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The Shad fisheries of the Chesapeake Bay in Maryland have been declining since an 1897 peak of 7860 × 103kg. No periods of stability have been recorded. Data are presented to trace the decline not only as a function of specific areas within the Northern Chesapeake Bay but also in terms of environmental problems and recruitment overfishing.The problem is related to improving the commercial fishing yield. An estimation of a maximum effort of 200,000 man-hours is suggested if a stable yield is to be approached. Methods for obtaining this goal include alternating of closed fishing areas, adoption of rest days, enforcement of fisheries regulations and reduction of the number of metres of gill net used per fisherman. 相似文献
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Carvalho Thayslan Brosinsky Arlena Foerster Saskia Teixeira Adunias Medeiros Pedro 《Journal of Soils and Sediments》2022,22(9):2557-2577
Journal of Soils and Sediments - Soil erosion by water yields sediment to surface reservoirs, reducing their storage capacities, changing their geometry, and degrading water quality. Sediment... 相似文献
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14C-Methylene labelled phorate was added to nutrient solutions supplying the roots of young broad bean plants and after 24 h absorption, fresh nutrient was substituted. The radiolabel accumulated at the leaf margins was initially associated with solvent-soluble toxic metabolites. Radiolabel extracted from foliage which developed after the treatment was mainly in water-soluble form. The maximum concentration of toxic material occurred in the marginal areas of the leaves after 5 or 6 days and in the central portions after 2 days. The concentrations declined to half of their peak values in 4–6 days from leaf-margin tissue and in 2 days from the central tissues. Water-soluble label did not accumulate to any significant extent. Most was found in the marginal areas but concentrations built up slowly reaching a maximum after about 2 weeks, declining slowly thereafter. 相似文献
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The cabbage looper, Trichoplusia ni (Hübner) (Lepidoptera: Noctuidae) is a generalist species and an important pest of Brassicaceae worldwide. Egg parasitoids are a feasible alternative for the control of this species. We evaluated the suitability of T. ni eggs as hosts for three Trichogramma Westwood (Hymenoptera: Trichogrammatidae) species and their tolerance to survive and develop within a range of temperatures between 15 and 30°C under laboratory conditions. The species evaluated were Trichogramma pretiosum Riley, Trichogramma atopovirilia Oatman and Platner, and Trichogramma acacioi Brun, Moraes and Soares. Parasitism rate was affected by temperature, parasitoid species, and by the interaction between these two factors. Parasitoids developed and reproduced in the range of temperatures evaluated, but Trichog. acacioi failed to parasitize T. ni eggs at 30°C. The highest parasitism rates of Trichog. atopovirilia and Trichog. pretiosum occurred at 20 and 25°C and Trichog. acacioi at 25°C, with parasitism rate above 70% in the three species. Parasitoid emergence was not affected by temperature or species. The estimated thermal constant and lower temperature threshold were 134.6 DD and 10.6°C for Trichog. pretiosum and 130.1 DD and 11.2°C for Trichog. atopovirilia. The results demonstrated that Trichog. pretiosum and Trichog. atopovirilia are the most suitable species for the control of T. ni, as they can remain active throughout the year in subtropical regions. 相似文献
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Arlena Brosinsky Saskia Foerster Karl Segl José Andrés López-Tarazón Gemma Piqué Axel Bronstert 《Journal of Soils and Sediments》2014,14(12):1965-1981
Purpose
Knowledge of sediment sources is a prerequisite for sustainable management practices and may furthermore improve our understanding of water and sediment fluxes. Investigations have shown that a number of characteristic soil properties can be used as “fingerprints” to trace back the sources of river sediments. Spectral properties have recently been successfully used as such characteristics in fingerprinting studies. Despite being less labour-intensive than geochemical analyses, for example, spectroscopy allows measurements of small amounts of sediment material (>60 mg), thus enabling inexpensive analyses even of intra-event variability. The focus of this study is on the examination of spectral properties of fluvial sediment samples to detect changes in source contributions, both between and within individual flood events.Materials and methods
