File:Colorful Van Diemen Gulf, Northern Territory, Australia (MODIS).jpg

English: Gorgeous jewel-toned colors created a spectacular glow in the Van Diemen Gulf in June 2023. The Moderate Resolution Imaging Spectroradiometer (MODIS) on board NASA’s Terra satellite acquired a true-color image of the colorful Van Diemen Gulf on June 13. Similar colors tint the water north of the Tiwi Islands, especially the two main islands of Bathurst (west) and Melville (east). These islands sit so close together that they often look like a single, larger, island from above.

Sometimes nicknamed “the bathtub”, in honor of it’s warm and shallow waters, the Van Diemen Gulf spans the waters of Australia’s Northern Territory between Melville Island, the Cobourg Peninsula, and the mainland coast. The large semi-enclosed embayment has a surface area of about 16,000 square kilometers (6,178 square miles) and is less than 20 meters (66 feet) deep. It also has a large tidal range measuring around 3 meters (9.8 feet) overall in the rainy season and even greater in the southeast end, near where the Alligator Rivers pour into the Gulf. The large change in height between low and high tides create strong tidal currents, which scour sediment off the Gulf’s floor and keep it suspended in the waters, especially during or shortly after the rainy season. This is the same time that sediment load in the numerous rivers that reach the Van Diemen Gulf is at their peak.

The rainy season in western Top End typically begins in November and ends by mid-May. Given that waters are still high and rivers full in mid-June, the colors captured in Van Diemen Gulf in this image are almost certainly primarily from sediment. Sediment appears mud-colored near the surface but reflectivity changes as it sinks, so deeper sediment appears green and sometimes blue. Similar colors can be created by blooms of microscopic plant-like organisms called phytoplankton, but studies have shown that sediment concentrations can be high enough in the Gulf to suppress the growth of phytoplankton. The colors off of the Tiwi Islands are also likely primarily sediment, but conditions in the open water off the islands can also be favorable for phytoplankton growth.