The Rio de Ia Plata Estuary, Argentina-Uruguay
189
!suRFACE SALINITY FRONT I
,-! PARTIALLY STRATIFIED] ,
0.4 0.4 0.3 0.3 0 .2
0.1
17
v
v
v
"
50
100
200
A
DISTANCE (km)
c
0
1000 2000
MEN HADE~
SALPS CRUSTACEANS
1
e)'
EGGS(n m)
e
:l;3
.... n. w
os ~
CTENOPHORES
7
81 %
0
5000 10000 0
300
600 0
50
100
COPEPODS
BACTERIA
SUSPENDED
(n m" )
SEDIMENT
(mg 1 ' 1 )
e
:l;3
.... n.
COPEPODS
w
o s
Fig. 13.2A-C. Salinity section along the major axis of the estuary under typically stratified conditions. Numbers above upper axis represent the stratification parameter after
Hansen and Rattray (1966) (values > 1 correspond to a salt wedge condition). A Vertical
microdistribution of biological and physical properties at the head of the salt wedge. B
Scattering layer due to zooplankton at the halocline (modified from Madirolas et a!.
1997). C Mesozooplankton (copepods) and macrozooplankton aggregations at the surface salinity front. (Modified from Mianzan and Guerrero in press)
13.3.1 Freshwater Environment
The phytoplankton community of low salinity waters (0.2-5.0) is dominated by the diatom Aulacoseira sp. and coccal green algae, with the diatom
Stephanodiscus hantzschii and cyanobacteria (i.e., Microcystis aeruginosa)
occurring at polluted sites (Gomez and Bauer 1998). Clorophyll-a values
of oligotrophic (71 %) and mesotrophic (27 o/o ) environments are < 4
and 4-10 mg m-3, respectively (Rodolosi 1997). The species Heleobia piscium, Corbicula fluminea , Limnoperna fortunei, and Chilina fluminea