The Peruvian Coastal Upwelling System
235
respond to an increase in food by intensifying egg production, although
their response in terms of growth rate is much slower. Juveniles (nauplii
and first copepodites) remain close to the sea surface, whereas older stages
undertake downward migrations into the subsurface layer and are transported back to the coast. Vertical migration is impeded where hypoxic conditions prevail in the subsurface layer. Zooplankton grazers have developed behavioral responses that maximize grazing time on phytoplankton
blooms and transport them further offshore with surface layer waters,
although blooms of primary producers and small crustacean grazers do
not necessarily coincide. Different phyto-zooplankton communities, one
close to upwelling centers and the other further offshore, appear to constitute an early and a mature stage of succession in the Peruvian upwelling
cycle, respectively (Vinogradov and Shushkina 1984). Re-circulation in a
poleward direction, favored by the Peru Countercurrent and by eddies of
the upwelling centers, prevents the loss of juvenile stages towards the equator.
16.3.3 Benthic Organisms
Intertidal and shallow subtidal zones of Peru and northern Chile display
high species diversity ( 432 decapods and stomatopods, 348 polychaetes,
972 mollusks). Several commercially important filter-feeding bivalves
(Mesodesma donacium, Donax marincovichi, Gari salida, Semele salida,
Protothaca thaca, Aulacomya ater, Argopecten purpuratus, Venus antiqua,
Tagelus dombeii, Ensis macha) form large populations. The community of
Aulacomya ater (up to 40,000 g m- 2 ) off Pisco hosts 186 species. Populations of the mussel Argopecten purpuratus, the gastropod Thais chocolata,
the decapod crustaceans Cancer setosus, Cancer porteri, and Platyxanthus
orbignyi, the sea urchin Loxechinus albus, and the kelps Lessonia spp. and
Macrocystis pyrifera develop high density and biomass. Kelp forests cover
rocky bottoms down to about 15m and provide niches and refuges for
numerous associated species, as do intertidal mussel beds where Perumytilus purpuratus and Semimytilus algosus communities harbor about 87
and 77 species, respectively. The structure of the floral and faunal communities tends to be controlled by grazers (sea urchins, chitons, limpets, and
other gastropods) and predators (sea stars, brachyurans, crabs, and fish),
respectively. Macroalgal (i.e., Macrocystis pyrifera) and bivalve (i.e., Aulacomya ater, Argopecten purpuratus) population dynamics are influenced
by the success of larval recruitment and by warmer and cooler conditions,
which are determined by upwelling periodicity and the ENSO cycle (Wolff
1988; Arntz and Tarazona 1990). Dominant macrobenthic organisms of
235
respond to an increase in food by intensifying egg production, although
their response in terms of growth rate is much slower. Juveniles (nauplii
and first copepodites) remain close to the sea surface, whereas older stages
undertake downward migrations into the subsurface layer and are transported back to the coast. Vertical migration is impeded where hypoxic conditions prevail in the subsurface layer. Zooplankton grazers have developed behavioral responses that maximize grazing time on phytoplankton
blooms and transport them further offshore with surface layer waters,
although blooms of primary producers and small crustacean grazers do
not necessarily coincide. Different phyto-zooplankton communities, one
close to upwelling centers and the other further offshore, appear to constitute an early and a mature stage of succession in the Peruvian upwelling
cycle, respectively (Vinogradov and Shushkina 1984). Re-circulation in a
poleward direction, favored by the Peru Countercurrent and by eddies of
the upwelling centers, prevents the loss of juvenile stages towards the equator.
16.3.3 Benthic Organisms
Intertidal and shallow subtidal zones of Peru and northern Chile display
high species diversity ( 432 decapods and stomatopods, 348 polychaetes,
972 mollusks). Several commercially important filter-feeding bivalves
(Mesodesma donacium, Donax marincovichi, Gari salida, Semele salida,
Protothaca thaca, Aulacomya ater, Argopecten purpuratus, Venus antiqua,
Tagelus dombeii, Ensis macha) form large populations. The community of
Aulacomya ater (up to 40,000 g m- 2 ) off Pisco hosts 186 species. Populations of the mussel Argopecten purpuratus, the gastropod Thais chocolata,
the decapod crustaceans Cancer setosus, Cancer porteri, and Platyxanthus
orbignyi, the sea urchin Loxechinus albus, and the kelps Lessonia spp. and
Macrocystis pyrifera develop high density and biomass. Kelp forests cover
rocky bottoms down to about 15m and provide niches and refuges for
numerous associated species, as do intertidal mussel beds where Perumytilus purpuratus and Semimytilus algosus communities harbor about 87
and 77 species, respectively. The structure of the floral and faunal communities tends to be controlled by grazers (sea urchins, chitons, limpets, and
other gastropods) and predators (sea stars, brachyurans, crabs, and fish),
respectively. Macroalgal (i.e., Macrocystis pyrifera) and bivalve (i.e., Aulacomya ater, Argopecten purpuratus) population dynamics are influenced
by the success of larval recruitment and by warmer and cooler conditions,
which are determined by upwelling periodicity and the ENSO cycle (Wolff
1988; Arntz and Tarazona 1990). Dominant macrobenthic organisms of