What factor causes seasonal changes in salinity levels in coastal regions?

Seasonal changes in salinity levels in coastal regions are primarily influenced by rainfall and river discharge. During periods of heavy rainfall or increased river flow, freshwater enters the coastal waters, diluting the salinity. This dilution can significantly lower the salinity levels, especially in estuarine areas where rivers meet the ocean.

Conversely, during dry seasons or periods of lower river discharge, the concentration of salt in the water increases as evaporation may occur and the influx of freshwater diminishes. This fluctuation creates a dynamic environment in terms of salinity, which directly impacts the local aquatic ecosystem, including the types of species that can thrive in these varying conditions.

While other factors like industrial waste, ocean currents, and urban runoff can influence water quality and salinity in certain scenarios, rainfall and river discharge are the main drivers of seasonal salinity changes in coastal regions. This makes them the most relevant factor in understanding how salinity levels can fluctuate throughout the seasons.