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Abstract

The Dead Sea is a hypersaline lake where only few types of organisms can grow. Recently, abundant and diverse microbial life was discovered in biofilms covering rocks and permeable sediments around underwater freshwater springs and seeps. We used a newly developed salinity mini‐sensor (spatial resolution 300 μm) to investigate the salinity environment around these biofilms in a flume that simulates an underwater spring. Compared with the hypersaline bulk water, salinity at the sediment surface decreased to zero at seeping velocities of 7 cm s−1. At similar flow velocities, salinity above rocks decreased to 100–200 g L−1 at a distance of 300 μm from the surface. This depended on the position on the rock, and coincided with locations of natural biofilms. The salinity reduction substantially diminished at flow velocities of 3.5 cm s−1. We suggest that locally decreased salinity due to freshwater input is one of the main factors that make areas around underwater freshwater springs and seeps in the Dead Sea more favourable for life. However, due to frequent fluctuations in the freshwater flow, the locally decreased salinity is unstable. Therefore, microorganisms that inhabit these environments must be capable of withstanding large and rapid salinity fluctuations.