The photo above was taken on May 4, 2013 by the Moderate Resolution Imaging Spectroradiometer (MODIS) on the Terra satellite. It shows a huge bloom of phytoplankton; they occur these every year and can last for weeks or months. The image below was taken just a few days before, on April 20th.
These blooms can be beneficial, but if supercharged by agricultural run-offs, they can be deadly. NASA writes:
Blooms can be a blessing to other marine species, as these tiny floating plants often feed everything from zooplankton to fish to whales. But some algae and plankton blooms can turn dangerous, either through the production of chemical toxins or by severely depleting the oxygen supply in the ocean and creating “dead zones” that suffocate marine creatures.
That's why we must improve our agricultural practices to reduce the use of fertilizers and better control how nutrient surpluses run-off into rivers and lakes. It makes no sense on one side to waste tons of precious fertilizer and nutrients, and on the other side destroy marine ecosystems and put strain on global food chains. By solving the problem at the source, we can minimize all of the knock-on effects.