Chlorophyll Numbers That Mean Something
“Look for the green-blue edge” is the most common chlorophyll advice in the fishing category, and it's not wrong, but it skips the number underneath. 0.2 mg/m³, 1.0 mg/m³, and 3.0 mg/m³ are three genuinely different kinds of water, and knowing roughly where you sit on that scale tells you more than the color alone.
The scale itself
Chlorofishy's chlorophyll layer runs a fixed logarithmic scale from 0.01 to 100 mg/m³ that never changes with the view, unlike SST, which restretches its color range per region. Because it's logarithmic, equal steps in color correspond to multiplying the concentration, not adding to it: the visual jump from 0.1 to 1.0 mg/m³ is the same size as the jump from 1.0 to 10 mg/m³, even though the second jump is ten times larger in raw units. That's deliberate: chlorophyll concentration in the ocean spans four orders of magnitude, from essentially nothing to bloom conditions, and a linear scale would crush almost everything into one end of it.
What 0.2, 1.0, and 3.0 mg/m³ actually represent
Ocean-color science divides chlorophyll concentration into trophic bands that are widely used across NASA's ocean-color products: waters under roughly 0.1 mg/m³ are classified oligotrophic: nutrient-starved, essentially open-ocean desert. From roughly 0.1 to 1 mg/m³ is mesotrophic: moderately productive, a common open-water baseline outside a bloom or a front. Above roughly 1 mg/m³ is eutrophic: genuinely nutrient-rich, elevated productivity, the range a real upwelling event or bloom pushes into. Against those bands:
- 0.2 mg/m³ sits just above the oligotrophic threshold: low mesotrophic water, relatively clear, unremarkable productivity. This is common baseline blue water offshore.
- 1.0 mg/m³ sits at the mesotrophic/eutrophic boundary: five times the concentration of 0.2, a real step up in productivity, the kind of reading that marks the edge of a front or shelf break worth investigating.
- 3.0 mg/m³ is solidly eutrophic: an order of magnitude above baseline oligotrophic water, consistent with active upwelling or a developing bloom, not just background variation.
On the map, low values render toward the violet end of the chlorophyll colormap and climb through blue, green, and yellow toward red at the highest, bloom-level concentrations, so the same visual color shift you already read by eye (see how to read water color offshore) is the exact scale described above, just quantified.
Why the gradient matters more than the value
A single reading of 1.0 mg/m³ tells you almost nothing on its own: it could be a stable regional baseline you'd see anywhere nearby, or it could sit right next to a sharp drop to 0.2 or a sharp rise to 3.0. What actually matters for finding structure is the chlorophyll edge: how fast the value changes over a short distance, not what the value is at any one point. A flat 1.0 mg/m³ across fifty miles is uninteresting water; a jump from 0.3 to 2.5 mg/m³ over a few hundred meters is a real convergence zone.
Why upwelling green is not runoff green
Two very different mechanisms can push a reading from oligotrophic into eutrophic territory, and the chart alone doesn't label which one produced a given number. Upwelling (see what is upwelling) brings genuinely nutrient-rich deep water to the surface, which fuels real phytoplankton growth: the productivity is real, and the food web built on top of it is real. A river plume carries sediment, dissolved organic matter, and often nutrient runoff from land, which can register as elevated chlorophyll (or simply as turbidity that confounds the chlorophyll retrieval) without representing the same kind of offshore biological productivity.
The distinguishing clues are mostly about context, not the number itself: upwelling-driven productivity typically sits alongside a measurable SST drop (cold water rising from depth) and tracks along a coastline or current boundary rather than radiating out from a river mouth. Runoff-driven elevation is anchored to a river mouth or estuary, often fades with distance offshore in a predictable plume shape, and doesn't carry the same cold-water signature. See combining SST and chlorophyll for how to check both signals together before deciding which kind of green you're looking at.
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