What is a reason for loss of buoyancy while descending?

The reason for loss of buoyancy while descending is related to suit compression. As a diver descends into the water, the increasing external pressure causes the air in the neoprene or dry suit to compress. This compression reduces the volume of air that provides buoyancy, leading to a decrease in buoyant force. Consequently, the diver may feel heavier and may require additional ballast to maintain neutral buoyancy at greater depths.

In contrast, the other options revolve around different physical principles or scenarios. Suit expansion can occur while ascending when pressure decreases, which can cause buoyancy to increase. Gas expansion typically signifies a phenomenon that would occur while ascending rather than descending, as gases are more likely to expand due to reduced pressure. Increased temperature can affect the density of gases but is not a direct cause of loss of buoyancy during the descent phase itself. Thus, suit compression stands out as the primary reason for losing buoyancy while descending.