Introduction
Our knowledge of ocean life is limited to big fishes and salty water. However, with new discoveries it is now evident that ocean life is mainly kept in balance by microorganisms. Billions of micro, nano and singular celled organisms called planktons are the primary producers of the marine fish web. Any living being in order to survive needs energy in the form of food and on earth the sole supplier of energy is the Sun. Be it ocean food chain or terrestrial food chain it always start with the sun. In case of terrestrial chains plants and weeds are the primary producers while for ocean life phytoplankton are the primary producers of food chain. Marine life is very different than the life on the land. The chemistry is different, the animals are different and the basic environment is different. This essay will first touch upon the marine environment to discuss upon the important characteristics for animals to survive in that environment and then will move on to the discussion about the marine food chain and food web comparing it with the terrestrial food, simultaneously highlighting the energy efficiency of the food chain.
Marine Environment
Marine environment is divided into many zones based on five primary factors called depth, light, temperature, substrate and salinity. The basic zonation is based on substrate. Pelagic zone is the exclusive water environment and on the other hand benthic zones are zones near to the bottom surface of the ocean. Pelagic zones are mainly divided into two zones; neritic zone which includes the areas around continental shelf, and oceanic zone. Oceanic zone is further divided into two main categories. Photic zone is the upper surface where sunlight can penetrate whereas aphotic zones are dark part of the ocean.
Fig 1: (State College of Florida, 2013)
There are several factors affecting the marine life. Light is one of the most important among them. Light is required for photosynthesis which is the primary way to convert solar energy inside a living cell. The whole food chain survives because of light which makes photosynthesis happen. Light generally can penetrate up to about 150 meter but it can vary depending on wavelength and turbidity (State College of Florida, 2013). Most of the ocean life actually lives in this zone because food is abundant in this layer of the ocean. Apart from light from the sun, in deep water some marine organism produces light by biochemical reactions known as bioluminescence.
Another important factor for marine life is temperature. Metabolic rates of living bodies depend on temperature. Higher temperature means higher metabolism rates. Higher metabolism rate means more energy production. As most of the living being lives within the first 150 meter of the marine surface where temperature variation is not much so marine bodies are not fit to survive huge temperature variations.
Nutrients in the sea water are extremely important for the marine life to grow and survive. The main nutrients required by the marine life are nitrogen and phosphorous and to a lesser extent calcium and phosphorous (State College of Florida, 2013). Marine plants recycle these nutrients to keep balance in the water. In fact there are certain oceanic areas in the world where because of the abundance of nutrients marine life is seen in large numbers. Another very important nutrient for the marine bodies is iron. Iron directly may not have much impact on the marine life but it indirectly helps nitrogen to assimilate in the phytoplankton which essentially helps in the photosynthesis process.
Salinity is another main factor important for the sustainability of life in the oceanic environment. Salinity varies from 6-40 ppt (State College of Florida, 2013). This huge variation can be primarily contributed to factors like evaporation rates, fresh water supply rates and sea ice formation. Salinity mainly varies at the surface level of the ocean. However, deep ocean level salinity varies far less. Salinity is important because of the way different species process water. Most of the marine species are isotonic. This means that the salinity of water inside the body of species is exactly same as that of the ocean. This is ideal to survive in any level of salinity. However, some fishes like bony fish are hypotonic. Water inside the body of bony fish is almost pure containing no salinity. Hence they continuously lose water and are threatened by dehydration. To prevent that bony fish continuously drinks and processes water in their gills to take out the salt. Salinity also means greater density of the water which helps in flotation. Most of the marine organisms are lighter than salt water density so they naturally float. However, most of the fishes are slightly heavier but they use gas filled swim bladders to maintain buoyancy.
Primary Producers
Fig 2: (Harris, 2013)
In nature everyone needs energy in some form or other to survive. The sole source of energy in earth is the sun but most of the organisms cannot directly convert solar energy into a form which can be used by other animals. These organisms are known as the primary producers in the food chain (Stewert, 2005). On the land trees, weeds, mosses and grasses are the primary producers. In the ocean phytoplankton and seaweeds are the only ones who can do photosynthesis from the solar energy. Planktons are a community of organisms that are mainly identified and categorized by their locomotion. Planktons live in the photic zone as they require sunlight to do the photosynthesis (Corey and Beutel). Phytoplanktons are plants and zooplanktons are animals. They can be of many types starting from soft bodies to hard cells to a single cell organism. Phytoplanktons are responsible for almost all types of primary productivity of ocean life. Phytoplanktons along with seaweeds are the only organisms which can create their own food and get energy from that and that is why they are called autotrophs. All other types of marine lives are heterotrophs meaning they cannot generate their own food and are dependent on others for nutrient and energy.
