Off the southern coast of Western Australia lies a cluster of islands known as the Recherche Archipelago. On its largest island, Middle Island, rests a natural phenomenon in the form of a saltwater lake with an unusual color.
The lake is named Lake Hillier. Unlike typical lakes that reflect the blue color of the sky, the water in the lake is pink.
The lake sits directly adjacent to the coastline of the Southern Ocean. A narrow strip of land consisting of white sand dunes and tree vegetation acts as a barrier separating the lake water from the sea.
Geographical Uniqueness of Lake Hillier on Middle Island
Lake Hillier measures approximately 600 meters in length and 250 meters in width. Its scale allows the entire expanse of the lake water to be fully visible from the elevated areas on Middle Island.
The depth of the lake is relatively shallow compared to other coastal lakes. The water depth mostly ranges between one and two meters under normal volume conditions.
The lake is rimmed by white salt crusts and a tree canopy dominated by melaleuca and eucalyptus. The trees grow evenly along the land strip that divides the lake area from the coastline.
From above, the surface of the lake stands out clearly with its pink hue. The water color is separated from the waters of the Southern Ocean by the sandy land gap.
The environment on Middle Island itself has no permanent settlements or infrastructure. Most of the island’s landmass consists of granite rock formations and coastal scrubland.
Causes of the Natural Pink Color in the Lake Water
The pink color of the lake stems from the high concentration of salt in the water. The extreme conditions create a suitable habitat for Dunaliella salina algae and salt-tolerant bacteria.
Beyond algae, microbial populations from the Salinibacter ruber group and halophilic Archaea also thrive in the dense waters. The microorganisms adapt to the high-salinity environment through the formation of cellular pigments.
When exposed to sunlight, the microorganisms produce carotenoid pigments such as beta-carotene and bacteriorhodopsin. The pigments serve to protect the cellular structure of the microbes from ultraviolet radiation.
The accumulation of pigments from billions of the microorganisms colors the entire expanse of the lake water naturally. The presence of the pigments remains stable against temperature and climate fluctuations around the island.
Because the pigments are dissolved and integrated into the water, the color does not turn clear when a sample of water is collected in a container.
High Salinity Conditions in the Coastal Island Area
The high salt content in Lake Hillier is driven by the structure of the lake basin, which is isolated from terrestrial water systems. Seawater trapped within the land depression has no natural drainage channels leading outward.
The lake receives no direct inflow from rivers. The sole additions to its water volume come from periodic rainfall and seawater seepage filtering through the porous beach sand.
The salt concentration level within the basin reaches approximately 30 percent. The level of salinity is exceptionally high when compared to open ocean water, which typically averages around 3.5 percent.
The high salinity renders the lake waters uninhabitable for fish species or other aquatic vertebrates. The ecosystem within the lake is purely dominated by communities of halophilic microorganisms.
During the summer, high temperatures trigger large-scale water evaporation. The evaporation process leaves behind concentrated mineral and salt deposits that settle into a thick white crust along the edges and bed of the lake.
