Oceanography
Effect of Salinity on Marine Life
Name________________________
Water’s excellent ability to act as a solvent implies that wherever water exists on Earth, it is carrying materials
with it. Ocean water carries a lot of materials- but from where do the materials, including salt, come?
1. From land: Fresh water in streams and rivers contains salts. Rivers flow into the ocean. It is estimated
that about 4 billion tons of slat are carried by rivers into the ocean each year. Over time, more and more
salts are added to the ocean. In essence, salts are constantly entering the ocean because rivers and
streams carry them there. Also, through evaporation, water leaves the ocean and goes into the
atmosphere. When water evaporates, alts are left behind.
2. From inside the Earth: volcanoes, vents, and other processes at the seafloor spew minerals from below
Earth’s crust into the ocean.
3. From the atmosphere: Particles, including minerals, can be carried by wind and deposited into the
ocean. There are both natural and human-made substances in the atmosphere. Minerals transported by
the atmosphere can be deposited from the atmosphere directly into the oceans.
Salinity and Marine Organisms
Water and its properties, which you have investigated, support life in the ocean and on Earth. Marine organisms
are well suited to living in salt water. Their body structures are equipped to handle the water’s salinity. In
saltwater solutions, salt will diffuse from areas of high concentration to areas of low concentration. Conversely,
if a membrane blocks the salt from moving, water will diffuse across the membrane from areas of high to low
concentration. This is known as osmosis. Cell membranes of aquatic organisms are usually permeable to water,
but not to salt. Seawater has a higher salt concentration than the body fluids of marine organisms. Freshwater
has a lower salt concentration than the body fluids of fish.
1. Draw a diagram showing how water moves in or out of freshwater fish. Freshwater fish cells contain
approximately 85% water, while freshwater bodies are usually around 98% water.
2. Draw a diagram showing how water moves in or out of saltwater fish. Saltwater cells contain
approximately 85% water, while saltwater bodies are usually around 70% water.
3. Which explanation is true of freshwater fish? Which is true of saltwater fish?
a. Fish constantly lose water to their surroundings. These fish have to drink continually and urinate
very little.
b. Fish constantly gain water from their surroundings. These fish never drink water and urinate
constantly.
4. What would happen to freshwater fish if they drank water?
5. What would happen to fish in saltwater that did not drink a lot?
6. You decided you want to start an aquarium with Nemo and his friends from the Coral Reef. Can you fill
up your aquarium will water from the sink? Why or why not? (Talk about the effects of osmosis in your
answer.)
We generally consider water to be salty or fresh. Water that is somewhere in between salty and fresh is referred
to as brackish. Brackish water can be found in many coastal ecosystems such as salt water marshes and
mangrove forests. Both of these ecosystems are where rivers meet the sea and are known as estuaries. Estuaries
are incredibly important to the ocean because they are places where the young of many seabirds, fish, shellfish,
and other organisms can develop in a protected area. In other words, estuaries are home to many juvenile
organisms. Many commercial fish species rely on estuaries as habitats for some portion of their life cycle.
Migrating bird species often use estuaries as a resting place. Estuaries also protect coastlines by preventing
erosion (the movement of sediment).
Most aquatic organisms can tolerate either fresh water or salt water. Plants and animals that live in estuaries
must endure wide ranges in salinity because estuaries are always changing. Estuarine organisms have unique
strategies for coping with differing conditions.
7. One example of an estuarine organism is the bull shark. Read the article and describe how bull sharks
can tolerate living in estuaries (mixture of saltwater and fresh water).