In any ecosystem, a food chain or a food web essentially defines which organism eats what or whom. Feeding relationships often depend on the body size of the organisms, for example, the bigger fishes feeding on the smaller ones. However, in aquatic food webs, the body size of the organisms change during their life cycles many a time. That’s how the food webs tend to get a lot more complicated than they may seem at first. Let’s look at the various levels in the food web and how they are linked to each other.
Organisms in food webs are generally classified into trophic levels which can be explained in a trophic pyramid depending on their role in the food chain. The various levels include primary producers at the base of the food pyramid, then the herbivorous consumers, the 1st level carnivorous consumers, followed by the 2nd level and 3rd level carnivorous consumers, and finally the top carnivores like the sharks and dolphins. The truth is, food chains overlap at many points as animals don’t just feed on one species alone but actually feed on multiple organisms, giving rise to complexities in the food web. On an average, 10% of the energy from an organism gets passed on to the other organism feeding on it. The remaining 90% gets lost as waste.
These form the base of the food web pyramid and are called producers as they are responsible for making their own food. Through the process of photosynthesis, they convert the energy of the sun into chemical energy. Phytoplankton and seaweed are the most widespread primary producers on which the entire food web and the aquatic ecosystems rely. Not only do they provide energy to the other organisms, they become food themselves for some, and also provide shelter to smaller organisms in the world’s oceans and protect them from being preyed upon.
Consumers are organisms that cannot prepare their own food using the process of photosynthesis and they rely on other organisms to feed upon. Consumers are mainly categorized into herbivores and carnivores. Examples of primary consumers include zooplankton, ducks, tadpoles, and small crustaceans. There are three levels of carnivores in the trophic pyramid that are based on the body size of the species found in the world’s oceans. Primary consumers like zooplankton and crustaceans get eaten up by the 1st level carnivores which include species like jellyfish. The 2nd and 3rd level carnivorous species feed on larger fishes, frogs, and octopus. The top carnivores or predators like the shark and dolphin can eat primary as well as secondary consumers. In fact, the top predators can even be omnivores.
Decomposers mainly include bacteria that are needed to break down the dead bodies of plants and animals in the ocean. If not for bacteria, we would have an ocean full of dead matter floating around. Horrid, isn’t it? Bacteria can break down organisms as big as the whale or as microscopic as the microorganisms that float in the water bodies. By breaking down organic molecules, they make the nutrients available for use by other organisms, such as plants.
Effects of Climate Change and on Food Webs
Rising carbon dioxide levels in the ocean leading to ocean acidification and other global alterations like the climate changes are creating a major disruption in the functioning of the aquatic ecosystems. With an increase in the carbon dioxide levels, the temperate noncalcifying plankton increases its primary production whereas the tropical plankton decreases its food production because of the acidification. The temperature also increases the consumption and metabolic rates of the herbivores that feed on the primary producers, but it does not translate into greater secondary production. This causes a mismatch in the species of the food web as the carnivores and top predators continue to consume more. Ocean acidification, as well as global warming, are responsible for the reduction in the calcification in tropical as well as temperate reef-building species. Because of these issues, there is a decline in the levels of dimethyl sulfide production by the plankton found in the ocean which is said to help in the formation of clouds and maintenance of the heating system of the Earth’s environment.
In an experiment that was conducted by some researchers, a 2000-litre tank was transformed into a self-contained ecosystem (also known as a mesocosm). It contained algae as primary producers, herbivores (invertebrates), and fishes as the predators. The results of this experiment showed that carbon dioxide enrichment helped algal development but the herbivorous species did not grow. And since herbivores remained the same, and the higher metabolic rates of the top predators (in this case, fish) consumed more herbivorous prey, it resulted in a crumple of prey populations creating a wide gap in the food chain.