Aquatic ecosystems are water-based environments where organisms interact with each other and with factors such as light, temperature, salinity, oxygen, and nutrients. They include freshwater systems like lakes, rivers, and wetlands, as well as marine systems like oceans, coral reefs, intertidal zones, and the deep sea. These ecosystems matter because they support biodiversity, regulate climate, cycle nutrients, and provide food and water resources for humans.
Understanding their structure helps explain why different organisms live in different zones.
Key Facts
- Freshwater ecosystems usually have salinity below 0.5 ppt, while marine ecosystems average about 35 ppt.
- Photosynthesis in water is limited mostly by light, so the photic zone supports most algae and aquatic plants.
- Dissolved oxygen decreases when water warms because warm water holds less O2 than cold water.
- Net primary productivity can be written as NPP = GPP - R, where GPP is gross primary productivity and R is respiration.
- Energy transfer between trophic levels is often about 10 percent, so Energy to next level = 0.10 x energy at previous level.
- Wetlands filter water, store floodwater, and provide nursery habitat for many fish, amphibians, birds, and invertebrates.
Vocabulary
- Salinity
- Salinity is the concentration of dissolved salts in water, usually measured in parts per thousand.
- Photic zone
- The photic zone is the upper layer of water that receives enough sunlight for photosynthesis.
- Benthic zone
- The benthic zone is the bottom region of an aquatic ecosystem, including sediments and organisms living on or in them.
- Plankton
- Plankton are small drifting organisms, including photosynthetic phytoplankton and animal-like zooplankton.
- Eutrophication
- Eutrophication is nutrient enrichment of a body of water that can cause algal blooms and oxygen depletion.
Common Mistakes to Avoid
- Treating all aquatic ecosystems as the same is wrong because salinity, flow, depth, light, and oxygen create very different habitats.
- Assuming deeper water always has more life is wrong because light decreases with depth, limiting photosynthesis and food production in many deep zones.
- Confusing wetlands with lakes is wrong because wetlands are shallow, often plant-dominated systems that may be saturated or flooded only part of the year.
- Thinking algal blooms are always beneficial is wrong because rapid algae growth can block light and lead to decomposition that removes dissolved oxygen.
Practice Questions
- 1 A marine sample has a salinity of 35 ppt. How many grams of dissolved salts are in 2.0 kg of seawater?
- 2 A lake receives 50,000 kJ of energy in producers. If about 10 percent transfers to herbivores and 10 percent of that transfers to small fish, how much energy reaches the small fish?
- 3 A river, a wetland, and a coral reef each support different organisms. Explain how water movement, salinity, light, and nutrients help determine which organisms can survive in each ecosystem.