squee!:
if iron is indeed a limiting factor in the oceans, and likely specific microorganisms subsist off of it (and are consequently held in check because of it being scarce), won't dumping several hundreds of tons of it possibly lead to a population explosion of these particular microbes, and is it unreasonable to guess that they may outcompete other organisms that they had previously coexisted with before (assumming of course that their competiton does not also derive benefits from iron)?
These same concerns were brought up during the ocean fertilization trials. It's not considered a big deal, once one understands just how radically different oceanic primary production differs from that of the terrestrial.
Phytoplankton are evolved to be experts at selective absorption. If there's something in the water that they need, they suck it in, and suck it in FAST. Their uptake of nutrients and trace minerals exceeds terrestrial and benthic plants by
orders of magnitude. During the iron fertilization experiments, tons of the more soluble powdered stuff was dumped into selected spots in the ocean, and monitored over a short period (days to weeks). What happened was, the endemic phytoplankters sucked the iron out of the water quick as lightning, and then bloomed. And
then they crashed and went back to normal levels. There wasn't time for more specialized microbes to get a "piece of the action". The local ecology suffered a small blip.
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Now with something large like a ship, you have a long-term iron supply, but only
small amounts are leached out into the water column. It can take decades to centuries for hulls to dissolve away. The process is so slow, its effect on the plankton is miniscule. Now for organisms attached to the metal, they can uptake much more of the stuff, and even sequester in in their tissues... assuming they HAVE tissues. Critters like this (macroalgae, barnacles, soft corals, brozoans) don't have the frightening growth rates of phytoplankton, so they'll just "sit" on their iron repository like Scrooge. In fact, if enough encrusting organisms cover over a wreck, it'll delay the iron leaching out of the wreck. This won't slow the ship's degradation, however. Overgrowth will collapse a wreck due to sheer weight.
My microbiology and chemoynthetic references are in the office, but as I recall iron-reducing bacteria are primarily an anaerobic clade. So they're not going to be around much on ships, 'less the ships are buried in the mud. Anaerobic respiration is pretty inefficient as you likely know, so even when happy, anerobes don't grow very fast.
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Back to ships as reefs, I've thought of great long-term ideas. TITANIUM SHIPS. Yes, these suckers will last near forever! The Russian
Alfa and
Sierra submarines had titanium hulls... lets' sink those babies!
I also recalled the "concrete Liberty ships", which were built in limited numbers. They weren't very big and had a tendency to split open or something, but as reefs they'd last a long time. I got to see one of these up close in Bimini last year, the old
Sapona. Considering all the abuse that wreck has withstood over the decades, it's in surprisingly good shape!
