Why our lakes and slow rivers sometimes turn into pea-green soup, which microbes are behind it, what makes them dangerous, and what we can actually do about it — the science behind every number on the dashboard.
A harmful algal bloom (HAB) is a rapid, dense overgrowth of algae or algae-like microbes in water — so dense it can paint the surface green, blue, or even reddish and form a paint-like scum.
In fresh water like the Charles River and the ponds around it, the usual culprits aren't true algae at all — they're cyanobacteria, also called blue-green algae. Cyanobacteria are ancient photosynthetic bacteria: they make their own food from sunlight, the same way plants do, and they've been doing it for over two billion years (they're the reason Earth's atmosphere has oxygen at all).
A little bit of them is completely normal and healthy. A bloom is when conditions line up so perfectly that they multiply faster than anything can eat or out-compete them. The trouble is that many bloom-forming species can release toxins, and even non-toxic blooms can choke a waterbody of oxygen as they die and rot.
Blooms are a recipe, not a single cause. When several ingredients show up together, cyanobacteria take off. These same ingredients are exactly what the dashboard's risk score watches for.
Cyanobacteria thrive in warm water (roughly above 18–25 °C) and out-compete other algae as it heats up.
Dashboard: Water temperaturePhosphorus and nitrogen from fertilizer, sewage, pet waste, and stormwater are food. Too much = eutrophication.
Dashboard: Nutrient-flush precipLow flow lets buoyant colonies float up and pile into surface scums instead of being flushed downstream.
Dashboard: Flow stagnationMore sun = more photosynthesis = faster growth, especially in calm, clear, stratified water.
Dashboard: SunshineA heavy bloom super-saturates oxygen by day, then crashes it at night and as it decays — stressing or killing fish.
Dashboard: Dissolved oxygenOf all the ingredients, nutrient pollution is the one humans control most directly. Rain washes fertilizer off lawns and farms, carries phosphorus from soil and pavement, and overflows aging sewers — a pulse of nutrients a few days before a warm, sunny, calm stretch is a classic bloom setup. That's why the dashboard treats recent rain as a "nutrient flush" signal.
Climate change tilts the whole system toward blooms: warmer summers, warmer water, longer growing seasons, and more intense downpours that flush more nutrients all at once.
A handful of genera cause most freshwater HABs in the northeastern U.S. Here's how to tell them apart and what each can produce. (Illustrations are stylized diagrams of each microbe's shape — see the sources below for microscope photos.)
Not every bloom is toxic, and you can't tell by looking — but blooms are harmful in two main ways.
Even a non-toxic bloom is dangerous. By day the bloom floods the water with oxygen; at night it consumes it, and when the bloom dies, bacteria decomposing it strip oxygen from the water — causing fish kills and "dead zones." That's why a big swing in dissolved oxygen is a strong late-stage warning sign.
Solutions fall into three tiers. The cheapest and most effective work happens on land, long before a bloom ever forms.
• US EPA — Cyanobacterial HABs
• CDC — Harmful Algal Blooms & health
• Mass.gov — Cyanobacteria & cyanotoxins
• NOAA — What is a harmful algal bloom?
• US EPA — Nutrient pollution (eutrophication)
This page is an educational overview, not health or safety guidance. For advisories about a specific waterbody, always check your state or local environmental and public-health agencies.