Three useful groups
Planktonic algae
Microscopic algae suspended in the water often give productive ponds a green or olive color and form the base of aquatic food webs.
Filamentous algae
This stringy growth—often called pond moss—may form floating mats and can commonly be lifted with a stick, rake, or fishing hook.
Cyanobacteria
Blue-green algae are photosynthetic bacteria. Some species can produce toxins harmful to people, pets, livestock, and wildlife. Treat suspicious surface scums with caution and keep animals away.
Runoff, warmth, and oxygen
Rainfall can move sediment and nutrients into ponds. Warm conditions and available nutrients may support rapid growth. Dense blooms can reduce oxygen overnight, and decomposition after a bloom dies can consume enough oxygen to stress or kill fish.
Healthy ponds are not necessarily algae-free. They are balanced ecosystems.