Trophic comes from the Greek word for food or feeding, so trophic relationships in marine life are about what eats what in the ocean. Almost all food chains on land or in the ocean begin with plants, the producers. On land, plants are big and obvious, but, in the ocean, it is much harder to see where the food chains begin.
Near the shores, where the bottom is still in the photic or light zone, large plants do occur. The largest of these are the kelps, a form of brown seaweed. These plants do not have true roots, stems or leaves, and in fact are related to the smallest and simplest plants, the algae. They form forests that provide food and habitat for a multitude of marine animals, but they are restricted to a very small area of the ocean. Other multicelled algae are also found in this zone and they are all important producers but the majority of photosynthesis that takes place in the ocean is performed by single-celled algae called phytoplankton, and these plants are basis of most ocean food chains.
Phytoplankton means floating plant and this is what allows them to exist throughout the world's oceans. They must still live in the top 30 or so meters where light can penetrate, but they do not have to be anchored to the bottom, which may be kilometers below them. Instead they float along on the currents, turning water and carbon dioxide into sugars using the energy of the sun and forming the first and most important trophic level in the ocean. As well, some scientists calculate that the phytoplankton, the forests of the sea, are as important as terrestrial rainforests for the production of the world's oxygen supply.
The phytoplankton are consumed by the next trophic level, the herbivores or first-order consumers. The smallest herbivores are also part of the plankton. Because they are animals they are called zooplankton, and they can be single-celled protozoans or they can be multicelled animals, such as molluscs, crustaceans and small fish.
The second- and third-order consumers are the predators of the oceans, the carnivores and the omnivores. Little fish eat plankton, bigger fish eat little fish, and these in turn are eaten by really big fish and sharks. If they eat both types of plankon then they are omnivores (both herbivore and carnivore). If they eat only fish and zooplankton then they are true carnivores.
In this order of things, there is a size gradation from smallest to biggest but the very biggest animals in the sea skip the middle levels altogether. These are the baleen whales and the whale sharks, which feed entirely on phytoplankton, zooplankton and small fish. These make some of the shortest food chains in the world: producers (phytoplankton) are eaten by herbivores (zooplankton), and both are eaten by the largest animals to ever live, the blue whales. The only things to eat blue whales are sharks that attack the newborn calves and the bacteria that feed on the decomposing bodies of dead adults.
This brings us to the last and, in many ways, the most important trophic level after the producers: the decomposers. Deep ocean food chains have no producers, so how do they support such diverse life forms? They are dependent on a 'rain' of organic material that floats down from the surface levels. Phytoplankton and zooplankton that die without being eaten, remains of larger animals that are incompletely consumed by their predators - all this material floats down to be consumed on the way by deep ocean fish and other life forms or eventually to reach the bottom where many worms, molluscs and other bottom-dwellers wait to finish the job.
So most of the trophic relationships in the ocean are similar to those on land: plants are the producers, animals eat the plants and other animals and bacteria and other scavengers recycle them all. On land the soil organisms perform the role of decomposers, and then plant roots bring the nutrients back to the surface to be recycled into new animals forms.
The situation is more difficult in the ocean where so many of the nutrients settle so far below the surface. Luckily, in some of the most productive ocean regions, a phenomenon called upwelling brings some of these nutrients back to the surface where they can be reused. Upwelling occurs when warm and cold waters meet. Cold water sinks and warm water rises, bringing nutrients up with it. Where this occurs, phytoplankton blooms and predators from fish to sharks and whales converge to share the bounty.
In additon, there are communities in the ocean that are totally independent of the sun for their energy. Deep in the ocean where earth's tectonic plates split apart, magma and gases rise to the surface along the mid-ocean ridges. Undersea volcanoes and black smokers spew out chemicals that are used by bacteria in a process called chemosynthesis to produce the organic molecules needed for life. These bacteria then provide food for consumers in a food chain that needs no plants at all. These communities give us insight into what life was like billions of years ago before the first plants evolved.
The food chains on land and in the sea are also connected.
Nutrients produced by terrestrial plants and animals can run off via rivers to promote plankton blooms. Once the nutrients reach the ocean there are a few ways for them to return to land. Seabirds are the main carriers of ocean nutrients back to the land. They fly over the ocean searching for fish and then return to land to breed, nest and feed their young. In the process they deposit vast amounts of feces or guano on islands and seashores around the world. These nutrients can then stimulate plant growth on islands that are otherwise barren of the nutrients needed for plant growth.
One tree on the Barrier Reef, the Pisonia, has gone one step further. It produces sticky seeds that trap young terns and kill them. When their bodies drop to the forest floor, the trees can use their nutrients. Pisonia trees are of course primarily producers, but they are also rather grisly predators and consumers as well. Tourists to these islands are often distressed by the sight of young birds trapped and dying but it is evolution's way of providing for a healthy ecosystem.
Another way that nutrients return from sea to land is via fish. Salmon in the Northern Hemisphere are spawned in rivers and then journey to the sea where they feed on plankton until they mature sexually. Then they journey back up the rivers to complete the cycle and deposit another generation of eggs. Instead of returning to the sea, the exhausted adults die and their bodies provide food for bears and other terrestrial animals, plus vital nutrients that the trees need. A number of other fish species either travel from fresh water to sea or sea back to rivers and streams and thus move nutrients from one ecosystem to another.
The trophic relationships of marine life are complicated. Energy passes from the sun to the plankton to a multitude of consumers and then on to another multitude of decomposers and scavengers, supporting numerous life forms deep below the photic zone.
Nutrients are washed from terrestrial ecosystems to help nourish marine life and, in turn, nutrients are returned to the land by sea birds and fish. Other food chains depend on chemosynthetic bacteria for their producers, and some of this energy is returned to the surface via upwelling. There is still much to be learned about the trophic relationships of marine life.