STUDY OF MORPHODYNAMIC AND SEDIMENTOLOGICAL CHANGES IN THE OUALIDIA LAGOON (MOROCCO) USING BATHYMETRIC DATA: FIRST INVESTIGATIONS AFTER THE SEDIMENT TRAP DREDGING

Coastal lagoons are highly dynamic and physically complicated systems. They are environmentally productive and socio-economically valuable. Contemporary global development and management pressures require a better understanding of their dynamics and sustainability. The present study focuses on the problem of water confinement in the Oualidia lagoon (Atlantic coast of Morocco). This lagoon is characterized by an asymmetric tidal propagation, with a shorter duration of the flood (rising tide) than the ebb (falling tide). In the long term, this contributes to the reduction of depths and the confinement of water upstream. After extensive studies, a sediment trap was created in 2011 to trap the finest sediment in the upstream part of the lagoon. This study aims to analyze the morphodynamical and sedimentological changes in the lagoon of Oualidia, after the sediment trap dredging. For this purpose, bathymetric surveys covering 6 years between 2006 and 2012 were analyzed, providing sufficient data to identify the morphological changes that the lagoon has undergone during this period. The data analysis was followed by a study of the lagoon bed dynamics using profile lines extracted from the bathymetric data in a GIS environment. As a result, the findings partly show that over 6 years, an average height of +0.65 m was gained by the lagoon, while the average change in the eroded areas was estimated to be −0.42 m. In addition, the eroded area in the lagoon was estimated to be about 1,513,800 m2 with an erosion volume of 633,383 m3, while the accumulated area found was about 2,699,396 m2 with an accumulation volume of 1,765,866 m3. These changes can be related to the large input of marine sediment, mainly caused by tidal currents and waves, but also to the creation of a sediment trap in the upstream area of the lagoon.

Evidence of Coastal Changes in the West Coast of Naxos Island, Cyclades, Greece

Coastal lagoons and dunes are of great ecological importance, not only for providing habitats for rare flora and fauna, but also for protecting coastal areas from rising sea levels and storms, etc. Even though these features are unique for sustainable development and are extremely important to the natural environment and economy, they are one of the most vulnerable and threatened ecosystems due to their small size. The western coasts of Naxos (central Aegean Sea) represent an example of such phenomena, with various wetlands bordered by low-lying sand dunes. The aim of our study is to assess the vulnerability and pressures of the western coasts of Naxos Isl. by taking into consideration both natural and anthropogenic factors. We used a variety of approaches for the environmental and geomorphological study of the coastal zone to achieve this goal, including: (a) study of aerial photography from 1960 until today, (b) systematic seasonal aerial monitoring by drone, since 2015, (c) use of DGPS for the mapping of the coastal zone and for obtaining detailed topographic sections, and (d) dune mapping. Our findings reveal the vulnerability of the western coasts of Naxos due to sea level rise, tectonic subsidence, and a variety of anthropogenic pressures, such as uncontrolled parking, vegetation uprooting, sand extractions, and heavy touristic traffic. Therefore, the protection of the sand dunes, their restoration where they have been eroded, the definition of a zone of control, and restriction of human activities must be a key priority.

Aspectos de la dinámica poblacional del ostión Crassostrea virginica en tres lagunas costeras de Tabasco, México / Aspects of the population dynamics of the oyster Crassostrea virginica in three coastal lagoons of Tabasco, Mexico

Se realizó una caracterización poblacional del ostión americano Crassostrea virginica existente en tres lagunas costeras de Tabasco. Para esto se determinó la biometría, la estructura poblacional, los parámetros de crecimiento poblacional y el rendimiento durante un ciclo anual. Mensualmente se recolectaron al azar 200 organismos determinándose: frecuencia de tallas, pesos, tasas de crecimiento, expresadas L∞ y K y el rendimiento en carne. Las longitudes totales oscilaron entre 7 y 66 mm, entre 7 y 80 mm y entre 27 y 90 mm, para El Carmen, Machona y Mecoacán, respectivamente, en la mayoría de los meses los organismos fueron mayores a 65 mm, sin embargo, en la laguna El Carmen no alcanzaban la talla de captura permisible para la especie que es de 70 mm. Con relación al peso húmedo total, los organismos de mayor peso, se presentaron en la laguna Mecoacán. El rendimiento en carne mensual presentó su valor mínimo en Enero (51.98 ± 5.23 %) en la laguna de El Carmen y un valor máximo en Mayo (106.29 ± 19.32 %) para la laguna Mecoacán, detectándose diferencias significativas mensuales (Fs = 79.31; P 0.001).

