Which of the following is associated with changes in oxygen levels in water?

Fertilizer runoff is closely linked to changes in oxygen levels in water primarily due to the introduction of excess nutrients, such as nitrogen and phosphorus, into aquatic ecosystems. When fertilizers are washed into lakes, rivers, or oceans, they stimulate algae blooms through a process known as eutrophication. These blooms can rapidly proliferate and eventually die off, leading to increased decomposition by bacteria.

As bacteria break down the decaying algae, they consume significant amounts of dissolved oxygen in the water. This depletion of oxygen can create hypoxic conditions, where oxygen levels drop critically low, which can lead to fish kills and loss of biodiversity in the aquatic habitat. Thus, fertilizer runoff directly impacts the oxygen dynamics in aquatic environments, making it the correct answer.