Freshwater Salinization of Seasonal Ponds: High Salinity and Stratification Threaten Critical, Overlooked Habitats

Nearly a century of road salt use
in the snowbelt region of North
America has led to substantial increases in salinity levels in freshwater
habitats. Salt pollution in lakes and rivers is well characterized.
Lacking are broad insights for seasonal ponds. As critical habitats
for many endemic species, these small and often poorly flushed surface
waters are especially vulnerable to accumulating high levels of salts
and other pollutants. Here, we measured salinity in 165 seasonal ponds,
characterizing salt pollution patterns across space, through time,
and over depth within ponds. We found that 70% of ponds within 37
m of a road contained salinity levels exceeding Canadian federal guidelines.
54% of ponds within 25 m exceeded less conservative US federal guidelines.
Within ponds, the water column was stratified due to the combined
density effects of salt and temperature. Bottom waters of polluted
ponds were about 57% saltier than near-surface waters, though many
were much saltier than this. Compared to lakes and rivers, far more
seasonal ponds appear to be compromised by deicing salt, and overall,
the concentration of salt appears to be substantially higher. Among
aquatic habitats, seasonal ponds are experiencing the most severe
impacts of freshwater salinization, with consequent impacts on sensitive
aquatic organisms.