Late Quaternary biotic and abiotic controls on long-term sediment flux in a northern Australian tropical river system     

Annegret Larsen  Jan-Hendrik May  Xavier Carah 《地球表面变化过程与地形》2019,44(12):2494-2509

The modern distribution of monsoonal rainforest in the Australian tropics is patchy and is mainly associated with river corridors and groundwater springs, which indicates a strong dependence on hydrologic and geomorphic conditions. While their present distribution is well known, very little data exists on past spatial and temporal dynamics of these ecosystems, or their medium- to longer-term controls. Factors such as (i) fire frequency and type, and/or (ii) hydroclimatic conditions (e.g. droughts) have been proposed to control riverine corridor rainforest extent. Recent observations, however, also suggest an additional (iii) geomorphic control induced by alluvial knickpoint migration. Sediment sequences provide valuable archives for the reconstruction of longer-term (a) floodplain sedimentary dynamics, (b) local vegetation history, and (c) catchment-wide fire histories. This study investigates such a sediment sequence at Wangi Creek, and shows that a phase of aggradation, lasting ~4000 years, was recently disrupted by channel incision and floodplain erosion. The aggradational phase is characterized by sand deposition with average vertical floodplain accretion rates of 0.8 cm/yr and includes phases of soil development. The recent incisional phase has changed hydro-geomorphic conditions and caused widespread degradation of vegetation, erosion and lowering of the macro-channel surface. While there is no evidence in our data for an erosional event of similar magnitude since the onset of late Holocene floodplain aggradation, Wangi Creek experienced significant erosion and incision immediately before ~4000 years, providing the first evidence for a tropical cut-and-fill river system. We hence argue that phases of aggradation mainly controlled by biotic processes alternate and depend on feedbacks with incision phases controlled mainly by abiotic processes. The results show that eco-hydro-geomorphic feedbacks may play a crucial role in the medium- to longer-term history of tropical fluvial systems and need to be considered when interpreting fluvial archives with regards to climate, fire or human induced change. © 2019 John Wiley & Sons, Ltd.  相似文献   

Low densities of sea urchins influence the structure of algal assemblages in the western Mediterranean     

Cruz Palacín  Gonzalo Giribet  Susanna Carner  Luis Dantart  Xavier Turon 《Journal of Sea Research》1998,39(3-4)

Numerous studies of interactions between urchins and algae in temperate areas have shown an important structuring effect of sea urchin populations. These studies focused almost wholly on the effect of high urchin densities on laminarian forests. In contrast, algal communities below 5–6 m depth in the northwestern Mediterranean are characterised by low sea urchin densities (<5 ind m⁻²) and the absence of laminarian forests. No previous research has addressed sea urchin/algal interactions in this type of community. To determine the effect of the most abundant echinoid species, Paracentrotus lividus, on well-established algal communities in this area, we performed a removal–reintroduction experiment in rocky patches located between 13 and 16 m depth in the northwestern Mediterranean, where sea urchin densities ranged between 0.9 and 3.4 ind m⁻². After 6 months, the cover of non-crustose algae was significantly higher in the plots from which sea urchins had been removed than in control plots (84 vs 67% cover). These removal plots reverted to their original state upon reintroduction of sea urchins. The non-crustose algae consisted of turfing and frondose forms, with the former representing some 70% of the non-crustose algal cover. Change in the cover of turfing algae was responsible for the significant increase in algal development in the sea urchin removal plots. The response of frondose algae to the treatment varied between algal species. It is concluded that grazing by P. lividus exerts a significant effect on habitat structure, even in communities with low sea urchin densities, such as those found in vast areas of the Mediterranean sublittoral.  相似文献   

Modelling of drainage dynamics influence on sediment routing system in a fold‐and‐thrust belt     

Marc Viaplana‐Muzas  Julien Babault  Stphane Dominguez  Jean Van Den Driessche  Xavier Legrand 《Basin Research》2019,31(2):290-310

