As we said, biomes can be both terrestrial and aquatic, representing large patterns of climate and species coexistence that can be divided into distinct regions.
Aquatic biomes are ecosystems classified by the presence of water (freshwater or marine) and the type of organisms that live in them.
Freshwater biomes include lakes, ponds, rivers, and streams. With low salinity in the water, these bodies support a variety of plant and animal life, including fish, amphibians, reptiles, mammals, and birds.
Marine biomes include oceans, coral reefs, and estuaries. These biomes have high salinity and support a diverse array of plant and animal life, including fish, shellfish, seaweeds, and marine mammals.
Aquatic biomes are important for the health of the planet, as they provide a habitat for a wide range of plant and animal species and play a key role in the global water cycle. They are also important for human activities, such as fishing, tourism, and recreation.
Aquatic Biomes
Streams and Rivers
Streams and rivers both have flowing fresh water. Rivers are usually wider and carry larger amounts of water than streams, although almost all streams will eventually join back into a river.. Since the water is constantly flowing, algae and aquatic plants are rare, and most organic matter is often from terrestrial biomes (like a fallen leaf). Since these biomes have fast-moving water, they usually have higher amounts of oxygen, which supports the presence of freshwater fish.
Lakes and Ponds
Lakes and ponds have standing freshwater, with an end deep enough to prevent emergent vegetation (where aquatic plants grow onto land). Lakes are generally larger than ponds.
Lakes are classified by how productive they are.
Lakes with a high level of productivity are called eutrophic lakes.
Moderately productive lakes are mesotropic. 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 of water near the shore where most of the photosynthesis occurs.
The deeper open water is called the limnetic zone, which goes as deep as the sunlight can penetrate.
Below the limnetic zone is the profundal zone, which is too deep for the sunlight to reach and therefore supports very few organisms.
The muddy bottom of the lake is the benthic zone.
Freshwater Wetlands
Freshwater wetlands are one of the most productive global biomes. They are submerged in water for part of or all of the year, but they are shallow enough in most parts to support emergent vegetation. Their soils are often saturated partially or fully in water, which allows for specialized living conditions and lots of present nutrients. They prevent flooding and drought with extensive rainfall intake, filter pollutants from water, and give birds a place to migrate for breeding.
Salt Marshes
Salt marshes are marshes that contain non woody emergent vegetation and are flooded either partially or entirely with salt water as opposed to freshwater. They are often found along the coast in temperate climates. Salt marshes are often located along estuaries, where freshwater rivers meet salt water. Because these rivers carry nutrient-rich material to the ocean, salt marshes are extremely productive biomes for plants and algae. They can also prevent flooding and extreme coastal erosion.
Mangrove Swamps
Mangrove swamps are similar to wetlands -- they contain trees whose roots are submerged in water. These roots are dense and can help preserve natural infrastructure as well as prevent natural disaster effects from hurricanes or storms. However, mangrove trees can survive in salt water as well, so mangrove swamps are located either in estuaries or in shallow salt water areas. This biome is located in tropical and subtropical areas.
Intertidal Zones
Intertidal zones are narrow bands of coastline between the levels of high tide and low tide. During high tide, the zone is submerged in water, so conditions are relatively stable. However, during low tide, the water recedes, which exposes organisms to harsh conditions. Intertidal zones are home to many organisms such as barnacles, sponges, sea stars, and crustaceans that are able to cling to their rocky surface. Species that live in this zone must be used to constantly changing conditions such as salinity level adjustment and exposure to sunlight.
Coral Reefs
Coral reefs are found in warm, shallow waters. The corals are made up of tiny animals that produce limestone to form a hard external skeleton. Corals are always found in warm, shallow waters that sunlight can penetrate because it relies on photosynthetic algae for food.
Coral reefs are Earth’s most diverse marine biome, but pollutants have created a growing problem in coral reefs. Coral bleaching is a phenomenon in which the algae inside corals die, which causes the corals to die as well. The ocean is currently experiencing acidification due to climate change, which lowers the pH of the ocean water and causes erosion, or bleaching, of coral.
The Open Ocean
The open ocean contains deep ocean water that sunlight cannot penetrate. The open ocean is home to many marine organisms that live within its depths.
