Over the past decades, as Grand and Georges Bank's fish stocks have fallen, along with zooplankton biomass, satallite photos show phytoplankton biomass to be increased. While at first this seems surprizing, Debbie MacKenzie has explained how this can happen: In an imaginary pasture, taller grass doesn't mean faster grass growth, if less cows are now grazing the grass.
But with less fish reducing zooplankton biomass, and more phytoplankton to graze, why don't zooplankton levels rebound? Debbie shows how oceanic zooplankton fill a unique role, accompanying their unique, and previously puzzling deep dawn dive from, and nightly return to, the surface, where they feed. In diving and returning in this way, zooplankton ferry needed nutrients from the deep to sunlit surface phytoplankton, which use these nutrients to photosynthesize. With less zooplankton, phytoplankton get less ferried nutrients, and can not do as much photosynthesizing. This reduces the amount of sunlight energy entering the Bank's food chains, so there is less to feed fish, and reduced fish stocks and harvests result.
But why are zooplankton doing less ferrying - why are their numbers and biomass reduced? What is missing that once was there? Debbie suggests that heavy harvesting of fish has actually removed such large amounts of important nutrients from the Banks that the zooplankton have less to ferry, no matter how numerous they are, so the phytoplankton are starving. The only reason there aren't less phytoplankton, instead of more, is that zooplankton are starving even more, and so leave more, but hungrier phytoplankton behind.
One supporting fact here is the scarcity of nitrogen, phosphorus and iron in oceanic ecosystems. Phosphorus tends to form solids and precipitate out of aquatic environments; it and nitrogen are major plant nutrients, and nitrogen tends to directly limit Banks productivity.
Iron is very, very rare in deep ocean waters. Because iron is central to enzymes which catalyze fixation of dissolved nitrogen from air, it may play a key role in the failure of Bank's fish stocks to recover as expected. By allowing bluegreen algae and such to fix nitrogen from the air, replenishing the iron removed by our fish harvesting may trigger a resurgence in the Bank's sealife, and at a very affordable cost. And then again it might not, but because iron replenishment would be affordable, and the research to confirm iron's role here would also be both affordable and safe, we should test it.
If iron is a limiting nutrient on the Banks, then we are very lucky - small amounts of iron could significantly increase Bank's water's iron concentrations, but the amount of nitrogen needed to significantly affect concentrations on the Banks would cost a lot of money, and require enormous amounts of energy to make - nitrogen fertilizer production is very energy intensive.