A case study of mollusc and polychaete soft-bottom assemblages submitted to sedimentary instability in the Mediterranean Sea     

Andrea Cosentino  & Salvatore Giacobbe 《Marine Ecology》2006,27(2):170-183

In the Strait of Messina (Sicily, Central Mediterranean Sea) strong tidal currents, very irregular hydrological regime and related solid load, and local seismic activity cause sediment instability; this area represents therefore a case‐study of a naturally disturbed soft‐bottom environment. In this paper, mollusc and polychaete assemblages of the northern part of the Strait were described. Composition, community structure, eco‐ethological features, trophic guilds and sediment‐type relationships were studied in 64 stations sampled located between 3.5 and 50 m depth in October 1992 by means of a Van Veen grab. A total of 131 species (65 molluscs and 66 polychaetes) were identified. A dense population of the tubicolous polychaete Ditrupa arietina was recognized, together with the occurrence of other species tied to a high sedimentation rate, such as Corbula gibba and Tellina distorta, as well as widespread, mud‐tolerant species (e.g.Chone spp., Hyalinoecia tubicola). Diversity (H′) showed a peak at intermediate depths (10/20 m) and a clear decrease beyond this depth, corresponding to the Ditrupa core population. Multivariate comparison between sediment features and community composition throughout the bathymetric gradient showed a narrow ecocline between two environments subjected to opposite hydrodynamic constraints. In the shallower zone, a wide typology of trophic‐ethological guilds was related to community patchiness, in contrast to a greater functional uniformity of the deeper assemblage, dominated by sessile, semi‐infaunal suspension feeders. A possible role played by a phase of increased rainfall to increase bottom instability, locally emphasized by a previous human activity, is also discussed.  相似文献   

Analysis, synthesis and modelling of high-resolution observations of salt-marsh eco-geomorphological patterns in the Venice lagoon   总被引：1，自引：0，他引：1  

Marco Marani  Enrica Belluco  Sergio Ferrari  Sonia Silvestri  Andrea D'Alpaos  Stefano Lanzoni  Alessandra Feola  Andrea Rinaldo   《Estuarine, Coastal and Shelf Science》2006,69(3-4):414

The present paper describes observations, analyses and models of salt-marsh channel network and vegetation patterns with the aim of contributing to the development of predictive models of ecological and morphological co-evolution. Existing and new observations are described, with particular emphasis on remote sensing and ancillary field surveys, which are shown to allow reliable, accurate and repeatable quantitative characterizations of landform and vegetation properties over the spatial scales of interest. The observed channel network morphological characters are then used as the basis and validation of models describing the emergence of channel network and vegetation spatial patterns. In particular, with reference to observations performed in the Venice Lagoon, the note describes: (i) new, 2-cm resolution, characterizations of channel network geometry obtained from “proximal sensing” photographic observations; (ii) the reliable quantitative maps of salt-marsh vegetation which may be retrieved from hyperspectral remote sensing data and field ancillary observations; (iii) a synthesis of recent and new analyses of the statistical properties of vegetation and landform spatial organization, that may be inferred from the maps so derived; (iv) recent and new conceptual and quantitative ecological and geomorphic models developed and validated by remote-sensing and field observations. A coherent observational and theoretical eco-morphodynamic framework is then proposed.  相似文献   

Gas hydrate disturbance fabrics of southern Hydrate Ridge sediments (ODP Leg 204): Relationship with texture and physical properties   总被引：1，自引：0，他引：1  

Elena Piñero  Eulàlia Gràcia  Francisca Martínez-Ruiz  Juan Cruz Larrasoaña  Alexis Vizcaino  Gemma Ercilla 《Geo-Marine Letters》2007,27(2-4):279-288

Soupy and mousse-like fabrics are disturbance sedimentary features that result from the dissociation of gas hydrate, a process that releases water. During the core retrieval process, soupy and mousse-like fabrics are produced in the gas hydrate-bearing sediments due to changes in pressure and temperature conditions. Therefore, the identification of soupy and mousse-like fabrics can be used as a proxy for the presence of gas hydrate in addition to other evidence, such as pore water freshening or anomalously cool temperature. We present here grain-size results, mineralogical composition and magnetic susceptibility data of soupy and mousse-like samples from the southern Hydrate Ridge (Cascadia accretionary complex) acquired during Leg 204 of the Ocean Drilling Program. In order to study the relationship between sedimentary texture and the presence of gas hydrates, we have compared these results with the main textural and compositional data available from the same area. Most of the disturbed analyzed samples from the summit and the western flank of southern Hydrate Ridge show a mean grain size coarser than the average mean grain size of the hemipelagic samples from the same area. The depositional features of the sediments are not recognised due to disturbance. However, their granulometric statistical parameters and distribution curves, and magnetic susceptibility logs indicate that they correspond to a turbidite facies. These results suggest that gas hydrates in the southern Hydrate Ridge could form preferentially in coarser grain-size layers that could act as conduits feeding gas from below the BSR. Two samples from the uppermost metres near the seafloor at the summit of the southern Hydrate Ridge show a finer mean grain-size value than the average of hemipelagic samples. They were located where the highest amount of gas hydrates was detected, suggesting that in this area the availability of methane gas was high enough to generate gas hydrates, even within low-permeability layers. The mineralogical composition of the soupy and mousse-like sediments does not show any specific characteristic with respect to the other samples from the southern Hydrate Ridge.  相似文献   

