How do nutrients and heat affect dissolved oxygen?
Nutrients effect oxygen levels indirectly in a process called eutrophication. If the nutrient happens to be a limiting nutrient (such as phosphate in lakes), then the carrying capacity of the photosynthetic organisms is raised. This allows for a population growth, commonly seen as algal blooms. The population of decomposers is then boosted too, since they feed (decay) on the phytoplankton/algae. As more and more organic material settles to the benthic zone, decomposers increase and need more oxygen for respiration. This results in oxygen depletion in the ecosystem, making life for marine animals such as fish difficult/impossible.
