms the surface layers, the shady area underneath remains cool and, since warmer water is lighter, it floats on top of the cooler, heavier water. At some point, there is a well defined temperature separation called the 'thermocline'. Oxygenated water from the surface cannot push through the thermocline.
Water below the thermocline begins to process the organic build up without oxygen. The result being an increase in toxic gases of hydrogen sulphide and carbon dioxide.
Okay, so what does this have to do with you? Well, plenty. This is life under water in your pond, my pond, everyone's pond and, to understand how best to manage your water, you should know how things work so you can develop a thoughtful strategy to counter the processes.
The addition of dissolved oxygen in lakes and ponds comes through both plant photosynthesis and diffusion from atmospheric air absorption.
The amount of dissolved oxygen in a body of water can vary considerably from pond to pond and from hour to hour. However, oxygen concentrations are typically lowest at dawn and highest during the late afternoon, with the amount of oxygen water can hold being dependent on atmospheric pressure, salinity and temperature.
The amount of oxygen fish need compared to humans is substantially different. We breathe air with almost a 21% oxygen content, whereas fish abstract oxygen from water that, when totally saturated, contains only 8 parts per million, (expressed as 8 milligrams per litre) of oxygen, not much at all.
So, how in the world can a pond 'not' keep enough oxygen to keep fish alive forever and ever?
Water has its limits and juggles a lot of processes. Chemical transformations and interactions are where the actual magic takes place.
Start with nitrogen transformation. Of the many combined forms of nitrogen present in a body of water, the most important are ammonia and nitrate. Both can be assimilated (absorbed and used) to produce amino acids to be used by both algae and weeds. After all, nitrogen is a nutrient for plants! Stir in some phosphorous (from the fertiliser that washed in from the rain storm event) and we have the makings of a natural soup that all plants love.
Decomposition of organic matter (dead algae and plants) results in release and accumulation of ammonia.
Under aerobic (with oxygen) conditions, ammonia is oxidised in a two stage process called nitrification (stage 1 to nitrite; stage 2 to nitrate).
However, under anaerobic (without oxygen) conditions, the nitrification of ammonia to nitrate does not occur, and ammonia accumulates at the bottom of the pond.
Enhancing nitrification of ammonia to nitrate, and subsequent use of nitrate in de-nitrification, can stabilise and reduce phosphorus loading, phosphorous being the food source for algae and aquatic plants!
Here is where all of this science ties together. Oxygen loss is directly related to the process of eutrophication. Add sediment phosphorous release, which feeds algae and weeds, and the potential for a lake to be stripped of oxygen, resulting in a potential fish kill, quickly becomes reality.
The solution is the addition of a well designed aeration system, be it from surface spray units or lake bed units, to relieve the natural symptoms by increasing the rate of dissolved oxygen input and increasing the aerobic respiratory capacity.
Aeration affects almost all aspects of the lake in:
• Nutrient cycling
• Heat distribution
• Aerobic (good) bacteria populations that break down muck on the pond bottom
• Decreased phytoplankton populations that block the sun's rays
• And, increases fish health and populations
Aeration allows the pond to breathe and speed up aerobic digestion (good digestion) or help the pond respirate and ensure achievement of aerobic benefits.
Lake and pond owners can choose from many mechanical aeration and circulations options available today.
Surface aerators are floating units with a pump or motor mounted beneath the float.
They are efficient as an emergency aerator, or to keep a constant oxygen supply in the water body.
In a deeper body of water, a surface aerator may need the addition of a 'draft tube' to have the ability of drawing water from below the thermocline.
As a rule of thumb, a typical display aerator, with a draft tube, requires 1-1.5hp per surface acre for proper aeration.
Lake bed aeration is a method of compressing atmospheric air and pumping it to the lake bottom, allowing it to flow through a series of lake bed positioned 'air diffusers', which create micron-sized bubbles that act as an airlift.
The misconception with lake bed aeration is that oxygen is transferred to the water through the bubble. Whereas, in reality, less than 5% of oxygen is transferred this way.
As bubbles rise through the water column they expand. Cascading bubbles entrain cold, dense, oxygen starved water below the thermocline and lift it to the surface, allowing the hydrogen sulfide and carbon dioxide gases to escape to atmosphere.
At the same time, oxygen is absorbed into the surface water, and eventually circulated throughout the entire water column. The bottom of the pond becomes aerobic and metals such as iron are precipitated out of the water, while phosphates are again rendered unavailable for the algae to use as a food source.
Unlike surface units, lake bed aeration uses an unconfined airlift technique, with the amount of energy required to circulate the entire volume of the lake or pond reduced.
To size a system using this technique, the entire water volume of the water body is calculated, together with the number of bottom placed diffusers to 'turnover' the entire water volume once per day.
Your lake or pond, and its fish, depend on oxygen for life. Just like you and me, if the oxygen supply is cut off, for even fifteen minutes, fish die. It does not matter how high the level was earlier in the day, you must be concerned with extremes as well as the chronic low levels that may not necessarily kill the fish, but definitely stress them.
As low oxygen conditions continue, the build up of harmful gasses continues, thermal stratification will become worse and the potential of a deadly mixture, that can result in a fish kill, increases.
In a fertile green water pond, algae will typically draw more oxygen at night than will the fish.
A heavy accumulation of sediment can also draw more oxygen than the fish. This, combined with the green water at night, may result in seeing dead fish spread across your pond in the morning.
Here are some actual numbers:
- 1kg of fish will consume about 0.3 grams of oxygen per hour
- Ten thousand litres of 'green water' will consume about 190 grams of oxygen per hour
- Sediment oxygen demand for a mucky pond is 2.27 grams per square metre.
- With no mechanical aeration, and relying on the wind, still water rate of oxygenation from the atmosphere at night is about 0.0075 grams per square metre per hour
In summary, if left alone, nature will try to turn your lake or pond into a marshland.
You have the ability to slow down and even reverse that process. An aeration system, be it a display aerator or lake bed system, is the life support for your pond.
If your lake or pond has gone 'bad', then aeration may be your best rescue solution.
For more information on lake and pond aeration systems contact Hydroscape Ltd. Telephone: 01425 476261 or email: firstname.lastname@example.org