In Situ Measurement of Pinna nobilis Shells for Age and Growth Studies:
A New Device     

Jos Rafael García‐March  Antonio Manuel García‐Carrascosa  lvaro Luís Pea 《Marine Ecology》2002,23(3):207-217

Abstract. Pinna nobilis Linnaeus 1758 is an endemic bivalve mollusc in the Mediterranean Sea, where it inhabits seagrass meadows, especially Posidonia oceanica (L.) Delile. It is the largest bivalve in the Mediterranean, reaching lengths up to 120 cm. In its natural habitat, P. nobilis lives with the anterior part of the valve buried in the seabed, attached to Posidonia rhizomes by byssus threads.
This habit makes it impossible to measure its total length directly in situ. As the only way to determine the individual age is the relationship between age and total length, several equations have been proposed to estimate total length by relating it to the unburied parts of the shell. Such measurements are essential to ecological studies that consider age, growth, and population dynamics, and that evaluate the environmental factors that affect this species.
Accurately estimating total length depends on the accuracy and precision of the method employed to measure the unburied shell parts. In this paper, we point out the lack of precision of the instruments and methods used until now; we also demonstrate the reason for this imprecision. A new device to measure unburied parts of Pinna nobilis with a precision comparable to that obtained when measuring extracted valves is described. This device is unaffected by substratum type and reduces measurement time. The latter is a very important feature, because these procedures are usually performed whilst SCUBA diving. Finally, a growth equation has been fitted to the measurements obtained with the new device from a population located in Moraira (Alicante, western Mediterranean).  相似文献   

Microphytobenthos vertical migratory photoresponse as characterised by light-response curves of surface biomass     

Joo Serdio  Helena Coelho  Snia Vieira  Snia Cruz 《Estuarine, Coastal and Shelf Science》2006,68(3-4):547

The migratory response of intertidal microphytobenthos to changes in irradiance was studied on undisturbed estuarine sediments. Two non-destructive optical techniques were used to trace variations in vivo of surface biomass: PAM fluorometry, for measuring the minimum fluorescence level (F_o); and spectral reflectance analysis, for quantifying the normalized difference vegetation index (NDVI). Following the formation of a dense biofilm at the surface, replicated sediment samples were simultaneously exposed to six different irradiance levels, ranging from 50 to 1500 μmol m⁻² s⁻¹, during a period of 120 min. The migratory photoresponse of the biofilms was characterised by constructing biomass vs. light curves (BLC), relating the accumulation of microalgal biomass after that period (estimated by F_o or NDVI) to the irradiance level incident on the surface. BLCs allow characterising the main features of the migratory photoresponse of intact biofilms. Typical BLC showed a clear biphasic pattern, with an increase in microalgal accumulation under irradiances below 100 μmol m⁻² s⁻¹, maximum values under 100–250 μmol m⁻² s⁻¹, and a gradual decrease of surface biomass under higher irradiances, indicating a strong photophobic downward migratory response. Similar BLC patterns were obtained when measuring F_o or NDVI. The construction of BLCs for biofilms from intertidal sites with distinctive sediment characteristics and diatom taxonomic composition allowed to detected significant differences in the migratory photoresponse. Biofilms from a muddy sediment exhibited considerably larger amplitude in the migratory photoresponse than the biofilms from a sandy mud site, especially under high irradiances. The photophobic migratory response to high light was found to vary among diatom species, particularly in the case of the biofilms from the muddy sediments.  相似文献   

Numerical modeling of nonlinear water waves over heterogeneous porous beds   总被引：1，自引：0，他引：1  

Eric C. Cruz  Qin Chen   《Ocean Engineering》2007,34(8-9):1303-1321

The transformation of nonlinear water waves over porous beds is studied by applying a numerical model based on Chen's [2006. Fully nonlinear Boussinesq-type equations for waves and currents over porous beds. Journal of Engineering Mechanics, 132:2, 220–230] Boussinesq-type equations for highly nonlinear waves on permeable beds. The numerical model uses a high-order time-marching solution and fourth-order finite-difference schemes for discretization of first-order spatial derivatives to obtain a computational accuracy consistent with the model equations. By forcing the wave celerity and spatial porous-damping rate of the linearized model to match the exact linear theory for horizontal porous bed over a prescribed range of relative depths, the values of the model parameters are optimally determined. Numerical simulations of the damped wave propagation over finite-thickness porous layer demonstrate the accuracy of both the numerical model and governing equations, which have been shown by prior theoretical analyses to be accurate for both nominal and thick porous layers. These simulations also elucidate on the significance of the higher-order porous-damping terms and the influence of the hydraulic parameters. Application of the model to the simulation of the wave field around a laboratory-scale submerged porous mound provides a measure of its capability, as well as useful insight into the scaling of the porous-resistance coefficients. For application to heterogeneous porous beds, the assumption of weak spatial variation of the porous resistance is examined using truncated forms of the governing equations. The results indicate that the complete set of Boussinesq-type equations is applicable to porous beds of nonhomogeneous makeup.  相似文献   