Metales pesados en agua y sedimentos de dos lagunas costeras de Tabasco, México / Heavy metals in water and sediments of two coastal lagoons of Tabasco, Mexico

constituyen un peligro para la biota acuática y el ser humano y un factor de deterioro de la calidad hídrica y ambiental. Los sedimentos ribereños, uno de los principales reservorios de estos elementos, actúan como fuentes secundarias de contaminación de los cuerpos de agua, por lo que resulta importante evaluar la concentración de metales como una herramienta que permita rastrear el origen de los contaminantes en el medio y predecir los impactos que pueden producir en los ecosistemas acuáticos. En el presente trabajo se analizó por cuadruplicado por espectrofotometría de absorción atómica la concentración de Cd, Cr, Cu, Pb y Zn en agua superficial y en sedimentos de dos lagunas costeras durante un año. Las muestras fueron colectadas mensualmente, a partir de tres estaciones de muestreo por cada una de las lagunas, El Carmen y Machona. Las concentraciones detectadas de Cd, Cr, Cu, Pb y Zn fueron mínimas y no sobrepasaron los LMP en la norma mexicana; en ninguna de las dos lagunas estudiadas.Se encontró que se presentan niveles significativamente superiores (p 0.05) de metales pesados en sedimento comparado con los observados en agua. El contenido de Cd y Cr en sedimentos no presentó diferencias significativas (p0.05) entre los bancos ostrícolas de ambas lagunas. La distribución de metales no fue homogénea, ni presentó un patrón geográfico marcadamente definido, pudiéndose encontrar altos niveles distribuidos a lo largo de los bancos estudiados.

On the Understanding of the Hydrodynamics and the Causes of Saltwater Intrusion on Lagoon Tidal Springs

Springs are common features on the Yucatán coast. They can discharge either under the sea (submarine) or inland in coastal lagoons and wetlands. Previous observations of a coastal lagoon located on the northern Yucatán Peninsula (La Carbonera) reported sea water intrusion on a spring that discharge on a coastal lagoon (lagoon tidal spring). The saltwater intrusion occurs when the tide is at its lower level, which is the opposite to what has been reported for submarine springs in the Yucatán Peninsula. In this study, the hydrodynamics of the spring is analyzed and the driving forces controlling the seawater intrusion are identified and discussed. Time series of water levels, salinity, and velocity measurements in the lagoon, the aquifer, and the spring are analyzed by means of tide component decomposition and cross-correlations analysis of the tide signals. Results show that the main driving forces causing the intrusion are the density differences and pressure head gradients, and the mechanisms influencing the driving forces driving those differences are the tides, the friction in the lagoon, and the confinement of the aquifer; other mechanisms are discussed to present a complete idea of the complexity of the interactions between the coastal aquifer, the coastal lagoons, and the sea.

Mercury and selenium concentrations in the oysters Saccostrea palmula and Crassostrea corteziensis from four coastal lagoons of NW Mexico: health risk assessment

Mercury (Hg) and selenium (Se) concentrations in the soft tissue of the oysters Saccostrea palmula and Crassostrea corteziensis from four coastal lagoons (Altata, Macapule, Navachiste, El Colorado) of Northwest Mexico were determined. A total of 2520 specimens of S. palmula and 3780 of C. corteziensis (72.15 ± 4.95 and 73.57 ± 5.31 mm, respectively) were sampled seasonally from summer 2019 to spring 2020. The higher Hg concentration (wet weight) for S. palmula (0.13 ± 0.03–0.11 ± 0.02 µg g−1) and C. corteziensis (0.12 ± 0.02–0.11 ± 0.03 µg g−1) were obtained in summer−autumn 2019; Hg level was similar among the coastal lagoons and did not exceed the limit established by Mexican legislation and by the Food and Drug Agency. The higher Se concentration occurred in spring 2020 for both oyster species in El Colorado lagoon (4.55 ± 0.02 and 4.08 ± 0.05 µg g−1, respectively). The Se/Hg molar ratio of S. palmula and C. corteziensis ranged from 48.76–149.59 and 31.72–155.37, respectively; while the Se health benefit value was 19.23–42.28 and 17.82–35.30, respectively. The hazard quotient for Hg -estimated as methylmercury- and Se in both species of oyster was below 1. The high molar ratio obtained (Se/Hg > 1) indicates that the concentration of Se was sufficient to neutralize the possible toxicity of the Hg, therefore, the consumption of S. palmula and C. corteziensis from the four coastal lagoons studied does not represent a risk.