The thermocline relates to the stratification of water in a body of water, such as an ocean or lake. Colder and more saline water tends to sink to the bottom layers, with shallower layers being warmer and less saline. Phytoplankton need to occupy the shallower layers as they need sunlight for photosynthesis. However, they also need nutrients to grow, and these nutrients typically sink to the bottom layers. One important nutrient that limits phytoplankton growth in the ocean is iron (Fe). Sometimes the thermocline can break down due to upwelling, wind or other forces, and cold, nutrient-rich water will rise to the shallower layers. This will result in a phytoplankton bloom. Interestingly, some scientists have debated 'seeding' the ocean with iron to try and increase phytoplankton growth as a way to capture atmospheric carbon.
