A fast-flowing river sweeps soil from the riverbed so that plants cannot grow there. On the other hand, there is more oxygen dissolved in the water, so that fish such as salmon thrive. Rivers in areas where the soil is peaty often have very little wildlife, because acid from the soil washes into the water.
Unlike temperature and dissolved oxygen, the presence of normal levels of nitrates usually does not have a direct effect on aquatic insects or fish. However, excess levels of nitrates in water can create conditions that make it difficult for aquatic insects or fish to survive.
Algae and other plants use nitrates as a source of food. If algae have an unlimited source of nitrates, their growth is unchecked. So, why is that a problem?
A bay or estuary that has the milky colour of pea soup is showing the result of high concentrations of algae. Large amounts of algae can cause extreme fluctuations in dissolved oxygen. Photosynthesis by algae and other plants can generate oxygen during the day. However, at night, dissolved oxygen may decrease to very low levels as a result of large numbers of oxygen consuming bacteria feeding on dead or decaying algae and other plants.
Eutrophication – “The process by which a body of water acquires a high concentration of nutrients, especially phosphates and nitrates. These typically promote excessive growth of algae. As the algae die and decompose, high levels of organic matter and the decomposing organisms deplete the water of available oxygen, causing the death of other organisms, such as fish.
Anoxia is a lack of oxygen caused by excessive nutrients in waterways which triggers algae growth. When the plants die and decay, oxygen is stripped from the water, which then turns green or milky white and gives off a strong rotten egg odour. The lack of oxygen is often deadly for invertebrates, fish and shellfish.