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11.
Examination of gut contents of the northern pipefish,Syngnathus fuscus, revealed that gammarid amphipods, caprellid amphipods, isopods, and calanoid copepods were the dominant food items during the sevenmonth study period.Gammarus mucronatus, calanoid copepods, andErichsonella attenuata were the seasonally dominant prey items in the spring, summer, and fall, respectively.G. mucronatus and calanoid copepods were consumed in approximate proportion to their numerical abundance in the environment, whileE. attenuata, present in rather uniform densities throughout the study period, was extensively consumed only in the late summer and fall. An ontogenetic pattern of prey consumption was evident, in addition to the seasonal pattern. Comparison ofG. mucronatus andE. attenuata size ranges from the field and in pipefish guts revealed thatS. fuscus preyed upon the smaller size classes of each species, and that mean size of prey consumed was positively related to fish size. 相似文献
12.
Historical pollution trends in a disturbed, estuarine sedimentary environment, SW Sweden 总被引:3,自引:0,他引:3
The objective of this article is to determine whether pollution trends can be reconstructed from sediments in the outer Göta älv estuary. Detailed interpretation of heavy metal profiles in the recent sediment, in combination with profiles of 210Pb- and 137Cs-activity, organic carbon content and grain size distribution, allows estimation of historical trends. This is based on the assumption that only certain intervals, which can be identified, are disturbed. The sediment data are compared with archive data on historical trends in emissions of heavy metals. Special attention is given to mercury, which shows high enrichment over long periods of time. It is found that the most polluted sediments were deposited between 1930 and 1980, but that episodic discharges have also occurred more recently. Nevertheless, the effects of remediation since the 1970s are apparent. 相似文献
13.
Karin E. Limburg 《Estuaries and Coasts》1996,19(4):794-813
Juvenile fishes often face conditions that force them to experience fitness trade-offs (e.g., foregoing a rich food patch because of high risk of predation). In this study, three aspects of the environment of juvenile American shad: food availability, predation risk, and “thermal risk” (defined here as the probability of the onset of adverse temperatures; ≤9°C, the temperature at which feeding ceases), are evaluated empirically with data from the Hudson River estuary in New York State. The evaluations are then used in dynamic programming models to determine when juvenile American shad should switch habitat (upper versus middle versus lower estuary), and, in combination with a simple bioenergetic model, to determine growth trajectories for fish spawned at different times in the spawning season. Comparisons of simulations with real data suggest that scenarios in which predation risk is highest in the lower river produce the most realistic patterns of habitat use. High upriver food availability in June promotes use of the upriver habitat; however, by September most size classes of fish utilize the middle estuary, and by late October, fish move to the lower estuary (even in the face of higher predation risk), due to a combination of lower food resources and thermal risks in the upper and middle estuary. 相似文献
14.
Habitat selection capabilities of the recruiting larval stages of marine invertebrates are limited, in part, by their ability to maneuver in flowing water. Distributional and experimental evidence suggest that blue crab (Callinectes sapidus) megalopae may preferentially settle into vegetated habitats. However, the behavior and swimming capabilities of megalopae in flowing water have not previously been investigated. Laboratory experiments were conducted in a small, recirculating seawater flume to determine the swimming response of megalopae to varying flow velocities. Nighttime trials were conducted at six flow velocities: 0, 1.9, 3.6, 4.8, 6.3, and 9.3 cm s?1. Behavior and swimming velocities of field-collected C. sapidus megalopae were video recorded. Megalopae exhibited negative phototaxis and were found in the water column at all flows in the dark. The maximum sustained swimming speed observed was 12.6 cm s?1 and the mean swimming speed in still water was 5.0 cm s?1, with short bursts in excess of 20 cm s?1. Megalopae frequently oriented into the current and were capable of swimming upstream against the current at flow speeds <4.8 cm s?1; at greater velocities they were not able to do so. The results suggest that at low to moderate current velocities C. sapidus megalopae have the ability to actively move in search of settlement sites and to maintain their positions in desirable sites rather than relying strictly on passive movements by currents. 相似文献
15.
Jean-Franois Pastre Agns Gauthier Sbastien Nomade Paul Orth Ana Andrieu Franck Goupille Herv Guillou Stphane Kunesch Stphane Scaillet Paul Randall Renne 《Comptes Rendus Geoscience》2007,339(16):987-997
The Alleret maar (Massif Central, France) provides a long lacustrine sequence (40.6 m) attributed to the early Middle Pleistocene. Sediment, pollen and diatoms analysis of its upper part (AL2 core, 14.6 m) indicates two temperate phases marked by high lake levels, forest development and vegetation expansion. They are separated by a cold period during which lake level drops, coarse sediment input increases and steppic and xerophilous plants develop. Pollen data suggests that this sequence belongs to the upper part of the Cromerian complex. These results are in agreement with the 557 ± 3 ka (±12 ka, including all errors) 40Ar/39Ar age obtained from an interbedded tephra layer emitted by the Mont-Dore/Sancy strato-volcano and establish that this sequence probably covers the MIS 15 substages. 相似文献
16.
17.
