The Coastal Lagoon Cienaga Grande de Santa Marta, Colombia
35
Circulation patterns in the lagoon are a consequence of the interactions
between east-northeasterly trade winds and river discharge. Due to the
opposing direction of winds and freshwater runoff, the mixing process of
the different water masses is restrained, thus horizontal physico-chemical
gradients are common. During the rainy season, the increased freshwater
levels in the lagoon cause an outwelling current to the open sea; during the
dry season, seawater enters through Boca de la Barra (Inderena-Sodeic
1987).
Sedimentary loads of the Magdalena River (30-100 x 10 3 m 3 day- 1 ) and
those from tributaries of Sierra Nevada (180 x 10 6 tons year- 1 ; Deeb Sossa
1993) have decreased the lagoon depth at several sites from 2.3 to an
average of 1.5 m over the last 30 years (Bernal 1996). Muddy-day sediments with a surficial component of detrital matter predominate in the
lagoon, though in some areas gravel represents an important fraction, and
sand and shell fragments of marine origin tend to increase toward the
sandbar.
The CGSM can be considered a euhaline-mixohaline system, with mean
annual water temperature of about 30 oc. Nevertheless, pronounced temporal and spatial salinity gradients are common, resulting from variable
freshwater runoff, seawater intrusion, rainfall, and evaporation. During the
dry season, salinity in the inlet is about 40 o/oo (parts per thousand), and
still may exceed 30 o/oo 2 km up the tributaries. During the short wet season
salinity values decrease to about 15-20 o/oo in the center of the lagoon, but
during the long wet season salinity may be close to zero even near the inlet
(Botero and Mancera-Pineda 1996). Furthermore, every 6 or 7 years the
Magdalena River has high discharge and, for extended periods, the entire
CGSM experiences salinities close to zero (Kaufman and Hevert 1973).
Because of the lagoon shallow depth and constant winds, the water column
of the CGSM is normally well mixed. Nevertheless salinity stratification
may occur in some areas during the rainy seasons.
Dissolved oxygen concentrations in the lagoon vary between seasons
and with location but they generally demonstrate an inverse relationship
with salinity distributions. The maintenance of mean dissolved oxygen
levels (6.2mg 1- 1 ) in the lagoon can be attributed to mixing of shallow
waters by winds, while reduced dissolved oxygen levels (2 mg 1- 1 ) are related to pulses of organic matter decomposition and to the introduction of
urban or agricultural wastewater. In general, total suspended solids (TSS)
in the water column show a clear seasonal pattern and vary as a consequence of interactions between sedimentary loads from the Magdalena
River, intrusion of seawater, and the influence of winds. During the dry season, when the water column is well mixed, TSS may reach concentrations of
360 mg l- 1 (mean Secchi depth 70 em). During rainy seasons, due to stratifi-
35
Circulation patterns in the lagoon are a consequence of the interactions
between east-northeasterly trade winds and river discharge. Due to the
opposing direction of winds and freshwater runoff, the mixing process of
the different water masses is restrained, thus horizontal physico-chemical
gradients are common. During the rainy season, the increased freshwater
levels in the lagoon cause an outwelling current to the open sea; during the
dry season, seawater enters through Boca de la Barra (Inderena-Sodeic
1987).
Sedimentary loads of the Magdalena River (30-100 x 10 3 m 3 day- 1 ) and
those from tributaries of Sierra Nevada (180 x 10 6 tons year- 1 ; Deeb Sossa
1993) have decreased the lagoon depth at several sites from 2.3 to an
average of 1.5 m over the last 30 years (Bernal 1996). Muddy-day sediments with a surficial component of detrital matter predominate in the
lagoon, though in some areas gravel represents an important fraction, and
sand and shell fragments of marine origin tend to increase toward the
sandbar.
The CGSM can be considered a euhaline-mixohaline system, with mean
annual water temperature of about 30 oc. Nevertheless, pronounced temporal and spatial salinity gradients are common, resulting from variable
freshwater runoff, seawater intrusion, rainfall, and evaporation. During the
dry season, salinity in the inlet is about 40 o/oo (parts per thousand), and
still may exceed 30 o/oo 2 km up the tributaries. During the short wet season
salinity values decrease to about 15-20 o/oo in the center of the lagoon, but
during the long wet season salinity may be close to zero even near the inlet
(Botero and Mancera-Pineda 1996). Furthermore, every 6 or 7 years the
Magdalena River has high discharge and, for extended periods, the entire
CGSM experiences salinities close to zero (Kaufman and Hevert 1973).
Because of the lagoon shallow depth and constant winds, the water column
of the CGSM is normally well mixed. Nevertheless salinity stratification
may occur in some areas during the rainy seasons.
Dissolved oxygen concentrations in the lagoon vary between seasons
and with location but they generally demonstrate an inverse relationship
with salinity distributions. The maintenance of mean dissolved oxygen
levels (6.2mg 1- 1 ) in the lagoon can be attributed to mixing of shallow
waters by winds, while reduced dissolved oxygen levels (2 mg 1- 1 ) are related to pulses of organic matter decomposition and to the introduction of
urban or agricultural wastewater. In general, total suspended solids (TSS)
in the water column show a clear seasonal pattern and vary as a consequence of interactions between sedimentary loads from the Magdalena
River, intrusion of seawater, and the influence of winds. During the dry season, when the water column is well mixed, TSS may reach concentrations of
360 mg l- 1 (mean Secchi depth 70 em). During rainy seasons, due to stratifi-