Question

The number one factor for determining what living things can survive in a body of water is

Answer 1

This question either has a very simple answer or it is something much more complex. However, if an organism (think a certain species fish for example) wants to survive under water, it needs to stay in such water that corresponds to a certain degree of salinity, similar to the one of its native environment.

In short - salinity determines what living things can survive in it. 

Answer 2

The most important factor which determines whether the living things can survive in a body of water s its salinity. The salinity level of the water is the factor at play in deciding which of the living organisms live inside the water. The salinity of the organisms has to be similar to the salinity level of the water in order for them to survive, otherwise, they will be dehydrated as a result of osmosis.

Hence, the answer is 'salinity'.