Short version:
Fertilisers lead to pond death.
I like the sound of the word – YOU-TRO-FIH-KAY-SHON, though it’s a pretty hideous thing.
Large scale farming requires large scale amounts of inorganic fertilisers. Inorganic in Chemistry means not containing carbon, which basically means not derived from living matter. Inorganic fertilisers are cheap and easy to transport. You can see how inorganic fertilisers are made in ‘Fertilisers’ and why their production is worrisome. Of course, fertilisers are required now for most of our farming methods – to provide cheap accessible food. The ions (atoms or molecules with a charge) in the fertilisers are water soluble (dissolve easily in water) which is great as it means the plants are able to take them up through their roots. However, water soluble also means they can be washed away (leached) from the soils. This means that the ions go into the waterways and end up in lakes, rivers and ponds. This increase of nutrients in water is called eutrophication.
Hey, that doesn’t sound so bad right? Fertilisers in water means more plant growth, which means more food in the food chain, right? Afraid not:
The most soluble ions are the nitrates (NO3–). Fertilisers are high in nitrates as these are required by the plants to make plant proteins and needed to make chlorophyll (required for photosynthesis). Nitrates encourage large amounts of leafy growth. In ponds this includes surface algae. If you see a pond that looks green on top, this would be the algal bloom. The algae covering the surface stops light from reaching the plants below the surface. These plants can no longer photosynthesise and will therefore die. No photosynthesis means that oxygen is no longer being produced. The decomposing bacteria then break down the dead plants and reproduce rapidly. The decomposition of the plants gives off more nitrates, compounding the problem. The respiration of any organisms in the water, along with the increasing number of bacteria uses up the oxygen. As oxygen is used up, life in the water dies. Fish and everything else go belly up. The algae will also eventually die.
With the surface covered, any plant life below the surface will not get enough light to photosynthesise
This tends to happen to ponds. Moving water will constantly be diluted so the effects of the nitrates are less damaging. As water moves it gets aerated and therefore oxygen levels increase (basically things like waterfalls, fountains, rushing water, etc allows for more oxygen to dissolve in the water).
Here the water was fed from a stream so there is water movement and therefore less algae
In hot weather all the metabolic processes are sped up and water can evaporate (which increases the concentration) so everything happens quicker.
Organic fertilisers (like manure) has nitrogen containing compounds that are less soluble so leach more slowly.
Not a Horticulturist 
3 thoughts on “Eutrophication”