Phytoplankton are plants, ranging from organisms invisible to the naked eye up to seaweed and algae. They prefer the warmer surface waters of the ocean. Zooplankton are animals and include all tiny fish and crustaceans, such as krill and smaller jellyfish. They frequent the deeper, darker parts of the ocean where sunlight doesn't reach.
At the bottom of the ocean's food chain is phytoplankton. It spends daylight hours near the surface, where it can photosynthesize sunlight into energy. In turn, it releases oxygen into the atmosphere. The next step up on the food chain is zooplankton, which falls into two groups. Primary consumers will rise to the surface to feed on phytoplankton while the secondary consumers will remain in the depths to feed on smaller zooplankton.
Plankton is very sensitive to environmental conditions and therefore is a very good indicator of the health of the marine environment. Rising water temperatures cause phytoplankton to multiply rapidly and form algae blooms, which block sunlight. If excess phytoplankton aren't eaten, they die and fall to the ocean bottom, where they are consumed by bacteria. This depletes oxygen levels in deeper waters.
Both phytoplankton and zooplankton are found in freshwater environments as well as oceans. A nutrient-dense environment is characterized by a murky blue-green-gray color while nutrient-poor waters, such as tropical seas, have clearer water. Typical distribution occurs in the highly productive coastal temperate oceans--the North Atlantic and North Pacific--and the polar seas--the Arctic Ocean and Southern Ocean. All plankton are consumed by fish and shellfish, especially during the larval phase. One herring may consume thousands of zooplankton in a single day. Some fish and whales will consume plankton their entire lives.