Let us take a look at all issues we may encounter

Weather conditions are the first issue encountered when operating any infrastructure on high seas. So let us first choose the only ocean not to be prone to cyclones, typhoons or other hurricanes, South Atlantic. Besides, it is the poorest ocean both in terms of fisheries and maritime traffic, so we should find an unoccupied and safe place there.

Tropical cyclones
(Source: NASA Earth Observatory)

The lack of mineral nutrients for phytoplankton growth is the second issue on high seas in general, and in South Atlantic in particular. Except in some eastern areas, where constant wind from land cause upwelling, algae bloom and related important fish production, it is a real desert out there. Anyhow, we have to content us with the desert areas, because the naturally rich ones along the African coast are already exploited and occupied. Farther west, the nutrients present in the photic zone - above a 100 meters depth where light comes in - are quickly completely consumed by phytoplankton's photosynthetic activity. In these conditions, the phytoplankton cannot multiply enough to feed many zooplankton. That is why the high seas are biological deserts. No existing wind is strong enough here to mix the layers, and the minerals are not massively renewed from the deeper layers, where they stay present in quantity.

seawifs annual NASA
(Source: SeaWifs Project)

The challenge now is to bring the existing nutrients from the depth and the light from above together.

Doing the math, there are only two possibilities, bring light down or bring nutrients up. Let us have a look at both.

Bring light down :

Due to the aggressiveness of ultraviolet radiation from the sun, and to the constant adaptation of the naturally sinking algae, the optimal wavelength for its photosynthetic activity is within the blue range, the one that have the highest water penetration coefficient. Luckily, thanks to today’s LED technology, we are able to produce just the needed blue light with very few energy. Furthermore LED can easily be deployed in high pressure environments, because, unlike light bulbs, they are not hollow and cannot implode. But, even if we manage to illuminate properly a portion of the dark aphotic zone - below 100 meters depth - with strings of blue LED, we still need to survey, handle and fix a production process in these depths… This is feasible, but too constraining.

Light penetration in open ocean
(Source: NOOA Ocean Explorer)
LEDs Super Bright
(Picture: LEDs Super Bright)

Bring nutrients up :

Unfortunately, nutrients are diluted in great amounts of water, far too much to be pumped up with external energy and create an artificial upwelling. It would not be economically viable to spend so much energy. So can we bring up colossal amounts of nutrient-rich deep-sea water without energy ? Yes we cansmiley common sea good

Antarctic Intermediate Water (AAIW) is a nutrient rich, and low salinity water body, which characteristics are due, among other factors, to the mixture of seawater with mineral rich and fresh meltwater from the southern ice cap. These are the waters, where the Antarctic life explosion takes place every summer. AAIW flows slowly northerly in every oceans. In the South Atlantic, it meets warmer and saltier sub-Antarctic water at the convergence zone, 50°-60°S. There, it sinks to a depth of approximately 1000 meters (3280 feet), gliding over cold, salty and very dense bottom water. A large part of it is flowing northeasterly to the South Atlantic Gyre, where it loses its characteristics by mixing. But a little part of it is flowing due north on Atlantic's west side, until crossing the Vitoria Trindade Chain east of Brazil. AAIW lays at this depth, and does not mix with the upper and under layers, even in this tropical region, where surface water becomes heavier, due to evaporation and induced saltiness increase. The sea stays stratified because the diffusion between the different water masses is too low, no storms are strong enough in this region to mix the layers, and no constant wind from land forms an upwelling.

AAIW in the Atlantic Ocean
(Courtesy: Earth' Climate: Past and Future, written by William Ruddiman)

These conditions are ideal to set up a Perpetual Salt Fountain, an "oceanographic curiosity", as first described by Henry Stommel et al. in 1956. If you have a warm and salty water mass above a colder and fresher one, put a vertical pipe between this two layers, pump it up until the pipe is full with the deep water, and then you can stop pumping. The up flow will last perpetually, without any other external energy spending. This is due to the fact that the heat difference between the water masses is conducted through the pipe walls, but the salinity difference remains unchanged. This property has been recently validated on open sea by Shigenao Maruyama et al. from the Institute of Fluid Science, TOHOKU University. In our previous example, we have very salty and warm water above very little salted and colder water. The salinity having the largest influence on density, the salt fountain should be very strong. Here is our solution to bring nutrients up without energy spending.

Perpetual Salt Fountain concept
(Courtesy: Institute of Fluid Science, TOHOKU University)

Furthermore, AAIW, found today at 1000 meters depth in the tropical zone of South West Atlantic, was in contact with the atmosphere about 300 years ago, in other words, long before the Anthropocene, and the wide dissemination of its pollutants. Even with the inevitable transfers between water layers, caused for example by the daily vertical migration of zooplankton, the concentration of anthropogenic pollutants in AAIW should remain insignificant. By considering the present state of seafood contamination with heavy metals and POPs, these waters constitute a very interesting aquafeed production environment. Since several years now years, who recommends to eat fish for its healthy omega 3 content, but not more than twice a week, because of its pollutants content. Just before Christmas 2016, a leading French consumer association revealed also, that organic farmed salmon has higher contaminant levels than conventional farmed fish (both luckily well below WHO standards), due to a higher fishmeal inclusion in their feed. On my view, the (still relatively low) contamination of fishmeal is not due directly to wild forage fish, but either to the 30% sourcing of trimmings from higher trophic level fishes, packaged in the filleting factories. In any way, it is time to take serious account of the current contamination of farmed fish with heavy metals and POPs, and to work on it.

Another issue is to build a floating infrastructure with, at least, a 1000 meters deep anchorage, or with dynamic positioning. This is quite preposterous if you consider the scale of the area, and the production we are aiming to. Help is provided here by seamounts, and more specifically guyots with a flat top, extinct volcanoes rising up from the seafloor, sometimes almost to the surface. Such a guyot is perfectly suited to support an infrastructure on its top and pipes along its slopes. In addition, due to Taylor columns effects, seamounts have the particularity to let the isotherms rise a bit, and form a vortex that retains the surrounding waters. This phenomenon allows us to find AAIW at less deep level and, after being raised through the Perpetual Salt Fountain pipes, to keep it a while above the seamount for exploitation.

A last issue is the needed energy supply so far away from the continent. Overlooking fossil fuels which have no more real future, let us have a look on renewable powers.

  •  Solar power: solar panels, deployed on a large area, are certainly too vulnerable in an open ocean environment.
  • Wind power: wind mills have already been anchored off shore, but provide only a non-constant supply.
  • Wave power: Wave Energy Converter are fragile mechanical devices and are also unable to provide a regular supply.
  • Current power: marine turbines need a very strong current to be efficient and it is better to avoid it in our case .
  • Osmotic power: membranes are expensive, cleaning chemicals are needed and the system is very complex to handle.
  • Temperature gradient power: Ocean Thermal Energy Conversion plants are also very sophisticated systems and need chemical refrigerants.

What else ? I think on Earth's internal heat power. Yes, geothermal energy on high seas smiley common sea good !
Why not, being located just above an extinct volcano, rising up from the seafloor almost to the surface.

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