Guides
Detecting Algae Blooms with Satellites
May 6, 2025
Detecting Algal Blooms with FAI, MCI & CI
TL;DR — Green chlorophyll pops in the Floating Algae Index (FAI), cyanobacteria glow in the Cyanobacteria Index (CI), and heavy blooms spike the Maximum Chlorophyll Index (MCI). Pick the right one and you catch a harmful bloom before it reaches the beach.
Basic Indices in Plain English
Index | Reveals | Quick formula* |
|---|---|---|
FAI (Floating Algae Index) | Surface scums & early bloom fronts | (Red‑Edge − (NIR + Red)/2) |
MCI (Maximum Chlorophyll Index) | Peak chlorophyll concentrations | (NIR − (Red‑Edge + SWIR)/2) |
CI (Cyanobacteria Index) | Cyanobacteria‑rich mats | log10(NIR/Red) − log10(Green/Blue) |
*Band numbers depend on the sensor. See NASA/ESA band tables for details.
Which One, When?
Your question | Best index | Free sensor |
|---|---|---|
“Is a bloom starting to form?” | FAI | Sentinel‑2 (10 m) |
“Is chlorophyll peaking right now?” | MCI | Sentinel‑2 (10 m) |
“Is this bloom cyanobacteria?” | CI | Sentinel‑2 (10 m) |
Real‑World Win: Lake Rotoaira, Taupō District
Orbica processed Sentinel‑2 imagery of Lake Rotoaira weekly through summer 2024. On the 15th of January, FAI values climbed significantly above the seasonal baseline, this would trigger an automatic alert for whoever is managing the lake. Field crews can then verify a bloom and take action—saving significant costs in preventing a larger bloom and ensuring public safety.
Three Pro Tips for Effective Bloom Monitoring
Mask clouds first — A thin cirrus veil can inject false positives. Orbica’s preprocessing engine automatically masks cloudy pixels.
Set alerts instead of babysitting dashboards — Let the system ping you the moment FAI or MCI crosses a risk threshold. Our email/SMS alerts fire within minutes of new satellite passes.
Keep your AOI strictly on the lake surface — Shoreline pixels mix land reflectance with water, distorting bloom metrics. Orbica’s polygon tool allow you to be selective, so your calculations stay water‑pure.
Orbica combines all three—automated masking, real‑time alerts, and custom AOIs—into a single workflow, delivering decision‑ready bloom maps while keeping the science transparent.
Why Orbica?
100+ indices on tap: Beyond FAI, MCI and the Cyanobacteria Index, Orbica processes over one hundred additional spectral metrics—from turbidity proxies to vegetation‑stress indicators—so you always use the metric that best answers your question.
Automatic cloud masking & preprocessing: Clean, analysis‑ready scenes without a line of code.
Visual overlay & smart alerts: Fade any index to true colour, draw AOIs that snap to water pixels, and set threshold alerts that ping you the moment a bloom begins.
Ready for Proof on Your Waterway?
👉 Book a free 15‑minute demo and watch FAI, MCI or CI unfold over your lake—live.