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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.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Monica Cellio Dec 18 '17 at 3:18
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The ocean is a dynamic and very large place, so it's unlikely to have many large-scale effects unless humans overreact. I'll focus on the local, immediate effects of this saucer from a physical, biological, and chemical perspective.

Some things to consider about the location of the saucer- it's suspended in the middle of the Atlantic Ocean, almost directly above the Mid-Atlantic Ridge. We're a little bit above the equator and thus miss the strong, surface equatorial currents and instead have a slow surface velocity of 1-2 km/day. We have strong, driving trade winds from the northeast and are almost at the edge of the ITCZ.

Physics

Let's explore the physics of such an object before we start working with the ecosystem. The biggest change will be the shadow cast by such an object- given that it absorbs all light, it's going to cast a pretty big shadow. Because it's pretty close to the surface relative to the size of the object, we'll mainly have a solid umbra- total darkness. Near the edge, we'll have the penumbra, and finally we get back into normal light conditions.

However, the ship also probably has some mass- this may affect tides and will certainly affect sea height nearby, but as I don't have any idea about the density or thickness of such an object, I can't do more than mention it.

Biology

The best way to model this addition is to treat the area within the umbra as suddenly a part of the aphotic zone. This is normally the deepest portion of the ocean, where no light reaches, and is home to all of our favorite little beasties. Suddenly, the aphotic zone extends to the surface in a single location, which causes a restructuring of the local ecosystem.

Phytoplankton

Phytoplankton require sunlight to live. The sun powers photosynthesis, and without light, they'll rapidly die. It's worth noting that these creatures wouldn't simply swim outside of the shadow- they're unable to swim at all, which is why they're considered plankton (literally, "drifter"). So all the plankton in our shadow die within a day or two and deposit a huge amount of carbon onto the seafloor, but as that's a one-time event, I won't spend too much time on it. Essentially, we no longer have a primary producer in the area, but that only reinforces the deep-sea model.

Given that we have surface currents of 1-2 km/day and the saucer is about 40km wide, that's plenty of time for any poor phytoplankton pulled underneath to starve- especially the little guys.

Zooplankton

At first, I assumed that zooplankton would be pretty happy about this event. The big change for them here is their diel vertical migration. Zooplankton feed at the surface during the nighttime when they can't be seen and eaten, and descend to the deep sea during the day. Initially, this implies that they'd suddenly have a horizontal daily migration as well, but they aren't designed for that. Plankton move easily and readily vertically in the water column because they're able to control their buoyancy, not because they're strong swimmers. Even though they would love to move in and out of the shadow at will, they simply don't have that capability.

Interestingly, the very first time your saucer appeared, it would cause this same daily vertical migration because the zooplankton would think it's nighttime- essentially emulating the effects of a solar eclipse. They'd feast voraciously, run out of food, and shortly die.

Nekton

Bigger creatures, on the other hand, would be stoked. "Nekton" is a term that includes all creatures that move under their own power, and they've been given a huge present by this saucer- the ability to move between night and day at will. Normally, creatures in the ocean have very different activities during the day and night, usually as a predation/predation avoidance mechanism. Now, they're able to swim out into the sunlight, feed on clearly visible organisms, and disappear back into the shadow within a few minutes, not 12 hours.

This behavior is why fish are often found under piers, boats, or rafts of seaweed- it serves as a place to hide from predators both above and below.

Benthic creatures

Essentially wouldn't care. They're too deep in the ocean to notice the difference between day and night, so they wouldn't notice that the diurnal cycle has stopped. They aren't even affected by the sudden disappearance of the marine snow from above because particles sink much more slowly than the ocean currents in this location, so they're still being fed by carbon from the NADW.

Seabirds

I'm not a bird person, so there may be more helpful information about this group coming from somewhere else. We wouldn't expect to find many birds to begin with, but they may start colonizing your saucer if it doesn't have some way of keeping them off- which might be a good idea for the aliens to do anyway. Nothing can ruin the aesthetics of a mysterious levitating saucer like bird poop. If the birds were able to land on it, they'd probably be pretty excited- it's a good spot to stop for a rest if you're migrating, and there will be some interesting marine life behavior below that they may be able to capitalize on.

