Why aren't the oceans covered in floating seaweed?

[Neil9327]

This is a question that has puzzled me for some time. It is that on land in every part of the world where there is abundant water and sunlight, there is vegetation everywhere (rainforests typically).
But on the oceans in the same parts of the world (tropical and equatorial regions) there is typically very little, if any, floating seaweed.
Given that floating seaweed is able to live in salt water, has access to abundant amounts of water (obviously), and has the same access to the air to support photosynthesis, why hasn't floating seaweed grown to the same extent as the plants on land?

Now, there is one type of seaweed that does seem to accumulate  - Sargassum. But not to anything like the extent that plants do on land. And then only in one part of the Atlantic Ocean - the Sargasso sea (named after it). But even then, judging from this photo of it, there doesn't seem to be very much of it as a percentage of the ocean surface (less than 5% I think):

https://en.wikipedia.org/wiki/Sargasso_Sea

 

[Sensei]

You're forgetting about availability of nutrients. There are known cases of "eruption" of algae as a result of pollution of sea by human made nutrients used in agriculture.

 

Here is example of natural enrichment of ocean by volcanic activity:

https://www.sciencenews.org/article/kilauea-lava-eruptions-fed-massive-ocean-phytoplankton-bloom

"From June 3 to August 6, 2018, Kilauea spewed 50 to 100 cubic meters of metal- and nutrient-rich lava daily into the Pacific Ocean (SN: 1/29/19). Three days after lava first entered the ocean, satellite images showed a patch of water enriched in chlorophyll-a — the pigment that can make plants and algae green — off the island of Hawaii. Once the lava stopped flowing into the ocean, the patch dissipated within a week."

 

Algae and microalgae are the main source of food of the all higher level sea living animals.

 

[mistermack]

Most of the deep ocean is technically desert, and the reason is lack of nutrients. Over time, stuff sinks to the bottom when it dies, and you end up with water so clear that it can't support the basic photosynthesis process. 

The productive seas are usually shallow, around obstacles like continental shelves and underwater mountains, where the deep currents are forced upwards, bringing up nutrients from down below. You get spectacular productivity in those areas, but they are only a tiny percentage of the ocean surface. 

I've argued for ages that pumping up sediment from the ocean floor with special ships would pay for itself in fish stocks, and would remove huge amounts of dissolved CO2 from the oceans, as well as relieving extinction pressure on the rarer fish species.  

[StringJunky]

The warm seeas, above 13c, have thermoclines, which is an intermediate temperature layer that prevents the nutrient-laden deeper water mixing with the upper layer where the  photosynthetic marine life is. 

https://oceanservice.noaa.gov/facts/thermocline.html

 

[Neil9327]

On 9/12/2019 at 10:06 PM, Sensei said:

You're forgetting about availability of nutrients. There are known cases of "eruption" of algae as a result of pollution of sea by human made nutrients used in agriculture.

 

Here is example of natural enrichment of ocean by volcanic activity:

https://www.sciencenews.org/article/kilauea-lava-eruptions-fed-massive-ocean-phytoplankton-bloom

"From June 3 to August 6, 2018, Kilauea spewed 50 to 100 cubic meters of metal- and nutrient-rich lava daily into the Pacific Ocean (SN: 1/29/19). Three days after lava first entered the ocean, satellite images showed a patch of water enriched in chlorophyll-a — the pigment that can make plants and algae green — off the island of Hawaii. Once the lava stopped flowing into the ocean, the patch dissipated within a week."

 

Algae and microalgae are the main source of food of the all higher level sea living animals.

 

Ah yes, nutrients. That would explain it.

Thanks!
 

 

Now that example you have given, about lava added to water giving rise to chlorophyll, is interesting.
"Once the lava stopped flowing into the ocean, the patch dissipated within a week"
I wonder why this was?
One reason might be that the plant growth used up all of the nutrients, leaving none left to support additional growth. But I think the more likely explanation, perhaps, is that the nutrients diffused away in every direction until they became too dilute for the plants to use - and growth stopped.

