1.3 Aquatic Biomes

Aquatic biomes are water-based ecosystems split into freshwater systems, like streams, rivers, ponds, lakes, and freshwater wetlands, and marine systems, like oceans, coral reefs, marshes, and estuaries. In AP Environmental Science, you should connect aquatic life and marine resources to salinity, depth, turbidity, nutrients, and temperature.

Aquatic Biomes APES

In AP Environmental Science, aquatic biomes are water-based ecosystems grouped into freshwater and marine systems. Freshwater biomes include streams, rivers, ponds, lakes, and freshwater wetlands. Marine biomes include oceans, coral reefs, marshland, and estuaries.

For Topic 1.3, focus on the environmental factors that explain where organisms and marine resources are found: salinity, depth, turbidity, nutrient availability, and temperature. If a question asks why fish, algae, or coral are distributed unevenly, connect the answer to one of those conditions.

Questions can ask you to predict how a change in water conditions affects an aquatic system's productivity. The freshwater versus marine distinction also carries into later units on water pollution, eutrophication, and wetland loss.

Salinity and Light Shape Aquatic Life

• Freshwater biomes include streams, rivers, ponds, lakes, and freshwater wetlands; they have low salinity.
• Marine biomes include oceans, coral reefs, salt marshes, and estuaries; oceans, coral reefs, and salt marshes have high salinity, while estuaries are brackish, with salinity between that of fresh and salt water.
• Algae are the major photosynthetic organisms in aquatic biomes.
• Algae in marine biomes produce a large share of Earth's oxygen and also absorb carbon dioxide from the atmosphere.
• The distribution of marine resources like fish depends on salinity, depth, turbidity, nutrient availability, and temperature.
• Sunlight only reaches the upper layers of water, so photosynthesis is limited to the photic zone near the surface.
• Estuaries, where freshwater meets salt water, are highly productive because rivers deliver nutrients.

Freshwater Biomes

Freshwater biomes have low salinity and support fish, amphibians, reptiles, mammals, and birds.

Streams and Rivers

Streams and rivers both have flowing fresh water. Rivers are usually wider and carry more water than streams, though most streams eventually join a river. Because the water keeps moving, algae and aquatic plants are often less abundant, and much of the organic matter comes from nearby terrestrial biomes (like a fallen leaf). Fast-moving water tends to hold more dissolved oxygen, which supports freshwater fish.

Lakes and Ponds

Lakes and ponds have standing fresh water, with a section deep enough to prevent emergent vegetation (where aquatic plants grow up and out of the water). Lakes are generally larger than ponds.

Source: RMB Env Lab

Lakes are classified by how productive they are:

• Eutrophic lakes have a high level of productivity.
• Mesotrophic lakes are moderately productive.
• Oligotrophic lakes have low levels of nitrogen and phosphorus and are the least productive.

Lakes and ponds can be divided into several zones:

• The littoral zone is the shallow area near the shore where most photosynthesis occurs.
• The limnetic zone is the deeper open water, reaching as far as sunlight can penetrate.
• The profundal zone sits below the limnetic zone, too deep for sunlight, so it supports few organisms.
• The benthic zone is the muddy bottom of the lake.

Freshwater Wetlands

Freshwater wetlands are among the most productive biomes on Earth. They are submerged for part or all of the year but stay shallow enough in most areas to support emergent vegetation. Their soils are often saturated with water, which creates specialized living conditions and holds many nutrients. Wetlands help reduce flooding and drought by absorbing heavy rainfall, filter pollutants from water, and give birds a place to migrate and breed.

Marine Biomes

Marine biomes have high salinity and support fish, shellfish, seaweeds, marine mammals, and more.

Salt Marshes

Salt marshes contain non-woody emergent vegetation and flood partially or entirely with salt water rather than fresh water. They are often found along coasts in temperate climates and near estuaries, where freshwater rivers meet salt water. Because those rivers carry nutrient-rich material to the ocean, salt marshes are extremely productive for plants and algae. They also help prevent flooding and reduce coastal erosion.

