If weeds are grown due to pollution they may no be the environmentally sustainable value chain 

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sea weeds are becoming popular as Plant growth promoter as well source of micro-nutrients to some extent, public/corporate research on this aspect is showing in the commercial markets. However, we have not come across reports on LCA of these products, may be some of the industries supplying this material hsve initiated this kind of studies

We find LCA of fertilizers/pesticides production by some corporates involved in fertilizers/pesticides manufacture

This aspect is important and should be evaluated for environmentally sustainable usage.

regards

The sustainability of ruminant production at the rate which it is needed globally seems to be a contested issue, considering that one of our global priorities is lowering greenhouse gases, which they copiously produce. A better question might be, are there any viable (and tasty) alternatives to producing ruminants at the industrial scale in the first place?

Do we have sufficient information about the variety, the geographic spread and the growth rate under different conditions of most seaweeds? This information also needs to be understood through the lens of climate change and impacts such as increasing ocean acidification. If humans begin harvesting seaweeds in greater numbers for food production, there is the distinct possibility of creating problems with the ocean ecosystem before we've fully understood it. In spite of this, there are many areas of the world where it seems that - with further study - they could provide a sustainable food supply. Areas like Japan and Korea as well as Ireland and many other locations have augmented their diets with seaweed ingredients for centuries.

Do we have sufficient information about the variety, the geographic spread and the growth rate under different conditions of most seaweeds? This information also needs to be understood through the lens of climate change and impacts such as increasing ocean acidification. If humans begin harvesting seaweeds in greater numbers for food production, there is the distinct possibility of creating problems with the ocean ecosystem before we've fully understood it. In spite of this, there are many areas of the world where it seems that - with further study - they could provide a sustainable food supply. Areas like Japan and Korea as well as Ireland and many other locations have augmented their diets with seaweed ingredients for centuries.

Intuitively, I shudder.  Lets compare the processes: 

Extract plant matter from the ocean, in a manner that does not significantly alter the local ecosystem, using underwater equipment.  Treat it in some way to remove the salt (unless osmotic pressure is maintained by a very high sugar content, instead).  Ensure that the pollution that we release into the oceans does not accumulate in the animals that eat the vegetation, probably by an extraction process that removes just the wanted chemistry from the vegetation..  Dump the waste vegetation into a composting facility, to ensure that the remaining cellulose and hemi-cellulose doesn't degrade to methane. Mix the extracted chemistry with the animals' food stock, testing it to ensure there is the targeted balance of nutrients and sugars.  Dump the remains back into the ocean to become the next generation of plants.

OR

Let animals eat the vegetation that grows in places where we can't use efficient planting and harvesting techniques (eg, pasture land). 

One of these things is simpler than the other, with a much lower embodied energy, waste products, and with far fewer unknowns.  I don't know how you would be able to predict the 'shadow' of this process, eg, identifying all of the aspects of the ecosystem that would be negatively affected by the process - maybe this is simpler than it appears. But I'm not seeing it. 

Intuitively, I shudder.  Lets compare the processes: 

Extract plant matter from the ocean, in a manner that does not significantly alter the local ecosystem, using underwater equipment.  Treat it in some way to remove the salt (unless osmotic pressure is maintained by a very high sugar content, instead).  Ensure that the pollution that we release into the oceans does not accumulate in the animals that eat the vegetation, probably by an extraction process that removes just the wanted chemistry from the vegetation..  Dump the waste vegetation into a composting facility, to ensure that the remaining cellulose and hemi-cellulose doesn't degrade to methane. Mix the extracted chemistry with the animals' food stock, testing it to ensure there is the targeted balance of nutrients and sugars.  Dump the remains back into the ocean to become the next generation of plants.

OR

Let animals eat the vegetation that grows in places where we can't use efficient planting and harvesting techniques (eg, pasture land). 

One of these things is simpler than the other, with a much lower embodied energy, waste products, and with far fewer unknowns.  I don't know how you would be able to predict the 'shadow' of this process, eg, identifying all of the aspects of the ecosystem that would be negatively affected by the process - maybe this is simpler than it appears. But I'm not seeing it. 

The Crown Estate in the UK (which owns large tracts of coastal waters) has looked into the possibility of seaweed aquaculture for biofuel production, including this report (see link) on the potential carbon footprint. We had to base it all on secondary data, for obvious reasons, but it gives an idea of the potential hotspots associated with seaweed. Note - it doesn't include drying (as it's fermented directly, close to where it's landed either to biogas or bioethanol in this system), the energy for which also has the potential to be a hotspot.

The other thing to consider with ruminant feeds is the digestible matter content, as this has a direct effect on the methane produced when the animal's digest it. From my experience looking at the carbon footprint of milk production in the UK, the enteric emissions of the cattle were usually a far greater contributor to the footprint than the upstream impacts of the feeds. Using 'low carbon' feeds might actually increase total carbon footprint if the cows produce more methane as a result of eating it.

Crown Estate Report (http://www.thecrownestate.co.uk/media/5559/carbon_footprint_of_seaweed_as_a_biofuel.pdf)

Dear Escribano, I revisited your question, because it just kept rolling around in my mind. My previous answer, although attending to the big picture, does not actually answer your question. So I kept pondering.

There may be an answer within your question. You asked for a sustainable way to do this. Perhaps if the activity is kept local, and in a circular feed pattern, it could be viable. The two references to scholarly papers below show that it is possible, but may prove difficult. If you watch this TED Talk about the issue of overfishing worldwide, after 11:30, the ichthyologist Paul Greenberg begins to discuss alternatives. I recommend watching from this point on, because you will see my logic. 

What I suggest as a landscape planner and biologist, is that a circular system be created, where the ruminant and agricultural runoff has created dead zones, be populated with a system beginning with freshwater plants or land plants at the riverbanks for phytofiltration of dangerous compounds and chemicals, the water moving further into an artificial mangrove for filtration (and fish propagation) around the rivermouth, and finally leading to seaweeds in the saltwater areas, all for the purpose of rhyzofiltration for the purpose of filtering the water and using the fertilizer for growth. In this way, the seaweeds and selected land plants or freshwater plants, can contribute to feeding the livestock which produced the runoff in the first place. It is a very large, complex version of a wastewater leechfield. This system would not only contribute to pollution reduction, and reducing the cost of transportation, it would solve ocean biodiversity problems and loss of mangrove forests facing us at a global level. Were the systems for collecting the plants cleverly integrated into the landscape, a minimal effect on the artificially created biosphere could be acheived.

Key to this, is that the local people be involved in the bioremediation, and be able to benefit from supporting it, both economically and environmentally. Planners and local government must be involved in the process. This is no minor endeavor, but could lead to very positive outcomes worldwide.

Although, again, perhaps we should be minimizing our consumption of the worst greenhouse gas polluter on the planet, and finding a tastier, more sustainable alternative.

All of my best wishes in your research for a better tomorrow. -Sofia Penabaz-Wiley

https://www.ted.com/talks/paul_greenberg_the_four_fish_we_re_overeating_and_what_to_eat_instead

http://agresearchmag.ars.usda.gov/2000/jun/soil

http://link.springer.com/article/10.1007%2Fs10666-005-9039-x

Dear all, and dear Sofia, thanks very much for your answers and information.

Regards,

Alfredo J. Escribano.