Gary,
You've clearly thought about this, but I'm surprised that you're so confident that lifting all that water wouldn't take much energy. The deep water is there because it is more dense than that at the surface. You're not discussing small amounts of water.
Surface waters in temperate regions warm in summer, water expands as it warms and the structure of the water becomes more stable. It takes more energy to exchange the low-density surface water for the high-density deeper water. The phytoplankton remove what nutrients are available
Consider temperate waters in the winter. Strong gales have mixed the waters. There's little sun, so not much heating, and not much photosynthesis. With spring comes the sun. Photosynthesis takes off, plants grow and zooplankton respond to increased availability of food and warmer temperatures.
The water close to the surface gets warmer. That means it gets less dense and more buoyant. It resists being mixed with the colder water that is more dense. The surface water becomes depleted of nutrients but low concentrations are sustained by recycling provided by the bodily functions of the animals grazing on the phytoplankton and higher trophic level. Occasional spring gales sometimes disturb the upper waters enough to bring in some of the deeper water with new nutrients. This brings a temporary reprieve and can stimulate another burst of activity that spreads up from the phytoplankton.
In the fall the sun is reduced, the surface water cools and is more susceptible to the strong gales of fall. There is often another burst of activity as the thermal structure starts to break down while there's enough sun to use the nutrients that are made available. Eventually though, the gales destroy the thermal stability and the upper waters become indistinguishable from those much deeper.
Now you might be considering the tropics, but the water has to be below the top mixed layer, which varies form about 30 to 200m deep. Gales or even hurricanes don't have enough energy to bring this water up. You want to pump the water up. If you replace the water in the top, say just the top 30m, then you have to keep replacing it because in a week or two the nutrients will be consumed.
Finally, stimulating photosynthesis doesn't necessarily remove Co2. Whatever fraction of the phytoplankton get eaten and metabolized is returned as C02. Any fish caught and consumed will represent carbon returned to the atmosphere. Not that stimulating fish production is a bad thing - it's just that it doesn't reduce Co2. Only faecal material and decaying detritus that falls into deep anoxic zones can truly be considered removed.
Ashley |
