[Intro]

So, last time we hiked up a volcano to explore the ways the geosphere affects the biosphere.  Now, let's go down.  We're going to dive deep to explore another connection between two of the Earth's spheres.  But this time, things might get a little soggy.  Can you guess what we're off to explore?  Water.  Let's take a look at how the hydrosphere affects the biosphere.

You remember the hydrosphere.  It includes all of the water on the planet, and of course the biosphere includes all the living things.  Now, we know that the features of the geosphere can affect things that live nearby.  As you climb up a mountain, the air gets thinner and cooler, creating an entirely different ecosystem at the summit than the one at the base.  And the same is true with water.  Lakes, rivers, and oceans are all habitats, and different plants and animals that live in water need different conditions to survive too.

Now, maybe you're thinking, "Duh, a humpback whale couldn't live in a lake.  Got it."  But it goes deeper than that.  When we look out on the ocean, we see rolling waves that stretch out toward the horizon.  It's beautiful, yes, but there is a lot more going on below the surface.  The ocean has layers.  As you move deeper into the water, the temperature, pressure and even salt content all change.  And as conditions change, the types of creatures that live there, change as well.

But don't just take my word for it.  Got your snorkel on?  Good.  Let's go swimming.  

From the surface of the sea to about 200 meters is the sunlight zone.  As the name suggests, it's shallow enough for the light to penetrate the water.  And I know you remember what kind of life needs sunlight--plants.  Through photosynthesis, plants turn energy from the sun into chemical energy.  

Here, sea grass, seaweed, and phytoplankton, which are microscopic plants, algae, and bacteria, form the base of the ocean's food chain.  Since this zone is the very top layer, it doesn't have much pressure.  That means marine animals, like most whales, seals, and manatees, are able to live here.  Any more pressure and their air-filled lungs would get squished. 

But the sunlight zone is just a tiny fraction of what makes up the ocean.  Let's dive.  As we, enter the next layer, the party starts to thin out.  It gets darker, chillier, and there's more pressure.  This space is called the twilight zone, because it receives some sunlight, but not enough for plants to grow.  So, plant eating fish can live here, but they have to survive on the scraps that fall from above.

You start to see some animals with some funky adaptations down here.  Some fish have really big eyes to collect every bit of light they can.  And some animals light up, not to see, but to hide.  Seen from below, an animal would look like a dark shape against the light water above.  But with spots of light on their bodies, some animals can better blend in and hide from predators.  

Now, think about this.  The deepest that a person has ever gone on a scuba dive was 305 meters, just barely entering the twilight zone. But we're not done yet.  To the deep.

Welcome to the spooky sounding, midnight zone.  You can probably guess what that means.  It's pitch black down here.  This zone goes about 4000 meters down and the animals down here are pretty weird.  They might have huge mouths and sharp teeth so they can eat anything that comes along.  Food is pretty scarce here, so their not very picky.  Sometimes it's a passing fish.  Other times a shark carcass falls from the sunlight zone above, providing a feast.  

We're nearing the bottom now.  This zone covers most of the sea floor.  Here the water temperature is just above freezing and it's extremely salty.  These conditions mean only a few creatures live down here.  Most of these are invertebrates, like basket stars and tiny squids.  But there's one more layer to the ocean to explore.  The trenches.

For a while, oceanographers thought no life could exist in such an inhospitable place.  The water pressure's like having fifty huge airplanes piled on top of you.  But even so, animals like starfish and tube worms can thrive at these depths.  Ok, back to the surface.  

Moving through the water, you can see how changes in temperature, pressure, and light affect the organisms that live there.  And that is just one important way that the hydrosphere acts on the biosphere.  From the top of a mountain to the bottom of the sea, I bet you never thought that the spheres could be so full of adventure.