The ocean is divided into three distinct layers. The photic zone is the layer of the ocean that sunlight can penetrate, allowing photosynthesis to occur. The aphotic zone is below the photic zone, where photosynthesis can no longer take place. Just like in lakes, the muddy bottom of the ocean is called the benthic zone. According to the National Oceanic and Atmospheric Administration (NOAA), less than 10% of the ocean has been explored.
🎥 To learn more about biomes, watch this stream.
Frequently Asked Questions
What are the main types of freshwater biomes and how are they different?
Freshwater biomes in the CED are streams and rivers, ponds and lakes, and freshwater wetlands. Streams/rivers are flowing systems—they have higher oxygen where water’s fast, clear, and cold (headwaters) and more turbidity/nutrients downstream; organisms are adapted to current. Ponds and lakes are standing water with vertical zones: littoral (shallow, plants), limnetic (open, photic, phytoplankton), profundal (deep, aphotic, lower oxygen), and benthic (bottom). Light, depth, temperature, and nutrient levels change across those zones, affecting algae and other producers (EK ERT-1.C.3; EK ERT-1.C.1). Freshwater wetlands (marshes, swamps, bogs) are saturated soils with high productivity, nutrient filtering, and flood control—they support diverse plants and slow water flow, trapping sediments. Key differences: flow vs. standing water, depth/light gradients, oxygen and nutrient levels, and dominant producers. For more AP-aligned detail, check the Topic 1.3 study guide (https://library.fiveable.me/ap-environmental-science/unit-1/aquatic-biomes/study-guide/Ka0nsiIoWMSAbSKgVUqC) and Unit 1 overview (https://library.fiveable.me/ap-environmental-science/unit-1).
Why do different fish species live in different parts of the ocean?
Different fish live in different ocean parts because species need specific combinations of salinity, temperature, light, depth, turbidity, and nutrients (CED EK ERT-1.C.4). For example, photosynthetic food (phytoplankton) is abundant in the photic zone (roughly the top ~200 m), so many small schooling fish and their predators live there. Deepwater (aphotic) fish tolerate cold, high pressure, low light and often feed on detritus or chemosynthetic food—traits not shared by surface species. Salinity and estuary gradients favor hardy, euryhaline species near coasts, while clear, nutrient-rich upwelling zones support big fisheries because nutrients boost primary productivity. Turbidity affects feeding/vision, and benthic species live on/near the seafloor while pelagic species occupy open-water layers. On the AP exam, expect questions linking these factors to fish distribution and resource patterns. For a focused review on aquatic biomes, check the Topic 1.3 study guide (https://library.fiveable.me/ap-environmental-science/unit-1/aquatic-biomes/study-guide/Ka0nsiIoWMSAbSKgVUqC) and practice questions (https://library.fiveable.me/practice/ap-environmental-science).
What's the difference between marine and freshwater biomes?
Freshwater and marine biomes mainly differ in salinity and the types of habitats they include. Freshwater biomes (streams, rivers, ponds, lakes, freshwater wetlands) have low salt, distinct zones like littoral, limnetic and profundal, and are dominated by freshwater algae and emergent plants. Marine biomes (oceans, coral reefs, estuaries, salt marshes) are salty, include large pelagic and benthic zones plus photic vs. aphotic depth differences, and are dominated by phytoplankton, macroalgae, and reef-building organisms. Key ecological factors that vary between them—and drive global distributions of marine resources—are salinity gradients, depth, turbidity, nutrient availability (upwelling), and temperature (CED EKs ERT-1.C.1–4). On the AP exam you might be asked to describe these global distributions or explain how those factors affect productivity and species ranges (learning objective ERT-1.C). For a focused review see the Topic 1.3 study guide (https://library.fiveable.me/ap-environmental-science/unit-1/aquatic-biomes/study-guide/Ka0nsiIoWMSAbSKgVUqC) and practice questions (https://library.fiveable.me/practice/ap-environmental-science).
Can someone explain what makes coral reefs different from regular ocean areas?
