Oceans are affected by climate, but some of the properties of water make the worldwide differences in oceans smaller than the worldwide differences on land. One key property is that water has a high thermal inertia, or specific heat. This means that water does not change its temperature easily. Picture two containers that are exactly alike, but one is filled with hot or cold water and the other is filled with hot or cold air. Even if the starting temperatures are the same, the air-filled container will get to room temperature long before the water-filled one. (You can actually test this yourself.) So ocean temperatures vary from the poles to the equator, but not nearly as much as land temperatures.

Light only gets through the top layer of water in the ocean. As light hits anything, even tiny particles, it is scattered. So below a certain depth (how far depends on how clear the water is), no light at all gets through, leaving the bottom of any ocean dark and cold. You might think that the warm water at the top would mix with the water at the bottom, warming it up. That would happen only in very shallow waters with a lot of wave action to do the mixing. To understand why, let’s look at density.

Density is the mass of something (which we usually measure as weight) divided by the volume of it. In other words, density is the the amount of stuff divided by the space the stuff takes up. The more stuff per unit of space, the greater the density. Think about those two equal-size containers again, one filled with water, one with air. The one filled with water will be about 20 times heavier. Low-density things float on high-density things. That’s why beach balls float on water. Warm water is less dense than cold water, so warm water actually floats on cold water. Did you ever swim in a lake on a hot summer day? The surface water may be pretty warm, then your foot drops down a little bit and the water down there is really cold. The low-density surface water does not mix well with the high-density cold water unless there’s a lot of wind and wave action. This all means that the depths of the ocean are dark and cold almost everywhere.

You might be wondering why the bottom of the ocean doesn’t freeze. Here’s another amazing thing about water, when it’s a solid (ice), it’s less dense than when it’s a liquid. That’s just weird. Water gets more dense as it cools, then, BAM!, as soon as it freezes, it gets less dense. I don’t know of anything else like that. It’s why ice floats. You can see it in a glass of ice water. The reason is that when water freezes, the molecules set up in a structure that pushes them apart. Having things like salt dissolved in the water makes it difficult for the structure to form. That’s why people put salt on icy sidewalks and why salt water can be colder than the normal freezing point of water, but still be liquid. Sea water can freeze, but it has to be really cold and the ice will be at the top.

I’ve written a lot about water, so let’s get to how this affects living things. Photosynthesis needs light and water. Getting water is no problem in the ocean, but producers like phytoplankton can only do photosynthesis up in the light layer. They can’t reproduce and grow without nutrients like phosphorus, nitrogen, and iron. These nutrients tend to fall to the bottom of the water. Dead organisms and waste also sink, carrying nutrients with them. So the layer with light and the layer with nutrients are far apart. That means there’s a lot less life per unit of ocean volume than for land volume. Whales tend to poop higher than they feed, so they help bring nutrients to the surface, but there are only so many whales in the world.

Where waters are warm and shallow, you’ll find the incredibly diverse and productive coral reefs. This zone is near the equator. As you go further north or south of the equator, it’s too cold for coral, but there are diverse, productive kelp forests (areas with giant seaweed). Where rivers empty into the ocean, they carry in lots of nutrients to coastal water. Water can get too many nutrients. Then algae reproduce like crazy, die, then are consumed by bacteria that use up all of the oxygen. These are called “dead zones” because fish and other animals have to leave or die.

Rain mostly has its effect by increasing the flow of rivers. Water is sometimes called the universal solvent because most things dissolve in it. The faster it flows, the more it can carry, so heavy rains erode a lot of soil and carry nutrients into the ocean. Water actually falling on the ocean doesn’t usually have a big effect because there’s so little water in the rain compared to water in the ocean.

Do you think rainwater has a higher or lower density than seawater? Can you think of a way to test this for yourself?

There’s a whole lot more to explore here. If you want to spend your life looking at amazing things like this, you may be interested in a career in marine ecology.

Different species of animals and protozoans live at different temperatures. This is just as true in the ocean as it is on land, and it is true for the same reasons in the ocean as it is on lands.

Rain is a bit more complicated. Life in the ocean doesn't need rainwater, because they're living in water already. Rain does lower the salt content of the ocean, however, which can poison life-forms used to living in saltier water. Evaporation without rain does the opposite. The effects of rainfall on the ocean are not as important as on land, however.