Chemistry

Now that we've killed off the phytoplankton, one might expect that we'd see $O_2$ concentrations plummet in the surface ocean. However, that would only happen for a few days or months, until all the respirers are dead as well. What would happen over 20 years is the transfer of oxygen into the water that would slowly raise the concentrations again. We can see that best on a concentration-depth profile:

O2 concentration with depth

$O_2$ actually increases in the deep sea because there's no longer much consuming it. This is the same thing we'd expect to happen under our alien saucer's shadow.

Recap

What your saucer has done is essentially take the normal vertical light distribution in the ocean and make it horizontal. Rather than moving from a euphotic to a disphotic to an aphotic zone by travelling vertically in the water column, the same sequence happens horizontally. This doesn't affect much on the 20-year scale because only nekton would be able to use it to any advantage. Essentially, expect the ocean to do what the ocean does best- change a little in a lot of ways.

Some thoughts, brought up by commenters and other answers

Pressure differential

Deep sea fish are famous for living under pressure, but they don't need it to stay alive. Fish have control over the amount of gas in their swim bladders, and as long as they're given enough time to rise to the surface they won't experience barotrauma. As to whether or not deep sea creatures actually colonize the surface, I don't know- we don't have a ton of data on this phenomenon. I find it hard to imagine that over 20 years there would be no interaction, but it probably won't be a full-scale surface invasion. Deep-sea fish are pretty happy to stay in the deep sea.

Light requirements

A couple comments and an answer have pointed out that it's not going to be pitch black underneath. I agree with that! However, it's not just any light that's useful. Photosynthesis requires rather a large amount of direct sunlight or it simply won't operate at all. In the ocean, this boundary is the difference between the euphotic zone and the so-called "disphotic zone". This depends on the turbidity of the water, but in the open ocean is about 100m. Additionally, photosynthesis operates best with blue light- the same wavelength that is scattered best by air. If the light is coming in at an angle, a lot of the blue light is removed and no longer able to power photosynthesis. I'd argue that any area directly under the saucer is more accurately modeled by the disphotic zone.

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    $\begingroup$ One criticism - only the very weakest birds ever stop during a long migration. Reason is that they make very substantial metabolic changes in preparation for the migration that take weeks to repeat. If there is any reasonable expectation of being able to complete the migration (and yes, birds DO know where they are, the mechanisms are unclear) they will continue uninterrupted. Any birds that do stop may actually decline to continue; and since self-selected for migratory weakness, likely diverge rapidly from the original population. $\endgroup$ – Pieter Geerkens Dec 12 '17 at 21:50
  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Monica Cellio Dec 17 '17 at 4:25
  • $\begingroup$ @MonicaCellio Um, that was a lot of info to move elsewhere, some of which was still relevant to the answer- and I didn’t think there was one, continuous discussion going on but rather the large amount of comments was due to the popularity of the question. Is it possible to move some of those back? I’m thinking specifically about Pieter’s comment on seabirds, which would be good to have here since I admit that I’m not a bird person $\endgroup$ – Dubukay Dec 17 '17 at 5:15
  • $\begingroup$ I restored that one. Most of the comments were 2 days old (or older), and a lot were from you. The best way to respond to requests for clarification or constructive feedback is to edit the post. Otherwise, after a time they're often presumed obsolete. There were 28 comments on this post and the odds of anybody being able to find valuable info in a pile that large are low. Thanks for understanding. $\endgroup$ – Monica Cellio Dec 17 '17 at 5:23
  • $\begingroup$ @MonicaCellio Totally! I agree that it has gotten out of hand- was just worried that some good info I hadn’t thought of had also disappeared. Thanks! $\endgroup$ – Dubukay Dec 17 '17 at 5:41
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Why is it that in questions like this about something blocking light, everyone seems to forget that the Earth rotates? To simplify the geometry, we'll use the following assumptions: the saucer has a flat bottom (it doesn't curve to follow the Earth), so the 2 km altitude is measured from the center of the saucer; the saucer is located on the equator, and it's the equinox, so the sun passes directly overhead the center of the saucer; the sun is point light source so we aren't worried about umbras, just either shadow or not; and we'll ignore the curvature of the Earth.