I wonder whether there is a way to provide nutrients onto the ocean surface, in such a way that it is kept concentrated?

One way might be to manufacture tiny spheres made of a material that allows water to diffuse through, but nutrients not. But this might be a big ask given the high degree of salinity.

Another approach might be to grow the types of surface-free-floating seaweed that survive in the oceans, but to grow them initially in a lab in water that is artificially very high in nutrients. This might make their internal composition very high in nutrients too. Or you could even inject the nutrients in somehow.

Then you could release them in the ocean and they might perhaps grow, say, to 10 times their current weight before the concentration of nutrients available to use becomes too low for growth to continue.

I appreciate this is just a thought experiment and probably wrong in most respects!

 

[mistermack]

My own idea is to sink horizontal plastic sheets to an ideal depth below the surface of the ocean, so that the nutrients that you add can't sink to the bottom. Wave motion might be enough to keep the nutrients circulating, while not being enough to damage the sheeting. You could then pump jets of water at the bottom of the ocean, and pump up the nutrient-rich clouds of silty stuff to the surface, to fertilise the surface layer that you've created. 

Seed the layer with fish fry and whatever other life can live off the algal bloom that will result. 

[Neil9327]

On 9/15/2019 at 1:32 AM, mistermack said:

My own idea is to sink horizontal plastic sheets to an ideal depth below the surface of the ocean, so that the nutrients that you add can't sink to the bottom. Wave motion might be enough to keep the nutrients circulating, while not being enough to damage the sheeting. You could then pump jets of water at the bottom of the ocean, and pump up the nutrient-rich clouds of silty stuff to the surface, to fertilise the surface layer that you've created. 

Seed the layer with fish fry and whatever other life can live off the algal bloom that will result. 

Maybe. How would you hold the sheets in position though, and stop them either rising to the surface (where they might dry out) or moving deeper into the ocean?

 

[mistermack]

13 hours ago, Neil9327 said:

Maybe. How would you hold the sheets in position though, and stop them either rising to the surface (where they might dry out) or moving deeper into the ocean?

 

It should be pretty easy. You design a floating buoy with a vertical pole that extends down and is attached to the sheet, and keeps it from rising. You would need some clever mechanism to take the up and down wave action out of the equation. 

For the sheet to rise in one place it would have to sink equally in another, because the water would have to go somewhere. If you had some vertical sheets at intervals, creating cells and preventing sideways circulation, the whole thing could be kept pretty stable. It would be the sort of thing that you improve over time, with experience. For a lot of engineering projects, you have to be doing it and seeing what works to evolve a design. 

[John Cuthber]

On 9/12/2019 at 10:59 PM, mistermack said:

Most of the deep ocean is technically desert, and the reason is lack of nutrients.

Somewhere is "technically a desert" if it doesn't rain or snow much. Nothing to do with nutrients.

 

 

"Why aren't the oceans covered in floating seaweed?"
Much of it is, but they are rather small.

https://en.wikipedia.org/wiki/Phytoplankton#/media/File:Plankton_satellite_image.jpg
from
https://en.wikipedia.org/wiki/Phytoplankton

 

 

[mistermack]

36 minutes ago, John Cuthber said:

Somewhere is "technically a desert" if it doesn't rain or snow much. Nothing to do with nutrients.

I meant an ocean desert. I imagined that people would get that, but I guess there's always one who needs it spelt out.

https://www.sciencealert.com/in-the-heart-of-the-ocean-lies-a-desert-and-scientists-just-found-what-lives-in-it

[Neil9327]

On 9/19/2019 at 8:42 PM, John Cuthber said:

"Why aren't the oceans covered in floating seaweed?"
Much of it is, but they are rather small.

https://en.wikipedia.org/wiki/Phytoplankton#/media/File:Plankton_satellite_image.jpg
from
https://en.wikipedia.org/wiki/Phytoplankton

 

 

That's interesting. Quoting from that Wikipedia page, it says: "Phytoplankton account for about half of all photosynthetic activity on Earth".

