Sunlight is for the first question, Salinity for the second, and Density for the last one. Now you just have to explain why they relate to each other. Example: Some sea stars live on the bottom of the ocean where there is no sunlight and thus food is not the same as it would be in shallower waters. Due to the lack of sunlight, there are scarce amounts of nutrition that falls to the ocean floor for sea stars to eat. Now you just have to relate the other two questions with their properties.