| Autotroph | - | primary producer requiring sunlight and inorganic nutrients to make organic matter |
| Heterotroph | - | consumer requiring pre-formed organic material to make organic matter |
| Trophic level | - | a trophic level in a food web contains organisms that obtain their nourishment in a similar way and from a similar source |
| Omnivory | - | the ability of an organism to obtain nourishment from more than one trophic level |
| Protozoa | - | single-celled animals |
| Metazoa | - | multi-celled animals |
| New nitrogen | - | nitrate. Supports the classic food web |
| Regenerated nitrogen | - | ammonium. Supports the microbial food web |
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.
Nitrogen is required for protein synthesis in phytoplankton and is taken up in the form of ammonium, nitrate and nitrite. Cyanobacteria can also fix molecular nitrogen. Nitrate is released from sediments into the overlying water as organic matter decomposes and is trapped in the bottom water below the thermocline until wind-mixing or upwelling conditions allow it to be brought to the surface where it can be accessed by phytoplankton. Nitrate is called new nitrogen and phytoplankton production that is supported by nitrate leads to an increase in fish which can be harvested by humans. Both protozoa and bacteria can excrete ammonium and this can be quickly taken up by phytoplankton and used for growth, this type of nitrogen is called regenerated nitrogen and supports the microbial food web.
(Lalli and Parsons, 1997)
Light is one of the two major physical factors controlling phytoplankton productivity in the sea. The second includes those physical forces which bring nutrients up from the deep water, where they accumulate, into the euphotic zone. These two features together largely determine what type of phytoplankton develop and how much primary productivity occurs in any part of the world’s oceans. They are also major factors in determining the amount and type of marine animals that are produced, including fish which are caught commercially.
The amount of light decreases from the Equator towards the poles. On the other hand, the amount of wind mixing, which brings nutrients up to the surface, increases from the tropics (where water is vertically stabilized by solar heating) towards the poles. Thus the abundance of light and the abundance of nutrients in the euphotic zone form an inverse relationship which largely determines the pattern of phytoplankton productivity in different latitudes. In polar regions, a single pulse of phytoplankton abundance occurs during the summer when light becomes sufficient for a net increase in primary productivity. In temperate latitudes, primary productivity is generally maximal in the spring and again in the autumn when the combination of available light and high nutrient concentrations allows plankton blooms to occur. In the tropics, where intense surface heating produces a permanent thermocline, the phytoplankton are generally nutrient-limited throughout the year, and there are only small and irregular fluctuations in primary production due to local conditions.
This is a general representation of the annual cycle of phytoplankton production in the world’s oceans. However, there are many physical features that affect nutrient levels in the euphotic zone and thereby greatly modify the general pattern. These include fronts, which are relatively narrow regions characterised by large horizontal gradients in variables such as temperature, salinity and density, and eddy-formations such as rings and large-scale gyres, which have characteristic rotational patterns of circulation. These modifying physical features may be thousands of kilometres wide (e.g. gyres) or only a few kilometres long (e.g. tidal and river-plume fronts). The size depends on the topography and ocean climate of any particular location. The common feature of all these structures is that there is some mechanism involved for bringing nutrients up to the euphotic zone from deeper water, on time scales which may range from days to months. These mechanisms are superimposed on the seasonal wind mixing that partly generals the global pattern of phytoplankton production. Some of the nutrient-enhancing processes can result in ‘oases’ of plankton production during periods of the year when the production of phytoplankton would otherwise be low.
Up until a few decades ago, marine pelagic food webs were depicted as containing 3 major groups of organisms: algal cells were the primary producers at the bottom of the food chain, they were grazed by mesozooplankton (0.2 mm – 20 mm in size), which in turn were grazed by fish (Fig. 1). This paradigm is called the classic food web or the herbivorous food web and results in high productivity that supports the fisheries of the world.
