I guess we should start with why they go bad and what bad really means. Once your pond is finished and filled up its goal is to fill back in and become a swamp or grassy meadow once again.
Many circumstances are involved in this process. From uses of the pond to the watershed that fills the pond and how much nutrient loading comes into play, along with plants, animals, and weather patterns.
There are 4 stages of Pond and Lake life cycle explained below. These terms do have many meanings and have various explanations depending on the water body. So, I will make it short and sweet
Oligotrophic having the least amount of biological productivity, “good” water quality for swimming.
Mesotrophic having a moderate level of biological productivity, “fair” water quality)
Eutrophic having the highest amount of biological productivity, “poor” water quality. Usually, the stage when fishing is great. Better chance of fish kills. Closer to the Hypereutrophic lake.
Hypereutrophic lakes are very nutrient-rich lakes characterized by frequent and severe nuisance algal blooms and low transparency, mucky bottoms, and higher thermocline. High risk of fish kills.
These terms do go into greater detail in their explanation but just wanted to show the life cycle of what could be happening in your pond. Lakes, like the Great Lakes are large and would be hard to help turn them around, where smaller ponds are much smaller, and they can be helped to survive longer with the right sized aeration system and adding beneficial pond bacteria regularly. Beneficial Pond Bacteria is a healthy bacteria all natural and for a scientific term could be called Bioaugmentation. More on those option later.
How fast will a pond go bad? A lot depends on the nutrient loading, size, and depth of the pond. Taking a quick look at on size and depth and the life span we can see where a 6’ deep and shallower pond will age much quicker than say a 20’ deep pond. Why? The 6’ and shallower, sunlight is able to reach the bottom to charge any roots, seeds and of course algae making the pond sort of a quick grower. The more growth the more mass of plant debris that eventually turns into muck, sludge, and nutrients that over time fills in the pond. Ponds are clay bottoms and rooted plant will grow quit well in the pond, just as they do on land.
Looking at deeper pond the sunlight may not be able to reach the bottom keeping it cool and dark. Less photosynthesis would mean less plant growth. Not all plants are equal, such as Water Shield can grow in depths of 9’ deep and have heard of other species that can grow from deeper depths.
Whether it is shallow or deep they do face nutrient loading, either internal loading or external loading. Internal loading is what happens in the pond, fish waste, algae growth, and plants. External nutrient loading is anything that washes into the pond, the rain itself to the watershed around the pond that washes into the pond after snow melt or rainstorms. Looking at the watershed, any land above the pond that captures rain and runs to the pond is the watershed. Look around and see what is coming into the pond. For example, homeowner association are built up and all the surface water flows to a retention pond. This means all pet waste, lawn fertilizers, grass clippings, automobile drips and road debris are all supposed to go to the pond and then metered out in the event of a 100-year storm.
Personal pond like farm ponds have the same situation, maybe not all the road debris but still fertilizers, decomposing lawn trimmings, leaves, animal waste, such as geese cows, horse, dog, and other wildlife can contribute to the pond nutrients. Rain event also wash dirt particles into the pond. I am sure you either seen it in your own pond or driving over a bridge and seeing the brown water after a rainstorm. Getting grass to grow around the new pond is a must to help slow the erosion.
The muddy brown water is simply small particles or dirt and clay that may have fertilizer on them, even the particles themselves are nutrients that will help the growth of algae and plants.
Back to the shallow versus deep pond. Yes, the deeper the pond the longer it will take to fill in and become Eutrophic. While the shallower pond will get there much faster due to the depth. Even building a pond with steep side and deep you will end up with shoreline plants such as cattails and bullrush. Hopefully not Phragmites. Cattails will usually grow out into about 4’ depth of water. They will try to go further by their root system, but if the 4’ area drop off quick they won’t grow after that.
How do plants get into the pond? Either we had planted them, or geese and other waterfowl can bring them into our ponds. Weather sometime can be a factor, such as a hurricane, tornado, or other such wind events.
Usually when these weeds / plants start to grow we do not see them for a while. They may be lurking on the bottom and eventually one year they will get it together and come to the surface. That is why most folks say I never had this problem but all the sudden I have all these weeds. Hopefully, they can be caught before this stage and be taken care of or at least come up with a plan to get them under control. If not, it becomes a longer and more expensive process to get the wees under control.
Allowing ponds and lakes to build up with muck, sludge and nutrients will decrease its lifetime quicker than taking a proactive approach to mediate the muck build up nutrient loading. Using a proper sized aeration system to promote beneficial bacteria, adding beneficial pond bacteria’s, slowing, or changing routes of incoming water to drop out the heavier nutrients allowing the shoreline to grow high to also filter out and drop out heavier nutrient will over time help the pond or lake live longer.
Aeration and beneficial pond bacteria can slow or reverse the pond aging process. Aeration gets oxygen to the bottom sediment of the pond. With the oxygen not at the floor beneficial bacteria can break down and consume the muck and sludge. When we add beneficial pond bacteria to the pond this help to accelerate the consumption process.
Slowing the external nutrient loading coming into the pond is one good way to help keep the pond living longer and with aeration and adding beneficial pond bacteria will help with the internal loading and external loading that washes into the pond