Now, I wonder how efficient that process is? It occurs to me that for ocean phytoplankton to photosynthesize light, that light has to first travel through a certain distance of water. But (from: https://en.wikipedia.org/wiki/Electromagnetic_absorption_by_water), we know that a fair amount of the light will instead be absorbed by the water and dissipated as heat, before it has a chance to reach a phytoplankton cell.

I wonder whether the efficiency of this process could in principle be improved, perhaps quite substantially, by artificially placing phytoplankton in high concentrations, in a thin layer on the surface of the oceans so that all of the light reaches it?

[John Cuthber]

Water is pretty nearly transparent to visible light- as used by plants.

So plankton a few metres down are not troubled by the absorption of light by water; they are, however shadowed  because the plankton above them soak up the light.

A thin layer of concentrated plankton will absorb the same fraction of the light as a thick layer of dilute plankton.

https://en.wikipedia.org/wiki/Beer–Lambert_law

 

But supply  of nutrients may be easier in a less crowded environment.

Of course, there's also other stuff in the water which absorbs lioo.

 

[Neil9327]

On 9/22/2019 at 4:49 PM, John Cuthber said:

So plankton a few metres down are not troubled by the absorption of light by water

That's true - but slightly misses the point - probably because I haven't explained it properly.

Climate change. Could you grow plankton or other biomass on the surface of the oceans to cover them completely, with the aim being to absorb carbon dioxide from the atmosphere at a much greater rate than algae can under the water? For two reasons:

1. At the surface there is greater availability of CO2 from the air compared to the water, because fresh air carrying new CO2 arrives constantly by the wind, whereas dissolved CO2 has to make its way to the plankton via diffusion, which is a slower process.

2. There is more power from the sun at the surface, power to drive photosynthesis, compared to under the water (due to as stated heating of the water).

If the nutrient availability problem could be solved, could mankind cover the oceans with floating biomass? And leave it there to "hoover up" some or even all of the excess CO2 that is being produced by mankind's burning of fossil fuels? Basically to leverage the huge power from the sun that is currently being wasted on heating up water in the ocean.

 

[Moontanman]

I've grown sargassum weed in an aquarium, water movement is important as are nutrients. If you really wanted to increase the nutrients you could anchor a bouy in very deep water and pump down air to make a rising current to bring up nutrient rich water. This is easy to do but it changes the local environment and that might not be desirable... 

[CharonY]

56 minutes ago, Moontanman said:

I've grown sargassum weed in an aquarium, water movement is important as are nutrients. If you really wanted to increase the nutrients you could anchor a bouy in very deep water and pump down air to make a rising current to bring up nutrient rich water. This is easy to do but it changes the local environment and that might not be desirable... 

A big issue in using feeding to create carbon sinks is that it requires the biomass to essentially get buried in order for it be effective. Initial attempts at iron feeding were done with the hope that algae blooms would result in biomass sinking to the floor and thus remove carbon from the cycle. However, so far only one study found that desired effect. The other fear is of course that algae blooms can deplete oxygen and thereby create or expand dead zones in the ocean. There are also a host of other (mostly ecological) concerns with such strategies, you mentioned. There is also a healthy discussion regarding the types of nutrients that affect various increases in biomass (C, N, S) relative to iron and what impact shifting such cycles may have. As a whole the huge uncertainty with regard to feasibility and potential consequences make it a rather difficult proposition to follow up. At minimum, more research is needed.

[mistermack]

15 hours ago, Moontanman said:

I've grown sargassum weed in an aquarium, water movement is important as are nutrients. If you really wanted to increase the nutrients you could anchor a bouy in very deep water and pump down air to make a rising current to bring up nutrient rich water. This is easy to do but it changes the local environment and that might not be desirable... 

I can't see that being easy or cheap to do. The pressures at ocean depths are phenomenal, so you would need extremely powerful pumps and thick walled piping, and a lot of energy, to do it that way. 

