File:Joseph Bonaparte Gulf, Northwest Australia (MODIS 2017-05-25).jpg

English: The waters off the coast of northwestern Australia glowed with greens, golds, tans, and blues in late May 2017. The Moderate Resolution Imaging Spectroradiometer (MODIS) aboard NASA’s Aqua satellite captured this true-color image of the spectacular scene on May 20.

The vibrant colors seen in the offshore waters are created by suspended sediment. As rivers and run-off sweep sediment from the land into the oceans and bays, it floats near the surface at first before submerging to deeper water layers. When close to the water surface, sediment usually appears tan or golden, much like it would on land. As it sinks into deeper water, the reflectivity of the sediment changes and the color – as seen from space – takes on a green cast, or sometimes a blue tone.

Sediment also carries nutrients into coastal waters, and flushes of nutrients may spur growth of microscopic, plant-like organisms that live in those waters. Known as phytoplankton, such organisms live in the waters year-round, but can grow robustly when conditions (sunlight, temperature, and nutrients) are right, creating colorful green or blue blooms which can be seen from space. It is possible that some of the color changes in this image may be growth of phytoplankton as well as sediment.

The most sediment-rich section of this image is Joseph Bonaparte Bay, where the waters are quite tan from particles carried from the Victoria and Keep Rivers, the two major sources of fresh water which flow into the Bay. The Victoria River lies in the east, and the Keep River in the west. The land in between these two rivers has been designated as an Important Bird Area (IBA) by BirdLife International. It is made up primarily of mudflats and hypersaline salt flats, with seasonally inundated wetlands and mangrove forests which support large populations of unusual and rare species of breeding and migrating bird. The waters here are part of Joseph Bonaparte Gulf Commonwealth Marine Reserve, and provide home to threatened and migratory species, such as marine turtles (green and olive ridley) and the Australian snubfin dolphin.