An aquatic ecosystem is a water-based environment that supports significant biological activity. There are several types of aquatic ecosystems including freshwater systems like lakes and rivers, groundwater, brackish water, and marine environments. Aquatic ecosystems provide important services like water purification, biodiversity, and breeding grounds for many species. Key factors that determine the organisms in an aquatic ecosystem include temperature, sunlight, oxygen, and nutrients. Plankton, nekton, and benthos are the main groups of aquatic organisms. Natural and human-caused changes can impact aquatic ecosystems, such as eutrophication from excess fertilizer runoff.