An isosmotic planktonic organism (one whose internal osmotic pressure matches seawater, so there is no salinity/osmotic driven buoyancy effect) relies primarily on the density contrast between its body and the surrounding water to control sinking or floating. Since viscosity affects the rate of passive sinking (Stokes' Law drag) rather than the energetic cost of active buoyancy regulation, and since the organism is already osmotically balanced (removing salinity as a direct lever), density is the key parameter it must regulate (via lipid storage, ion substitution, gas vacuoles, etc.) to minimize energy expenditure in maintaining vertical position.