Why can't we see plankton organisms easily?

Protoplankton, phytoplankton, bacterioplankton, and zooplankton, the protozoa, algae, bacteria, and small animals that swim or float in water, are small. If the floating forms were not small, they would sink too easily. Being small is a good way to avoid detection by predators! Small is also advantageous for absorbing mineral nutrients from water. Zooplankton and some phytoplankton are just visible with the unaided eye, but we can observe much more with a microscope. A microscope magnification of 400X is sufficient to see bacterioplankton, protoplankton, and most phytoplankton.