The passage describes the ecological impacts of agricultural runoff and domestic waste disposal on aquatic systems. Complete the passage by writing a suitable word or words in each of the spaces.
When heavy rainfall occurs, soluble minerals from synthetic fertilisers are carried into watercourses via [1] _______________\text{\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_}_______________. This raises the levels of key nutrients, particularly [2] _______________\text{\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_}_______________ ions. The resulting rapid proliferation of [3] _______________\text{\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_}_______________ blocks sunlight, preventing benthic plants from photosynthesising.
Separately, the discharge of raw domestic sewage adds large amounts of organic compounds, such as [4] _______________\text{\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_}_______________, to the stream. Saprobiontic [5] _______________\text{\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_}_______________ multiply rapidly as they metabolise this organic matter. These organisms carry out aerobic [6] _______________\text{\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_}_______________ at an extremely high rate, which drastically lowers the concentration of dissolved [7] _______________\text{\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_}_______________ in the river, leading to the death of fish.
