Tidal Flats & Algal Mats
Elevation of the flats behind South Padre Island is generally less than 1.0 meter (3 feet) above sea level. These flats are regularly flooded by tides driven by the prevailing southeasterly winds. But flats on the mainland side of Laguna Madre, which are underlain by mud, are as high as 1.5 m (5 feet) above sea level and are flooded only by tropical storms and hurricanes. The low elevation combined with an extremely gentle slope, allows lagoon water to be pushed by the winds to quickly flood large areas. Fluctuating winds and related tides preclude establishment of a stationary shoreline position since it can shift several kilometers within hours.
Sediments transported to the flats include wind driven sand from the adjacent dunes, and the suspended clays brought by the high tide. These are redistributed and smoothed by swift tidal currents, resulting in a broad, low gradient unvegetated flat area. Wind-tidal flats are mostly barren of rooted vegetation, due to a combination of irregular flooding, hot summer temperatures, little freshwater inflow, and salty soils. They occupy the niche that coastal marshes occupy in the more humid middle and upper parts of the Texas coastal zone.
Felt-like mats of microscopic, single-celled, blue-green algae or algal mats are prolific on the flats. They are most extensive on areas that alternate exposure and submergence in regular cycles. On the ground the algal mats appear as very dark areas surrounding the lagoon. With each tidal inundation, thin layers of clay are deposited from suspension. The algal mats that develop on the moist surface commonly crack and peel from the underlying sand and clay during drier periods between tidal floods.
As the footprints of wading birds indicate these are important locales for feeding and breeding shorebirds. Good examples of wind-tidal flats and their associated resilient algal mats are found along Boca Chica Boulevard, 3 kilometers before the beach access.
Dentro del árido clima del Sur de Texas, los ventosos humedales de marea constituyen un ecosistema singular conformado por amplias llanuras baldías en la periferia de la Laguna Madre que se inundan con las mareas. La elevación de estas llanuras detrás de la Isla del Padre es por lo general un metro inferior al del nivel del mar, por lo que las mareas acrecentadas por los vientos preponderantes del sureste las inundan de manera frecuente.
Los sedimentos que se depositan en estas llanuras incluyen arena transportada por el viento desde las dunas adyacentes, así como arcillas en suspensión introducidas por la marea alta. Las bruscas corrientes de la marea redistribuyen y allanan estos sedimentos, dejando en su retirada una amplia llanura de escasa pendiente y desprovista de vegetación. Este tipo de humedales de marea son baldíos y carentes de vegetación arraigada, debido a la combinación de inundaciones irregulares, altas temperaturas estivales, escasa aportación de agua dulce y suelos salobres. Estas formaciones ocupan el lugar propio de los marjales costeros y marismas en el litoral mucho más húmedo del centro y norte de Texas.
En estas llanuras de inundación suelen proliferar auténticos mantos aterciopelados de algas unicelulares microscópicas de color azul verdoso, dándose de forma más extensa en aquellas zonas donde se alternan con mayor regularidad los ciclos de exposición e inmersión. A nivel de suelo, estos mantos de algas se identifican como zonas de color muy oscuro alrededor de la laguna. Con cada pleamar, el agua de la marea deposita finas capas de arcillas en suspensión, pero los mantos de algas que crecen en la superficie húmeda se desgajan y se separan de la arena y las arcillas durante los períodos secos entre cada inundación.
Se pueden observar buenos ejemplos de estos ventosos humedales de marea con sus resistentes mantos de algas junto al Bulevar de Boca Chica, en Brownsville, tres kilómetros antes del acceso a la playa.
Escucha en Español – los humedales costeros de marea y los mantos de algas
Wind tidal flats are a unique ecosystem on the arid south Texas coast consisting of extensive barren flats on the periphery of Laguna Madre, flooded by the tides.
The elevation of the flats behind South Padre Island is generally less than 3 feet above sea level. These flats are regularly flooded by tides driven by the prevailing southeasterly winds.
Sediments transported to the flats include wind-driven sand from the adjacent dunes, and the suspended clays brought by the high tide. These are redistributed and smoothed by swift tidal currents, resulting in a broad, low gradient unvegetated flat area. Wind-tidal flats are mostly barren of rooted vegetation due to a combination of irregular flooding, hot summer temperatures, little freshwater inflow, and salty soils. They occupy the niche that coastal marshes occupy in the more humid middle and upper parts of the Texas coastal zone.
Felt-like mats of microscopic, single-celled, blue-green algae or algal mats are prolific on the flats. They are most extensive on areas that alternate exposure and submergence in regular cycles. On the ground, the algal mats appear as very dark areas surrounding the lagoon. With each tidal inundation, thin layers of clay are deposited from suspension. The algal mats that develop on the moist surface commonly crack and peel from the underlying sand and clay during drier periods between tidal floods.
A good example of wind-tidal flats and their associated resilient algal mats are found along Boca Chica Boulevard, 3 kilometers before the beach access.
Listen to the English podcast - Algal Mats and Wind Tidal Flats