Each day, ships carry about 20 million barrels (3 million cubic meters) of crude oil—nearly a quarter of global petroleum liquids consumption—through the Strait of Hormuz. It is one of the world’s most important oil passages. Recently something more than ships was passing through the strait.

On October 16, 2019, the Operational Land Imager on the Landsat 8 satellite captured this image of swirls of phytoplankton in the Strait of Hormuz.

The image appears like a watercolor painting—a blend of art and science. Like a photographer adjusting lighting and using filters, Norman Kuring of NASA’s Ocean Biology group works with various software programs and color-filtering techniques to draw out the fine details in the water. The detailed swirls in the chlorophyll-rich water are all quite real; Kuring simply separates and enhances certain shades and tones in the data to make the biomass more visible.

Phytoplankton are plant-like organisms that serve as the center of the aquatic food web. Phytoplankton abundance depends on the availability of sunlight and nutrients, but can be influenced by other factors like water temperature, salinity, and depth; winds; and the number of marine animals grazing on them. When conditions are right, phytoplankton populations can grow explosively, a phenomenon known as a bloom.

Phytoplankton blooms—drawn into thin, swirling ribbons by turbulent eddies—commonly occur in this region. Nestled between the Arabian Gulf and Gulf of Oman, the Strait of Hormuz is adjacent to some of the most biologically productive waters in the world. In the Arabian Gulf (also known as the Persian Gulf), strong winds and extratropical weather systems often influence water circulation and mixing, which can cause blooms. Phytoplankton productivity in the Gulf of Oman is heavily influenced by the coming and going of the monsoon and related upwelling.

Although the Strait of Hormuz sits at the boundary of these two systems, past research showed phytoplankton species in the strait are more similar to those in the Gulf of Oman. The two waterways contain a high number of dinoflagellates, which can produce a neurotoxin that affects muscle function in marine life and humans. These blooms—called harmful algal blooms—have increased in the area due to a high number of nutrients pouring into the sea from coastal areas.

NASA image by Norman Kuring/NASA's Ocean Color Web, using Landsat data from the U.S. Geological Survey. Story by Kasha Patel.