How would ecology be affected if 100km² of an ocean surface was covered by a floating object?
The vast majority of light only penetrates the top 200 metres of the ocean, the euphotic zone, so your answer is roughly &quot;what ever lives in the euphotic zone which requires light.&quot; In all likelihood I would expect this to cause:

anything  which is not highly mobile but, is dependent on sunlight (phytoplankton etc.), would die.

the more mobile (&quot;free swimming&quot;) which rely on either sunlight or the (now dead) slower resources would move out of the area.

reduced flow of nutrients in to the lower parts of the ocean - biomass from the upper regions dies and sinks to the lower levels, if it no longer grows in the upper 200m then there will be less down flow after time.


Overall, the impact would be low because the area of the pacific is &gt;165,000,000 km2 so the density of biota would only increase marginally in the top 200 metres of the pacific (the sunlit water volume decreases from 33,040,000,000,000 cubic metres to 33,039,080,000,000 cub. metres). Also, if we assume that the platform is free floating that means it would drift thus only causing short term loss of sunlight for the less mobile organisms.
EDIT:
Upon reading @Kotatsu's answer I feel they have made a very good point and I should add the following:
By providing a solid surface it could also benefit oceanic life by increasing area available to live for sessile organisms (just like shipwrecks, oil rigs etc.). Further if the panels were broken up (I assume this would be the likely design because sitting a huge sheet across the ocean will leave it prone to twisting by wave motion) light would be able to pass between the gaps. I agree with @Kotatsu, and this &quot;floating panel&quot; could even be beneficial to the ocean ecosystem.

note: I am not an expert on this so it is purely an application of logic and my best guess