40
J. Polania et a!.
older. Most of the birds (91 %) in the CGSM are secondary consumers, 19
species preferentially feed on insects, and 14 on fish and aquatic invertebrates. Only five species, which include the ruddy ground dove ( Columbina
talpacoti), orange-winged parrot (Amazona amazonica), brown-throated
parakeet (Aratinga pertinax), and the hummingbird (Trochilidae), feed
directly on seeds, fruits, and nectar (Botero and Marshall1994).
2.5 Human Impact and Fisheries
During the last 40 years, human populations have profoundly modified the
entire CGSM, leading to a general degradation of the ecosystem and to a
substantial decrease in the abundance and diversity of fauna and flora. The
discharge of ever increasing volumes of untreated sewage into the CGSM
has elevated fecal coliform concentrations from 4,2501- 1 in 1972 to 46,0001- 1
in 1995, when Salmonella was detected in oysters ( Crassotrea rhizophorae ).
Heavy metals (i.e., cadmium, zinc, lead, nickel) are added to the CGSM with
suspended matter from the rivers of Sierra Nevada and during overflows of
the Magdalena River. Although metal concentrations vary between seasons,
copper concentrations in oysters have increased from 5.9-42.8 flg per g dry
weight in 1982 to 50.2-179.1 flg per g dry weight in 1988.
The Colombian government constructed a coastal highway along the
entire length of Isla de Salamanca between 1956 and 1960, thereby eliminating several important outlets through the Barrier Island and restricting
water exchange with the Caribbean Sea to one bridged opening (100m) at
Boca de la Barra. To drain wetlands and to prevent floodwaters from reaching agriculture lands, earthen dikes were built along the eastern bank of
the Magdalena River during the 1970s. As a result, the flow from several tributaries of the Magdalena River to the mangrove forests was reduced or
eliminated. Furthermore, the diversion of water for agricultural irrigation
has reduced freshwater runoff, while the erosion of deforested watersheds
has led to ever-increasing sediment loads from the rivers. The modification of hydrologic patterns and sedimentation processes in the CGSM has
induced a gradual increase in salinity and total suspended solids (Fig. 2.2)
and has altered the composition, distribution, abundance, and diversity of
organisms (Botero and Mancera-Pineda 1996). The discharge of wastewater, the addition of agricultural fertilizers, and phosphate-enriched runoff
from the rivers of Sierra Nevada have also started a process of eutrophication in the lagoon complex. Changes in the structure of the phytoplankton
community caused a reduction of species diversity by more than 10%
while the density of some groups, like diatoms (2-20 x 10 6 cells 1- 1 ) and
J. Polania et a!.
older. Most of the birds (91 %) in the CGSM are secondary consumers, 19
species preferentially feed on insects, and 14 on fish and aquatic invertebrates. Only five species, which include the ruddy ground dove ( Columbina
talpacoti), orange-winged parrot (Amazona amazonica), brown-throated
parakeet (Aratinga pertinax), and the hummingbird (Trochilidae), feed
directly on seeds, fruits, and nectar (Botero and Marshall1994).
2.5 Human Impact and Fisheries
During the last 40 years, human populations have profoundly modified the
entire CGSM, leading to a general degradation of the ecosystem and to a
substantial decrease in the abundance and diversity of fauna and flora. The
discharge of ever increasing volumes of untreated sewage into the CGSM
has elevated fecal coliform concentrations from 4,2501- 1 in 1972 to 46,0001- 1
in 1995, when Salmonella was detected in oysters ( Crassotrea rhizophorae ).
Heavy metals (i.e., cadmium, zinc, lead, nickel) are added to the CGSM with
suspended matter from the rivers of Sierra Nevada and during overflows of
the Magdalena River. Although metal concentrations vary between seasons,
copper concentrations in oysters have increased from 5.9-42.8 flg per g dry
weight in 1982 to 50.2-179.1 flg per g dry weight in 1988.
The Colombian government constructed a coastal highway along the
entire length of Isla de Salamanca between 1956 and 1960, thereby eliminating several important outlets through the Barrier Island and restricting
water exchange with the Caribbean Sea to one bridged opening (100m) at
Boca de la Barra. To drain wetlands and to prevent floodwaters from reaching agriculture lands, earthen dikes were built along the eastern bank of
the Magdalena River during the 1970s. As a result, the flow from several tributaries of the Magdalena River to the mangrove forests was reduced or
eliminated. Furthermore, the diversion of water for agricultural irrigation
has reduced freshwater runoff, while the erosion of deforested watersheds
has led to ever-increasing sediment loads from the rivers. The modification of hydrologic patterns and sedimentation processes in the CGSM has
induced a gradual increase in salinity and total suspended solids (Fig. 2.2)
and has altered the composition, distribution, abundance, and diversity of
organisms (Botero and Mancera-Pineda 1996). The discharge of wastewater, the addition of agricultural fertilizers, and phosphate-enriched runoff
from the rivers of Sierra Nevada have also started a process of eutrophication in the lagoon complex. Changes in the structure of the phytoplankton
community caused a reduction of species diversity by more than 10%
while the density of some groups, like diatoms (2-20 x 10 6 cells 1- 1 ) and