Phytoplankton are algae that swim or float in water. Freshwater phytoplankton live in ponds, lakes, rivers, and streams. They include single-cells, colonies of cells, and filaments (linear strings of cells) that are usually capable of photosynthesis. Many protozoa are closely related to algae, so the distinction between protozoa and algae is artificial. Floating or swimming cyanobacteria, which are prokaryotes (lacking a true nucleus), are often regarded as phytoplankton, because they are photosynthetic and many are as large as eukaryotic algae. But they could also be regarded as bacterioplankton.