To some extent, this is mapped out, but the easier way to tell is by the color of the water.
Ocean deserts are characterized by less plankton and thus Chlorophyll. Therefore, the water in an ocean desert will be bluer and less green than the water in an area with plenty of nutrients and oxygen and plankton.
Since nutrients are the limiting factor in algae growth, the more nutrients the more productivity. First how do we know how much productivity there is? There are several ways of measuring this. First, in general, the higher the productivity in an environment, the greater the abundance of life. So one way of measuring productivity is to quantify the mass of living organisms---say organisms per cubic meter, for example (square meter if you’re talking about land). Since animals usually move around, this is most effectively done with plants. In the ocean this means counting algae cells.
A new and easier way of estimating oceanic productivity is by satellite measurement. Satellites can measure ocean color. The greener the surface ocean water, the more algae, therefore the higher the productivity. Thus global maps of productivity such as the one below can be produced.
So the ocean is more productive in temperate zones. This also explains why the tropics have that pretty clear blue water. There’s not much life in it!
Productivity and Nutrients
Over time, areas with less mixing show reduced productivity, less phytoplankton, and so less chlorophyll. Polovina said, “Regions that have the lowest level of chlorophyll are akin to biological deserts; there’s less energy propagating up through the food web.”
An ocean full of deserts
And Jeff Polovina, at a National Marine Fisheries Service lab in Hawaii, has been watching by satellite as that greenery in the middle of the ocean is fading away.
"The regions that are showing the lowest amount of plant life, which [are] sometimes referred to as the biological deserts of the ocean, are growing at roughly 1 to 4 percent per year," Polovina says.
Outlook for Oceans Bleak as Sea 'Deserts' Grow