We're assuming the Earth to be flat in this case because simple trigonometry (assuming the Earth to be a sphere 6,371 km in radius) shows that if the center of the saucer is at a height of 2 km, the edge of the saucer is at a height of 2.03 km above the surface, which is a trivial difference and can be ignored.

So, that all said...

From a point on the surface of the ocean under the center of the ship, it spans 168.6 degrees of the sky, which means 11.4 degrees of visible sky (obviously 5.7 degrees on either side). Given 12 hours for the sun to cross the sky, that's 45 minutes of daylight, minimum.

However, also something to take into account: Roughly 5% of the light you get on a clear, sunny day doesn't come from the sun directly, it comes from scattered light due to the atmosphere. While a good portion of the sky will be blocked so you only a cylinder of airglow around the horizon, that's still light. It might not seem like much, but for comparison the full moon is only 0.00025% as bright as the sun, and a full moon provides a surprising amount of light once your eyes adapt.

Even underneath the center of the ship, with the most sky blocked, you'll get 45 minutes of direct sunlight and some residual light the rest of the day in the worst-case (ignoring weather, of course). As you go away from the center, the effect rapidly decreases, very quickly for the north and south. By the time you go 20 km east or west, the worst case gives you 6 hours of direct sun.

So, how about going north or south? If you go 5 km north or south (and again assume the sun goes directly overhead for simplicity), the ship covers 151 degrees of the sun's path, giving you almost two hours of direct sunlight. Go 10 km north or south, it's 4 hours of direct light. Go 15 km, 6.5 hours of direct sunlight (and increasing sky brightness). And of course at 20 km, basically a full 12 hours.

Long story: the actual area under the ship where it's dark is never completely dark for 24 hours, and doesn't actually occupy a very large area compared to the total area of ocean directly under the ship. Ecological effects are fairly minimal because air and water currents are going to rapidly dissipate cooling effects (so no real weather effects) and sweep phytoplankton and other light-requiring organisms into the area of near-perpetual dark, so it will not be lifeless. You'll see a higher prevalence of organisms that come to the surface at night in the center of the zone, but it's so comparatively small as to be measurable but largely insignificant.

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    $\begingroup$ The central area will only get sunrise/sunset though, when the light is not very strong as it must go through a lot of atmosphere. @Mindwin if you want more obscurity just make the saucer hover at 100 meters rather than 2 kilometers high. $\endgroup$ – Nicolas Raoul Dec 13 '17 at 3:41
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    $\begingroup$ I live above the Arctic Circle where, from the first week of December to the second week of January, the sun doesn't come above the horizon. At noon, even on December 21st, shortest day of the year, it's still bright enough at noon to turn off streetlights and read outside. $\endgroup$ – Keith Morrison Dec 13 '17 at 4:13
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    $\begingroup$ The angle sunlight comes through the atmosphere has a huge impact on the energy it imparts. Enough light to read/drive by is very different from enough light to sustain life. Even if the light intensity didn't change with the angle, 45 minutes of sunlight vs. 12 hours of sunlight, that's a 93% reduction at the center. $\endgroup$ – Mr.Mindor Dec 13 '17 at 18:28
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    $\begingroup$ This whole analysis assumes the sun passes directly overhead of the ship. It will only do so 1 day per year. The rest of the year the earth's tilt will be different and more light will reach further under the ship. $\endgroup$ – Dunk Dec 13 '17 at 19:13
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    $\begingroup$ @Dunk, that's why I kept specifying things like worst case scenario. The further off the equator, the more light will get underneath. $\endgroup$ – Keith Morrison Dec 13 '17 at 21:47
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In the middle of the Ocean there is just a lot of water and then, after going some thousands meter deep, ground.