Fig. 1. The classic marine pelagic food web
This view of marine pelagic food webs was challenged during the 1970s and 1980s by scientists such as Pomeroy and Azam who showed that there was an alternative pathway of carbon flow that led from bacteria to protozoa to metazoa, with dissolved organic matter (DOM) being utilized as substrate by the bacteria. DOM can enter the pelagic environment from a variety of sources: excretion of dissolved organic carbon (DOC) by algal cells, algal cell lysis, ‘messy eating’ by mesozooplankton or diffusion from fecal pellets. This food web paradigm was called the microbial loop and under the original definition did not contain autotrophs (Fig. 2). This food web is driven by recycled carbon.
Fig. 2. The microbial loop
Recent paradigms of pelagic food webs link the classic food web to microbial food webs which include nanoplankton (2-20 µm in size) and picoplankton (0.2-2 µm in size). Only 10% of the energy available for an organism in a particular trophic level is available to an organism in the trophic level above it. Thus, the additional trophic level in the microbial food web will lower the energy available for fish compared to the classic food web.
There is not always the same decrease in size between organisms at different trophic levels in microbial food webs as compared to classic food webs. For instance, dinoflagellates can eat prey larger than themselves by engulfing them in a feeding veil and covering prey items with digestive enzymes to decompose them.
There is a lively debate in scientific circles about what type of interactions can take place between the micro-organisms in pelagic food webs. For instance, young copepod life stages and ciliates may compete for the same food sources, but adult copepods have been observed to graze preferentially on ciliates and so may give their offspring a competitive advantage. It has been discovered that omnivory is very common. For example, both copepods and ciliates can be primary and secondary consumers, with ciliates preying on autotrophic and heterotrophic flagellates as well as cyanobacteria and bacteria, and copepods grazing on phytoplankton and ciliates. There is therefore a blurring of trophic levels in microbial food webs that further complicates interactions in pelagic food webs. Changes in the flow of carbon in pelagic food webs may have large repercussions further up the food chain and may determine how much carbon is available for export from a pelagic ecosystem in the form of fish catches. See Gismervik et al. (1996) for an interesting discussion about pelagic food webs and the impact grazers can have on them.
Conditions present in coastal and oceanic ecosystems can lead respectively to herbivorous or microbial food webs becoming dominant. In the open ocean oligotrophic conditions often prevail with a permanent thermocline preventing nutrients from mixing up from deep water leading to low productivity, this encourages the development of a recycling food web. In coastal regions there can be seasons of high productivity. In temperate latitudes, winter mixing of the sea on continental shelves ensures that surface waters have a rich supply of nutrients. The spring bloom is initiated as winds slacken creating partial stratification of the water column trapping nutrients and phytoplankton in the surface waters where light is available for photosynthesis. Legendre and Rassoulzadegan (1995) proposed that six observations could be made to determine whether coastal or oceanic conditions were dominant in an ecosystem and that there was a continuum between the two extreme conditions. Oceanic ecosystems are more likely to develop when waters are stratified, light is limiting, production relies mainly on regenerated nitrogen, the dominant microplankton (pelagic organisms less than 200 micrometers in size) are cyanobacteria, bacteria and flagellates, and a strong microbial food web is present. Coastal ecosystems are more likely to develop when waters are well mixed, growth is nutrient limited, production is supported mainly by new nitrogen, the phytoplankton are dominated by large species, and a strong herbivorous food web occurs is evident.
Legendre and Rassoulzadegan (1995) proposed four pelagic food webs that may occur under different conditions: the herbivorous food web, where mesozooplankton are the dominant grazers; the multivorous food web, where mesozooplankton and protozoa are equally dominant; the microbial food web, where dissolved organic nitrogen (DON) is plentiful and recycling of nutrients is occurring; the microbial loop, where DON is scarce and bacteria are competing with phytoplankton for ammonium. They proposed that the herbivorous food web and the microbial loop were transitory ecosystems and that multivorous and microbial food webs should be more stable and persist for longer. Herbivorous food webs will run out of nutrients that support the growth of large phytoplankton cells needed to feed metazoa and will be replaced by smaller cells which could select for protozoan grazers and the development of a multivorous food web, this could then further develop into a microbial food web as nutrients are recycled by the protozoa. The microbial loop may evolve into a microbial food web as protozoa numbers increase and there is enough recycled nitrogen to nourish small phytoplankton. It is proposed that in this way a continuum of food webs can occur in an ecosystem depending on changing conditions.
