Saturday, October 09, 2004

Can we replenish Northwest Atlantic Banks' fish stocks by chumming with iron?

Restoring the Banks' fish stocks to levels of two centuries ago might be possible now, and affordable, due to science and ecology turning a corner of understanding this past year.

Reportedly, declines in Banks' productivity over past decades have paralleled declines in dust from the Gobi desert falling across the North Pacific, North America and the North Atlantic. This North Asian desert is said to be the most important dust source for the temperate lattitudes.

But how can a little dust in a vast ocean matter? The link is iron, which is very rare in the oceans. Blue-green algae use iron in their nitrogen-fixing enzyme, which allows them to use dissolved air's nitrogen to form proteins and such. A little iron allows them to fix a lot of nitrogen. Since growth of Banks' sealife is usually limited by lack of nitrogen, and many organisms feed directly or indirectly on blue-green algae, blue-green algae and added iron might help feed all sealife there the nitrogen-rich food they lack.

If iron will help Banks' productivity, it will be noticable quickly, since plantlike phytoplankton are said to double in a day, given the right conditions. If blue-green algae double for three days with added iron, and zooplankton, which eat them, double the day after that, wandering herring, mackerel and such that eat zooplankton might soon stay to feast, attracting cod, haddock and other top-value fish. Fishermen might find it worth their while to spread iron over an area at the beginning of the week, then return at the end to the week to net the gathered fish.

There are many iron and steel wreaks on and near the Banks, yet the Banks don't produce as they did. Most of the iron in these wreaks is hard for sealife to get to - there is little surface area to allow access by sealife, compared to windblown dust. Experiments in other oceans have found iron sulphate works well, when spread across surface waters by ship.

Iron is rare in the oceans because it tends to precipitate out and settle down out of the sunlit surface waters. But because the Banks are shallow and turbulent, and rich in sealife, iron is recycled quickly on the Banks, and added iron would get used over and over, as opposed to other parts of oceans, where most added iron settles out within days. So on the Banks a little iron might go a long way.

Debbie MacKenzie has pointed out the important role other sealife plays in returning nutrients like nitrogen to the photosynthesizing phytoplankton floating at the sunlit surface. If Banks sealife was continually recycling iron, previously gathered over centuries from blown-in dust, and if heavy fish harvests this century have removed much of that iron, replenishing the iron might restore the Banks' ability to make fish out of the sunlight and seawater that come there, and thus yield enough fish for seals, whales, trawlers and anglers alike.

But if adding iron causes 'red tide' to bloom across the Banks, it might be a huge disaster. There are reports of blooms of blue-green algae, also called cyanobacteria, sickening fish in middle Atlantic states, in estuaries like the Chesapeke and Pamlico Sound, but these occur in the absence of water turbulence and the presence of much dissolved organic matter.

For better or worse, marine science may allow us to change the Banks as we now know them.

Brian Cady

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