1.3 - Aquatic Biomes

Learning Objectives

Global Significance of Aquatic Biomes

• Freshwater biomes like lakes provide an important source of drinking water globally.

• Marine biomes (ocean based) are critical because they’re a huge global supply of oxygen and an important sink for carbon dioxide. Algae and phytoplankton in the ocean produce a vast portion of the oxygen needed on Earth, and they absorb $\text{CO}_2$ from the atmosphere, which helps reduce the effects of global warming.
• NOAA estimates that the ocean produces at least half of the oxygen made by photosynthesis on Earth, mostly from phytoplankton. Marine life also uses up about as much oxygen as it makes.

Key Characteristics of Aquatic Biomes

1. Salinity (salt content): The concentration of salt in the body of water. Salinity dictates which species can survive and determines whether the water is suitable for humans to drink.

2. Depth: Water depth influences how much sunlight can penetrate the surface. Sunlight is necessary for plants below the surface to carry out photosynthesis.

3. Flow (movement): Water movement determines which plants can live there. Rapidly moving water like rivers and streams mixes air and water efficiently, which leads to higher amounts of dissolved oxygen (DO).

4. Temperature: This is vital because as water gets warmer, its capacity to hold dissolved oxygen decreases. Since aquatic animals require high levels of oxygen from the water, warming lowers the number of animals the water can support.

5. Turbidity (cloudiness): how much sediment and other particles are floating in the water. Cloudy water blocks sunlight, so less photosynthesis happens and less food is available to everything above the producers. Coral reefs only grow in clear water, and soil runoff from farms and construction can smother them.

6. Nutrient availability: nitrogen and phosphorus feed the algae and phytoplankton at the base of aquatic food webs, so more nutrients usually means more fish (up to a point: too much causes eutrophication, see 1.6). The richest fishing grounds are upwelling zones, where winds push surface water away from a coast and cold, nutrient-rich water rises from the deep. The upwelling off the coast of Peru supports the Peruvian anchoveta, one of the largest fisheries in the world.

Freshwater Biomes

Rivers and Lakes

• Rivers: Characterized by water movement, or flow. That movement mixes air and water, which typically means high dissolved oxygen levels. Rivers carry sediments along their bottom that contain high levels of nutrients.

• Lakes: Standing bodies of fresh water. Because they lack salinity, they’re a critical source of drinking water across the globe.

• Streams: small, fast-moving channels that feed into rivers. Fast, cold, shallow water holds a lot of dissolved oxygen, so streams support species that need it, like trout and mayfly larvae.

• Ponds: small, shallow bodies of standing water. Sunlight often reaches the bottom, so rooted plants can grow across most of the pond.

• Drinking water: lakes, rivers, and reservoirs supply much of the world's drinking water. The Great Lakes alone hold about 84% of North America's surface fresh water and about 21% of the world's surface fresh water (EPA).

Lake Zones

Freshwater Wetlands

• Plant adaptations: A key distinguishing factor is that wetland plants like cattails, reeds, and lily pads need unique adaptations to survive with their roots submerged in water. Most non-wetland plants would “suffocate” because they can’t get enough oxygen with completely submerged roots.

• Ecosystem services: Wetlands are extremely productive and provide a lot of benefits.
1. They reduce flood damage by storing excess water during storms.
2. They recharge groundwater by absorbing rainfall, which helps refill aquifers.
3. They provide natural filtration. Plant roots trap pollutants carried in stormwater runoff, preventing them from contaminating the groundwater below.
4. They offer a diverse and productive habitat thanks to high water availability and nutrient rich organic sediments.

Transitional and Marine Biomes

Estuaries (Mixing Zones)

• High productivity: They’re highly productive biomes because of the high levels of nutrients in the sediments the river carries and deposits. River floodplains and deltas, where rivers open to the ocean, have very fertile soil as a result.

• Salt marshes: Found in temperate coastal climates. These are vital breeding grounds where fish and shellfish species swim in to lay eggs before heading back to the ocean.

• Mangrove swamps: Found along tropical and subtropical coasts (for example, southern Florida), where it's too warm for frost. These habitats feature mangrove trees with long, stilted roots that tolerate salt water. Those roots stabilize the shoreline, anchor the soil against high winds or storm surges, absorb excess water to prevent flooding, and provide essential habitat and protection for many fish species.

Marine Biomes

• Found in warm, shallow waters beyond the shoreline.

• They’re the most biodiverse ecosystem in marine biomes, and among the most biodiverse on Earth.

• The reef structure, an exoskeleton of calcium carbonate, is built by coral, which is an animal.

• The ecosystem relies on a mutualistic relationship ($+/+$) between the coral and tiny algae called zooxanthellae, which live inside the coral's tissues. (These algae are single-celled protists, not plants.) The coral provides a home and carbon dioxide from its respiration, which the algae use for photosynthesis to produce sugars. Those sugars are the coral's main energy source. Reef-building corals can't survive long without their algae, which is why coral bleaching (corals expelling their algae when the water gets too warm) is so deadly to reefs (see 9.6).

• A narrow strip of coastline between the high tide and low tide marks.

• Organisms here must be adapted to survive crashing waves and the stress of being exposed to sunlight and drying out when the tide goes out.

• Adaptations: Species like crabs, sea stars, and barnacles need features that let them firmly attach to the rocks so they aren’t pulled away by the water. They also need a tough outer shell or skin to prevent desiccation (drying out) from harsh sunlight. Different organisms are adapted to survive at specific levels of the intertidal zone.

• The open ocean has very low productivity per unit of area. It’s too deep for most plants to survive, so only algae and phytoplankton can exist across much of the vast ocean.
• Why it's low: sunlight only reaches roughly the top 200 m, and nutrients in dead organisms and waste sink below that layer. Unless upwelling brings them back up, surface water stays low in nutrients, which limits phytoplankton growth (see 1.8).

• Despite that low productivity density, the ocean’s massive size means its tiny inhabitants generate a huge amount of Earth’s necessary oxygen.

• The ocean is critical for mitigating global warming because it acts as a sink for carbon dioxide, absorbing large amounts of the gas from the atmosphere.

• Ocean zones:
• Photic zone: The area near the surface that sunlight can reach, so photosynthesis can occur.
• Aphotic zone (includes the deep abyssal zone): Too deep for sunlight to penetrate, so no photosynthesis takes place.

• Abyssal adaptations: Organisms in the dark abyssal zone need specific adaptations to survive harsh conditions. They have to withstand extremely high pressure from the water above, and many use bioluminescence (glowing) to provide light for navigation and sight.

Big picture: The consistent message across all aquatic biomes is that the organisms living in them have to be uniquely adapted to the specific conditions found there. The need for submerged roots in a wetland is just as essential as the need for tough shells in the intertidal zone. You can view aquatic biomes like apartments in a specialized building, where each floor (zone) has different environmental requirements. The organisms are tenants who need specific gear, like a built in oxygen tank for the warm basement or suction cups for the wave battered entryway, just to survive in their particular apartment.