Lake Notes
Having been part of the lake community for the past six years I have seen the passion the lake community has for their lakes. With a background as a research scientist and with more than 25 years of teaching biology, environmental science and agriculture I thought I might be able to provide a little insight into a lake ecosystem and why things are occurring in lakes today.
Below are four readings. The first, “What’s in a Lake?” describes the basic lake ecosystem. The second reading, “How are Lakes Classified”, briefly describes three types of lake ecosystems. The next reading, “Oh No My Lake is Eutrophic”, addresses some of the causes of eutrophication and problems associated with eutrophication. The final reading, “What’s up with Cyanobacteria?”, explains why so many lake system world wide are experiencing cyanobacteria blooms.
What is in a Lake?
For many of us lakes are a place for fun and relaxation. Whether it’s trying to catch a fish of our dreams, tubing behind a boat, riding a jet ski over the wake of a boat or maybe just sitting on the lake enjoying a beautiful sunset. But as we enjoy all the offerings of lakes we need to understand that lakes are some of the most complex ecosystems on earth. Lakes are living systems comprised living and non-living components. Most of us are familiar with many of living organisms found in a lake. Aquatic plants, algae, aquatic insects and fish are organisms we have all seen in a lake. However, organisms such as zooplankton, microscopic animal-like organisms, or phytoplankton, plant-like microscopic organisms, along with many different types of bacteria inhabit lakes. All the organisms found in a lake from the smallest bacteria to the largest fish make up what scientists refer to as the food web of the lake. Food webs are large food chains that show how the organisms in the lake depend on each other for survival. At the bottom of this food web are the organisms that use the sun to make their food, primary producers. Aquatic plants, algae and cyanobacteria convert sunlight into the energy through a process called photosynthesis. They use this energy to grow and reproduce. These types of organism are the primary producers of the lake. They produce their own food. During photosynthesis these organisms take carbon dioxide into their cells, convert it to sugar and releases oxygen into the water. This increases the oxygen level in the lake allowing organisms like zooplankton, fish and bacteria to survive.
Just like in your garden, the aquatic plants, algae and cyanobacteria all need nutrients such as nitrogen and phosphorus. Lakes get their nutrients from within the lake when good bacteria breakdown dead matter, leaves, dead fish, plants, and from the weathering of rocks in the lake. Lakes also get nutrients from outside sources such as fertilizer run-off from yards or farms, grass clippings, and animal waste from farm animals, humans and wildlife. In a healthy lake system there is a balance between the aquatic plants, algae, cyanobacteria and the nutrients in the lake. Since the primary producers are at the bottom of the food web they determine how many consumers can inhabit the lake. Consumers are organisms that eat, consume, other organisms. So, organisms like fish, waterfowl, zooplankton depend on the producers for their food.
Just as in any living system, a delicate balance that must be maintained in a lake in order for the lake to remain. But also, as with any living system, lakes can usually adapt to changes that occur within the lake. However, when these changes become too extreme the lake cannot naturally correct its self and human intervention is required. Unfortunately, lakes throughout the world are now experiencing excessive amounts of nitrogen, phosphorus, and carbon causing the lakes to become so unbalanced that humans must intervene in order to keep the lakes healthy.
How are Lakes Classified?
The first installment of Lake Talk explained what was in a lake and how primary producers (aquatic plant, algae, cyanobacteria) start the food web in a lake. So, these organisms determine how productive a lake will be. The more producers a lake has the more consumers (fish, zooplankton, waterfowl, bacteria) can live in the lake. Since both producers and consumers are living they are used to determine the biological productivity of a lake. Remember producers need sunlight and nutrients in order to grow and reproduce. So, nutrients like nitrogen and phosphorus are also very important factors in determining how biologically productive a lake is. The more nitrogen/phosphorus in a lake the more producers the lake can support. The more producers in a lake the more consumers a lake can support. Science refers to the complex interaction of nutrients, producers and consumers in a lake as the trophic state index of the lake. Trophic state indices are used to classify lakes into one of three categories; oligotrophic, mesotrophic and eutrophic.
Oligotrophic lakes are deep cold lakes usually with rock shoreline, hard bottoms and clear water. These lakes have few producers, nitrogen and phosphorus levels are low and sunlight penetrates deep into the water. Because these lakes are deep and cold the oxygen levels (dissolved oxygen) in the water are high. The consumers in these lakes are usually larger and found in the deeper water due to temperature and dissolved oxygen levels. There is less dead matter (detritus) found on the bottom of these lakes. With less detritus the lake will have less bacteria. Bacteria use the detritus as food. As the bacteria breakdown the dead matter they use oxygen. Because there are fewer good bacteria in the lake the dissolved oxygen levels remain high.