Swimming animals will experience something like night, regarding plants, well, thousands meters of water are already enough to stop light, so the saucer is not going to affect them any worse.

For floating algae the effect will be transitory, as currents will move them away before they are significantly affected.

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Since its not technically over coastal waters technically it has no or virtually no effect.

Algae wouldn't be able to thrive in that spot but at the same time that doesn't matter because algae have the whole ocean to live in.

If the saucer were closer to coastal waters it would kill off corals but that is limited to such a tiny area that it is negligible to mans ability to kill off marine life.

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Looking at the effect of a solar eclipse might be helpful. The blocked light will make the inner surface waters (under the center) cooler. With the edges being hotter, there should be some interesting wind patterns in the region. This also will affect the water temperature, so a convective pattern should be set up, with a column of water sinking in the middle.

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In the novelization of the late 1970's movie "Close Encounters of the Third Kind" by Steven Spielberg, the alien mothership was of a similar size. Speilberg wrote of the mass of the massive ship hovering over the canyon partially canceling the gravity for the people below, rendering them about 40% weightless. Yes the earth is much bigger, but the ship is closer (and perhaps denser) than most of the earth. I haven't done the math on this, but surely there would be some gravity/tidal effects, such as a bulge in the water's surface below your ship.

Also keep in mind that the earth's surface, including the ocean, is curved and therefore the horizon distance comes into play. If you were on a large watercraft, 20 meters above the surface of the ocean, under the center of the spaceship, the visible horizon would be about 16 km distant. This would have massive cooling effects - that's why the arctic is cold, after all - it's not farther from the sun, it just doesn't get as much sunlight.

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  • $\begingroup$ The mass of the Earth is about $6\times10^{24}$ kilograms, which can be treated as if it’s $6\times10^{6}$ meters away. Let’s say that there’s another spherical massive body directly “above” a person. Let’s say the ship weighs as much as a mountain ($10^{16}$ kg) and is 1000 meters in radius. We’d expect it to exert about one hundredth as much force as the Earth. Not a negligible force (you probably could perceive it), but it would have to be larger to cancel out most of one’s body weight. $\endgroup$ – Obie 2.0 Jan 22 '18 at 0:06
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While 40km is pretty darn huge put at the scale of human made objects (even more so objects that are able to float in mid-air!) it is quite neglegible in planetary terms. A single raincloud is approximately the same size, and life does not end every time one comes by.

The same is true for the topmost 50-100 meters of the ocean which will be affected by the shadow disk. From our point of view, 100 meters is pretty darn deep, but for an ocean which is approximately 7,000 meters deep at the approximate location indicated (Romanche trench), it's nothing.

Yes, some plankton will probably die, and a few dolphins and other animals near the surface may get irritated. Temperature (both air and surface water) will be lower within the shadow cone, and because of that there will be some currents. But, whatever, compared to what happens every day in weather, that's just nothing. The planet couldn't care less, nor could the overwhelming majority of animals that live in regions where it's dark all day anyway.

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Q: What would be the effects its (the UFO) shadow cause on the ocean and the oceanic life beneath it?

As mentioned here 'An object is opaque because of scattering, or because of absorption. All "opaque" means is that light doesn't travel through the object. Thus, an object which appears to our eyes to be black just isn't allowing any photons in the visible range to reach our eyes.'.

Keeping this in mind, The light from the Sun would not pass through the UFO and reach the ocean below it. Also, the large body of air present under the saucer, and the water below it, will experience significant cooling effects. If we do the math, there is at most, ~2514 Cubic Kilometres of air below the UFO that will stay in the shadow of the saucer. Katabatic winds would form on the edges of the saucer shaped object, and rapidly cool and contract, possibly creating localized rain and wind, providing the regional humidity is high. In all possibility, the salinity in this region might be slightly lower due to constant rain fall. Because cool air is much denser than hot air, it has a higher carrying capacity for water, and this region would be a low pressure zone. In the opposite scenario, one could create a "Hot House" by enveloping a container of air with glass. In this situation, a "Cool Zone" is created instead, right below the saucer. Thermal conduction properties, and IR heat dissipiation of the UFO was not specified, so, assuming the UFO would absorb and dissipate IR energy similar to an object at room temperature (as stated), heat eminating from the saucer would be negligible and could be considered constant. Similarly, the ocean too, provides very steady IR dissipitation. Climate scientists state the worlds oceans are responsible for and regulate the earths temperatures and climate. However, here, we have a localized "Micro Climate".

