MAP Technical Reports Series No. 106 UNEP

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A significant number of macrophyte species that recruit from many families inhabit
humid environments, i.e. wetlands such as swamps, marshes, reeds, which are vital habitats
for water fowl, spawning grounds for anadromous fish, and other animals closely associated
with the water milieu. Macrophyte communities are important primary producers in and around
estuaries and lagoons, as well as in sublittoral seacoast beds that receive sufficient light to make
photosynthesis possible. Among typical marine species that form extended submersed
communities are: Zostera, Posidonia, Cymodocea.
3.1.2 Secondary producers
This compartment includes all those species called heterotrophic that need for their
metabolism and survival organic substances acquired in the form of food through predation on
other species. Secondary producers in the marine environment derive from all phyla and classes
of the animal reign covering a very wide range of biotypes, habitat occupancy, food preference,
etc.; fresh waters are less rich in this respect. This wide variety makes it difficult to characterize
the category 'secondary producers' in a succinct way. Body size is among the more significant
criteria (Peters, 1983). Characteristic size class spectra that encompass the whole range of
biotypes present in aquatic communities reflect fundamental structural properties of aquatic
systems generally (cf. below).
Plankton (that operationally includes both, zoo- and phytoplankton, and hence, is not
a term referring to secondary producers only) has been typified as: (a) net plankton with two
subcategories (a.1) mesoplankton (>200 µm; mostly zooplankton), (a.2) microplankton (200-64
µm; includes also most of the larger phytoplankton), (b) nanoplankton (200 µm), (b) microplankton (200-20 µm), (c)
nanoplankton (20-2 µm) and (d) picoplankton (2-0.2 µm).
Actively swimming organisms of larger size (cm range and up) are generally
categorized as nekton, but many of the smaller species, both phyto- and zooplankton, have the
ability of active locomotion.
As regards food preference species are categorized as herbivores, carnivores, and
omnivores. These categories are somewhat artificial either, as not all types of secondary
producers can be allocated unequivocally to this scheme: e.g., detritus feeders, which fulfil an
important role in both, aquatic and terrestrial ecosystems. 'Herbivorous' plankton feeders obtain
their feed from living phytoplankton (some species seems to be able to be selective in their
choice), but also from the large pool of organogenic seston. Also, it has been claimed that some
species cover part of their energy need by osmotic uptake of dissolved organic substances.
Secondary producers may also be characterized by the mode of feed acquisition: (a)
species showing 'raptorial feeding' (active or passive random encounter between predator and
prey, most carnivorous species), (b) 'filter feeding' species concentrating their food by
appropriate sieving mechanisms (e.g., certain cladocera and copepoda species like Calanus,
mollusc, etc.), and (c) 'diffusion feeding' (accidental collision and sticking). Of particular
importance also in terms of eutrophication are species of the second category, because the kind
and size of the feed they can retain depends on the fine structure of their filter system.