Jochen Knies Jens Matthiessen Christoph Vogt Jan Sverre Laberg Berit O. Hjelstuen Morten Smelror Eiliv Larsen Karin Andreassen Tor Eidvin Tore O. Vorren 《Quaternary Science Reviews》2009,28(9-10):812-829
Based on a revised chronostratigraphy, and compilation of borehole data from the Barents Sea continental margin, a coherent glaciation model is proposed for the Barents Sea ice sheet over the past 3.5 million years (Ma). Three phases of ice growth are suggested: (1) The initial build-up phase, covering mountainous regions and reaching the coastline/shelf edge in the northern Barents Sea during short-term glacial intensification, is concomitant with the onset of the Northern Hemisphere Glaciation (3.6–2.4 Ma). (2) A transitional growth phase (2.4–1.0 Ma), during which the ice sheet expanded towards the southern Barents Sea and reached the northwestern Kara Sea. This is inferred from step-wise decrease of Siberian river-supplied smectite-rich sediments, likely caused by ice sheet blockade and possibly reduced sea ice formation in the Kara Sea as well as glacigenic wedge growth along the northwestern Barents Sea margin hampering entrainment and transport of sea ice sediments to the Arctic–Atlantic gateway. (3) Finally, large-scale glaciation in the Barents Sea occurred after 1 Ma with repeated advances to the shelf edge. The timing is inferred from ice grounding on the Yermak Plateau at about 0.95 Ma, and higher frequencies of gravity-driven mass movements along the western Barents Sea margin associated with expansive glacial growth. 相似文献
18.
We show that a sufficiently energetic impact can generate a melt volume which, after isostatic adjustment and differentiation, forms a spherical cap of crust with underlying depleted mantle. Depending on impact energy and initial crustal thickness, a basin may be retained or impact induced crust may be topographically elevated. Retention of a martian lowland scale impact basin at impact energies ∼3 × 1028-3 × 1029 J requires an initial crustal thickness greater than 10 km. Formation of impact induced crust with size comparable to the martian highlands requires a larger impact energy, ∼1-3 × 1030 J, and initial crustal thickness <20 km. Furthermore, we show that the boundary of impact induced crust can be elliptical due to a spatially asymmetric impact melt volume caused by an oblique impact. We suggest the term “impact megadome” for topographically elevated, impact induced crust and propose that processes involved in megadome formation may play an important role in the origin of the martian crustal dichotomy. 相似文献
19.
Neoproterozoic chemostratigraphy 总被引:3,自引:0,他引:3
Chemostratigraphy has diverse applications to investigating the rock record, such as reconstructing paleoenvironments, determining the tectonic setting of sedimentary basins, indirect dating, and establishing regional or global correlations. Chemostratigraphy is thus an integral component of many investigations of the ancient sedimentary record. In this contribution, we review the principle inorganic geochemical methods that have been applied to the Neoproterozoic sedimentary record. Analysis of the traditional stable and radiogenic isotope systems, such as δ13C, δ18O, δ34S, and 87Sr/86Sr, is routine, particularly in successions rich in carbonate. These mainstay applications have yielded invaluable data and information bearing on the chronology and evolution of this eventful era in Earth history. Alongside the growing database of traditional data, a series of novel geochemical techniques have given rise to important new models and constraints on Neoproterozoic biogeochemical change. In particular, a range of proxies for water column redox, mainly obtained from black shales, have shed light on the pace and tempo of Neoproterozoic oxygenation and its link to the appearance of early animal evolution. Increased integration of diverse geochemical, sedimentological, and paleontological datasets, and the gradual radiometric calibration of the stratigraphic record promise to bring the details of the evolution of the Neoproterozoic Earth system into ever greater focus. 相似文献
20.
Karin M. Kettenring Melissa K. McCormick Heather M. Baron Dennis F. Whigham 《Estuaries and Coasts》2010,33(1):118-126
The invasion and expansion of the introduced haplotype of Phragmites australis across North America is of growing concern. Previous studies in the Chesapeake Bay region found that Phragmites was more abundant, had higher foliar nitrogen, and produced more viable seeds in brackish wetland subestuaries with more
anthropogenic development of the watershed. Here, we focus on a different scale and address issues related to the invasion
of Phragmites within a single subestuary, the Rhode River. We evaluated patterns in seed viability, foliar nutrient concentrations, patch
size, and genetic variation in ten Phragmites patches in wetlands that occur in the side of the subestuary that is surrounded by forest and 10 patches in wetlands that
are in the side of the subestuary that has extensive anthropogenic development. Seed viability varied from 0–60% among the
20 patches but did not differ significantly between patches on the developed vs. forested sides of the Rhode River. Foliar
nutrients also did not differ between patches on the two sides of the Rhode River. Seed viability, however, was negatively
related to foliar nutrients. Most Phragmites patches consisted of >1 genotype. Larger patches had multiple genotypes, and patches with more genotypes produced more viable
seeds. Our study indicates that the Rhode River subestuary behaves as one system with no differences in the measured Phragmites variables between the forested vs. developed sides of the watershed. Our findings also suggest a cyclical process by which
Phragmites can spread: larger patches contain more genetic diversity, which increases the chances for cross-fertilization. The subsequent
increased production of viable seeds can increase local levels of genetic diversity, which can further facilitate the spread
of Phragmites by seed. 相似文献