Req 6 — Life in Seawater

For marine life, seawater is not just a place to live. Its temperature, salt content, light levels, oxygen, and movement decide where organisms can survive. This requirement is about the living side of oceanography and the major groups scientists use to organize ocean life.

Biologically Important Properties of Seawater

Salinity

Marine organisms are adapted to certain salt ranges. A fish or invertebrate that does well in normal seawater may struggle in an estuary where fresh river water lowers salinity.

Temperature

Temperature affects metabolism, growth, migration, and where organisms can live. Coral reefs, for example, depend on fairly warm water, while polar organisms are adapted to cold conditions.

Dissolved oxygen

Animals need oxygen in the water just as land animals need oxygen in air. Low-oxygen water can stress or kill marine organisms.

Light

Sunlight powers photosynthesis, but only the upper layer of the ocean gets enough light for most phytoplankton growth. That is why the surface zone is so important to food webs.

Nutrients

Nutrients such as nitrate and phosphate act like fertilizer for marine producers. Areas where nutrient-rich deep water rises can become especially productive.

Three Big Groups of Ocean Life

Plankton

Plankton are organisms that drift with currents more than they swim against them. Some are tiny plants or plant-like organisms, and some are tiny animals. Examples include phytoplankton, copepods, and the drifting young stages of many larger animals.

Nekton

Nekton are active swimmers that can move through the water on purpose. Fish, squid, dolphins, sea turtles, and whales are all examples of nekton.

Benthos

Benthos are organisms that live on or in the seafloor. Sea stars, clams, crabs, sea cucumbers, anemones, and many worms are benthic organisms.

Examples You Can Mention

Plankton examples

• diatoms
• dinoflagellates
• copepods
• krill

Nekton examples

• tuna
• sharks
• dolphins
• squid
• sea turtles

Benthos examples

• sea stars
• crabs
• clams
• corals
• sea anemones

Why Phytoplankton Matter So Much

Phytoplankton are tiny photosynthetic organisms drifting in the upper ocean. Even though each one is microscopic, together they are one of the most important foundations of life on Earth.

They matter because they:

• make organic food from sunlight, carbon dioxide, and nutrients
• feed zooplankton, which then feed larger animals
• support marine food chains all the way up to fish, seabirds, and whales
• produce a large share of Earth’s oxygen
• help move carbon from the atmosphere into ocean systems

Connecting Chemistry to Biology

This requirement also connects strongly to Req 5. If seawater loses nutrients, gets too warm, or has too little oxygen, marine life changes too. Oceanography works best when you connect physical conditions and chemical conditions to living systems.

Next you get to choose a hands-on project and use oceanography in action.