Marine Food Web
Fig 3: (Britannica, 2005)
The food chain in marine life starts from the phytoplankton. The most common form of phytoplankton is the diatoms. Diatoms take energy from the sun and through photosynthesis convert that energy to nutrients and store it. Phytoplankton are the primary producers in the marine food chain. Zooplanktons then eat the phytoplankton. Zooplanktons, some kind of shrimps, are the primary consumers as they only survive on the primary producers. The most common forms of zooplanktons are pteropods and copepods (Corey and Beutel). Zooplanktons are the primary food for small fishes. Small fishes are the secondary consumers. In some cases big zooplanktons are also food for some marine mammals like whales. Because of the sheer volume and biomass in the whole food chain copepods are the most important link between primary producers and the secondary consumers. Copepods make the largest animal mass in the ocean. Among the secondary consumers amphipods and alewife fish and menhaden fishes are voracious eaters of zooplankton and if their population increases in the ocean then that will create a dent in the zooplankton population creating havoc in the whole food web (Stewert, 2005). However, that is prevented by the next layer of secondary consumers. Bigger fishes like bluefish eats menhaden and alewife fishes thus preventing unprecedented increase of menhaden population in the ocean. Further down the food chain bigger fishes hunt smaller fishes. The hunt at this level is cutthroat and there is no singular relationship of food chain. This means that at this level fishes even can eat each other. For example, Bluefin tuna is targeted by whales, swordfish, sharks and even other types of tuna. Apart from this type of predation, larval forms are often a target by many other types of fishes (Corey and Beutel). For example, squid feeds on bluefish larva but squid on the other hand is a prey for the adult bluefish. Apart from this standard food cycle energy circulates in other forms as well. For example, large whales and sea turtles are not targeted by any other animals in the ocean but they often generate waste as part of excretion or dead tissue. Bacteria work on them and release nutrients from the waste which then is used by the plants to start the food cycle again.
Fig 4: (LGFL, 2005)
Organisms higher up the food chain are larger in size but fewer in number than lower level. This is mainly because of the loss of energy while going from one trophic level to another. The energy efficiency is only 10% and 90% of the energy is lost with every trophic level (LGFL, 2005). That is why most of the food chains do not have more than a few levels. It is very unlikely to see food chains with more than 6 trophic levels. For example, if a phytoplankton generates 100 Calorie from photosynthesis then by eating one phytoplankton, zooplankton will get only 10 calorie. Subsequently, by eating a zooplankton an amphipod will only get 1 calorie of energy (LGFL, 2005). It can be seen that only within 3 levels the energy level has come down to only a single calorie. That is why the food chain lengths generally are not high, further the population of the animals as we go up the food chain are much less than that of primary producers and primary consumers.
Terrestrial vs. Marine Food Web
Terrestrial and Marina food chains and webs are similar in many aspects but also have many differences. Apart from the difference in the medium which is water in case of marine food chain and land in case of terrestrial food chain, there are significant differences between the two food webs.
The main primary producers in case of terrestrial food chain are crops, trees and other green vegetation and forests. In almost all the cases these are well developed multi cellular complex organism. On the other hand the primary producer in the marine food web is the phytoplankton. Most of the primary producers are single celled, small and primitive organisms. Even the primary consumers in case of marine life are small primitive organism in many cases. For example, copepods are the largest primary consumers in the ocean life and they are primitive organism. In case of terrestrial food chain, most of the primary consumers are well developed animals. For example, in case of terrestrial food chains, deer is one of the primary consumers of primary producers.
Land food chains have less trophic level than ocean food chain. Almost all ocean food chains have minimum 4 trophic levels. On the other hand, many of the terrestrial food chains and web have only three trophical levels.
Effects of Overfishing on Marine Food Web
Fig 5: (Scheffer, 2005)
As we know that for any marine food chain due to loss of energy as we go up the food chain the number of organisms up the chain decreases. For example, number of zooplanktons should always be much more than the small fishes in ocean as small fishes require large quantities of zooplankton to survive and get the required energy. That’s why as we go up the chain the number should come down. However, marine food webs are not that simple. For example, as per the above theory phytoplankton should be the highest in ocean (Scheffer, 2005). However, that is not the case. The amount of zooplankton in the ocean is more than the amount of phytoplankton. Phytoplankton may be smaller in number than zooplanktons but they have quick replenishment time. That is when phytoplankton grows very fast. If some of the phytoplanktons are eaten by zooplanktons then phytoplanktons can grow back quickly to fulfill the void. This way the balance in food chain is maintained between primary producers and primary consumers in marine food web. Furthermore, the weight of big fishes compared to small fishes in the oceans is more. This is also contrary to our basic understanding of the food chain as primary food for the big fishes are the small fishes. This also is somewhat compensated by the fact that small fishes can grow back very quickly. In case of big fishes, the number is high because they hunt for each other as well. For example, squids hunt the larvae of blue fish; sharks eat tuna and so on.
