Wrack Removal as Short-Term Disturbance for <Emphasis Type="Italic">Talitrus Saltator</Emphasis> Density in the Supratidal Zone of Sandy Beaches: an Experimental Approach

A distinctive feature of sandy beaches is the stranded wrack, which represents a key element in the maintenance of their biodiversity and ecosystem function. However, these materials are commonly removed from beaches worldwide. In October–November 2012, a field experiment following an M-BACI design was conducted to assess the effect of wrack removal on sandhopper populations on two sandy beaches on the Atlantic southwest coast of Spain. The beaches, Levante (36°33′N, 6°13′W) and Cortadura (36°28′N, 6°15′W) differed in their morphodynamics (dissipative and intermediate, respectively) and in the dominant wrack sources (seagrasses vs. macroalgae). Wrack removal diminished the density of the sandhopper, Talitrus saltator, a dominant species in the supratidal zone especially under wrack, but the difference between impacted and control plots was short-lived (several days). Juvenile sandhoppers appeared more sensitive to this disturbance than adults, although this was found only at Levante. This study highlights the adaptability of this sandhopper to a single wrack removal event and shows that the magnitude of the effect is likely dependent on the particular characteristics of individual beaches. The consequences of frequent wrack removal from these beaches on sandhopper populations remain to be determined.     

Les rhyolites hyperalcalines (pantellérites) du lac Tchad. Composition et signification tectonomagmatiqueThe peralkaline rhyolites (pantellerites) of the Lake Chad. Composition and tectonomagmatic significance

Remnants of rhyolite lava-dome and of alkaline microgranite extrusions emerge, as inselbergs, in the alluvial plain, south of Lake Chad. The peralkaline, arfvedsonite- and acmite-bearing rhyolites are determined as pantellerites. They can be related to a tectonomagmatic stage, dated of the Late Cretaceous, of the western central Africa rift system. They may constitute the NNE extension of a N50° to 30° elongated succession of small alkaline complexes, from the Gulf of Guinea to the North-Cameroon, which initiated the magmatic activity of the Cameroon Line. To cite this article: J.-P. Vicat et al., C. R. Geoscience 334 (2002) 885–891.     

Influence of a Drought Event on Hydrological Characteristics of a Small Estuary on the Amazon Mangrove Coast

The effects of atypical climatic conditions, such as those of a drought event, are remarkably accentuated in minor estuaries filled with sediments and with reduced or sporadic freshwater input, where the salinity intrusion is pronounced. To understand these effects, hydrological and hydrodynamic parameters were evaluated during a drought period in a small estuary located on the eastern Amazon coast in northern Brazil. Five campaigns were conducted between June 2012 and June 2013. Samples were collected from the surface and bottom layer every 3 h over a 25-h period at three stations of the Taperaçu Estuary. To compare drought and post-drought periods, in terms of salinity and chlorophyll-a, data was recorded until June 2015. Taperaçu is a relatively shallow estuary of the Amazon coastal zone, which is characterized by the absence of any direct fluvial discharge, although it does receive freshwater input from adjacent wetlands, as well as less saline waters from the Caeté Estuary through the Taici Creek. Hydrological variables were controlled by rainfall levels and the tidal range, and the water became more saline and more oxygenated, with reduced dissolved nutrient and chlorophyll-a concentrations when precipitation decreased. A significant variation was found between the months of June 2012 (most intense drought) and 2013 (less intense drought). The connection to the neighboring Caeté Estuary and adjacent mangroves and wetlands contributed to the influx of nutrient-enriched waters. Overall, then, it is hoped that the results of this study can contribute to the understanding of the effects of drought events in other minor estuaries on the highly indented Amazon coast, as well as in other areas of the equatorial zone with similar environmental characteristics.     

Assessment of the geometry and volumes of small surface water reservoirs by remote sensing in a semi-arid region with high reservoir density

Water fluxes in highly impounded regions are heavily dependent on reservoir properties. However, for large and remote areas, this information is often unavailable. In this study, the geometry and volume of small surface reservoirs in the semi-arid region of Brazil were estimated using terrain and shape attributes extracted by remote sensing. Regression models and data classification were used to predict the volumes, at different water stages, of 312 reservoirs for which topographic information is available. The power function used to describe the reservoir shapes tends to overestimate the volumes; therefore, a modified shape equation was proposed. Among the methods tested, four were recommended based on performance and simplicity, for which the mean absolute percentage errors varied from 24 to 39%, in contrast to the 94% error achieved with the traditional method. Despite the challenge of precisely deriving the flooded areas of reservoirs, water management in highly reservoir-dense environments should benefit from volume prediction based on remote sensing.