Mesotrophic lakes are lakes that are in a transition stage between oligotrophic lake and eutrophic lake. Mesotrophic lake are not as deep as oligotrophic lake. The temperature difference between the surface and deeper water becomes greater during the summer causing the lake to stratify. When a lake stratifies surface water containing higher levels of dissolved oxygen cannot mix with deeper water. The deeper water begins to become lower in dissolved oxygen. Mesotrophic lakes have a higher population of producers requiring a greater supply of nitrogen and phosphorus to support the growth of these producers. The amount of algae increases slightly decreasing water clarity. Eventually there are not enough nutrients to support all the producers so some die. As these producers die they sink to the bottom of the lake where they become food for bacteria. With excess food the bacteria begin to reproduce rapidly. The increase in bacteria along with the stratification of the lake water causes a greater decrease in the dissolved oxygen levels in the deeper lake water. Eventually the deeper water becomes hypoxic (very low in oxygen) or possibly anoxic (without oxygen). This causes fish to move to shallower water. This is why mesotrophic lakes support more cool and warm water fish species (bass, sunfish, crappie, perch, chain pickerel, musky) than cold water fish (trout, salmon, whitefish).
Eutrophic lakes are similar to mesotrophic lakes. They tend to be shallow to moderately deep, the surface warms greater than deeper water during the summer months causing these lakes to stratify. The bottom of these lakes are soft and contain a large amount of decomposing organic matter (muck). Eutrophic lakes are classified as highly productive lakes due to the high number of producers and high levels of nitrogen and phosphorus. The higher levels of nitrogen and phosphorus in these lakes is due to several reasons. One reason is external nutrient loading. External nutrient loading refers to nutrients that enter the lake from outside sources. Examples of external nutrient loading would be fertilizer run-off, animal waste, inefficient septic systems, atmospheric nitrogen, grass clipping and leaf debris. Another reason for excess nitrogen and phosphorus is internal nutrient loading. Internal nutrient loading refers to nutrients that come from within the lake. Examples of internal nutrient loading would be the nitrogen and phosphorus released by bacteria as they breakdown dead matter. A more important source of internal nutrient loading is legacy nitrogen and legacy phosphorus. Legacy nitrogen and phosphorus refer to nitrogen and phosphorus that has accumulated in the muck at the bottom of the lake. This accumulation occurred over years due to farming practices, over fertilizing yards and fields, phosphates in cleaning products, organic matter and excess animal/human waste. After hundreds of years a mass amount of excess nitrogen and phosphorus have been deposited in the lake’s muck. With an unlimited supply of nitrogen and phosphorus producers grow and reproduce uncontrollably. The lake begins to fill with weed, the lake turns green with algae and cyanobacteria blooms occur. This sets up what science calls a positive feedback loop. Remember as producers die they sink to the bottom of the lake where bacteria break them down to release more nitrogen and phosphorus. Giving the producers even more nutrients allowing them to grow and reproduce at an even faster rate. As more and more producers eventually die the bottom of the lake becomes thicker with dead matter. More dead matter leads to even higher levels of bacteria. Higher levels of bacteria lead the release even more nutrients. It also leads to less oxygen at the bottom of the lake. Eventually the bottom of the lake becomes anoxic (without oxygen) and the good bacteria that breakdown the dead matter die because of too little oxygen. This prompts a shift in the type of bacteria at the bottom of the lake. Anaerobic (without oxygen) bacteria (bad bacteria) now become the bacteria breaking down the dead matter. This type of bacteria is inefficient at breakdown dead matter. They also produce toxic gasses as they break down the muck. As this happens the dead organic matter continues to build up increasing the depth of muck. Another important event occurs when the bottom of the lake becomes anoxic. Chemical reactions begin to take place that favor the release of even more legacy phosphorus. You know the story by now - more phosphorus more aquatic plants, more algae, more cyanobacteria and more muck.
Eutrophication is a natural process simply due to the natural accumulation of organic matter in lakes. A lake’s depth, temperature, elevation, latitude and human activity determine how long it will take natural eutrophication occur. Without legacy nitrogen/phosphorus or high levels of external nutrient input of nitrogen and phosphorus it takes most lakes hundreds to thousands of years to reach eutrophic conditions.
Oh No, My Lake is Eutrophic!