The location of the UFO is roughly over the Mid-Atlantic Ridge, and far away from any land masses. It is well known that large metallic deposits, especially blue stone and iron ore, can increase the electrical content of the air significantly. If the UFO contained large deposits of refined metal, this could significantly alter the magnetic field of the earth at this point. It is well known that large deposits of shallow iron ore, or blue stone rock, can twist and warp the magnetic field for kilometres from the point of origin. This twisted magnetic field can cause large electric field gradients to occur resulting in violent storms. For example, in Venezuela, a famous bay is the home to very regular and violent thunderstorms. Most scientists agree it is due to the level of methane gas being emitted by the water (and oil deposits below). Large magnetic field variations could affect sea life, and fish that use magnetic guidance for migration.

A +40 kilometre shadow cast over the ocean would cool the water slightly, and lower the level of Methane Hyrdate, and decrease gas boiloff. This in turn would reduce the methane expelled into the atmosphere. Surface ocean water would evaporate less quickly in the absence of direct sunlight. Likewise, the water would cool slightly, as the ocean surface currents in this region are quite gentle. Denser water carries more dissolved minerals with it, and in turn, hydrothermal vents on the sea floor around the ridge would grow a little bit faster. The water would also contain more dissolved gases, and could be more oxygenated, which would affect predator-prey relationships in fish life.

Light plays an important part in Ocean animal and plant life. Light can be divided up into 3 zones.

  1. Euphotic (above 200 meters depth)
  2. Dysphotic ( 200 to 1000 meters depth)
  3. Aphotic ( below 1000 meters depth)

Only a small amount of sunlight penetrates below 200 metres, which contains the vast amount of commercial fisheries. Placing these zones into a shadowed area would significantly impact light levels. These 3 zones would essentially be in perpetual darkness. Perhaps the first metres of water would contain tiny amounts of light. In essence, the majority of plant life relying of photosynthesis for life would either slowly starve or perish, depending on ocean currents. If the plankton are fewer, then less oxygen would be produced in this area. Large predator fish like Tuna and Swordfish would avoid this area due to lack of smaller prey fish. However, animals such as the Sea Squirt or Sea Brush would thrive due to increased water nutrients. Likewise, fish that are accustomed to dark, and cold waters, could increase in numbers in this area (if water pressure was neglected).

Lack of adequate sunlight would actually change the ocean colour. When sunlight and nutrients are abundant, Phytoplankton increase the opacity of the water and lend it their colour (green). However, in this case, the ocean water that is in the shadow will have a colour that is less green and more blue and red.

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  • $\begingroup$ You say that cool air has a higher carrying capacity for water. That is not correct: it's not the air density that matters, it's the partial pressure of water in gas phase for a given temperature, i.e. the vapour pressure of water, and it increases strongly with temperature. This is why indoor air is so dry in winter: the relative humidity drops a lot after heating it from outdoor temp to room temp. $\endgroup$ – Peter Cordes Dec 13 '17 at 12:05
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    $\begingroup$ "If we do the math, there is at most, ~2514 Cubic Kilometres of air below the UFO that will stay in the shadow of the saucer." No, it won't. The majority of the surface below the saucer will have several hours of direct light a day, and even the part farthest from the edges in the direct center will get 45 minutes a day, at least. $\endgroup$ – Keith Morrison Dec 13 '17 at 16:05
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In addition to wind and current effects, the heat capacity of the UFO might also have other effects. With the wind effects and cooling, there could be condensation on the underside of the UFO, with fresh water (rain) forming.

Also, if UFO absorbs more heat on its top surface than the water would, it will get hotter, making more complex thermals.

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