An oceanographic research station deploys a rigid, sealed cylindrical sensor casing of total volume \(V\) and total mass \(M\). The casing floats in translational equilibrium at the boundary between two unmixed seawater layers, with volume \(V_1\) submerged in an upper layer of density \(\rho_1\) and volume \(V_2\) submerged in a lower layer of density \(\rho_2\), where \(\rho_1 < \rho_2\). Warm water rising from a thermal vent plume reduces the density of the upper layer to a new constant value \(\rho_1' < \rho_1\), while the lower layer's density \(\rho_2\) remains unchanged. The casing remains partially submerged in both layers. Which of the following claims correctly describes the change in the submerged volume \(V_2\) in the lower layer, and provides a correct physical justification?