Plankton- Organisms that drift; they cannot swim against a current stronger than 1 knot (1 nautical mile/hour)
Plankton are organisms that drift; they cannot swim against a current any stronger than 1 knot (1 nautical mile/hour). Usually, plankton are very small, microscopic organisms but some larger animals, like certain jellyfish, are also considered plankton. Plankton are divided into two groups, plants (phytoplankton) or they can be animals (called zooplankton). Phytoplankton make their own food through photosynthesis (using sunlight to combine carbon dioxide and water into sugar), but zooplankton must ingest or eat food from the ocean.
Plankton are usually heavier than water. This is important because if a planktonic organism just floated on the surface of the water, it might not be able to get to food sources below it or it might get too warm or too much light from the sun (even phytoplankton can be "bleached" by the sun!). So plankton will tend to sink in the water column. But phytoplankton do need to stay where sunlight penetrates. Zooplankton feed on phytoplakton so the zooplankton want to stay where the phytoplankton are in the water column. One important note is that zooplankton are usually able to swim upward in the water column very slowly to maintain their position. But if they sink too quickly or are too heavy, they will go straight to the bottom of the ocean and not be able to get back up! Therefore, planktonic organisms will have adaptations that prevent them from sinking too quickly. These adaptations include the following:
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