Fishing in large scale creates some really dangerous changes in the marine food cycle. For example, most of fishing happens for large fishes. Especially the population of large fishes like tuna, cod, and salmon has gone down drastically. This is causing a decrease in the number of large fishes in the marine food cycle, leading to a top-down problem in the marine food chain. As the number of big fishes decreases the number of small fishes gets on a rise (Scheffer, 2005). Fishes like shrimp, crab, sunfish and lantern fish are on a rise. This increase in the population of small fish means that they need more zooplankton than before to survive. This is causing the zooplankton population to decrease. Especially there are many evidences that large bodied zooplanktons have decreased substantially. During fishing due to the usage of heavy fishing nets and crawlers used by the fishermen lot of zooplanktons are getting affected. This also adds to the reduction of zooplankton population. A decrease in zooplankton population means that the phytoplankton population is on a rise as there is not enough zooplankton to eat them (Scheffer, 2005). If this goes on then the marine food chain will be disrupted. This in the long run can change the marine environment completely if not addressed immediately. In fact to preserve the current balance in the marine food chain a good many countries have banned Cod fishing to encourage the fishing of small organisms like prawn.
Conclusion
Food chains or more accurately food webs are linear and circular sequence of organisms, each of which feeds on the preceding or in some cases feeds on each other. Primary producers are the main link in any food chain. Phytoplanktons are the primary producers in the marine food chain. They convert the energy from the sun into nutrients and then that is consumed by the primary and secondary consumers in the marine food chain. Phytoplanktons are crucial to marine food chain and so are the zooplanktons which are most consumed organisms in the ocean. Marine food chain is different than terrestrial food chain in some aspects. The marine food chains are generally longer than the terrestrial food chain. Modern fishing has enabled the fishermen to fish at a very large quantity. This is changing the ecology of the marine food chain drastically. Some measures have been taken to restrict the fishing of big fishes but much more need to be done if we want the marine food web to remain the way it is today.
References
Marine Environment and Primary Productivity (2013). State College of Florida (SCF). Retrieved on 22nd November 2013 from <http://faculty.scf.edu/rizkf/OCE1001/OCEnotes/chap11.htm>
Efficiency of Energy Flow between Trophic Levels. Boundless. Retrieved on 22nd November 2013 from < https://www.boundless.com/biology/ecosystem-function/movement-of-energy-between-trophic-levels/efficiency-of-energy-flow-between-trophic-levels/>
What happens to energy and biomass at each stage in a food chain (2013). London Grid for Learning (LGFL). Retrieved on 22nd November 2013 from <http://lgfl.skoool.co.uk/content.aspx?id=757>
Food Chains and Webs, Making a Food Chain (2006). Cornwall Wildlife Trust (CWT) UK. Retrieved on 22nd November 2013 from <http://www.cornwallwildlifetrust.org.uk/>
Valenti, Christopher. Malinda Schaefer Zarske and Denise Carlson (2004). Got Energy? Spinning a Food Web. Teach Engineering. Retrieved on 22nd November 2013 from <http://www.teachengineering.org/view_activity.php?url=collection/cub_/activities/cub_bio/cub_bio_lesson03_activity1.xml>
Gratton, Claudio (2012). Midges connect aquatic and terrestrial food webs. Retrieved on 22nd November 2013 from < http://gratton.entomology.wisc.edu/2012/05/02/midges-connect-aquatic-and-terrestrial-food-webs/>
Ecosystem: food chain in the ocean (2012). Encyclopedia Britannica. Retrieved on 22nd November 2013 from < http://www.britannica.com/EBchecked/media/119100/A-food-chain-in-the-ocean-begins-with-tiny-one>
Corey, Tony and Beutel, Dave. The Marine Food Web. Sea Grant, Rhode Island. Retrieved on 22nd November 2013 from < http://seagrant.gso.uri.edu/factsheets/foodweb.html>
Stewert, Robert R (2005). Marine Fisheries Food Webs. Department of Oceanography, Texas A&M University. Retrieved on 22nd November 2013 from < http://oceanworld.tamu.edu/resources/oceanography-book/marinefoodwebs.htm>
Scheffer, Marten (2005). Cascading effects of overfishing marine systems. Aquatic Ecology and Water Quality Management Group, Department of Environmental Sciences, Wageningen University. Retrieved on 22nd November 2013 from <http://www.sciencedirect.com/science/article/pii/S0169534705002752>
Harris, David (2013). Seeking sailors to help measure phytoplankton populations. PNAS. . Retrieved on 22nd November 2013 from <http://firstlook.pnas.org/seeking-sailors-to-help-measure-phytoplankton-populations/>