Chances are if you live by a smaller warm shallow lake surrounded by houses and farms the lake it is showing signs of eutrophication. Eutrophication of lakes is occurring worldwide causing water usage, recreation and economic concerns. As the eutrophication process in a lake progress the lake begins to show changes that maybe unpleasant or possibly dangerous. No one likes to see weeds, algae or cyanobacteria covering their lake. All which occur as a lake becomes more eutrophic. Scientists throughout the world are working on solutions to remediate, or prevent eutrophication. However, since lakes are living systems no lake is exactly like another lake. It’s kind of like lakes have their own unique DNA just like humans. This makes it difficult to come up with a single solution that will work on every lake.
The last installment of Lake Talk explained how excess nitrogen/phosphorus cause excessive growth in aquatic plants, algae and cyanobacteria. As these producers die and settle to the bottom of the lake the number of good bacteria increases because they now have more food. Eventually these good bacteria use up all the oxygen and the bottom becomes anoxic. This anoxic condition allows even more nitrogen/phosphorus to be release causing the producers to grow and reproduce at an even greater rate. The lake becomes less clear, aquatic plants begin to take over the lake and the dreaded cyanobacteria blooms become more common. Many lake people think that’s it my lake is doomed to become a swampy wetland. Not at all, there are things that can be done to help slow or even reverse the eutrophication process. But it takes the cooperation of the lake community to make this happen.
So what needs to be done to slow or reverse the eutrophication process? The solution may seem quite simple reduce the amount of nitrogen/phosphorus in the lake and you reduce the growth of producers. Done, the eutrophication process stops and the lake will get better. Although that is part of the solution it’s not easy to reduce the amount of nitrogen/phosphorus in a eutrophic lake. Although both nitrogen and phosphorus are important nutrients for the growth of lake producers phosphorus is usually more important, it’s called the limiting nutrient. The growth and reproduction of producers in the ecosystem are controlled by the limiting nutrient, it limits their growth. So, if you reduce the amount of available phosphate in a lake you can control the growth of aquatic plants, algae, and cyanobacteria. Reducing the amount of available phosphorus in a lake is not that easy. Remember there is a large amount of legacy phosphate stored in the muck at the bottom of the lake. When the bottom of the lake is in an anoxic state (without oxygen) chemical reactions favor the release of phosphate from the legacy store of phosphate in the muck. Anoxic conditions also promote the production of methane and sulfur both of which are toxic to the good bacteria.
So how can the amount of phosphate available to the aquatic plants, algae and cyanobacteria be reduced? Chemicals are available and approved for use in lakes to “lock -up” phosphate. One common chemical used is aluminum in the form of alum. The alum is spread over the lake, it sinks to the bottom where it undergoes a chemical reaction with phosphate. This chemical reaction renders the phosphate unavailable to the producers. Other chemical compounds that “lock-up” phosphate are also approved for use in eutrophic lakes. Chemical treatments can be successful in reducing the amount of available phosphate in the lake but how long the treatment last depends a lot on the lake’s chemistry, amount of muck in the lake and the external loading of phosphate. Using chemical treatment can cause unwanted harm to other organisms in the lake so these chemicals must be applied by certified applicators.
The other thing that still must be addressed with the use of chemical treatments is the lack of dissolved oxygen in the lake. With chemical treatment reducing the amount of available phosphate in the lake more aquatic plants, algae and cyanobacteria are going to die and accumulate at the bottom of the lake. Without the addition of oxygen, the muck will just continue to get deeper and deeper. So, it is necessary to add aeration to the lake. Bottom aeration systems do several things to help improve the lake’s water quality. As the bubbles from the aerators rise they help de-stratify the lake. This de-stratification allows higher oxygenated surface water to mix with the low oxygenated deeper water. Once the deeper water becomes oxygenated the good bacteria can begin to break down the muck. In some instances, the lake may need additional good bacteria added to help bring it back into balance. Over time and continual aeration, the lake can return to it natural balance, the level of muck will decrease, the number of weeds will be reduced and cyanobacteria blooms will dramatically decrease.
So, just because your lake is experiencing ongoing eutrophication all is not loss, it can be saved. However, there is a substantial financial cost associated with any lake treatment used to help remediate a eutrophic lake. With so many lakes undergoing eutrophication most governmental agencies simply do not have the money to treat all of these lakes. This is why it is so important for the lake community and lake users to become involved. The old adage the squeaky wheel gets the grease is true in this situation. The more information and data that can be used to support what is occurring in the lake the more attention the lake will receive. Become a “lake watcher”, become a lake advocate, get involved in fundraising, become a member of a lake association no matter what you do just become involve. It truly takes a community to remediate a eutrophic lake, but it can be done if you get involved.
