The physical characteristics that determine the exhistance of organisms in aquatic enrironments relate to its pH (which is whether it's acid, neutral or basic), the water clearance (which refers to the particles diluted in it, the presence of toxic substances, etc), the presence of predators (bio chain) and also if there's enough food for the organisms to live, to reproduce and if the water temperature allows them to subsist or if they need to migrate within a year, due to the natural changes.