Coral reefs are special parts of the marine biome—not just “regular” ocean. Reefs are built mainly by calcium-carbonate secreting animals (corals) that live with symbiotic algae (zooxanthellae), so they’re hotspots of biodiversity and primary production compared with most open-ocean waters. Key differences: reefs sit in the shallow photic zone (so lots of light for photosynthesis), have complex benthic structure (lots of niches), and high local nutrient/recycling efficiency (but low turbidity and relatively stable salinity and temperature). Open ocean (pelagic) is deeper, often aphotic below the surface, lower in macroscopic primary producers, and limited by nutrients, upwelling, salinity gradients, and depth. For the APES exam, link these ideas to productivity, photic/aphotic and benthic/pelagic zones, and factors like turbidity, temperature, and nutrient availability (CED EK ERT-1.C.2–4, EK ERT-1.C.3). Want a quick review? Check the Topic 1.3 study guide (https://library.fiveable.me/ap-environmental-science/unit-1/aquatic-biomes/study-guide/Ka0nsiIoWMSAbSKgVUqC) and more unit resources (https://library.fiveable.me/ap-environmental-science/unit-1) or practice questions (https://library.fiveable.me/practice/ap-environmental-science).
I'm confused about estuaries - are they freshwater or saltwater?
Estuaries are considered marine (saltwater) biomes, but they’re not purely ocean water—they’re brackish. They form where freshwater from rivers mixes with ocean saltwater, creating a salinity gradient that changes with tides, river flow, and depth. That mix makes estuaries nutrient-rich, highly productive, and often turbid, so they support lots of algae, fish nurseries, and wetlands. For the AP CED: estuaries are listed under marine biomes (EK ERT-1.C.2), and you should be able to describe how salinity, depth, turbidity, nutrient availability, and temperature affect their organisms (keywords: salinity gradient, turbidity, nutrient availability). Review the Topic 1.3 study guide for quick notes (https://library.fiveable.me/ap-environmental-science/unit-1/aquatic-biomes/study-guide/Ka0nsiIoWMSAbSKgVUqC) and try practice questions at Fiveable (https://library.fiveable.me/practice/ap-environmental-science) to prep for exam items that test these concepts.
How does salinity affect what organisms can live in aquatic biomes?
Salinity sets which organisms can live where by controlling water chemistry and how organisms regulate their internal fluids. Freshwater (<0.5 ppt) species (freshwater fish, pond algae, macrophytes) are adapted to take in salts and excrete excess water; marine species (ocean ~35 ppt: phytoplankton, coral reef organisms) are adapted to lose water and conserve/expel salts. Brackish/estuarine zones (salinity gradient) host tolerant species and many nursery habitats because organisms there can osmoregulate across changing salinities. Salinity also affects nutrient availability, turbidity, and community composition (so different fish and algae dominate in rivers vs. reefs). On the APES exam you should link salinity to biome distribution and resources (EK ERT-1.C.4) and mention estuaries as transition zones. For a targeted review, check the Aquatic Biomes study guide (https://library.fiveable.me/ap-environmental-science/unit-1/aquatic-biomes/study-guide/Ka0nsiIoWMSAbSKgVUqC) and practice questions (https://library.fiveable.me/practice/ap-environmental-science).
What role do algae play in aquatic ecosystems and why are they so important?
Algae (including microscopic phytoplankton and larger macroalgae/seaweeds) are the primary photosynthetic producers in aquatic biomes (CED EK ERT-1.C.3). They convert sunlight into organic matter in the photic zone, produce much of the oxygen in Earth’s atmosphere, and form the base of aquatic food webs that support zooplankton, fish, and higher predators. Algae also sequester carbon and cycle nutrients (N, P), so productivity is strongly affected by depth, turbidity, temperature, salinity, and upwelling. Too many nutrients, though, can trigger algal blooms that cause eutrophication and hypoxic “dead zones,” hurting fisheries. On the AP exam you may be asked to describe these roles or link productivity to factors like turbidity and nutrient availability (use CED keywords: phytoplankton, littoral/limnetic, photic/aphotic). For a focused review, see the Aquatic Biomes study guide (https://library.fiveable.me/ap-environmental-science/unit-1/aquatic-biomes/study-guide/Ka0nsiIoWMSAbSKgVUqC) and practice questions (https://library.fiveable.me/practice/ap-environmental-science).
Why don't the same fish species live everywhere in the ocean?
