Where we are in the unit

• Modules 1 to 7 showed that an aquatic ecosystem depends on a delicate set of conditions: the unique properties of water, dissolved gases, light, temperature, salinity and pressure, and that organisms are adapted to those conditions.
• This final module of Unit 1 asks: what keeps the whole system in balance, what disturbs it, and what can humans do about it? It also touches the unit's "included issues": water pollution, climate change, sustainability of water resources, conservation of biodiversity, water resources management, and population growth.

Ecological balance is a state of dynamic stability that occurs when living organisms within an ecosystem interact with the non-living components in a way that sustains the continuity of life.

• "Dynamic" means the system is constantly changing (organisms are born and die, nutrients are used and recycled, energy flows) but, in a healthy ecosystem, the changes tend to cancel out so that the main features stay roughly the same over time, like a bathtub with the tap and the drain both open at the same rate.
• The balance involves three things: the balance of nutrients, the diversity (and numbers) of living organisms, and the flow of energy through food webs.