Aquatic Ecosystems

Essential Question

How do sunlight, salt, and moving water shape life in water?

Water covers most of Earth’s surface, but a river, a lake, an estuary, and the open ocean support very different communities. In this lesson, you’ll learn how salinity, light, temperature, and the movement of water shape life in aquatic ecosystems, and how to use data to explain those patterns.

Fresh Water, Estuaries, and Ocean

• Salinity: the amount of dissolved salt in water
• Fresh water: rivers, lakes, ponds, and most wetlands
• Estuary: where a river meets the sea
• Ocean: the largest aquatic ecosystem

Aquatic ecosystems are sorted first by salinity, the amount of dissolved salt in the water. Fresh water, found in rivers, lakes, and ponds, has a salinity below about 0.5 parts per thousand. Ocean water averages about 35 parts per thousand. An estuary, where a river flows into the sea, has brackish water between the two, and its salinity changes with the tides and with river flow. Most organisms are adapted to a limited range of salinity, so the salt level strongly affects which species live in a water body.

Light Zones

• Sunlight weakens with depth
• Photic zone: enough light for photosynthesis
• Aphotic zone: too little light
• Producers live in the photic zone

Water absorbs and scatters sunlight, so light gets weaker with depth. The photic zone is the upper layer that receives enough light for photosynthesis. In clear open ocean it can reach a couple of hundred meters, but in a muddy river or a lake full of algae it can be only a few meters deep. Below it is the aphotic zone, where photosynthesis cannot happen. Most producers, such as algae and phytoplankton, live in the photic zone. Life below it depends mostly on organic matter, such as dead organisms and waste, that sinks from above.

Temperature and Dissolved Oxygen

• Aquatic animals need dissolved oxygen
• Colder water can hold more oxygen
• Warm water holds less oxygen
• Oxygen enters from air and photosynthesis

Fish and other aquatic animals take in dissolved oxygen, oxygen gas mixed into the water. Oxygen enters the water from the air and from photosynthesis by aquatic producers. The graph shows that the maximum amount of oxygen water can hold falls as the water warms: 14.6 mg/L at 0 °C but only 7.6 mg/L at 30 °C. Warm water holds less oxygen at the same time that the metabolism of animals speeds up and they need more, which is one reason warm summer conditions can stress fish. Real oxygen levels also depend on photosynthesis, respiration, decomposition, and mixing of the water.

Moving Water and Nutrients

• Rivers carry nutrients downstream
• Estuaries trap nutrients where river meets sea
• Upwelling brings deep nutrients to the surface
• More nutrients often means more producers

Moving water carries nutrients from place to place. Rivers carry nutrients from the land to lakes and the sea, and estuaries, where that water meets the ocean, are often rich in nutrients and serve as nurseries for many young fish and shellfish. In the open ocean, the surface sunlit water is often short of nutrients, because organisms near the surface use them up and dead material sinks. In upwelling, winds push surface water away and cold, nutrient-rich deep water rises to replace it. Where upwelling occurs, such as along parts of the coasts of California, Peru, and western Africa, phytoplankton tend to be abundant and the food web supports large fish populations.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Warm Water and Oxygen

Use the graph. A stream warms from 10 °C in spring to 25 °C in summer. How does the maximum dissolved oxygen the water can hold change? What might that mean for fish? Write a claim, evidence, and reasoning.

Show the solution

1. Read the graph: at 10 °C the water can hold 11.3 mg/L, and at 25 °C it can hold 8.3 mg/L.
2. Change: 11.3 − 8.3 = 3.0 mg/L less.
3. Claim: warmer water can hold less dissolved oxygen.
4. Evidence: the maximum fell from 11.3 mg/L at 10 °C to 8.3 mg/L at 25 °C, a drop of 3.0 mg/L.
5. Reasoning: the gas dissolves less in warm water. Animals in warm water also tend to use oxygen faster, so fish may be under more stress in summer, especially if oxygen is further lowered by decomposition.
6. Answer: It falls by 3.0 mg/L (from 11.3 to 8.3 mg/L), so warm water gives fish less oxygen.

Example 2: A Cloudy Lake

Use the light-zone model. Soil washing in from a construction site makes a lake very cloudy. Predict what happens to the photic zone and to the producers living near the lake bottom.

Show the solution

1. Cloudy water: particles scatter and absorb light, so light weakens faster with depth.
2. Photic zone: it becomes shallower, because the depth where light is too weak for photosynthesis moves up.
3. Bottom producers: plants and algae on the bottom of a lake that was once lit may no longer get enough light, so they are likely to decline.
4. Effect on the community: animals that depend on those producers for food or shelter may decline too.
5. Answer: The photic zone gets shallower, so bottom producers that no longer get enough light are likely to decline.

Example 3: Weaker Winds, Fewer Fish?

Use the upwelling model. In one year, the winds that normally drive upwelling along a coast are much weaker. Predict how phytoplankton and the fish that eat them are affected.

Show the solution

1. Normal case: wind pushes surface water offshore, and nutrient-rich deep water rises.
2. Weaker winds: less deep water rises, so fewer nutrients reach the sunlit surface.
3. Phytoplankton: with fewer nutrients, they are likely to be less abundant.
4. Fish: fish that eat phytoplankton, or eat animals that do, are likely to find less food, so their numbers may fall. Other factors, such as fishing and temperature, also matter.
5. Answer: Fewer nutrients reach the surface, so phytoplankton and the fish that depend on them are likely to decline.

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Practice

Classify water by salinity, use a light-zone diagram, use temperature and oxygen data as evidence, and predict the effect of changes in water movement. Try it on your own first, then check your work with the answer key.

• Aquatic Ecosystems Practice Worksheet (PDF)
• Aquatic Ecosystems Practice Answer Key (PDF)

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Lesson Slides

• 3.3 Aquatic Ecosystems Slides (PDF)

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Big Ideas

• Salinity sorts aquatic ecosystems into fresh water, estuaries, and ocean
• Light gets weaker with depth, and producers live in the photic zone
• Warm water holds less dissolved oxygen
• Moving water carries nutrients, and upwelling brings them to the surface
• Aquatic communities are shaped by salt, light, temperature, oxygen, and nutrients

Back to the essential question: in water, salinity, light, temperature, dissolved oxygen, and the movement of nutrients play the role that temperature and precipitation play on land. They decide where producers can grow and which animals can live there. Next, you’ll look at how species within a community interact, beginning with competition.

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