A3. Effects of an increase in dissolved oxygen

1. Supports respiration: aquatic organisms depend on dissolved oxygen to breathe; more oxygen improves their ability to breathe.
2. Improves metabolism: adequate oxygen supports metabolism and promotes growth.
3. Increases activity: organisms swim, hunt and reproduce more actively.
4. Maintains ecosystem balance: a healthy level of dissolved oxygen supports diverse populations of fish, invertebrates and plants and a stable ecosystem.

Research activity (my own summary): what lowers dissolved oxygen, and what are the effects?

Causes of oxygen shortage: high water temperature (lower solubility); eutrophication (excess nutrients from fertilisers and sewage → algal bloom → algae die → bacteria decompose them and use up the oxygen); organic pollution (sewage, food-factory waste); stagnant water without waves or flow; dense cloud or ice cover reducing photosynthesis; salinity increase; excessive stocking of fish farms.

Effects: fish and invertebrates become stressed, slow and stop feeding; mass fish deaths ("fish kills"); sensitive species (trout, mayfly larvae) disappear first; anaerobic bacteria multiply and produce foul gases (hydrogen sulfide); "dead zones" with almost no life form in lakes and some coastal seas; loss of biodiversity and of food chains.