Identifying meromictic coastal lakes influenced by marine interactions

Coastal waters can develop into meromictic lakes when seawater becomes trapped in deepwater layers, driven by postglacial isostatic rebound or occasional seawater intrusions. To explore these processes, we studied twelve coastal lakes in British Columbia to investigate how deep salt layers persist and maintain long-term meromixis.Physical, chemical, and isotopic analyses indicate that three lakes, Henderson, Pack, and Hesquiat, retain relic seawater at depth. Their strong stratification and anoxic salty deepwaters (13.1 to 19.7 ‰), enriched in nutrients and dissolved gases, confirm sustained meromixis without vertical mixing. Nine freshwater lakes instead have low conductivity and oxic deepwater, indicating seasonal convective overturn.We classify meromictic coastal lakes into three types: (1) high-elevation basins isolated from marine influence, (2) low-elevation basins receiving episodic seawater inflows during extreme events, and (3) tidal lakes with ongoing marine input.In all cases, limited deep freshwater inflow is essential to maintain stable saline layers. Geomorphic factors favouring meromixis include low elevation (< 10 m above sea level), small catchment-to-lake area ratio (< 12), proximity to the ocean (< 1 km), and sufficient depth (> 120 m). These results show that lake morphology and hydrology control the formation and persistence of meromictic conditions in postglacial coastal environments.