The image is a calculation.
Ocean-color products are assembled from a chain of corrections and assumptions, most of them made before anyone looked at the water.
A satellite bloom map looks like a photograph and is not one.
1. What arrives at the sensor
An orbiting radiometer records radiance at the top of the atmosphere in a set of visible and near-infrared bands. Over a typical ocean scene, the overwhelming majority of that signal is atmosphere: molecular scattering, aerosol, and specular reflection off the sea surface. The part that came out of the water is a small residual.
So the hardest step is removing everything that is not water-leaving radiance. Get the aerosol model wrong over a dusty coastline and the residual is wrong by a proportion no downstream algorithm can recover.
Two providers can publish different chlorophyll fields from the same overpass and both be defensible. They made different atmospheric assumptions.
2. From radiance to a pigment estimate
Standard chlorophyll algorithms use ratios between blue and green bands, calibrated against a global set of shipboard measurements. In clear open ocean, where phytoplankton dominate the optical signal, that relationship is strong and well tested.
Coastal water is different. Suspended sediment scatters, dissolved organic matter absorbs in the blue, and in shallow water the bottom reflects, each pushing a band ratio in directions that mimic chlorophyll.
Regional algorithms and machine-learned retrievals handle this better where they were trained. Ask where a coastal algorithm was tuned, and on what water.
3. The pixel problem
A pixel is an average. In a narrow inlet, a marina or a mangrove-fringed lagoon, one pixel may hold water, wet sand, vegetation and a jetty, and the retrieval returns one number for all of it. Bright land also scatters light into nearby water pixels, biasing the retrieval along exactly the shoreline people care about.
Finer resolution alone does not solve this: finer pixels usually mean a narrower swath, a longer revisit and a noisier signal. The right trade-off depends on the size of the water body you are watching.
4. Revisit, cloud, and the coverage figure that matters
Procurement documents quote revisit interval. Operations need clear-sky revisit, a much worse number.
A sensor with nominal daily revisit over a coast overcast, hazy or dust-laden for a fortnight delivers nothing for a fortnight. In several regions where blooms do the most damage, the worst atmospheric conditions coincide with bloom season, because both follow the same seasonal wind.
Before buying, pull two years of archive imagery for your own area and count the days with usable water pixels. That count is the real specification, and it is often half what the brochure implies.
5. What a well-specified operational index looks like
NOAA Coral Reef Watch converts satellite sea-surface temperature into Degree Heating Weeks, a cumulative index of thermal stress, maps it onto named alert levels, and states in plain language what each is expected to produce on a reef.
Alert Level 1 begins at four degree-Celsius-weeks, with risk of reef-wide bleaching. Level 2, from eight, adds mortality among heat-sensitive corals. Levels 3, 4 and 5 run from twelve, sixteen and twenty: multi-species mortality, severe multi-species mortality across more than half of corals, and near-complete mortality above eighty percent.
The index is defined, all five levels are published on one public product page, each says in words what it expects on a reef, and anyone can check the product afterward against what happened. Bloom products should be judged against that standard, and most fall well short of it.
6. A regional blank space
A 2021 study in the Journal of Marine Science and Engineering, working from MODIS chlorophyll-a data, observed that very few harmful algal bloom studies had been carried out in the Red Sea — a basin lined with desalination plants supplying drinking water to millions of people, and exposed to biofouling risk from high-biomass blooms.
The satellite record is the only long-baseline observation most of that coast has.
Beyond it, a program there builds its baseline from scratch. Budget for that. An instrumented program running continuously accumulates a baseline as a by-product; work bought for a single event produces a before and an after with nothing on either side.
7. Reading a supplier's claim
Four questions separate a real product from a demonstration.
Which sensor and which processing level. Where the coastal algorithm was validated, and against how many matched in-situ samples. What clear-sky coverage the product achieved over the client's own area last year. And what the false-alarm rate was when it was checked against sampling.
A supplier who answers all four quickly is worth talking to. One who answers with imagery is showing you a picture of the sea.
Notice what the third and fourth answers are actually describing. Clear-sky coverage over one area is the share of the period the product could see anything at all, and a false-alarm rate is the share of its calls that arrived without an event behind them. Together they set the distance between what a satellite product says happened and what happened at the quay. Anything built on top of the product inherits that distance, so it belongs on the page in numbers, alongside the comparator the skill was scored against and the date the scoring was last refreshed.
Sources
- NOAA Coral Reef Watch — the Degree Heating Week product, where all five bleaching alert levels are defined.
- Journal of Marine Science and Engineering, MDPI, 2021 — MODIS chlorophyll-a assessment of harmful algal bloom risk in the Red Sea.
- Alarivean, Inc. — data services and full-cycle monitoring, for how these products route intervention.
Asked in procurement, and afterward
Five questions about what a satellite can do
Can a satellite tell us which species is blooming?
Not on its own, and not reliably in coastal water. Multispectral sensors estimate pigment concentration, and almost all phytoplankton share the pigment.
Hyperspectral instruments can sometimes suggest a dominant group. Confirming a species still needs a microscope or a molecular assay.
Can it see a bloom below the surface?
Only the upper water column contributes to the signal, so a subsurface layer beneath clearer water is largely invisible from orbit.
Thin subsurface layers are common, and they are exactly what damages a deep intake or a net pen. Depth structure is a job for a profiler, a glider or a moored string.
What spatial resolution do we actually need?
It depends on the size of your water body, not on what is being marketed. A pixel of several hundred meters tracks a large offshore feature well and is useless inside a marina.
Ask how many usable water pixels the product yields inside your area at your typical tide. That is the specification. Nominal resolution is not.
Why does the satellite bulletin disagree with our buoy?
They measure different quantities at different scales: an average across a pixel and the first optical depth, against fluorescence at one point and depth.
Occasional disagreement is normal and informative. Persistent disagreement in one direction usually points at atmospheric correction over turbid water, or a fluorometer that needs cleaning.
Should we buy imagery or buy a service?
Buy imagery if you employ people who can maintain a processing chain and defend its outputs under scrutiny. Buy a service if you do not.
The expensive failure is the middle path: a program licenses data, builds a chain around one enthusiastic analyst, and loses the capability the month that person changes job.
Before the procurement closes
Have someone check the clear-sky number.
Send the area, the season and the products you are being offered. Alarivean will say what earth observation can contribute at that site, and where an in-situ layer would do more for less.