All life-forms need a supply of energy and food to grow and reproduce. Autotrophic organisms are able to manufacture organic materials using inorganic carbon and nitrogen, and obtain energy from sunlight or inorganic molecules. The rest need to consume other life-forms in order to obtain energy and nourishment, and these are called heterotrophs. Autotrophs are primary producers they provide all the original energy and materials that can be used by the heterotrophs (consumers) in the ecosystem that they support. They form the base of food chains or, more commonly, food webs and are grazed by herbivores which are in turn consumed by carnivores. Food webs are divided into trophic levels with primary producers as the first level, herbivores as the second, carnivores as the third, carnivores that eat carnivores as the next, and so on. As in all food webs, the 10 % rule applies - on average only 10 % of the energy contained in one trophic level is available to be used by the trophic level that consumes it.
Pelagic food web interactions are those that take place within the water column, and during this lecture we will only be concerned with marine pelagic food webs. Phytoplankton are the primary producers that support pelagic food webs. The consumers consist of herbivorous zooplankton that graze on the phytoplankton, these are grazed by filter feeding planktivores, predators can be piscivores - eating fish or carnivores - eating other types of animals. Omnivory is very common in pelagic food webs with organisms able to obtain food from more than one trophic level i.e. salmon eat juvenile fish which are planktivores but can also eat euphausiids which are zooplankton.
At the bottom of the food web, occupying the first trophic level, are the primary producers - phytoplankton. These are grazed by herbivorous zooplankton: euphausiids, copepods and pteropods at the second trophic level. Filter feeding planktivores in the form of juvenile fish, anchovies, hyperiid amphipods and lantern fish graze on the zooplankton occupy the third trophic level. Trophic level four contains the primary predators: salmon, squid and lancet fish. Top predators occupy two trophic levels, the first contains porpoises, seal and sea lions, mackerel shark and sea birds, and the very top predator, occupying trophic level six, is the killer whale.
Organisms which consume the same food source will be in competition when food becomes scarce. For instance, killer whales, porpoises and seals/sea lions may compete for salmon if other items of food become scarce. The impact of competition for a food source will be greatest for those animals which have only one type of prey such as seals/sea lions or lancet fish in this particular food web representation. Omnivory is prevalent in predators: killer whales prey on seals and mackerel shark in the trophic level below them but they also eat salmon and sea birds from the trophic level below this; sea birds consume squid from trophic level 4 and anchovies from trophic level 3, and so on.
A 'classical' food web is thought to dominate during the spring at temperate latitudes. Winter mixing of the sea ensures that nutrients are plentiful throughout the water column, however, the phytoplankton are light limited throughout the winter and so there is little biomass accumulation and food web productivity is low. The spring bloom is initiated as winter winds slacken and phytoplankton spend more time in the photic zone where increasing daylight is available for photosynthesis and excess nutrients are available for uptake and growth. Phytoplankton biomass becomes abundant culminating in the spring bloom and large chain-forming diatoms which can rapidly take up and store dissolved inorganic nitrogen are dominant. Soon zooplankton numbers increase, to graze on the large, luxuriant algae cells. Copepod life-cycles are closely synchronised to phytoplankton productivity so that their numbers are highest during the spring. Their numbers in the photic zone reflect phytoplankton density. Copepods descend to deep water to lay eggs and then rise to the surface developing through several nauplii and copepodite stages as they ascend. The number of generations depends on environmental conditions and generally increases southward from the poles to the tropics. Many benthic organisms have a life-stage in the plankton, and larvae are produced to coincide with the spring bloom of phytoplankton. Many fish, such as cod, spawn their eggs to coincide with the spring bloom period so that they are synchronised to hatch into juvenile fish as the zooplankton reach their peak.
The phytoplankton bloom only lasts for a short time, days to weeks, and then subsides due to nutrient depletion, grazing pressure, cell sinking or other causes. Warming of the surface waters during summer results in a thermocline developing and nutrients become scarce in the photic zone. Larger phytoplankton cells, such as diatoms, which need turbulence to continuously recirculate them up into the photic zone decline and are replaced by smaller phytoplankton which are more adapted to low nutrient, stratified conditions. Small cells have a higher surface to volume ratio with more nutrient uptake sites. Small flagellates dominate the phytoplankton, productivity is low. Mesozooplankton cannot directly access the tiny cells, instead single celled protozoan grazers increase in abundance. The phytoplankton are nourished by regenerated nitrogen in the form of ammonium, which is released by bacteria and protozoa. This type of food web is called the microbial loop and the organisms within it are able to recycled nutrients very efficiently so that very little organic matter is lost to the deeper waters. There is a rapid turn over of both prey and predators as small organisms have faster metabolic rates than larger life-forms. Protozoa and bacteria are detrivores and faecal pellets, debris from messy eating and dead organic material is consumed and recycled as inorganic nutrients that can be reused by the flagellates. Copepods and other zooplankton graze on the protozoa connecting the microbial food web back into the classic food web, but now an additional trophic level is added and summer productivity is lower than in the spring.
In tropical waters there is no light limitation, instead a permanent thermocline caused by heating of the surface waters prevents nutrient rich deep waters from reaching the sea surface and productivity is very low. There is no seasonality. The phytoplankton at the bottom of the food web are dinoflagellates and coccolithophores which are adapted to the low nutrient stratified conditions. No larger chain-forming diatoms can thrive here as they sink out of the water and are not able to survive under low nutrient conditions. The primary producers are grazed by a large variety of zooplanktonic organisms. These in turn provide food for a number of planktivorous fishes, such as flying fish at the surface and lantern fish in the mesopelagic. These are eaten by the larger, first-level predatory fishes and squids, which are then preyed upon by larger predators such as the marlin, swordfish and sharks. Finally, at the top of the food web are the largest sharks which attack tuna, swordfish and marlin.
If we compare the temperate and tropical food webs we can see that tropical food webs are more complex with more links and more trophic levels. This is partly due to the greater number of species present in the tropical waters and the general absence in the colder waters of the swifter predaceous fishes such as tuna and marlin. Another difference is that mammals and birds play a larger role in temperate food webs than in tropical food webs. Larger numbers of marine mammals occur in colder waters than in warmer tropical seas. The third difference is that the microbial food web is dominant in tropical waters (except in upwelling regions) due to oligotrophic waters caused by the permanent thermocline that isolates the warm, well-illuminated surface waters from the cold nutrient rich waters beneath.
Largely because nutrients are inadequate in the photic zone, the oceans are generally not very biologically productive. The two main exceptions to this are the shallow waters that cover continental shelves and upwelling regions. Both of these areas share a common trait - the regular influx of plant nutrients, especially nitrogen and phosphorus. In shallow coastal systems regular supplies of enhanced nutrients are input from anthropogenic activities such as agriculture and sewage treatment plants and from resuspension of bottom sediments by wave motion. Upwelling regions owe their high fertility to the slow but persistent upward flow of cold subsurface waters rich in nutrients. These regions support the highest average rates of primary production in the oceans. Not unexpectedly, some of the world's largest fisheries are located in upwelling areas.