If you had a submersed electric propeller and a thin walled pipe, you could pump up cloudy water that you have disturbed with a jet and the only energy needed is to overcome friction in the pipe. The wider the pipe, the lower the friction, so it should be doable that way. 

As far as the local environment is concerned, you would be doing it in an ocean desert, something that covers more than half of the planet, so I can't see much harm coming from it. If you can raise the productivity in the lifeless parts of the ocean, it takes the pressure off the threatened species elsewhere, by applying a brake to the price of fish worldwide.

[Moontanman]

22 minutes ago, mistermack said:

I can't see that being easy or cheap to do. The pressures at ocean depths are phenomenal, so you would need extremely powerful pumps and thick walled piping, and a lot of energy, to do it that way. 

If you had a submersed electric propeller and a thin walled pipe, you could pump up cloudy water that you have disturbed with a jet and the only energy needed is to overcome friction in the pipe. The wider the pipe, the lower the friction, so it should be doable that way. 

As far as the local environment is concerned, you would be doing it in an ocean desert, something that covers more than half of the planet, so I can't see much harm coming from it. If you can raise the productivity in the lifeless parts of the ocean, it takes the pressure off the threatened species elsewhere, by applying a brake to the price of fish worldwide.

Off the coast of south america cold water wells up naturally and supports a wide ecosystem from sardines to humboldt squid. A series of solar powered pumps could use thin tubes in clusters to carry the air. The thin tubes are more resistant to pressure than one large tube and the air pressure would support the tube against the water pressure.  Over time you could establish a similar ecosystem. Unusual currents have brought the humboldt squids as far north as washington state in recent years temporarily creating a new fishery for the huge squids. Hmmm squid steaks on the grill! Ecosystems can be manipulated and appropriate species will move in... 

 

[mistermack]

I think using air as you describe really is a non-starter, but pumping up water as in my model might well work.

Just a quick look at the required air pressures makes using air an impossible job. At 100 feet, you need 440 psi. At a thousand, you need 4,400 psi. I agree that at a thousand feet, the internal pressure in the pipe would balance the water pressure. But what about at the surface? What kind of pipe could hold 4,400 psi pressure at the surface? And what sort of pump could supply it? And imagine the heat energy being lost by that kind of pumping. It's an impossible project that way, but pumping up cloudy water from the deep is just overcoming the friction with the pipe walls, which in a wide enough pipe would be negligible. 

[Moontanman]

5 minutes ago, mistermack said:

I think using air as you describe really is a non-starter, but pumping up water as in my model might well work.

Just a quick look at the required air pressures makes using air an impossible job. At 100 feet, you need 440 psi. At a thousand, you need 4,400 psi. I agree that at a thousand feet, the internal pressure in the pipe would balance the water pressure. But what about at the surface? What kind of pipe could hold 4,400 psi pressure at the surface? And what sort of pump could supply it? And imagine the heat energy being lost by that kind of pumping. It's an impossible project that way, but pumping up cloudy water from the deep is just overcoming the friction with the pipe walls, which in a wide enough pipe would be negligible. 

I'm pretty sure that at one hundred feet you need about 45 psi... I am an open water scuba diver, pressure increases at the rate of 15 psi per 33 feet.. 

Edited October 2, 2019 by Moontanman

[mistermack]

4 minutes ago, Moontanman said:

I'm pretty sure that at one hundred feet you need about 45 psi... I am an open water scuba diver, pressure increases at the rate of 15 psi per 33 feet.. 

Yeh, sorry you're right. I put the decimal point in the wrong place. 

The principle still applies though, I don't think with those pressures you would have a workable system.

[Moontanman]

1 minute ago, mistermack said:

Yeh, sorry you're right. I put the decimal point in the wrong place. 

The principle still applies though, I don't think with those pressures you would have a workable system.

I disagree, clusters of small pipes would do the trick if the walls were supported by the pressure... 

[mistermack]

Just now, Moontanman said:

I disagree, clusters of small pipes would do the trick if the walls were supported by the pressure... 

