Phytoplankton

Phytoplankton are microscopic, photosynthetic organisms found in oceans, seas, and freshwater bodies. They are a crucial part of the aquatic food web and are often referred to as the "grass of the sea" because they serve as the primary producers, forming the base of the marine food chain.

Like plants, they use sunlight to convert carbon dioxide and water into glucose (food) and oxygen through photosynthesis.

They are usually tiny, often single-celled organisms, and require a microscope to be seen.

Zooplankton, in contrast, are very small or microscopic invertebrate animals.

Phytoplankton are a vital part of the ocean ecosystem, providing the basis on which virtually all marine life depends either directly or indirectly. They can present problems for farmed fish, however. Harmful algal blooms (HABs) can create toxins in the water and can also absorb so much oxygen that fish in the area can suffocate.

Also, some phytoplankton such as diatoms have a hard shell and can, in large numbers, damage fish gills.

Farmers can take action such as suspending feeding, oxygenating pens or deploying bubble curtains to protect fish from harmful phytoplankton.