English: Moon jellyfishes (Aurelia aurita) disturbing the top water layer of Gullmarn fjord at Sämstad, Lysekil Municipality, Sweden. The photo was taken on a calm, sunny morning after a rain, so it's likely that the layer is warmer water with some freshwater added. In this case that would make a thermocline coincide with a halocline and resulting in a very thin but visible pycnocline. This difference in density and the altered refractive index is what makes the water look "oily". The jellies are quite small and young, about 5–6 cm (2.0–2.4 in) in diameter, and since they are just below the surface and stirring up the pycnocline beneath them, that would make the layer of warmer freshwater on the fjord about 6–8 cm (2.4–3.1 in) thick at the time.