Search

Due to pronounced hydroclimatic fluctuations and rapid urban growth, coastal lagoonal systems in the Southern Mediterranean semi-arid areas undergo pronounced shoreline retreats and increased coastal floodings. The socioeconomic impacts of these alarming coastal changes remain poorly uncharacterized. To address this deficiency, we calculate the Socioeconomic Vulnerability Index (SVI) based on the decadal changes in shoreline retreat and land use occupation considering the study case of the coastal lagoon of Ghar El Melh in Tunisia which is representative of several lagoonal systems in the southern Mediterranean basin. We first monitor the diachronic shoreline changes from 1882 to 2016 using aerial and orbital photogrammetric scenes combined with topographic and bathymetric maps. Using the Digital Shoreline Analysis System (DSAS) and historic bathymetric records, we then measure the changes in shoreline position from 1882 to 2016. Secondly, the above is correlated with the Land Use/Land Cover (LULC) assessment using a supervised classification algorithm from multiple Landsat 5-8 orbital photogrammetric scenes. We assess the SVI using five socioeconomic parameters: (1) population density, (2) land use change, (3) road network, and (4) settlements. Our results reveal the occurrence of severe coastal erosion with a maximum Net Shoreline Movement of −1580 m (±6 m) between 1882 and 2016. Moreover, the changes in land use from 1985 to 2021 reveal the extension in urban growth around the lagoon, rising from 5.4 % to 13.5 %. Whereas the vegetation coverage is decreasing by 9 % over the same period. The resulting SVI map reveals that 85 % of the lagoon area is characterized by high to very high vulnerability. This high coastal vulnerability is found to be primarily due to the extensive change in land occupation over the last few decades and is aggravated by the increase in hydroclimatic extremes.

Background. The Gulf of Gabès supports an important fishery which is characterized by a great diversity of fish species. Despite this importance, there has been no attempt to examine the whole larval fish assemblage in this area. The aim of this study was to investigate the larval fish communities during summer and winter seasons in the Gulf of Gabès. Materials and methods. Two multidisciplinary surveys were carried out aboard the R/V Hannibal in 2009. A total of 80 and 70 stations were sampled during winter and summer surveys, respectively. Stations were arranged in a regular sampling grid of 10 × 10 nautical miles. Ichthyoplankton was sampled by oblique tows with a Bongo net of 60 cm mouth diameter, fitted with 335 µm mesh nets. Zooplankton dry weight was obtained by drying aliquots of the zooplankton sample in an oven at 60ºC, for 72 hours. Results. In summer 34 taxa representing 24 families, were collected, whereas in winter 35 taxa, representing 16 families, were collected. Larval fish communities were dominated by small pelagic species: sardinella and anchovy in summer and sardine in winter. The results also highlight the Gulf of Gabès as an important spawning ground for many large and medium pelagic fishes, both highly migratory (Thunnus thynnus) and resident species (Auxis rochei, Euthynnus alletteratus). The summer/winter differences observed in species composition and abundance of the larval fish communities reflected the seasonality in fish species spawning in the Mediterranean Sea, likely influenced by the highly contrasting environmental conditions between seasons. Thus, the seasonal changes in the hydrographic conditions of the Gulf of Gabès result in different scenarios that provide suitable spawning environments for a variety of fishes, allowing them to share the Gulf as spawning habitat. Conclusion. The Gulf of Gabès is an important fish spawning area for demersal, small pelagic, medium pelagic, and large pelagic (tunas). Larval species composition differed considerably between summer and winter surveys.

Archaean hydrothermal chert veins commonly contain abundant organic carbon of uncertain origin (abiotic vs. biotic). In this study, we analysed kerogen contained in a hydrothermal chert vein from the ca. 3.5 Ga Dresser Formation (Pilbara Craton, Western Australia). Catalytic hydropyrolysis (HyPy) of this kerogen yielded n-alkanes up to n-C22, with a sharp decrease in abundance beyond n-C18. This distribution ( ≤ n-C18) is very similar to that observed in HyPy products of recent bacterial biomass, which was used as reference material, whereas it differs markedly from the unimodal distribution of abiotic compounds experimentally formed via Fischer–Tropsch-type synthesis. We therefore propose that the organic matter in the Archaean chert veins has a primarily microbial origin. The microbially derived organic matter accumulated in anoxic aquatic (surface and/or subsurface) environments and was then assimilated, redistributed and sequestered by the hydrothermal fluids (hydrothermal pump hypothesis).

Background. The red mullet, Mullus barbatus L., is abundantly captured and targeted throughout the year in Tunisian waters but not all aspects of its biology have been studied well. One of them—the feeding behaviour of this fish may be important for stock assessment, ecosystem modelling, and understanding mechanism and processes which structure and influence the fish assemblages. The aim of this paper is to describe the diet of specimens from the northern coast of Tunisia, to analyse seasonal variations in its composition and to delineate feeding intensity in relation to sex and size. Materials and methods. Red mullet were sampled from different landing sites along the northern Tunisian coast and the Gulf of Tunis. Random samples were taken from both commercial and demersal trawlers from January to December 2005. A total of 472 specimens were examined following the commonly accepted procedures. The following parameters were analysed: vacuity index, percentage frequency of occurrence, percentage numerical abundance, percentage gravimetric composition, and index of relative importance. The trophic level for any consumer species was calculated, for total sample, but also in related to size, sex, and season. Results. Mullus barbatus mainly fed on crustaceans and polychaetes. According to the classification of fishes in functional groups based on their TROPH red mullet is an omnivorous feeder, animal species being preferential preys. Basing on data assembled by Stergiou and Karpouzi for the Mediterranean ichthyofauna, the estimated trophic levels for the red mullet ranged between 2.79 and 3.57 units. Such within-species differences in trophic levels might reflect the combined effect of the following factors: area, year, length structure, and differential fishing pressure between years and/or areas, because fishing removes the largest individuals of a species, which generally have higher trophic levels. Conclusion. The red mullet is not only omnivorous, but also opportunistic feeder with trophic level related to body size, season, geographic area and fishing pressure.