Salinity in the open ocean is typically in a tolerable range for most marine creatures living in normal seawater (about 3.5‰ [ppt]). However, salinity is variable near landmasses (i.e.: tide pools, river outlets and with depth).
Different organisms have different tolerances to salinity changes. For examples, some bull sharks can tolerate both freshwater and marine water settings. Oysters can't tolerate normal seawater because of predation and food supply. If drought shuts down the supply of freshwater, the water where the oysters are attached to the seabed may get too salty.
Many of the predators that feed on oysters can only tolerate normal seawater, and will only move in to feed on oysters when highest tides or storm surges flood oyster beds with normal seawater conditions.
|Figure 14.9. Bull shark||Figure 14.10. Oysters|