The Sargasso Sea is in the middle of the Atlantic Ocean, between the West Indies and the Azores. It is encircled by the Gulf Stream and the North Equatorial Current which cause the oval-shaped sea to move in a slow, clockwise direction. There is a permanent thermocline which prevents nutrients in the deeper waters from reaching surface water and so this is an oligotrophic ecosystem with concentrations of nitrate generally less than 0.1 mmol kg-1. Despite this, it covers an area equivalent to two-thirds of the landmass of the United States and contains approximately one-third of the Atlantic’s plankton.
The environmental conditions prevalent in the Sargasso Sea select for a microbial food web. Nanoplankton 2-4 mm in diameter dominate the phytoplankton, with some coccolithophores and pennate diatoms present. Recent scientific research (Sheridan et al., 2002) indicates that resource-rich particles which form around colonies of the cyanobacterium Trichodesmium spp. can be colonised by a range of micro-organisms (bacteria, fungi, diatoms, dinoflagellates, hypotrich ciliates, hydroids, juveniles and nauplii of copepods and juvenile decapods), and can act as an additional food source for plankton in this nutrient-poor environment. These enriched particles are called ‘marine snow’.
Eels from Europe, the Mediterranean and the United States annually migrate to the Sargasso Sea to mate, spawn and die, leaving their eggs in the plankton. Schnetzer and Steinberg (2002), found that diets of 3 mesozooplankton changed seasonally depending on availability of prey, but that diatoms were selectively grazed even when they were present in low numbers. The marine snow also formed an important component of the diet for all three species studied. Species specific feeding preferences occurred and there were differences in feeding selectivity.
An additional primary producer is a seaweed called Sargassum which floats on the surface and is colonised and utilized for nutrients by many invertebrates and detritus from it and its inhabitants will add to the DOM available to bacteria in the water column.
Fish are scarce, but some species of angler fish called Sargassum fish are present which feed on invertebrates. Angler fish are generally found in deep sea environments where food is also scarce. The adaptation is the same in both environments, the fish has a series of lures which attract invertebrates to it. Sargassumfish have adapted remarkable camoflage which makes them indistinguishable from the seaweed in which they lurk.
The food web is supported by regenerated nitrogen (ammonium) supplied by bacteria and protozoa. Small internal waves can sometimes cause nitrate to be brought up into the surface layer and phytoplankton biomass will increase in response to this allowing small fluctuations in productivity to occur. Occasionally bacterial biomass is greater than phytoplankton biomass and a microbial loop community dominates, but this is transitory and likely occurs at the end of one of these mini ‘blooms’ when for a short time detritus increases.
The Benguela upwelling occurs when Trade winds cause surface water to be advected away from the African coast allowing cold, nutrient-rich deep water to well-up to the surface. The combination of abundant light and new nitrogen (nitrate) causes a bloom of large celled phytoplankton which can be grazed directly by mesozooplankton. A large selection of diatom species dominate the phytoplankton, including very big cells of Coscinodiscus spp. Diatoms often form spores which sink rapidly, and this enables them to remain in the nearshore and nutrient-rich environments closer to upwelling centres.
The mesozooplankton, mainly euphausiids and copepods, have life cycles that enable them to be present in greatest numbers when the diatom bloom is taking place. They are grazed by anchovies or sardines resulting in a very short food web that is conducive to high fishery productivity.
The herbivorous food web dominates during upwelling periods, however there is evidence to suggest that recycling of nutrients also occurs. Hutchings and Field (1997) have suggested that grazing by copepods and euphausiids and the sedimentation of organic matter within the Benguela system cannot account for the decline in phytoplankton blooms after upwelled water has stabilised, so considerable recycling of organic matter appears to be taking place in the water column. Thus, despite the high rate of new nitrogen input to the shelf waters via upwelling, much of the shelf water is dominated by the microbial food web fuelled by recycled nutrients. Following the argument further, as it is new production that determines the productivity at higher trophic levels (fish), and as this food chain has been found to be much more complex (and less efficient) than the classical 20% energy tranfer applicable in a simple short-food chain, the presence of a strongly recycling food web may, to some extent, explain why the Benguela system yields considerably less fish than expected.