Stratification and mixing dynamics of hypersaline Lake Urmia (Iran)

Study region:

Lake Urmia, northwestern Iran

Study focus:

This study investigates the stratification and vertical mixing dynamics of Lake Urmia, a transboundary hypersaline lake under critical ecological stress. Using multi-year, high-resolution in-situ temperature and salinity measurements (2016–2019), we characterize seasonal mixing patterns and quantify the relative contributions of salinity and temperature to vertical water column stability. A lake-specific density formulation and thermal energy estimates are applied to assess buoyancy structure and overturn dynamics across contrasting seasonal phases.

New hydrogeological insights from the region:

Lake Urmia exhibits a dual-phase mixing regime, polymictic during the warm season with recurrent full-depth mixing, and inverse meromictic during the cold season due to salinity-enhanced density stratification. Salinity is shown to be the dominant stabilizing factor, while temperature plays a supporting role, primarily as a long-term tracer. The lake is classified as Hyperhalimictic, where mixing is governed by salinity rather than classical thermal stratification. A seasonal salinity pump mechanism is identified—winter brine rejection deepens stratification, while summer halite re-dissolution erodes it. Thermal inertia in lakebed sediments contributes to persistent cold-phase stratification. Findings indicate a trend toward increasing vertical decoupling, reduced overturning, and elevated risk of ecological stress. These insights support improved understanding of hypersaline lake behavior and can guide monitoring and resilience strategies in similar terminal lakes worldwide.