Ocean algae is over-growing, due to a combination of factors - warmer oceans and human pollution from agricultural chemicals. Fertilizer that runs off the land then runs into our waterways and enters the ocean. This provides a glut of food for algae. When algae "blooms", it uses up all the oxygen in the water, creating dead zones. There is a huge one in the Caribbean, just off the Mississippi river delta, as a direct result of American farmers and their chemicals.
In other parts of the ocean, algae takes over as water warms, killing off other life that is more temperature sensitive. This changes the essential composition of reefs, for instance, leaving bleached and dead coral but an abundance of algae.
There is so much on this topic, because it is a complex piece of the overall effect of human on our planet's oceans. A great source is Alanna Mitchell's book "Sea Sick" (I have written a hub on some of the ideas behind the book, but this is not comprensive - http://hubpages.com/hub/Sick-Seas-How-Alannah-Mitc... She is not a scientist: Mitchell is a reporter who went out talk to a variety of scientists in an effect to put all the pieces together. It is a powerful book.