Abstract
In the last sixty years many geomorphological changes have occurred in Costa Rica's Térraba-Sierpe National Wetlands. Changes in coastal geomorphology are generally associated with erosion or accretion of sediment, which has led to the removal of sections of mangrove forests or sediment banks colonized by mangroves. The aim of this study was to analyze sedimentation as a leading process in the dynamics of coastal morphology and its implications for mangrove forest cover in the Boca Zacate area of Térraba-Sierpe wetlands. The study was conducted in the sectors of Bocón, Brujo and Coco Island in Boca Zacate, from 2008 to 2013. The research was based on a multi-temporal analysis of coastal morphology using aerial photographs from the years 1948, 1960, 1974, 1978, 1984, 1992 and 2011. The following measurements were also performed: monthly sedimentation rate (g/cm2/day), and granulometric composition and content of chemical elements in the sediments of the study area. These last two measurements were performed once each in the dry and rainy seasons during the years of study. The results indicated that over the past 60 years, Boca Zacate has witnessed a process of sustained erosion; from 1948 through 2001, losing 10.6 % of its land and approximately 8.9 % of its forest cover. It has also experienced accretion in the area of Coco Island. The Brujo sector showed the highest sedimentation rate and the Camibar estuary, the lowest. The dominant type of sediment in all study sites was sand, followed by clay and silt. The most widespread chemical elements (mg/L) included magnesium, calcium and potassium; others, such as manganese, iron, aluminum, phosphorus, zinc and copper, were measured in smaller amounts. Transport, composition and quantity of sediment in Boca Zacate are crucial to the changes that have occurred on the coastal area of La Boca, where the presence of dead trees was evident. This geomorphological analysis holds great importance for future guidelines and actions for the conservation and integrated management of the mangrove in Térraba-Sierpe National Wetlands.
Highlights
Los ecosistemas costeros se caracterizan por constituirse en espacios dinámicos que poseen una estrecha relación con el tipo, cantidad de sedimentos y su depósito por medio de la acción de agentes fluviales y marinos; ocasionando la colonización de nuevos bancos de sedimentos durante procesos de acreción o progradación litoral (Woodroffe, 1992)
The results indicated that over the past 60 years, Boca Zacate has witnessed a process of sustained erosion; from 1948 through 2001, losing 10.6 % of its land and approximately 8.9 % of its forest cover
Composición y cantidad de sedimentos en Boca Zacate aportados por el río Grande de Térraba, asociados a las corrientes, mareas y oleaje, son factores determinantes en los cambios que han ocurrido en el litoral de la Boca, conllevando a un paisaje de bosque de mangle muerto en la zona
Summary
Área de estudio: La cuenca del río Grande de Térraba constituye la cuenca más grande de Costa Rica, fluye hacia la costa Pacífica drenando un área de 5 085 km (Cedeño, López, Villalobos, & Hernández, 2012, Rojas, 2011). Tasa de sedimentación en el canal principal de los esteros: Se presentaron diferencias significativas en la tasa de sedimentación entre las épocas del año (F1/438 = 8.5, p < 0.05), con el mayor aporte de sedimentos en la época lluviosa (promedio 0.8±0.6 g/cm2/día). Composición granulométrica de los sedimentos en el canal principal de los esteros: La arena fue el tipo de sedimento que se cuantificó en mayor cantidad en los canales principales de los esteros, luego la arcilla y en menor proporción el limo, presentándose variaciones entre las épocas seca y lluviosa. Composición química de los sedimentos: El magnesio (Mg), calcio (Ca), potasio (K) son los elementos químicos dominantes en el sedimento de los canales y en los sectores Bocón, Brujo, isla Coco y sitios de control, mientras que el manganeso (Mn), hierro (Fe) y aluminio (Al), fósforo (P), zinc (Zn) y cobre (Cu) se cuantificaron en cantidades menores.
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