Ponds and lakes are living systems. Inside that giant puddle of water, there is an intricate ecosystem with many important players coexisting and shaping the characteristics of the lake from the inside out. As easy as it would be to focus on the surface of the water, it is what lies beneath the surface that keeps the lake alive.
When managing a lake, it is crucial to view the lake as the ecosystem that it is. When problems arise, the entire system needs to be considered, not just the affected area. It is just like a human body. Each part of the body of water plays an important part for the overall health.
The Pond’s Digestive System
The benthic layer is the layer of mud and muck at the bottom of the pond or lake. It is made up of the sediment that settles from the water column, as well as microorganisms and bacteria. The benthic layer is the pond’s digestive system. Organic matter settles from the water column decomposes due to the benthic bacteria. During this decomposition, minerals and nutrients are released back into the water column for the other inhabitants to use, as well as gaseous by-products (carbon dioxide and methane, for example). When a pond’s digestive system is healthy, the benthic bacteria will be able to keep up with the amount of organic matter that settles to the bottom, and it will likewise release just enough nutrients to sustain the aquatic life.
The benthic layer will dictate the overall water quality. When the benthic layer is sick, it will not be able to keep up with the organic matter. This leads to a number of issues throughout the lake — most notably, an increase in algae growth and harmful bacteria. When the benthic layer is digesting too much, the harmful gaseous by-products will start to cause odors and potentially harm the other organisms living in the lake.
Keeping the benthic layer healthy is a challenge. There are many factors that can have an influence.
When slopes surround small lakes and ponds, water will naturally flow into the lake when sprinklers are running or during any rainfall. If fertilizers are used in the surrounding landscaping, then that water runoff will be saturated with nitrogen and phosphorus. When this nutrient-rich water enters the lake, it is like throwing a bunch of junk food into the benthic layer!
The same issues occur when using reclaimed and recycled water. Often times, the nutrient loading is far greater than that of potable water, and the salinity levels are also much higher. This upsets the balance in the aquatic ecosystem.
Certain systems may also become contaminated with street or storm drain runoff. This water runoff will contain a lot of sediment, heavy metals and oil from automobiles driving on the road. This is particularly hazardous to the benthic layer because these constituents cannot be digested. Contaminants in the water will cause a buildup of sediments, nutrients, biomass, heavy metals and oils. This will cause a big stomach ache for the lake.
The Impact of Plants & Wildlife on a Pond’s Digestive System
Aquatic plants will also have an effect on the benthic layer. Aquatic plants can strengthen the benthic layer by consuming some of the nutrients and minerals before they reach the benthic bacteria. Emergent and rooted floating plants will uptake nutrients from the soil, free-floating plants will consume from the water column itself and submersed plants will consume the nutrients from both the soil and the water column. All nutrients removed by plants are nutrients that the benthic layer no longer has to digest. As long as these plants are healthy and are in balance with the size and strength of the benthic layer, the two will be symbiotic.
Wildlife, while aesthetically pleasing, is detrimental to the benthic layer. Where there is wildlife, there is feces, and where there is feces, there is nutrient excess and fecal coliform. The nutrient excess, as you can surmise, is hard for the benthic layer to keep up with. Fecal coliform, if left to fester, can turn into a serious human health hazard. Various pathogens thcan be transmitted to humans via water (E. coli, swimmers’ itch, etc.). When there is abundant wildlife in a lake, you are increasing that risk to the people who frequent the body of water.
In many water systems, the lakes are surrounded by large trees and other landscaping. While this certainly increases the aesthetic value of the system, special attention needs to be paid to the foliage. The occasional leaf or grass clipping is not going to be detrimental to the system. However, when leaves cover the entire water surface, or when the lawn is freshly mowed and the clippings end up in the lake, it becomes problematic. Foliage is organic matter, which is what would flow into natural systems, so it is “healthy food,” so to speak. But too much healthy food can be a bad thing, too.
Diagnosis & Management of Benthic Layer Issues
There are several ways to identify issues with a benthic layer. The first are visual indicators. If there is noticeable sedimentation like islands, sandbars or a noticeable buildup on the shoreline, this is a sign that there is too much biomass building up in the system.
Another visual indicator would be bubbling or boiling, like a pot on a stove. This indicates excess gas production from the bacterial digestion process. Other indicators of this issue would be bad odors coming from the water, getting a headache or feeling light-headed when you are walking around the water. (This is a result of inhaling methane and carbon dioxide.)
One of the best indicators of healthy pond muck is the presence of frogs and dragonflies. They will only lay their eggs in areas with good, stable muck. Their presence is a sign that the system is in good shape.
If it does happen that your benthic layer is not healthy, there are several management options.
The first option is biological treatment. This is like taking your vitamins. Biological treatments are a mixture of enzymes and supplemental bacteria. Enzymes are proteins that break up substrate. They are specifically designed to break apart one particular kind of substrate (another type of protein), so it is important to have a wide blend of enzymes in any treatment. Once the substrate is broken up, it becomes exponentially easier for the bacteria to be consumed.