Preboreal climatic oscillations recorded by pollen and foraminifera in the southern Adriatic Sea     

Sonia Favaretto  Alessandra Asioli  Antonella Miola  Andrea Piva 《Quaternary International》2008,190(1):89

Core SA03-1 from the southern Adriatic Sea (EC-Eurostrataform project) provides new information about climate changes and palaeocirculation in the Adriatic region during the last deglaciation. The results of an integrated study based on pollen and foraminifera records of the part of the core spanning the late Pleistocene–early Holocene transition (including the late Younger Dryas, the Preboreal and the beginning of the Boreal) are presented. The major vegetation changes and the short-term oscillations occurred during the early Holocene warming in the southern Adriatic basin on the basis of a high-resolution pollen record are documented. Vegetation changes are correlated to short-term oscillations detected in the foraminifera record during the same interval. The two independent terrestrial and marine proxies indicate at least three short-term cold and dry oscillations occurring at 11.2–11, 10.8–10.4 and 10 cal ka BP, according to the age–depth model adopted in this study. Finally, adopting an event-stratigraphy approach, the comparison of these results with two western Mediterranean records of Preboreal short-term oscillations suggest the occurrence of synchronous bio-events in the Mediterranean basin.  相似文献   

Avian and mammalian bone taphonomy in southern continental Patagonia: A comparative approach   总被引：1，自引：0，他引：1  

Isabel Cruz   《Quaternary International》2008,180(1):30

In southern Patagonia, the occurrence of large seabird nesting colonies condition the features of the regional bone record, which is characterized by a large amount of bird remains, despite their small body size. Bird and mammal bones have remarkably different taphonomic histories in this region: mammal remains are less damaged by carnivores and resist weathering for long periods, whereas bird bones show a greater initial destruction by carnivore activities and faster deterioration by weathering. A comparison of these results with results from research conducted in several African parks (particularly Amboseli) shows that differences in both records are mainly due to differences in community composition and predator–prey dynamics in both ecosystems. Furthermore, while in both ecosystems bird bones weather more rapidly than mammal bones, destruction by this process is more rapid in Patagonia. Overall, the analysis reinforces the need to generate specific models to understand taphonomic histories of archaeofaunal assemblages from Patagonia.  相似文献   

Palaeolimnological studies of the eutrophication of volcanic Lake Albano (Central Italy)     

Andrea Lami  Frank Niessen  Piero Guilizzoni  Julieta Masaferro  Claudio A. Belis 《Journal of Paleolimnology》1994,10(3):181-197

We use palaeolimnological techniques to reconstruct the eutrophication history of a volcanic lake (Lake Albano, central Italy) over the past three centuries. The presence of annual varves down to the bottom of the core (c. 1700 A.D.) indicated the lack of bioturbation and likely long-term meromixis. Sedimentation rates were estimated by varve counts (calcite/diatom couplets), indicating a mean rate of 0.15 cm yr^–1. The reconstruction of eutrophication was traced using past populations of algal and photosynthetic bacteria (through their fossil pigment), and geochemistry, as well as fossil remains of chironomids. Phaeophorbidea and the red carotenoid astaxanthin were used to detect past zooplankton development.The first sign of trophic change related to human activities is datedc. 1870 A.D. From that period onward a sharp increase of authigenic CaCO₃, nitrogen, N:P ratio, and dinoxanthin, a characteristic carotenoid of Chrysophyceae and Dinophyceae, is observed.Chironomid analyses showed the near absence of a deep water fauna throughout the core length. The populations of chironomid larvae are restricted to oxygenated littoral zones. In fact, the few fossil remains found are primarily of littoral origin, representing shallow water midges that were transported to profundal waters. The reduction of total chironomid in the uppermost layers of the core is to be related to human land uses.  相似文献   

The role of flares, CMEs and CME shocks in the generation of solar energetic proton events   总被引：1，自引：0，他引：1  

R. Pérez Enríquez  B. Mendoza 《Solar physics》1995,160(2):353-362

We examined solar energetic proton (SEP) events associated with intense H flares. We located these flares on the solar disk and obtained their distribution in heliographic longitude as well as their angular distance distribution with respect to the neutral lines corresponding to the heliospheric current sheet at 2.5R. We found that the SEP-associated H flares tend to occur in active regions at the feet of those helmet streamers which form the heliomagnetic equator and are related to coronal mass ejections (CMEs) and CME shocks. We discuss the possible role of flares, CMEs and CME shocks in generating SEPs.  相似文献