Plants are well-adapted for living on land, unlike their protistan ancestors, the algae, which include seaweeds. However, plants can be found growing in the ocean habitat. Plants that grow in marine water have mechanisms for tolerating its high salt content and for getting oxygen to the plant. A few marine plants grow near the shore and in shallow water, but some can be found far from land, in the open ocean. Where the plant thrives in the ocean depends on what elements that region provides.
Seagrasses are flowering, grass-like plants that live their entire lives submerged in temperate and tropical waters. There are more than 50 species of seagrass worldwide, with some species reaching up to three feet long. Because they need sunlight to survive, they live in shallow regions of the ocean where they form thick meadows. These shallow regions can be in coral reef areas, with sand that has slowly built up almost to the surface of the water, that feel like "the middle of the ocean"; you can stand in a seagrass meadow miles from the shore, but the water is only knee-deep. Seagrasses are of ecological importance because they provide food for manatee and sea turtles, store carbon and offer shelter for a variety of marine life.
Mangroves are salt-tolerant trees found along the ocean’s coast in tropical and subtropical climates. They are identifiable by their tangle of roots which remove most of the salt before water is transferred up the trunk. Red mangroves (Rhizophora mangle) grow offshore with their roots constantly submerged, whereas white mangroves (Laguncularia racemosa) grow in intertidal areas with their roots alternating between submersion and exposure as the tide rises and fall. In mangroves, aerial roots provide oxygen to the plant, while submerged roots stabilize shorelines during storms and provide a nursery for crustaceans, fish and endangered species of sea turtles.
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Algae are photosynthetic organisms from the kingdom Protista in the five-kingdom system. Though algae are not plants, they have similar ecological roles due to their status as primary producers of nutrients and oxygen through photosynthesis. Phytoplankton are algae that are abundant in open ocean water. They float near the water’s surface where they filter nutrients from the water and gather sunlight to photosynthesize. Phytoplankton are important to the ocean environment because they produce a large portion of the oxygen used by other marine species, and, indeed, all organisms on earth, and they are a food source for many aquatic species. Dinoflagellates and diatoms make up two classes of phytoplankton. If left to grow out of control, phytoplankton can cause harmful algae blooms that result in fish kills and can have adverse effects on human health.
Kelp is another member of the algae, as are all the seaweeds. Unlike phytoplankton, these algae really resemble plants, at least superficially. A type of brown seaweed, kelp grows on rocky areas of the ocean floor and mimics a tree in stature. It prefers cold or arctic water and obtains energy through photosynthesis. The depth at which it grows is only limited by water clarity and the amount of light the species requires. Kelp, like all algae and in contrast to plants, does not have roots; instead it is held in place by a root-like holdfast and small air bladders at the base of each blade that allow it to float vertically in the water. (Anatomical features like roots and seeds are unique to plants; adaptations that allow plants to efficiently live on land.) Kelp is of importance because it provides food and shelter to a vast number of marine species and researchers use it to understand other ecological processes.