Turquoise swirls encircled the Chatham Islands, New Zealand in early December, 2013. These spectacular colors were caused by a large bloom of phytoplankton, which are microscopic organisms that make up the base of the marine food chain.

Such organisms are present in these waters year round, but normally in small numbers. It takes the perfect combination of nutrients, daylight and water temperature to encourage rapid growth – and when conditions are just right, growth can be explosive. The result can be huge blooms that stretch over many kilometers of ocean.

The waters surrounding the Chatham Islands provide excellent conditions for phytoplankton growth, especially in the spring and fall. The islands sit on the Chatham Rise, an underwater plateau that runs eat from the South Island of New Zealand to just beyond the Chatham Islands. To the north and south of the plateau, the ocean is very deep. In the depths north of the plateau the deep water is nutrient-poor, iron-rich water from the subtropics. To the south of the Chatham Rise the ocean is cold, nutrient-rich, but iron-poor water from the Antarctic. The two pools or water come together in a current that rides over the plateau, mixing cold water with warm. This mixing provides nutrients and iron fertilizers that spurs the growth of blooms when conditions are most favorable, typically in the spring and fall.