Complex sentence: The study assumes that whales tend to defecate more commonly in the upper part of the water column which they frequent for purposes of breathing and the feces tend to float whereas whales feed at deeper levels of the ocean where krill is found.
Split into simpler sentences: The study assumes that whales tend to defecate more commonly in the upper part of the water column, which they frequent for breathing; additionally the feces tend to float. Whales feed at deeper levels of the ocean where krill is found.