Earth's oceans are wide-reaching and teeming with life. One microscopic aquatic organism plays a major role in making life on earth possible: phytoplankton. If it weren't for phytoplankton, the earth, as we know it, would probably not be able to exist. Life on this planet pretty much depends upon phytoplankton. Light and the presence of nutrients in the surface waters make phytoplankton growth possible. Physical processes, like ocean circulation and upwelling, help to regulate the distribution and abundance of the plants. For the last decade, scientists have used satellite measurements of ocean color to determine the global distribution of phytoplankton...and to begin to link its variability to environmental factors. The beauty of studying the oceans from space, in a consistent way that satellites allow us to do...is that you can study phenomena both on different time and different space scales. Using data from an instrument called SeaWiFS, researchers observed that both natural and human-induced events...cause levels of phytoplankton to shift from their usual rhythms. Under certain conditions, excessive phytoplankton growth can result in an area known as a dead zone. Dead zones form when big balloons of phytoplankton at the surface trigger large quantities of organic matter, which then sink to the bottom. Bacteria break down the organic material, releasing carbon dioxide, but absorbing oxygen as they work. Most marine organisms need oxygen for survival, and dead zones prove fatal for many aquatic species. Many human activities on land trigger the seasonal appearance of dead zones. Fertilizers run off lawns and farmland. They seep into the network of rivers and streams that comprise the local watershed.....and provide a jolt of nutrients to phytoplankton that edge the shore. The Mississippi River watershed drains directly into the Gulf of Mexico, where dead zones frequently occur. Satellites cannot actually see dead zones, but they can monitor the distribution and abundance of phytoplanktonincluding conditions that may lead to phenomena, like dead zones. There is no question that the earth is changing. In the decades since its launch, SeaWiFS has enabled us for the first time to monitor the biological consequences of that change...to see how human activity, and how natural variability affect the earth's ability to support life.