Mangrove Swamps

Mangrove swamps contain trees whose roots are submerged in water. These dense roots help protect natural infrastructure and reduce damage from hurricanes and storms. Mangrove trees can survive in salt water, so these swamps grow in estuaries or shallow salt water areas. This biome is found in tropical and subtropical regions.

Source: Unsplash

Intertidal Zones

Intertidal zones are narrow bands of coastline between high tide and low tide. During high tide the zone is submerged, so conditions are relatively stable. During low tide the water recedes and exposes organisms to harsh conditions. This zone is home to barnacles, sponges, sea stars, and crustaceans that cling to rocky surfaces. Species here must handle constantly changing conditions like shifting salinity and exposure to sunlight.

Estuaries

Estuaries form where freshwater rivers meet salt water, creating brackish conditions. Rivers deliver nutrient-rich material here, which makes estuaries highly productive and important nursery grounds for many marine species.

Coral Reefs

Coral reefs are found in warm, shallow waters. Corals are made up of tiny animals that produce limestone to form a hard external skeleton. They live in warm, shallow water that sunlight can reach because they rely on photosynthetic algae for food.

Coral reefs are Earth's most diverse marine biome, but pollution and warming have become a growing problem. Coral bleaching happens when the algae inside corals die, which can cause the corals to die as well. The ocean is also undergoing acidification as it absorbs more carbon dioxide, which lowers the pH of the water and stresses corals.

The Open Ocean

The open ocean contains deep water that sunlight cannot fully penetrate. It is home to many marine organisms living at different depths.

The ocean can be divided into layers:

• The photic zone is the upper layer that sunlight can penetrate, allowing photosynthesis to occur.
• The aphotic zone sits below the photic zone, where there is not enough light for photosynthesis.
• The benthic zone is the muddy bottom of the ocean, just like in lakes.

According to the National Oceanic and Atmospheric Administration (NOAA), less than 10% of the ocean has been explored.

Describing Aquatic Biomes and Matching Their Conditions

Free Response: When a question asks you to describe an aquatic biome, name specific features instead of staying vague. For a marine system, point to salinity, depth, turbidity, nutrient availability, and temperature as the factors that shape which species live there. For freshwater systems, mention salinity, flow, dissolved oxygen, and productivity.

MCQ: Multiple-choice questions often test whether you can match a biome to its conditions. Watch for clues like "brackish water" (estuary), "warm shallow water with photosynthetic algae" (coral reef), or "low nitrogen and phosphorus" (oligotrophic lake).

Common Trap: If a question asks why photosynthesis drops with depth, connect it to light. Red light is absorbed in the upper 10 m of water, and even blue light only reaches about 200 m in the clearest water, so photosynthesis is limited to the surface layers.

Common Misconceptions

• Misconception: Large rooted plants, not algae, are the major photosynthetic organisms in most aquatic biomes.
  Actually: Algae, not large rooted plants, are the major photosynthetic organisms in most aquatic biomes. Don't assume seaweed or pond weeds do most of the work.
• Misconception: Eutrophic automatically means "healthy."
  Actually: Eutrophic does not automatically mean "healthy." High productivity from excess nutrients can lead to low oxygen and dead zones later on.
• Misconception: Estuaries are pure freshwater or pure salt water.
  Actually: Estuaries are not pure freshwater or pure salt water; they are brackish, where the two mix.
• Misconception: Coral bleaching is the instant death of the coral.
  Actually: Coral bleaching is the loss of the symbiotic algae, not the instant death of the coral. Corals can survive short bleaching but die if stress continues.
• Misconception: "Photic zone" refers to where the water is warmest.
  Actually: "Photic zone" refers to where light reaches, not where the water is warmest. Light, not temperature, is what limits photosynthesis with depth.

Related AP Environmental Science Guides

• 1.1 Introduction to Ecosystems
• Unit 1 Review: The Living World: Ecosystems
• 1.2 Terrestrial Biomes
• 1.4 The Carbon Cycle
• 1.5 The Nitrogen Cycle
• 1.6 The Phosphorous Cycle