Yes, but like I pointed out, the walls would be supported by the pressure at a thousand feet depth, but at the surface, you would have 450 psi inside the pipes, and just atmospheric pressure outside. And that doubles for 2,000 feet etc etc. 

The deep ocean is really deep. Sperm whales dive to 7,000 feet. At that depth, you would be pumping at 3100 psi. 

The energy needed to pump air at just 450 pounds at 1,000 feet would be prohibitive, even if you could get over the pipe problem. Also, narrow air pipes would incur far too much friction, at any realistic rate of flow. 

[CharonY]

6 hours ago, Moontanman said:

Off the coast of south america cold water wells up naturally and supports a wide ecosystem from sardines to humboldt squid. A series of solar powered pumps could use thin tubes in clusters to carry the air. The thin tubes are more resistant to pressure than one large tube and the air pressure would support the tube against the water pressure.  Over time you could establish a similar ecosystem. Unusual currents have brought the humboldt squids as far north as washington state in recent years temporarily creating a new fishery for the huge squids. Hmmm squid steaks on the grill! Ecosystems can be manipulated and appropriate species will move in... 

Even if we can manipulate current flow economically, there is a big issue in terms of the ecological effects. Say, if we were able to circulate from the ocean floor by magic means, then we would also move carbon back into the cycle. We could have more biological activity, but that also entails higher release of CO2. One of the reason why iron seeding was deemed attractive is that it would (in theory) target specific nutrient limitations, while leaving sunk carbon alone. In fact, the basic idea was that the newly produced biomass would sink before it can be oxidized. But it is still not clear whether that happens quantitatively. The other challenges of course is the interplay with existing currents. The deposition of carbon does not really happen locally. For example huge conveyor belts that move across the globe are instrumental in burying carbon, e.g. by downward movement of biomass rich warmer water once it moves to the colder region. In other words, effects are not only hard to impossible to contain, but sometimes it actually requires these long-distance transports to actually act as sinks. There are much more details to it but it would be way beyond my expertise. I admit that my eyes somewhat glaze over when they go full-bore on their research, but my takeaway from those seminars is always: "it's complicated" and "it is very global". 

The latter I suspect to justify all that travel for field work (and tans).

[Moontanman]

53 minutes ago, CharonY said:

Even if we can manipulate current flow economically, there is a big issue in terms of the ecological effects. Say, if we were able to circulate from the ocean floor by magic means, then we would also move carbon back into the cycle. We could have more biological activity, but that also entails higher release of CO2. One of the reason why iron seeding was deemed attractive is that it would (in theory) target specific nutrient limitations, while leaving sunk carbon alone. In fact, the basic idea was that the newly produced biomass would sink before it can be oxidized. But it is still not clear whether that happens quantitatively. The other challenges of course is the interplay with existing currents. The deposition of carbon does not really happen locally. For example huge conveyor belts that move across the globe are instrumental in burying carbon, e.g. by downward movement of biomass rich warmer water once it moves to the colder region. In other words, effects are not only hard to impossible to contain, but sometimes it actually requires these long-distance transports to actually act as sinks. There are much more details to it but it would be way beyond my expertise. I admit that my eyes somewhat glaze over when they go full-bore on their research, but my takeaway from those seminars is always: "it's complicated" and "it is very global". 

The latter I suspect to justify all that travel for field work (and tans).

Agreed...

[mistermack]

15 hours ago, CharonY said:

Say, if we were able to circulate from the ocean floor by magic means, then we would also move carbon back into the cycle. We could have more biological activity, but that also entails higher release of CO2.

Not really a problem, if you are producing high photosynthetic activity. The CO2 just becomes a useful source for the green algae. 

If CO2 capture was your motive, you would arrange your project in places where sinking matter falls into a low-oxygen zone, rather than the random way that nature does it. 

Personally, I wouldn't be aiming at CO2 fixing with this sort of project, but food production by making near-sterile oceans productive. Any carbon capture would be a bonus. 

But this kind of food production would have a beneficial effect on hard-pressed environments that we are currently fishing to death.