Different fish live in different parts of the ocean because environmental conditions change a lot across space—and most species are adapted to narrow ranges of those conditions. Key factors from the CED are salinity, depth (photic vs. aphotic zones), temperature, turbidity, and nutrient availability. For example, many reef fish need warm, clear, shallow, well-lit (photic) waters and the complex benthic structure of coral reefs, while deep-sea (aphotic) fish tolerate high pressure, cold temps, low light, and scarce food. Upwelling zones bring nutrients to surface waters, boosting phytoplankton and supporting different fish communities than nutrient-poor gyres. Because of those gradients, a species that thrives at 25°C and 35‰ salinity in shallow water won’t survive in 4°C, high-pressure deep water. This is exactly what the AP topic asks you to describe (EK ERT-1.C.4). For a quick review, see the Aquatic Biomes study guide (https://library.fiveable.me/ap-environmental-science/unit-1/aquatic-biomes/study-guide/Ka0nsiIoWMSAbSKgVUqC) and more unit resources (https://library.fiveable.me/ap-environmental-science/unit-1).
What factors determine where different marine organisms can survive?
Where a marine organism can live depends on several physical and chemical factors from the CED: salinity (tolerance to salt), depth (pressure and light), turbidity/light penetration (photic vs. aphotic zones), temperature, and nutrient availability (including upwelling zones). Those set the major habitat zones—littoral/pelagic vs. benthic, photic vs. profundal—and determine which algae (phytoplankton, macroalgae) or animals can photosynthesize, feed, or reproduce there. For example, coral reefs need warm, clear, shallow (photic) water and stable salinity; many fish track temperature and nutrient-rich upwelling. Estuaries add salinity gradients that favor tolerant species. These are the exact factors listed in EK ERT-1.C.4 that explain global distributions on the AP test. For a focused review, check the Topic 1.3 study guide (https://library.fiveable.me/ap-environmental-science/unit-1/aquatic-biomes/study-guide/Ka0nsiIoWMSAbSKgVUqC) and try practice questions (https://library.fiveable.me/practice/ap-environmental-science).
How does water depth affect what lives in aquatic biomes?
Depth controls light, temperature, pressure, and nutrient access—so it strongly determines which organisms live where. In shallow zones (littoral, limnetic, upper photic) sunlight supports photosynthetic producers like macroalgae and phytoplankton, so you get lots of plants, photosynthetic microbes, and the consumers that eat them. Below the photic zone (roughly below ~200 m in open ocean) is aphotic—no photosynthesis—so life relies on detritus falling from above or chemosynthesis; you'll find decomposers and specialized deep-water fish. Benthic zones (bottom) host organisms adapted to low light and higher pressure, while pelagic zones (open water) have swimmers and plankton. Depth also affects temperature and oxygen (deeper = colder, often less O2) and influences nutrient upwelling that boosts productivity. These depth-driven gradients explain why salinity, turbidity, nutrient availability, and depth shape marine resources (CED EKs and keywords: photic/aphotic, littoral/limnetic/profundal, benthic/pelagic, phytoplankton). For a targeted review, see Fiveable’s Topic 1.3 study guide (https://library.fiveable.me/ap-environmental-science/unit-1/aquatic-biomes/study-guide/Ka0nsiIoWMSAbSKgVUqC) and more practice questions (https://library.fiveable.me/practice/ap-environmental-science).
I don't understand turbidity - what is it and why does it matter for aquatic life?
Turbidity is how cloudy water is from suspended particles (sediment, plankton, pollutants). It’s usually measured in NTU (nephelometric turbidity units). Why it matters: high turbidity reduces light penetration into the photic/limnetic zones, so algae and phytoplankton get less light and primary productivity drops (EK ERT-1.C.3 & ERT-1.C.4). Less photosynthesis means less oxygen for animals, and organic particles can carry nutrients that fuel harmful algal blooms. Suspended particles also clog fish gills, reduce feeding efficiency for visual predators, smother egg/larval habitat, and change water temperature by absorbing heat. On the AP exam, turbidity is one of the abiotic factors that explains distribution and productivity in aquatic biomes (use that idea when answering free-response or multiple-choice). For a quick review and practice on aquatic biomes, see the Topic 1.3 study guide (https://library.fiveable.me/ap-environmental-science/unit-1/aquatic-biomes/study-guide/Ka0nsiIoWMSAbSKgVUqC) and try practice problems at (https://library.fiveable.me/practice/ap-environmental-science).
What's the difference between marshes and wetlands?
