Geoscience Reference
In-Depth Information
Laguna Madre is a shallow lagoon that
receives little runoff from the mainland and has
few connections to the open sea. Owing to
minimal circulation and high evaporation rates,
lagoon water is hypersaline. Water depth in
much of the lagoon is around 1 m; the deepest
point is only 2.5 m (Weise and White 1980). The
lagoon is virtually tideless; water l uctuations
result from wind-driven waves (Brown and
Huey 1991). Sediment of the lagoon consists
of mud, sand, and shell berms, much of
which is stablized by marine grass. Shoalgrass
(
Halodule wrightii
) is the most abundant
marine grass, which prefers shallow water (
1 m
deep) and tolerates high salinity, as its Latin
name suggests. Grassl ats are highly productive
environments that support abundant inverte-
brates (snails and clams) and are spawning
grounds or nurseries for many i sh and crusta-
ceans, such as shrimp and crabs (Weise and
White 1980).
Laguna Madre contains, in addition, patch
reefs composed of calcium carbonate and con-
structed by serpulid (annelid) worm tubes in the
Bafi n Bay vicinity (Weise and White 1980).
Reefs tops are commonly exposed at the water
surface or during low tides. The reefs are
currently degrading, and none are living
today; they may represent a wetter climatic past
(Tunnell 2002). These serpulid reefs are well
known for excellent i shing. The Laguna Atascosa
National Wildlife Refuge includes the southwest-
ern margin of Laguna Madre and adjacent main-
land (Fig. 15-29). This region has a complex
mixture of marine and terrestrial ecosystems at
the interface of temperate, subtropical and
desert environments (Fig. 15-30).
Both Padre Island and Laguna Madre are
modii ed by human activities. The Mansi eld
Channel cuts across the lagoon and island, con-
necting Port Mansi eld on the mainland directly
to the Gulf of Mexico (see Fig. 15-23). This
channel has altered the l ow and exchange of
water between the lagoon and open sea. Salinity
in Laguna Madre has moderated, and i sh can
migrate between the lagoon and ocean (Weise
and White 1980). Another channel is the Intra-
coastal Waterway, a canal running the entire
<
A
B
Figure 15-24.
Preparation for the annual autumn
i shing tournament at Padre Island National Seashore,
Texas. A. Temporary i shing camp on the beach;
vertical kite aerial photograph by S.W. Aber and J.S.
Aber. B. Ground view of surf i shing from the beach.
Photo by J.S. Aber.
UPPER SHOREFACE
BEACH
FORE-ISLAND
DUNE RIDGE
STABILIZED
BLOWOUT DUNES
VEGETATED BARRIER FLAT
BACK - ISLAND
DUNE FIELD
WIND-
TIDAL
FLAT
LAGOON-
MARGIN
SAND
GRASSFLAT
Gulf of
Mexico
Laguna
Madre
FORE-
BEACH
BACK-
BEACH
BREAKER
Padre Island
SWASH ZONE
PONDS AND MARSHES OCCUPYING
WIND - DEFLATION TROUGHS
BREAKER BAR
Figure 15-25.
Generalized section across Padre Island showing environmental zones. Adapted from Weise and
White (1980, Fig. 2).
