Guide
Chlorophyll and phytoplankton: reading the bottom of the food chain
Chlorophyll concentration is a measure of how much phytoplankton is in the water, read by satellites from the colour of the sea. Phytoplankton is the base of the marine food chain: it feeds zooplankton, which feeds baitfish, which feeds everything anglers target. Offshore, the edges between green and blue water are what matter.
Last updated
This is the reading most anglers have never looked at, and the one that explains why the fish were there in the first place.
What it measures
Phytoplankton are microscopic algae drifting in the upper layer of the sea. Like plants, they photosynthesise, and to do it they carry chlorophyll — the pigment that makes them green.
That pigment is visible from orbit. Ocean-colour satellites measure the wavelengths coming off the sea surface and infer chlorophyll concentration from them. More chlorophyll means more phytoplankton, and water that shifts from deep blue towards green.
So a chlorophyll reading is not an abstraction. It is a direct measure of how much food is in the water at the very bottom of the chain.
Why it reaches all the way up to your target species
The chain is short and reliable:
- Nutrients reach sunlit surface water, through upwelling, river outflow or mixing.
- Phytoplankton bloom on those nutrients.
- Zooplankton graze the phytoplankton.
- Baitfish and filter feeders eat the zooplankton.
- Everything you are trying to catch eats the baitfish.
Each step takes time, so the productive water is not always exactly where the bloom is densest — a strong bloom concentrates bait over the following days, often a little down-current of where it started.
Blue water, green water, and the edge between
Very clear blue water looks beautiful and is frequently a desert: clarity means little plankton, which means little food. Very green water has plenty of food but poor visibility, and sight-feeding predators struggle in it.
The productive zone is usually the boundary — the colour break, where clean blue water meets greener, plankton-rich water. Bait holds on the green side, predators hunt from the blue side, and the edge itself is often a sharp temperature and current break as well.
Offshore anglers have used these breaks for decades, traditionally by reading sea-surface temperature charts. Chlorophyll adds the other half of the picture: temperature tells you where the water masses meet, chlorophyll tells you which side of the line the food is on.
| Water | Chlorophyll | Typically means |
|---|---|---|
| Clear blue | Low | Excellent visibility, little bait; often unproductive on its own |
| Blue-green edge | Rising sharply | The break most offshore anglers are hunting |
| Green | High | Plenty of food, reduced visibility; bait holds here |
| Brown or heavily stained | Very high or sediment-laden | Often river outflow; can be excellent close in, poor offshore |
Where it matters most
Offshore and pelagic fishing. This is where colour breaks genuinely decide a day, and where a run of thirty miles is worth planning around them.
Nearshore after rain. River outflow carries nutrients and sets off blooms that can switch a stretch of coast on for a week.
Seasonal transitions. Spring blooms are the annual reset of the whole system, and much of the year's pattern follows from them.
A caution worth keeping
More is not always better. Extremely dense blooms can strip oxygen from the water as they die and decay, and some algal blooms are harmful outright. Water that is too green, for too long, can push fish out rather than draw them in. Read chlorophyll alongside dissolved oxygen rather than on its own.
Global Fishing League records chlorophyll and phytoplankton concentration on every catch, alongside dissolved oxygen, salinity and water temperature — the readings that only make sense together. Seeevery reading recorded with a catch.
Back to all guides.