[crimsonknight3 (OP)]

I saw a program a few years ago about ideas to help tackle global warming and one of the ideas was to use ammonia to help 'seed' beds of green sea algae to reduce carbon dioxide in the atmosphere, im wondering if there has been any new developments?

[CliffordK]

One of the problems with fertilizers is that they tend to cause algae overgrowth.

In the oceans, the algae overgrowth is at the top layers where there is the most sunshine.  As the algae dies, it sinks, and is attacked first by aerobic decomposers, both releasing much of the stored CO₂, but more importantly creating an oxygen poor layer in the ocean which is devastating to marine life, especially bottom dwellers. 

One other issue to keep in mind is that most of the nitrogen that we have easy access to is atmospheric nitrogen in the form N₂.  Energy is required to produce hydrogen gas to convert the nitrogen gas (N₂) to ammonia (NH₃). 

It is likely that the amount of hydrocarbons being consumed to produce the ammonia would be equal or greater than the amount of carbon that eventually falls to the bottom of the ocean and becomes buried, or sequestered in a form that has relatively long-term stability such as frozen methyl hydrates (which are at risk of releasing methane in some areas).

[crimsonknight3 (OP)]

See it never mentioned these issues on the program, though you did remind me that isn't there a microscopic sea creatures that consume carbon (in some way) then when they die float down to the bottome of the ocean? Im not sure if its plankton or something else? Though thinking about it using the sea to help us reduce the toxic gasses we create is a very fine line, if you made more plankton, it would create an over-abundance that would reverberate down the food chain, if you seed clouds it could cause droughts in places where there may not have been any and if you put reflective particles into clouds as another person suggested you could miss the mark and make the planet too cold pushing us into a new ice age sooner than the planet intended

[Rasimus]

I know this is quite an old post, but I thought I would give some input.
The only 'ocean seeding' that I have heard about did not involve adding nitrogen in the form of ammonia (too expensive to make it viable I would imagine) but used iron filings instead. The target was algae as you mentioned but not macroalgae (or seaweed) but microalgae, otherwise known as phytoplankton, which you mention in you last post. These little critters are often greatly ignored despite the fact that they are responsible for producing around half of the world's oxygen (via photosynthesis). As CliffordK mentioned - they grow in the photic zone, the top 100 - 200m of the ocean that receives light, and as they grow in this zone they suck up a lot of the nutrients from water that they need to grow. Nitrogen and phosphorus are the major nutrients they require but there is a range of nutrients they need in smaller doses and these are referred to as micronutrients. Iron is one of these micronutrients. Apart from near the coast, most of the available iron in the ocean water is from aeolian dust - dust that gets blown over the ocean by the wind and falls into the ocean. In oceanic regions that are far away from any continent (such as the central Pacific) this wind-borne dust simply doesn't make it and levels of iron are so low that despite there being plenty of nitrogen and phosphorus in the water, the algae simply cannot grow. Iron is the limiting factor. Thus, the theory expoused by some is that sprinkling iron filings around in these locations will result in a massive increase in phytoplankton growth until the nutrients have all been used up - with lots of carbon fixed in the process. This is sound thinking and some expeditions have done this and found a "bloom" of phytoplankton does result. However it is never as simple as it sounds. All sorts of other processes happens as secondary consumers (those beasties that feed on the phytoplankton...and the ones that feed on them etc.) do there thing which MAY actually result in a net RELEASE of carbon dioxide.
What CliffordK said about algal growth resulting in low oxygen (hypoxia) is correct - but only in reasonably enclosed embayments or rivers and estuaries - google the term "eutrophication". The volume of the open ocean is too large for this to be an issue. Deadzones do occur in areas such as the Northern Adriatic but that is because of the amount of crap (in some cases literally) we humans pump into these regions (fueling the bacteria) not the algae.

[fishmicon]

we humans newbielink:http://www.mmovegas.com/ [nonactive] pump into these regions