Drainage networks link erosional landscapes and sedimentary basins in a source‐to‐sink system, controlling the spatial and temporal distribution of sediment flux at the outlets. Variations of accumulation rates in a sedimentary basin have been classically interpreted as changes in erosion rates driven by tectonics and/or climate. We studied the interactions between deformation, rainfall rate and the intrinsic dynamics of drainage basins in an experimental fold‐and‐thrust belt subjected to erosion and sedimentation under constant rainfall and shortening rates. The emergence of thrust sheets at the front of a prism may divert antecedent transverse channels (perpendicular to the structural grain) leading to the formation of longitudinal reaches, later uplifted and incorporated in the prism by the ongoing deformation. In the experiments, transverse incisions appear in the external slopes of the emerging thrust sheets. Headward erosion in these transverse channels results in divide migration and capture of the uplifted longitudinal channels located in the inner parts of the prism, leading to drainage network reorganization and modification of the sediment routing system. We show that the rate of drainage reorganization increases with the rainfall rate. It also increases in a nonlinear way with the rate of uplift. We explain this behaviour by an exponent > 1 on the slope variable in the framework of the stream power erosion model. Our results confirm the view that early longitudinal‐dominated networks are progressively replaced by transverse‐dominated rivers during mountain building. We show that drainage network dynamics modulate the distribution of sedimentary fluxes at the outlets of experimental wedges. We propose that under constant shortening and rainfall rates the drainage network reorganization can also modulate the composition and the spatial distribution of clastic fluxes in foreland basins.  相似文献   

The Manila clam population in Arcachon Bay (SW France): Can it be kept sustainable?     

Cécile Dang  Xavier de Montaudouin  Mériame Gam  Christian Paroissin  Noëlle Bru  Nathalie Caill-Milly 《Journal of Sea Research》2010,63(2):108-118

The venerid clam Ruditapes philippinarum is the most prominent suspension-feeding bivalve inhabiting muddy intertidal seagrass beds in Arcachon Bay (SW France). It is exploited by fishermen, and Arcachon Bay ranks number one in France in terms of production and total biomass of this species. Previous studies revealed a decrease in the standing stock of R. philippinarum since 2003 and unbalanced length–frequency distributions with a lack of juveniles and of adults > 40 mm. Consequently, the population dynamics of this bivalve were studied at four intertidal sites and one oceanic site in Arcachon Bay. As clam size structure did not allow classical dynamics computations, field monitoring was coupled with field experiments (tagging–recapture) over two years. Monitoring of condition index and gonadal maturation stages highlighted a high variability in spawning number and intensity between sites. Recruitment events in the exploited area varied spatially but with uniformly low values. Von Bertalanffy Growth Function (VBGF) parameters (K, L_∞) were determined using Appeldoorn and ELEFAN methods. In the exploited sites in the inner lagoon, K was relatively high (mean = 0.72 yr^? 1) but L_∞ was low (mean = 41.1 mm) resulting in a moderate growth performance index (Φ′ = 2.99). Growth parameters were not correlated with immersion time and L_∞ was different between sites. Comparison of mortality coefficients (Z) between cage experiments and field monitoring suggested that fishing accounts for 65–75% of total adult mortality. Low recruitment, a low growth rate and a normal mortality rate led to low somatic production (4.1 and 8.7 g Shell-Free Dry Weight (SFDW) m^? 2 yr^? 1) and an annual P/B ratio from 0.44 to 0.92 yr^? 1. Under current conditions, the possibility of a sustainable population in Arcachon Bay will strongly depend on recruitment success and fishing management.  相似文献   

Holocene size variations in two diatom species off East Antarctica: Productivity vs environmental conditions     

Xavier Crosta   《Deep Sea Research Part I: Oceanographic Research Papers》2009,56(11):1983-1993

I here present a biometric investigation on two diatom species, Fragilariopsis kerguelensis (O’Meara) Hustedt and Fragilariopsis curta (Van Heurck) Hustedt, in Holocene samples from sediment core MD03-2601 from the Antarctic Continental Shelf off Adélie Land, East Antarctica. Apical valve length measurements of the two species are compared to their respective absolute and relative abundances as a proxy for the species productivity. Fragilariopsis kerguelensis valves were longer and more abundant during the warmer Mid-Holocene period and smaller and less abundant during the colder Late-Holocene period. Conversely, F. curta valves were smaller and less abundant during the warmer Mid-Holocene period and longer and more abundant during the colder Late-Holocene period. Mean apical valve length variations even follow centennial-to-millennial oscillations in the species abundances. Maximal valve length and minimal valve length were also larger during the warmer Mid-Holocene period and during the colder Late-Holocene period for F. keguelensis and F. curta, respectively. The observed positive size-abundance relationships are linked to the environmental conditions at the core location that stands today at the lower ecological limit for F. kerguelensis and upper ecological limit for F. curta. More favourable environmental conditions (warmer, less icy for F. kerguelensis and colder, icier for F. curta) allowed for sexual reproduction at the upper range of the sexually inducible size window and subsequent restoration of larger initial cells which, in turn, resulted in overall bigger size of the species populations, though vegetative multiplication was more frequent.  相似文献