A wetland is the broad category: any ecosystem where the soil is saturated or flooded long enough to support water-tolerant plants (hydrophytes) and anaerobic soils. Wetlands include marshes, swamps, bogs, and fens. A marsh is a specific type of wetland dominated by herbaceous plants—grasses, reeds, sedges—rather than trees. Marshes are usually shallow, have frequent flooding, high nutrient levels, and high primary productivity. Marshes can be freshwater (freshwater wetlands in the CED) or salt/brackish (salt marshes in coastal/estuarine zones). Key APES idea: know wetlands’ role (habitat, nutrient filtering, flood control) and how salinity, depth, turbidity, and nutrients shape aquatic biomes (EK ERT-1.C.1–2). For review, check the Topic 1.3 study guide (https://library.fiveable.me/ap-environmental-science/unit-1/aquatic-biomes/study-guide/Ka0nsiIoWMSAbSKgVUqC) and practice problems (https://library.fiveable.me/practice/ap-environmental-science).
Why do some areas of the ocean have more fish than others?
Some parts of the ocean have more fish because they have higher primary productivity and better habitat—things the CED lists as salinity, depth, turbidity, nutrient availability, and temperature (EK ERT-1.C.4). Phytoplankton (the main aquatic photosynthesizers) need light (photic zone) and nutrients. Upwelling zones (off coasts like Peru) bring nutrient-rich deep water to the surface, boosting phytoplankton → more zooplankton → more fish. Estuaries and coral reefs are also fish-rich because they’re shallow, warm, nutrient-rich, and structurally complex (lots of niches). Deep, aphotic, low-nutrient open ocean areas have far fewer fish per area. Turbidity, salinity gradients, oxygen levels, and temperature shifts (e.g., El Niño reducing upwelling) also change local fish abundance. For AP review, focus on links between primary productivity, upwelling, and habitat type—see the Topic 1.3 study guide on Fiveable (https://library.fiveable.me/ap-environmental-science/unit-1/aquatic-biomes/study-guide/Ka0nsiIoWMSAbSKgVUqC) and practice questions (https://library.fiveable.me/practice/ap-environmental-science).
How does temperature affect the distribution of aquatic organisms?
Temperature strongly controls where aquatic organisms live because it affects metabolism, oxygen, and habitat structure. Warmer water increases metabolic and growth rates of ectotherms (they need more food) and lowers dissolved O2 (solubility drops as temperature rises), so high temps can stress big, active species. Temperature-driven layering (thermoclines) creates distinct zones (photic vs. aphotic, littoral/limnetic/profundal) that limit vertical movement and photosynthesis by algae and phytoplankton. Cold, nutrient-rich upwelling zones support lots of fish; warm, stable waters favor coral reefs and macroalgae but can cause coral bleaching when temps rise. Global patterns of fish and marine resources vary with temperature plus salinity, depth, turbidity, and nutrients (CED EK ERT-1.C.4). For AP review, see the Topic 1.3 study guide (https://library.fiveable.me/ap-environmental-science/unit-1/aquatic-biomes/study-guide/Ka0nsiIoWMSAbSKgVUqC), the Unit 1 overview (https://library.fiveable.me/ap-environmental-science/unit-1), and practice questions (https://library.fiveable.me/practice/ap-environmental-science) to practice applying these ideas on the exam.
What makes an estuary special compared to rivers or oceans?
Estuaries are special because they’re transition zones where freshwater from rivers mixes with salty ocean water, creating a strong salinity gradient and highly variable conditions (brackish water). That variability (salinity, turbidity, nutrient load) makes estuaries extremely productive—lots of phytoplankton and macroalgae thrive—so they act as nutrient traps and carbon sinks. They also provide shallow, protected littoral/benthic habitat that serves as nurseries for many fish and shellfish, supporting diverse food webs and commercial fisheries. Compared to rivers (mostly freshwater, unidirectional flow) and open oceans (stable high salinity, greater depth, lower nutrient availability except in upwelling zones), estuaries combine high nutrients + sunlight + shelter, driving high primary productivity and biodiversity. Good to review these CED terms (salinity gradient, turbidity, phytoplankton) for AP questions—see the Topic 1.3 study guide on Fiveable (https://library.fiveable.me/ap-environmental-science/unit-1/aquatic-biomes/study-guide/Ka0nsiIoWMSAbSKgVUqC) and more practice problems (https://library.fiveable.me/practice/ap-environmental-science).