There is a great diversity of plant life in the ocean, ranging in size from microscopic plankton to 80 meter giant kelp. While some ocean plants are harvested directly for food by humans, or provide important industrial substances, every single ocean plant plays a crucial role in the environmental health of both the ocean and the atmosphere.
Some ocean plants are important economically. Seaweeds such as nori and wakame, for instance, are eaten in Japan, and seaweed cultivation is a major industry in Asia. Agar and carrageenan are seaweed extracts which are widely used in processed foods. Seaweed is also used in the cosmetic industry, and as fertilizer, while Kelp ash which is used in soap and glass production. Kelp is also a source of alginate, widely used in the food industry to thicken ice cream, jelly and salad dressing.
But more importantly, seaweeds such as sargassum and kelp play a major ecological role in the ocean. Kelp forests provide a habitat for a diverse array of species, and are as important for biodiversity in the ocean as rainforests are on land. Small fish hide in the forest to escape predators, while migratory mammals use it as a breeding ground. Clams and mussels feed on micro-organisms that live in the forest, and crabs and worms feed on dead organic matter on the forest floor. Predators eat the herbivores which feed on the plants, and are in turn eaten by larger predators.
Both land and ocean plants are vital to the health of the atmosphere due to the role they play in the carbon cycle through photosynthesis. This is the process through which green plants remove carbon dioxide from the atmosphere and fix it within their bodies as organic compounds. While the photosynthetic activity of seaweed is important for the levels of oxygen in the ocean, the photosynthesis of other ocean plants, the innumerable numbers of floating, microscopic plants known as phytoplankton, is crucial to oxygen levels in both the ocean beneath and the air above.
Phytoplankton are a universal food source for marine life. Other plankton, invertebrates, fish, and marine mammals all feed on plankton. They incorporate some of the carbon fixed by the plankton into their bodies as they grow, and excrete the rest to the ocean floor. Ultimately, they too will die and sink to the deep ocean. These processes constitute a vast carbon pump which annually transfers an estimated 10 gigatonnes of carbon from the atmosphere to the deep ocean.
Phytoplankton populations are extremely sensitive to light levels, temperature and nutrient availability, and these fluctuations may have serious repercussions. When phytoplankton levels drop during cloudy El Nino periods, for example, the food chain is disrupted and populations of fish, sea birds and marine mammals are adversely affected. Small changes in the growth of phytoplankton may also affect atmospheric carbon dioxide levels, and consequently global surface temperatures.
The relationship between man-made global warming, global dimming, and phytoplankton populations is an incredibly complex puzzle which is not adequately understood. It is therefore an area of active research, as scientists attempt to find the key to this puzzle.