Prairies - the rainforests of the temperate flatlands of the North American Great Plains. Lush and dense with life, they burst with vitality and multifarious discovered and likely undiscovered species, like rainforests in important ways but a lot closer to the ground. This natural phenomenon, now mostly gone, does not extend to the ocean surface. But it could, and in the fullness of time maybe it will.
The ocean surface is vast, with plentiful water and sunshine to nurture plants and animals from microscopic to blue whale-sized. The surface waters often contain plankton, mostly small organisms that drift with the currents. Algae use sunlight and dissolved nutrients and gases to grow, and the many tiny animals in turn eat algae and each other. If you are ever unfortunate enough to be cast away and adrift at sea, a survival tip is to use a nylon stocking to sift plankton out of the water, capturing a nutritious food substantial enough that the largest creatures to ever inhabit the earth (blue whales) eat little else but krill, the tiny shrimp-like arthropods that form part of the plankton community.
Although plankton live near the surface, the actual ocean surface itself is generally pretty clear. Looking down at the water, one usually sees mostly water, not a plant-covered surface. Oddly enough, the competition for sunlight that often seems to characterize plant life on land seems not to be nearly as important in the oceans. On land, plants that shoot up the fastest and tallest get more precious sunlight (solar energy that they use for growth), casting shade on neighboring, shorter plants and tending to impoverish them. That seems to be why trees are tall, and why rain forests grow as high as they do. Prairies display similar traits, though on a shorter scale, probably because periodic prairie fires tend to kill off tall vegetation with large above-ground structural investments with long-term height payoffs, like trees. "Grow fast, die young" seems to be the operative strategy on the highly competitive prairie. But less clear is why the race to grow higher than the competition has not taken over the seas. But maybe some day it will.
Fresh water. The value of taking over the water surface has sometimes been recognized in the fresh water arena. Fields of duckweed, the world's smallest flowering plant, sometimes cover the surface of stagnant ponds. Each plant's tiny rosette of leaves floats on the water, with small roots descending downward. They can proliferate to the point where the water appears from a distance to be covered with a green sheet.
Even more dramatic is the common water hyacinth (Eichhornia Crassipes). Not content to merely float at surface level, it can rise into the air, covering the water in such profusion that it looks like a solid meadow.
Source: US Army Corps of Engineers
Usually considered a vicious pest, in many areas it is illegal to transport. Yet it can purify polluted water and, when cooked, the young leaves, stems, and flotation pods are edible and nutritious. The pods may be deep fried. (Do not eat if grown in polluted water. Check local regulations before transporting to the dinner table due to restrictions on transportation.
The oceans. Mostly, below-surface plankton rules the waves, currently. Floating mats of algae occur sometimes. The Sargasso Sea (in the storied Bermuda Triangle) has historically been known as a repository of masses of floating seaweed, though its density is far below the levels, dangerous to ships, that have been the stuff of seafarers' legends.
###Add here: listing of various cases of floating seaweed, floating seaweed clumps, and floating seaweed rafts (thanks to Amanda for bringing this up...check for the seaweed alba).
Problems plants must solve. If floating seaweeds are to cover the ocean surface, thereby out-competing plankton and underwater seaweed in the struggle for sunlight, they will first have to solve some problems that they obviously have not solved heretofore on this Earth. The three biggies here are navigation, nutrition, and predation. The navigation problem occurs because, left to be buffeted by the whim of wind, wave, and current, a worldwide carpet of floating seaweeds would soon give way to giant swaths of cleared water as massed seaweeds pile up elsewhere to die. The nutrition problem derives from scarcities of life-supporting substances dissolved in the water (plants cannot live by pure water alone). And the predation problem arises because, by the inescapable nature of life as we know it, plants contain substances of interest to hungry animals. Appropriate changes in the plants' genomes would be needed to make these solutions happen.
The navigation problem. To have a hope of covering the ocean surface, floating seaweeds will need to develop the capability to control where they go. That way they can seek more sparsely populated patches of water and avoid densely populated areas less friendly to newcomers. The key to navigation is the ability to choose among modes of motion, because each mode is likely to cause motion in a different direction.
Wind is an obvious method which can be harnessed most efficiently with leaves sticking up like little sails. Wave energy for movement could be captured by plants with a floating component that moves up and down, and a sunken component not directly influenced by the ocean swells. Current is best harnessed by sinking under the surface as needed to achieve the desired movement, or with feathery roots to capture those under-surface currents. Differential currents at different depths can be used by sinking and rising using finely controlled flotation bladders (at night, getting higher than other plants to catch the sun isn't an issue). Active swimming using cilia or mechanical approaches pioneered by animals (e.g. fins as in fishes, water jets like the octopus, legs like crabs) would be a paradigmatic advance in higher plant function. It would (and some day maybe will) revolutionize the biosphere with far more drama than the invention of flowers 140 million years ago.
Such seaweeds would of course need to be able to choose navigation mode. This is not a big issue as current plants can easily raise or lower their leaves, and controlling flotation bladders would hardly be amazing. Of course, a control mechanism for deciding when to choose what mode is needed. Simple (and not-so-simple) criteria are already used by plants to decide when to grow, flower and fruit, so this does not seem to require radically new genetic capabilities.
Nutrients. The main things a plant needs - water, oxygen, and carbon dioxide - are readily available from the ocean and atmospher. Nitrogen is critical as well and is potentially obtainable from air (legumes do that on land already). Smaller quantities of various minerals are important as well. Yet every naturally occurring element is present in sea water. Indeed more gold than King Midas ever dreamed of is dissolved in every cubic mile of seawater! Not that plants need gold, but everything they do need is there to be extracted from the water.
Predation. Fish and other sea creatures like to eat plants. Of course, land animals eat land plants, and that obviously doesn't prevent thick profusions of plants in prairies, rain forests, and other land habitats. Protective mechanisms used by land plants, from poisons to prickers, are available. If sea prairies become a product of genetic engineering, the plants will probably be built to be edible to humans, who will either make them as inedible to other animals as possible, or eat the animals that eat the floating plants. If on the other hand sea prairies develop naturally, humans may be long gone by then though we may hope instead for the alternative.
Will Earth's wide oceans become a slowly undulating, solid green mass from shore to shining shore? Only time will tell.
References
"...when cooked, the young leaves, stems, and flotation bladders are edible and nutritious." See e.g. http://www.youtube.com/watch?v=V1kkn5Sz4MI (as of 11/1/09).
"Check local regulations before transporting to the dinner table due to restrictions on transportation." http://houstonwildedibles.blogspot.com/2008/10/water-hyacinth.html (accessed 1/1/09)