Req 5 — Seawater Chemistry

Seawater is much more than “water with salt in it.” It is a chemical mixture that carries minerals, dissolved gases, and nutrients needed by living things. This requirement helps you see the ocean as a giant chemical system that is always changing.

What Is in Seawater?

Main salts

The biggest dissolved salt in seawater is sodium chloride, the same basic combination found in table salt. Other important dissolved ions include magnesium, sulfate, calcium, and potassium.

You do not need to memorize a chemistry textbook. The key idea is that seawater contains a mix of dissolved ions, and sodium and chloride make up the largest share.

Main gases

The main gases dissolved in seawater include:

• oxygen — needed by most marine animals
• carbon dioxide — used by photosynthetic organisms and important in ocean chemistry
• nitrogen — present in dissolved form as part of the ocean’s chemical balance

Main nutrients

The most important nutrients for ocean productivity are usually:

• nitrate
• phosphate
• silicate

These nutrients help fuel the growth of phytoplankton and other marine plants or plant-like organisms.

Important Properties of Water

Water has several unusual properties that make life on Earth possible.

Water can store a lot of heat

Water warms and cools slowly. That helps the ocean moderate climate and keeps marine environments from changing temperature too suddenly.

Water is a good solvent

A solvent is a substance that dissolves other substances. Water can dissolve many salts, gases, and nutrients, which is why seawater can carry so much chemistry.

Ice floats

Solid water is less dense than liquid water, so ice floats. That protects aquatic life because lakes and seas freeze from the top down instead of turning into solid blocks.

Water molecules stick together

This property, called cohesion, helps form droplets and surface tension. It matters in everything from waves to how tiny organisms interact with the water surface.

How Marine Life Changes Ocean Chemistry

Ocean plants and animals do not just live in seawater. They actively change it.

Photosynthesis

Phytoplankton and marine plants use carbon dioxide and release oxygen. That changes the balance of dissolved gases.

Respiration

Animals, plants, and microbes also respire, using oxygen and releasing carbon dioxide. So oxygen levels and carbon dioxide levels can rise or fall depending on time of day, water depth, and how much life is present.

Shell and skeleton building

Many marine organisms, such as corals and shellfish, use dissolved calcium and carbonate to build hard parts. That changes local chemistry too.

Decomposition

When organisms die and decay, microbes break them down. This process can release nutrients back into the water and use up dissolved oxygen.

Evaporation, Precipitation, and Salinity

Evaporation removes water but leaves dissolved salts behind. That usually makes seawater saltier.

Precipitation adds fresh water. That usually makes seawater less salty.

River runoff and melting ice also lower salinity, while sea-ice formation can increase salinity in nearby water because salts are left behind in the liquid water.

The next requirement moves from chemistry to biology. You will see how seawater properties shape the living groups that fill the ocean food web.