Ocean productivity

Why is the clearest blue ocean water almost empty of life while murky coastal water teems with it?

The postcard blue of the mid-Pacific is the emptiest water on the planet, and the grey-green murk off a fishing coast is the richest. That inverts every instinct we have about clean and dirty. But before explaining it, notice that the clarity and the emptiness may not be cause and effect at all. What if they are the same fact, seen twice?

Start with the colour. Pure water absorbs red light and scatters blue, so water with nothing in it looks blue. Phytoplankton carry chlorophyll, which absorbs blue and red and reflects green; add plankton and sediment and the water turns green, then brown. The blue is not a cause of emptiness — it is a measurement of it, which is exactly why satellites read ocean colour. That disposes of the framing and leaves the real question: why is the open ocean short of plankton?

Not light — the subtropical gyres are the sunniest surfaces on Earth. What a plankton cell also needs is nitrogen, phosphorus, iron, silica, and here is the geometry that decides everything. Light reaches only the top hundred metres or so. Nutrients are mostly below, because everything that dies sinks, taking its nitrogen with it, and is remineralised in the dark. Light at the top, food at the bottom, and they must meet.

What keeps them apart is density. Sunlight warms the surface, warm water is lighter, and a warm layer floating on cold is extremely stable — the thermocline. In the middle of a subtropical gyre it is worse than stable: the wind pattern drives surface water to converge and sink there, pressing the boundary deeper still. The lit layer is sealed off from the reservoir beneath it.

Now the coast. Several deliveries arrive there that never reach the gyre: rivers, shallow bottoms stirred up by tide and swell, and above all upwelling. Wind blowing along a coast does not push surface water straight downwind; the Earth's rotation deflects the transport sideways, and on the eastern edge of an ocean that means offshore. Water pulled away from the coast must be replaced from below, so cold, nutrient-loaded deep water is drawn into the light. The murk is largely the consequence: plankton, plus the sediment the same energetic water keeps in suspension.

Here is the distinction that makes it click. Oceanographers separate new production, run on nutrients newly delivered into the lit layer from outside, from regenerated production, run on nutrients recycled within it. The gyre is not sterile; it is a tight, fast loop, where tiny cells are grazed almost as fast as they divide and the nitrogen is released and reused within days. A millilitre of gyre water holds on the order of a hundred thousand Prochlorococcus cells. What the gyre lacks is not activity but surplus — only new production supports biomass that leaves the loop, as sinking carbon or as fish.

Two honest complications. First, the missing nutrient is not always a major one. In the Southern Ocean, the equatorial Pacific and the subarctic Pacific, nitrate is abundant and chlorophyll is still low; the limiting element is iron, which mostly arrives as dust and is scarce far from land. That was a hypothesis until it was tested — more than a dozen open-ocean iron-addition experiments produced blooms, so the mechanism is established. What is not established is what happens to the carbon: whether fertilised blooms export meaningfully to the deep sea is contested and the evidence is weak. Note that the loudest voices on both sides are interested parties — ventures selling ocean carbon credits on one, researchers whose funding depends on the debate staying live on the other.

Second, "almost empty" is about density, not totals. Productivity per square metre in a gyre is low, but the gyres are enormous, so their share of global marine photosynthesis is large. Conversely the eastern boundary upwelling systems occupy on the order of 1% of the ocean's area while supplying something like a fifth of the world's fish catch — a figure to treat as an order of magnitude rather than a measurement.

Where this sits next to its neighbours: the collection's piece on lake turnover turns on the same density barrier and reaches the opposite fixed point — in a temperate lake the stratification breaks down every autumn and the nutrients come up on schedule; in a subtropical gyre it essentially never does. And the piece on dust-fed rainforests is the same limitation solved by remote delivery rather than by mixing from below.

Three refinements. First, the causal chain runs from nutrient supply rate to plankton to turbidity, with turbidity the last link, not the first — muddying clear water with sediment would not make it productive, it would shade it. Second, "productive" needs its units named: a gyre is unproductive per square metre and in new production, but it is not idle. Third, the barrier is a rate, not a wall — nutrients cross the thermocline continuously by mixing and by eddies, just slowly, which is why the gyre supports a steady small population rather than none.

And a fourth corner: the axes trap you at either end. Very turbid estuaries can be so full of sediment that light, not nutrients, becomes limiting — nutrient-rich water too dark to use.

• How the biological pump moves carbon to depth, and why so little of what sinks actually gets there.

Every term the collection defines is gathered in the glossary.