A guide for planners and managers - IUCN

PART III
Case Histories of Marine Protected Areas
There are some localised problems at the edge of the lagoon, such as
encroachment into wetlands, dumping of garbage, and a marina development with
clearing, felling, and filling 146 ha of prime mangrove and threatening effectively to
destroy about 700 ha of productive habitat.
The National Park was designed around the unaltered parts of the lagoon. The
village, the marina, and a second development close to the east end of the lagoon were
not included within the management boundaries. Management was confined to
boundaries that were determined more by convenience than ecological design.
The problems arising from activities outside the park’s boundaries, but inside
the lagoon’s ecological boundary, are those that most threaten the ecosystem. These
problems are outlined below.
The Functional Unit
The brackish water in Laguna de Tacarigua derives from mixing of freshwater—from
streams, groundwater seepage, surface runoff, and rains—with seawater entering
through the mouth and seeping in through porous areas of the coastal barrier. Lagoon
water may range from almost fresh to hypersaline. In the dry season the lagoon level
drops when water is lost through the mouth, and through evaporation and
evapotranspiration when they exceed all inputs. At this point tidal currents dominate
the estuarine currents at the mouth and salt-water flows into the lagoon.
If tidal flushing is insufficient to keep the mouth open, it is blocked by longshore
drift of sand. When the water level of the lagoon is high, flow of water from the lagoon
scours the inlet and maintains it open.
Clearly, the water balance of the lagoon is a function of forces operating on it
from outside, not inside, the limit of its shorelines. The shorelines of watersheds
delineate ecological boundaries of the Laguna de Tacarigua ecosystem. Similarly,
surrounding swamps that act as natural filters of silt borne down rivers, and connected
channels and waterways that influence overall lagoon productivity, are all part of the
same functional unit.
The Siltation Problem
The gravest, most obvious and most urgent problem in Laguna de Tacarigua is
siltation in the western zone called El Guapo. In 1964 the water from the Rio Guapo
was diverted to the lagoon via a channel, called Madre Casanas, to avert flooding
problems in low-lying villages on the Rio Guapo floodplain. Madre Casanas is much
shorter and its slope greater than the meandering Rio Guapo. Hence the flow of water
is faster and there is erosion of the channel bed, resulting in deposition of a vast amount
of silt in the estuary. Since 1964 a delta of 225 ha has developed in El Guapo at the
mouth of Madre Casanas.
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