44
Bastiaan Knoppers . Bjorn Kjerfve
along the axis of coastal lagoons (Copeland et al. 1968), although the seiche motion
in shallow lagoons is quickly overcome by friction.
In choked coastal lagoons, persistent winds can regulate the longitudinal salt distribution by driving oceanic waters into or out of the systems, as in the case of Patos 1.,
which becomes totally fresh within a few kilometres of the ocean entrance during times
of persistent winds from the north (Kjerfve 1986). On the other hand, when southerly
winds blow, brackish waters push northward and the 5 isohaline can sometimes extend to the innermost reach of the lagoon, more than 250 km from the ocean entrance
(Delaney 1963).
3.7
Salinity and Stratification
Salinity in the lagoons varies according to fresh water runoff, local climate, the
geomorphology of lagoon cells and ocean connection, and degree of tidal choking.
Apart from hypersaline Araruama 1., the freshwater Patos 1., and the polyhaline
Itaipu 1., all coastal lagoons in southeastern Brazil are oligohaline to mesohaline
(Table 3.2). Most of the exterior lagoons of these systems are mesohaline, and the interior lagoons are either oligohaline or slightly mesohaline. The individual cells of these
lagoon systems are usually homogeneously mixed land-sea breezes, and temperature
convection as a result of 3-5
0
C diurnal temperature changes. Wind-induced mixing
becomes more important during autumn and winter due to the frequent passage of
weather fronts from the south, which commonly cause drastic temperature drops of
up to 9
0
C within 2-3 days (Kjerfve et al. 1990).
However, vertical density (salinity) stratification may still occur in some lagoons.
The Patos 1. Estuary and the lower section of Cananeia-Igwlpe 1. are partially stratified (Abreu et al. 1994a,b; Miyao et al. 1986). The central basin of Concei<;:ao L. is highly
stratified because marine waters enter the lagoon, descend after some mixing to the
bottom, and are trapped within the basin by bottom sills (Knoppers et al. 1984; Odebrecht and Caruso 1987). Similar trapping occurs occasionally in Guarapina 1. during events of maximum tidal inflow (Knoppers and Moreira 1988). In both cases, bottom waters stagnate and turn anoxic during periods of constant surface water outflow. Renewed tidal intrusion as well as wind-induced erosion of the halocline can
result in nutrient-rich bottom waters mixing towards the surface. This feature of coastal
lagoons, with relatively deep external basins, has been documented by Mee (1978).
Although Araruama 1. harbours a basin 17 m deep, the water column remains homogeneously mixed by salt convection from evaporation and intense wind mixing
(Kjerfve et al. 1996).
The coastal lagoons exhibit pronounced annual cycles of salinity (Fig. 3.8).
Short-term salinity variability is most pronounced in the Patos 1. Estuary and is lowest in the hypersaline Araruama L. Salinity changes are always smallest in the internal cells of any lagoon system. Intense rain events may, however, induce drastic salinity changes, as in the case of Guarapina 1. and Piratininga 1., resulting in marked
biogeochemical and ecological responses (Knoppers and Moreira 1988; Carneiro
et al. 1993; 1994). Extreme rain events occur sporadically along the microtidal Brazilian lagoon coast.
Bastiaan Knoppers . Bjorn Kjerfve
along the axis of coastal lagoons (Copeland et al. 1968), although the seiche motion
in shallow lagoons is quickly overcome by friction.
In choked coastal lagoons, persistent winds can regulate the longitudinal salt distribution by driving oceanic waters into or out of the systems, as in the case of Patos 1.,
which becomes totally fresh within a few kilometres of the ocean entrance during times
of persistent winds from the north (Kjerfve 1986). On the other hand, when southerly
winds blow, brackish waters push northward and the 5 isohaline can sometimes extend to the innermost reach of the lagoon, more than 250 km from the ocean entrance
(Delaney 1963).
3.7
Salinity and Stratification
Salinity in the lagoons varies according to fresh water runoff, local climate, the
geomorphology of lagoon cells and ocean connection, and degree of tidal choking.
Apart from hypersaline Araruama 1., the freshwater Patos 1., and the polyhaline
Itaipu 1., all coastal lagoons in southeastern Brazil are oligohaline to mesohaline
(Table 3.2). Most of the exterior lagoons of these systems are mesohaline, and the interior lagoons are either oligohaline or slightly mesohaline. The individual cells of these
lagoon systems are usually homogeneously mixed land-sea breezes, and temperature
convection as a result of 3-5
0
C diurnal temperature changes. Wind-induced mixing
becomes more important during autumn and winter due to the frequent passage of
weather fronts from the south, which commonly cause drastic temperature drops of
up to 9
0
C within 2-3 days (Kjerfve et al. 1990).
However, vertical density (salinity) stratification may still occur in some lagoons.
The Patos 1. Estuary and the lower section of Cananeia-Igwlpe 1. are partially stratified (Abreu et al. 1994a,b; Miyao et al. 1986). The central basin of Concei<;:ao L. is highly
stratified because marine waters enter the lagoon, descend after some mixing to the
bottom, and are trapped within the basin by bottom sills (Knoppers et al. 1984; Odebrecht and Caruso 1987). Similar trapping occurs occasionally in Guarapina 1. during events of maximum tidal inflow (Knoppers and Moreira 1988). In both cases, bottom waters stagnate and turn anoxic during periods of constant surface water outflow. Renewed tidal intrusion as well as wind-induced erosion of the halocline can
result in nutrient-rich bottom waters mixing towards the surface. This feature of coastal
lagoons, with relatively deep external basins, has been documented by Mee (1978).
Although Araruama 1. harbours a basin 17 m deep, the water column remains homogeneously mixed by salt convection from evaporation and intense wind mixing
(Kjerfve et al. 1996).
The coastal lagoons exhibit pronounced annual cycles of salinity (Fig. 3.8).
Short-term salinity variability is most pronounced in the Patos 1. Estuary and is lowest in the hypersaline Araruama L. Salinity changes are always smallest in the internal cells of any lagoon system. Intense rain events may, however, induce drastic salinity changes, as in the case of Guarapina 1. and Piratininga 1., resulting in marked
biogeochemical and ecological responses (Knoppers and Moreira 1988; Carneiro
et al. 1993; 1994). Extreme rain events occur sporadically along the microtidal Brazilian lagoon coast.