What’s up with Cyanobacteria?
Anyone living near a lake anywhere in the world has probably heard of cyanobacteria (cHABs) outbreaks or blooms. In some cases, the blooms are so great the government closed the lake to the public. In other cases, the public water supply had to be shut down. Why so much concern over cyanobacteria? In order to explain that we need a little more understanding of what exactly cyanobacteria are.
Cyanobacteria are one of the oldest organisms on earth and are found in soil and water throughout the world. It is estimated that cyanobacteria have been around approximately 3.0 – 3.2 billion years. In the biology world these organisms are quite unique. They are not true bacteria because unlike other bacteria they can produce their own food through photosynthesis That means they use sunlight and carbon dioxide to make sugar and release oxygen to the environment. It is believed that cyanobacteria helped develop the oxygen atmosphere on earth. Early earth had high temperatures, little to no free oxygen, high levels of carbon dioxide, methane and sulfur yet cyanobacteria thrived in conditions that would surely have destroyed most other organisms living today.
So why was it important to know a little about cyanobacteria? What allowed cyanobacteria to thrive in early conditions on earth allows them to flourish in lakes today. They are opportunistic. As conditions in a lake worsens cyanobacteria begin to out compete other producers for the available resources. Remember the conditions that allow a lake to become eutrophic begin to create harsh condition in the lake. Low to no dissolved oxygen, warmer water, high levels of phosphorus, anaerobic (without oxygen) breakdown of dead matter releasing sulfur and methane along with elevated carbon dioxide levels allow lake conditions to resemble conditions on early earth. Cyanobacteria are able to capture more carbon dioxide than algae which allows them to undergo photosynthesis at a faster rate. Also, higher atmospheric and water temperatures actually inhibit the growth of good algae. The good algae can no longer keep the cyanobacteria in check because conditions now favor the cyanobacteria. Therefore, they can grow and reproduce at a faster rate causing cyanobacterial hazardous algae bloom (cHAB). Another special feature of cyanobacteria allows them to avoid being trapped by lake stratification. Cyanobacteria have special gas chambers cells. They can control the amount of gas in these chambers allowing the cyanobacteria to move up and down in the water column. Therefore, they can move to the surface to take advantage of maximum sunlight for maximum photosynthesis. These factors combined with an unlimited supply of phosphorus has set up the perfect conditions for cyanobacteria blooms to occur throughout lakes worldwide.
What’s the deal with cyanobacteria hazardous algal blooms (cHABs)? Certain types of cyanobacteria are capable of producing cyanotoxins. These cyanotoxins in high concentrations are harmful to people, animals and the environment. Different types of cyanobacteria produce different types of toxins. It is not well understood when/how cyanobacteria release their toxins. It’s believed that most toxins are released as the cyanobacterial cells rupture. What is known is that some of the toxins released by cyanobacteria can cause serve rapid reactions in people and animals. It is also believed that the actual lake conditions where the cHABs form can influence the level of toxicity of the bloom. In a unique twist some cyanobacteria are being investigated for their possible pharmaceutical use in treating human disease.
How do you know if the lake you live on or visiting is experiencing cHABs? If you’re lucky you have someone trained to identify cHABs watching your lake. In many areas’ government agencies such as the Department of Conservation or Department of Environmental Resource patrol lakes and will notice the occurrence of cHABs and report it. On other lakes there are lake residence that have been trained by the state to identify cHABs. These citizens go out on the lake in search of cHABs or investigate possible sightings by other lake goers in order to verify it is a cHABs. If your lake does not have a trained resident or no government agency patrols your lake how can you identify a possible cHABs? There is no way for you to be absolutely sure that it’s a cHABs. However, if it appears as a greenish oily paint slick it best to avoid direct contact with this area of the lake. If the bloom occurs repeatedly you should contact the state agency in your area that is responsible for water resource. Again, this is part of becoming involved in your lake community. The more reports of cHABs the state receives the greater the odds are your lake will be able to secure some type of funding to help remediate your lake, the squeaky wheel.
While cyanobacterial blooms are becoming more common on almost every lake they should not prevent you from enjoying your favorite lake. It is still safe to catch and release fish from a cHABs area, it’s safe to kayak and boat. It is advisable to avoid direct contact with the cyanobacteria so swimming in the area of a cHABs should be avoided. If you do come in contact with cyanobacteria be sure to wash the area of contact well with warm soapy water. Don’t let a cHABs ruin your lake experience. Lakes are still a valuable resource for fun, relaxation and should be continued to be enjoyed.