21
controlled from inside. Plankton contains nothing comparable with the wellstructured transport system found in a forest. In trees, the major part of power that
brings up water and mineral nutrients to the leaves comes from the pull caused by
evaporation of water at the leaves; evaporation here plays an equivalent role to the
work done by turbulent energy or upwellings in aquatic environments (Margalef
1997). In natural plankton, control of nutrients transport is still entirely in the
physical environment, in the mobile structure of water masses, with cells of circulation and eddies of every size. In terrestrial plants, the transport system internalizes the nutrient cycle and places it under the plant’s control, unlike what occurs
in the sea (Margalef 1978, 1997) (Fig. 3.5). This underlines the ecological importance of marine fronts, characterized by upward water movements that fertilize the
illuminated zone.
3.2 Trophic Webs
While recognizing that there are some highly specialized marine predators, the
diverse diet of many species indicates that feeding at sea is often opportunistic and can be considered as less dependent on prey taxonomy than on prey size.
Contrasting with changes in species composition, the size spectra of marine ecosystems exhibit remarkably constant shapes. This observation suggests that,
beyond strict species interactions, size-based interaction controls the energy transfer in the marine environment (Cury et al. 2001).
With some notable exceptions (Sargassum for instance), most of the primary
organic production in the open sea is by single-celled plants and most herbivores
are small, but they are usually larger than the plants (Sheldon et al. 1977). In oligotrophic, oceanic waters the base of the food chain is composed of very small
cells. Smallness is usually seen as an adaptation to take up extremely low nutrient concentrations because of a favorable surface to volume ratio. Such kind of
phytoplankton is too small to be ingested by copepods thus most of primary production is channeled through the “microbial loop” (picoplankton-heterotrophic
nanoflagellates-ciliates). In these environments, copepods feed mainly on heterotrophic nanoflagellates and ciliates, thus adding links to the classical food chain
and raising the trophic level of zooplanktivorous fish. Whenever there is an input
of nutrients, such as in fronts, larger phytoplankters (i.e. diatoms; dynoflagellates)
become dominant. Most of such phytoplankton falls well into the food spectrum
of herbivorous copepods, and consequently those “classical” food chains consist
of fewer levels (Fenchel 1988; Kiorboe 1993; Sommer et al. 2002).
The size structure of phytoplankton depends not only on nutrients abundance
but also on hydrodynamic forces. Sinking of organic particles out of the euphotic
zone represents a major loss of organic matter to the deep ocean and eventually to
the sea floor; therefore, large cells depend on water motion to remain suspended.
Both empirical observations (Taylor et al. 2012) and models (Rodriguez et al. 2001)
indicate that the relative proportion of large phytoplankton cells increases with the
magnitude of the upward velocity. This suggests that mesoscale vertical motion
3.1 Biological Production
controlled from inside. Plankton contains nothing comparable with the wellstructured transport system found in a forest. In trees, the major part of power that
brings up water and mineral nutrients to the leaves comes from the pull caused by
evaporation of water at the leaves; evaporation here plays an equivalent role to the
work done by turbulent energy or upwellings in aquatic environments (Margalef
1997). In natural plankton, control of nutrients transport is still entirely in the
physical environment, in the mobile structure of water masses, with cells of circulation and eddies of every size. In terrestrial plants, the transport system internalizes the nutrient cycle and places it under the plant’s control, unlike what occurs
in the sea (Margalef 1978, 1997) (Fig. 3.5). This underlines the ecological importance of marine fronts, characterized by upward water movements that fertilize the
illuminated zone.
3.2 Trophic Webs
While recognizing that there are some highly specialized marine predators, the
diverse diet of many species indicates that feeding at sea is often opportunistic and can be considered as less dependent on prey taxonomy than on prey size.
Contrasting with changes in species composition, the size spectra of marine ecosystems exhibit remarkably constant shapes. This observation suggests that,
beyond strict species interactions, size-based interaction controls the energy transfer in the marine environment (Cury et al. 2001).
With some notable exceptions (Sargassum for instance), most of the primary
organic production in the open sea is by single-celled plants and most herbivores
are small, but they are usually larger than the plants (Sheldon et al. 1977). In oligotrophic, oceanic waters the base of the food chain is composed of very small
cells. Smallness is usually seen as an adaptation to take up extremely low nutrient concentrations because of a favorable surface to volume ratio. Such kind of
phytoplankton is too small to be ingested by copepods thus most of primary production is channeled through the “microbial loop” (picoplankton-heterotrophic
nanoflagellates-ciliates). In these environments, copepods feed mainly on heterotrophic nanoflagellates and ciliates, thus adding links to the classical food chain
and raising the trophic level of zooplanktivorous fish. Whenever there is an input
of nutrients, such as in fronts, larger phytoplankters (i.e. diatoms; dynoflagellates)
become dominant. Most of such phytoplankton falls well into the food spectrum
of herbivorous copepods, and consequently those “classical” food chains consist
of fewer levels (Fenchel 1988; Kiorboe 1993; Sommer et al. 2002).
The size structure of phytoplankton depends not only on nutrients abundance
but also on hydrodynamic forces. Sinking of organic particles out of the euphotic
zone represents a major loss of organic matter to the deep ocean and eventually to
the sea floor; therefore, large cells depend on water motion to remain suspended.
Both empirical observations (Taylor et al. 2012) and models (Rodriguez et al. 2001)
indicate that the relative proportion of large phytoplankton cells increases with the
magnitude of the upward velocity. This suggests that mesoscale vertical motion
3.1 Biological Production