Correct Response: D. Estuaries are areas where freshwater rivers and streams meet the salty water of oceans; therefore, the salinity in an estuary varies widely. Organisms that live in these areas have adaptations that allow them to cope with varying salinity within the general range that occurs annually. A problem arises when salinity is not within the general range for the area. If the salinity is too low, excess water enters an organism's cells, and it cannot be removed quickly enough for the organism to survive. If the salinity is too high, excess water is removed from the organism's cells, and the organism's survival is in jeopardy. While availability of light (A), leaching of nutrients (B), and erosion of substrate (C) are challenges for aquatic organisms, none of these are unique to estuaries.