There are two main categories of bacteria in aquatic systems, aerobic and anaerobic. Aerobic bacteria require oxygen to decompose substrate while anaerobic bacteria do not. Aerobic bacteria typically need to reside in the water column or the very top of the benthic zone to receive enough oxygen. Anaerobic bacteria thrive under the top layer of the benthic zone, where they do not face competition from aerobic bacteria. When choosing biological supplements, it is important to make sure that you have not only the blend of enzymes, but also the blend of aerobic and anaerobic bacteria. The main benefit of using biological management is that it is extremely gentle on the system and has very long-lasting effects. However, the drawback is that treatment can be slow with unpredictable results, as you are still dealing with living organisms.
The second treatment option is chemical. This is like taking prescription medication. You need to use chemical treatments when biological treatments are not strong enough, or if there are any pathogens present. Chemical treatments neutralize the benthic layer and degrade the organic matter with chemical reactions. One of the benefits is its high effectiveness; however, chemical treatments are harsher on the system. They will destroy the beneficial bacteria that you need to keep the benthic layer healthy and kill the pathogens. It could also take the ecosystem a long time to readjust from changes in chemical compositions and pH.
The final management option would be physical — like getting surgery. The premise of this method is very simple. You remove the unwanted or unhealthy muck from the system completely. The results from physical maintenance are instantaneous. Removing the sediment that has built up also reduces the effective age of the water body, meaning the water body will keep living strongly. Physical removal is extremely harsh on the aquatic environment, just like surgery is harsh on your body. It can also be quite expensive and take weeks of time.
Physical removal is definitely beneficial for some systems; however, it should not be confused with dredging. Dredging is scraping and scooping the mud and weeds with a dredge, whereas physical removal via vacuum extraction removes debris by layers. Dredging destroys the contour of the lake bed, which causes ecosystem problems later on, whereas vacuum extraction is more gentle and retains the lake contour.
Sometimes surgery is completely necessary. Just make sure you go to the doctor and not the butcher!
All in all, you need a blended system. You can’t get surgery every time your knee is sore, and taking Vitamin B-12 is not going to cure your pneumonia. You need all three management types to keep the pond’s digestive system healthy.
Preventative Maintenance Keeps the Pond’s Digestive System Healthy
Just as in healthcare, lake and pond management has preventative care. Preventative-care measures help maintain the overall health of the lake. As the lake gets healthier, it will be less susceptible to issues with the benthic layer, as well as a myriad of other issues. There are six key areas of preventative care: aeration, circulation, filtration, biological, chemical and water quality testing.
Aeration is the process of dissolving air into water. This makes sure that there is plenty of oxygen available for the aerobic bacteria, as well as the other organisms that need oxygen. There are two ways to aerate a lake — passive aeration, when the substrate and organisms are exposed to the air at the water’s surface, and active aeration, where you introduce the air into the water column using surface aerators or diffusers.
Circulation is the process of mixing the nutrients in the water so they are available to more organisms. Some systems have natural circulation that provides sufficient mixing; however, for water bodies with irregular shapes thanks to alcoves and fingers along the shoreline, it is more difficult to achieve equal circulation throughout the water. Installing features such as waterfalls will help with circulation while adding aesthetic value. There are also underwater circulating pumps that achieve very good circulation throughout the water column.
Filtration is the separation of solids from liquids. If your lake is prone to sedimentation issues, installing a filtration system would be of great benefit. Filtration will reduce the overall level of sedimentation and contaminants that would otherwise accumulate on the benthic layer.
Biological preventative care encompasses the enzymes and bacteria discussed previously. Even without an immediate issue at hand, supplementing the bacteria in the system can improve the overall lake health.
Chemical preventative care is beneficial to systems that have had ongoing algae or digestion problems. The basic idea is that you would dose your lake with very small amounts of chemicals at the end of winter when algae is first starting to grow. This will prevent the algae from blooming so drastically in the spring and summer, which would require a much higher chemical dosing. As stated, this option should only be taken with systems that have ongoing problems. You wouldn’t do this to a completely healthy lake, just like you wouldn’t take prescription medication that was not prescribed for you.
Water quality testing is like getting the lake’s blood work done. Generally, it is a good idea to test for heavy metals, pH, dissolved oxygen, alkalinity and temperature. These values are sensitive to disruptions in the aquatic environment, so they serve as good indicators for lake health.
In addition to shedding light on current problems, getting regular water quality tests can help predict future issues. If you notice the dissolved oxygen levels in your pond steadily dropping year to year, it may indicate an aeration system issue. It could also mean you have too much organic matter decomposing. The aerobic bacteria need more and more oxygen to keep up.
The benthic layer is a fascinating and vital part of any aquatic ecosystem. With a firm understanding and careful observations you can learn to better manage your water systems. Avoid trips to the doctor!
Patrick Semsgeiger is the founder and president of Diversified Waterscapes, Inc.,a provider of lake management services and aquatic treatment products. He is a licensed aquatic pesticide applicator, landscape contractor and certified lake manager.