Where does all that garbage come from? Where does it go? Much of it ends up on our beaches washed in with the waves and tides, some sinks, some is eaten by living things in the ocean mistaking it for food. Other forms of pollution changing the health of the ocean come from a single known thing, like an oil spill, or from the building up of many things that spread out over time, like fertilizer from our yards. Pollution is anything that is put into an environment that is bad for the things that live there. 
Plastic is one form of pollution. It does not break down in water and can interfere with marine life.
Kate Ter Haar/Flickr.com


Most of the pollutants going into the ocean come from things that are done on land. Things that happen by itself and things that people do, can change the health of our ocean. One case behind pollution comes when pollutants from many small things, like septic tanks, cars, trucks, and boat, lead to runoff. Runoff is when the water drains from the land to larger bodies of water, like the ocean. Different pollution can come from a one time case, like an oil spill. Often this kind of pollution can touch a lot of people and places, but, lucky for us, they take place less often. 

Things that Help Plants Grow and Algal Blooms
From time to time, it is not the kind of stuff, but how much stuff there is in one place that tells us if it is a pollutant. Here's how this works: nutrients are what help plants grow. However, if there is too much of any nutrient in a body of water, it can lead to too much algae to grow. An algal bloom is when there is too much algae growing in water. Some algal blooms are bad because they can hurt living things. 
An algal bloom is when there is too much algae in a water, which can harm living things.
Diana House/Flickr.com


When large amounts of algae sink and break down in the water, the oxygen that ocean animals need is used up. Most of the living things in the ocean either die or leave the place. When water doesn't have enough oxygen, it's hypoxic. Places around the world that would often have lots of life, lose many of the living things that live there, which is why they are often called dead zones. 

Ocean Garbage
Ocean garbage is another can of worms for our oceans. Ocean garbage hurts and kills ocean life, gets in the way of travel safety, and can be bad for our health. Our oceans and waterways, or path water moves along, are polluted with many different kinds of ocean junk, from soda cans and plastic bags to fishing gear left behind boats. Today, there is no place on Earth that is not hurt by this. Most of the garbage that covers our beaches comes from storm drains and sewers, as well as from the many fun things people do on the beach. Left behind fishing gear is also dangerous because this garbage can wrap up, hurt, and drown animals that live in the ocean. We call any kind of solid trash that does not belong in the ocean marine debris
Marine debris strewn across a beach.
U.S. Fish and Wildlife Service Headquarters/Flickr.com


Meaning for Seafood
Pollution can change on the food that we eat. Bad things can build up in seafood and make it bad for people and animals to eat. A large number of the shellfish growing in our country are hurt by pollution on beaches. 
This marine bird coated in oil is just one of the many tragic effects of ocean pollution.
Marine Photobank/Flickr.com

References

adapted from http://www.noaa.gov/resource-collections/ocean-pollution