212
G.M.E. Perillo et al.
Three different macrobenthic faunal associations occur in the estuary
(Elias 1985). An association of heterogeneous species is found in different
vertical substrates. The lower and middle mesolittoral is occupied by
Laeonereis acuta and Eteone sp., which are occasionally replaced by C. altimanus or bivalves. Littoridina australis, Cyrtograpsus altimanus, and
Tagelus gibbus inhabit the middle and upper mesolittoral and may be substituted by Scolecolepides sp. An association composed of tellinids, thracids, Buccinidae, amphipods, nemerteans, and polychaetes, which tend to
be deposit feeders, occupies unvegetated mudflats with Venus and Tellina
in the lower littoral of the southern estuary. Dense populations of the burrowing crab Chasmagnatus granulata represent a third association on
sandy substrate in the upper intertidal of salt marshes and mudflats ("cangrejal"). In Spartina-dominated marshes Chasmagnatus acts as a herbivore and on mudflats it largely feeds on organic deposits (Iribarne et al.
1997). The extensive burrows of the crabs tend to retain detritus, and thus
control organic matter flow between marshes and the open estuary (Botto
and Iribarne 2000).
14.4.2 Plankton
Diatoms are the principal components of the phytoplankton community,
which appears to have low specific diversity since only 36 taxa have been
cited for the estuary (Popovich 1996). Thalassiosira curviseriata, T. eccentrica, T. minima, Rhizosolenia delicatula, Skeletonema costatum, and
Cyclotella sp. are the most frequent (>50 o/o) species. Other representative
groups are Prasinophyceae, Crytophyceae, Euglenids, Cyanophyceae, and
Sillicoflagellates, while species like Ophiocytium sp. (Xanthophyceae) and
dinoflagellates (Protoperidinium sp., Prorocentrum micans) are occasionally responsible for red tides (Gayoso 1988). Neritic warm water forms
and oceanic elements (i.e., Actinoptychus, Coscinodiscus) may occasionally become dominant. Phytoplankton succession initiates in winter
(June/September) with species like Skeletonema costatum, Thalassiosira
hibernalis, T. anguste-lineata, and Chaetoceros debilis, followed by Rhizosolenia delicatula and Ditylum brightwelli in spring (September/October), and by "pulses" of flagellates (i.e., Pyramimonas sp.) in December
and January (Gayoso 1988). High nutrient levels and low zooplankton
grazing pressure in winter and spring favor blooms of Tallassiosira anguste-lineata, T. rotula, T. pacifica, and Chaetoceros sp. (Freije et al. 1980).
However, Tallassiosira curviseriata dominates because the species is well
adapted to low temperatures and high turbidity in the inner estuary
(Popovich 1996). The shallow euphotic zone of the estuary is compen-
G.M.E. Perillo et al.
Three different macrobenthic faunal associations occur in the estuary
(Elias 1985). An association of heterogeneous species is found in different
vertical substrates. The lower and middle mesolittoral is occupied by
Laeonereis acuta and Eteone sp., which are occasionally replaced by C. altimanus or bivalves. Littoridina australis, Cyrtograpsus altimanus, and
Tagelus gibbus inhabit the middle and upper mesolittoral and may be substituted by Scolecolepides sp. An association composed of tellinids, thracids, Buccinidae, amphipods, nemerteans, and polychaetes, which tend to
be deposit feeders, occupies unvegetated mudflats with Venus and Tellina
in the lower littoral of the southern estuary. Dense populations of the burrowing crab Chasmagnatus granulata represent a third association on
sandy substrate in the upper intertidal of salt marshes and mudflats ("cangrejal"). In Spartina-dominated marshes Chasmagnatus acts as a herbivore and on mudflats it largely feeds on organic deposits (Iribarne et al.
1997). The extensive burrows of the crabs tend to retain detritus, and thus
control organic matter flow between marshes and the open estuary (Botto
and Iribarne 2000).
14.4.2 Plankton
Diatoms are the principal components of the phytoplankton community,
which appears to have low specific diversity since only 36 taxa have been
cited for the estuary (Popovich 1996). Thalassiosira curviseriata, T. eccentrica, T. minima, Rhizosolenia delicatula, Skeletonema costatum, and
Cyclotella sp. are the most frequent (>50 o/o) species. Other representative
groups are Prasinophyceae, Crytophyceae, Euglenids, Cyanophyceae, and
Sillicoflagellates, while species like Ophiocytium sp. (Xanthophyceae) and
dinoflagellates (Protoperidinium sp., Prorocentrum micans) are occasionally responsible for red tides (Gayoso 1988). Neritic warm water forms
and oceanic elements (i.e., Actinoptychus, Coscinodiscus) may occasionally become dominant. Phytoplankton succession initiates in winter
(June/September) with species like Skeletonema costatum, Thalassiosira
hibernalis, T. anguste-lineata, and Chaetoceros debilis, followed by Rhizosolenia delicatula and Ditylum brightwelli in spring (September/October), and by "pulses" of flagellates (i.e., Pyramimonas sp.) in December
and January (Gayoso 1988). High nutrient levels and low zooplankton
grazing pressure in winter and spring favor blooms of Tallassiosira anguste-lineata, T. rotula, T. pacifica, and Chaetoceros sp. (Freije et al. 1980).
However, Tallassiosira curviseriata dominates because the species is well
adapted to low temperatures and high turbidity in the inner estuary
(Popovich 1996). The shallow euphotic zone of the estuary is compen-