A 40km diameter alien saucer is floating 2km above the ocean for a long time. What are the effects on the sea ecosytem below?

Twenty years ago, a 40km diameter alien saucer came to Earth, and stopped 2km above the Atlantic ocean, somewhere near the midpoint between Casablanca, Morroco and Natal, Brazil but over international waters. It is completely opaque, reflects no light or radar (but still irradiates IR as any object at room temperature) and has absolutely no lights on. Just a disk of blackness.

To be more precise, the means on how the disk hovers at that position and how it dissipates the radiation it absorbs is beyond the scope of the question.

It has been completely immobile since then. No contact. Today (in the fictional world) is somewhere in 2017. Earth is the same otherwise.

What would be the effects its shadow cause on the ocean and the oceanic life beneath it?

EDIT: To address several concerns commenters posted below,

I don't want to develop the whole scenario in one question. It would be very broad. So following stackexchange guidelines, I broke it up into smaller aspects. The other aspects are considered irrelevant for the scope of this particular question.

Therefore the following factors do not affect the ocean below:

• Propulsion: There is none. It has no thrusters and makes no wind downards.

• Irradiation: All the sunlight the disk absorbs is not irradiated outwards. Although it gives off the same amount of IR radiation as the surrounding air (at 2km high).

• Gravity: It does not significantly affects gravity at the surface of the ocean. So it has no tidal impact.

• Thickness: the disk is very thin. Its thickness when compared with its altitude and diameter is negligible.

• Stability: people have landed on top of it, and it didn't tilt a single minute (of a degree). It is very stable, and not even a hurricane could nudge it.

posted about 7 years ago

Mindwin‭

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