At low tide, the salinity is dominated by downstream river flow, whereas at high tide the inflow of sea water dominates the salinity. Reduced mixing with the open sea permits local river drainage or the precipitation-evaporation balance to affect sea water properties, e.g., salinity, temperature, and density. Salinity is the amount of dissolved salts in grams per 1000 g of sea water. In small, tidally mixed estuaries, salinity can change from fresh to completely marine in a tidal cycle. In larger estuaries, salinity gradients are more gradual, tidal motion is of lesser importance, and seasonal fresh water input affects salinity structure to a greater degree.
Species richness decreases up-estuary and reaches a minimum at the critical salinity.
This may be due to the decrease in numbers of fresh water species at this salinity range and the inability of many marine invertebrates to regulate specific ionic concentrations at and below the critical salinity. In daily estuaries, where salinity is tidally regulated, benthic organisms face a great physiological challenge because they experience both fresh and saltwater in a single tidal cycle. In larger estuaries, less influenced by tidal motion, acclimation is possible as salinity changes gradually over the course of the year. Seasonal changes in fresh water input, however, may change salinity at any point in the estuary throughout the year. In the open ocean, away from the poles, a stratified profile of temperature usually develops as a result of solar heating.