Diatoms are one of the most abundant types of marine phytoplankton. But a new 15-year long NASA study reveals global populations have declined. Diatoms, like all phytoplankton, have chlorophyll, the same photosynthesizing pigment as plants.They occupy the surface of the ocean where they harvest light from the sun. In large numbers, diatoms form colorful swirling blooms that can be seen from space. Scientists used NASA satellite data, water samples and computer modeling to determine how diatom populations have varied. This data visualization shows changes from 1998 - 2012. According to the study, significant decreases in populations shown here in red are mainly in the Northern Hemisphere. Scientists link this to a shift in the availability of nutrients. Diatoms rely on nutrients such as nitrate, silicate and iron to reach the surface layer where they live. What our study shows is that the availability of these nutrients has changed due to the way they cycle within the water column. Diatoms occupy the surface area of the ocean called the mixed layer. Nutrients collect on the ocean floor and are cycled up to this layer. Various physical forces can cause the depth of the mixed layer to become shallower so that fewer nutrients reach the diatoms. Without these nutrients, their populations decline. This map shows areas on the globe where the depth of the mixed layer shallowed. It's hard to pinpoint exactly why these changes have happened. Things like winds, circulation and temperature can affect the way these nutrients are brought into the surface layer. We hope a longer study can yield more information on whether these changes are in fact a trend or variability. music.