Sediment samples from the following three different origins were collected in the Isábena catchment (445 km2) in the central Spanish Pyrenees: (1) soil samples from the main potential source areas, (2) stored fine sediment from the channel bed once each season in 2011 and (3) suspended sediment samples during four flood events in autumn 2011 and spring 2012 at the catchment outlet as well as at several subcatchment outlets. All samples were dried and measured for spectral properties in the laboratory using an ASD spectroradiometer. Colour parameters and physically based features (e.g. organic carbon, iron oxide and clay content) were calculated from the spectra. Principal component analyses (PCA) were applied to all three types of samples to determine natural clustering of samples, and a mixing model was applied to determine source contributions.Results and discussion
We found that fine sediment stored in the river bed seems to be mainly influenced by grain size and seasonal variability, while sampling location—and thus the effect of individual tributaries or subcatchments—seem to be of minor importance. Suspended sediment sources were found to vary between, as well as within, flood events; although badlands were always the major source. Forests and grasslands contributed little (<10 %), and other sources (not further determinable) contributed up to 40 %. The analyses further suggested that sediment sources differ among the subcatchments and that subcatchments comprising relatively large proportions of badlands contributed most to the four flood events analyzed.Conclusions
Spectral fingerprints provide a rapid and cost-efficient alternative to conventional fingerprint properties. However, a combination of spectral and conventional fingerprint properties could potentially permit discrimination of a larger number of source types. 相似文献8.
Saskia Foerster Charlotte Wilczok Arlena Brosinsky Karl Segl 《Journal of Soils and Sediments》2014,14(12):1982-2000
Purpose
Many Mediterranean drylands are characterized by strong erosion in headwater catchments, where connectivity processes play an important role in the redistribution of water and sediments. Sediment connectivity describes the ease with which sediment can move through a catchment. The spatial and temporal characterization of connectivity patterns in a catchment enables the estimation of sediment contribution and transfer paths. Apart from topography, vegetation cover is one of the main factors driving sediment connectivity. This is particularly true for the patchy vegetation cover typical of many dryland environments. Several connectivity measures have been developed in the last few years. At the same time, advances in remote sensing have enabled an improved catchment-wide estimation of ground cover at the subpixel level using hyperspectral imagery.Materials and methods
The objective of this study was to assess the sediment connectivity for two adjacent subcatchments (~70 km2) of the Isábena River in the Spanish Pyrenees in contrasting seasons using a quantitative connectivity index based on fractional vegetation cover and topography data. The fractional cover of green vegetation, non-photosynthetic vegetation, bare soil and rock were derived by applying a multiple endmember spectral mixture analysis approach to the hyperspectral image data. Sediment connectivity was mapped using the index of connectivity, in which the effect of land cover on runoff and sediment fluxes is expressed by a spatially distributed weighting factor. In this study, the cover and management factor (C factor) of the Revised Universal Soil Loss Equation (RUSLE) was used as a weighting factor. Bi-temporal C factor maps were derived by linking the spatially explicit fractional ground cover and vegetation height obtained from the airborne data to the variables of the RUSLE subfactors.Results and discussion
The resulting connectivity maps show that areas behave very differently with regard to connectivity, depending on the land cover and on the spatial distribution of vegetation abundances and topographic barriers. Most parts of the catchment show higher connectivity values in August as compared to April. The two subcatchments show a slightly different connectivity behaviour that reflects the different land cover proportions and their spatial configuration.Conclusions
The connectivity estimation can support a better understanding of processes controlling the redistribution of water and sediments from the hillslopes to the channel network at a scale appropriate for land management. It allows hot spot areas of erosion to be identified and the effects of erosion control measures, as well as different land management scenarios, to be studied. 相似文献9.
P. Foerster 《Journal of Agronomy and Crop Science》1988,161(4):238-248
Periodic and Vertical Nitrate Distribution in Unsaturated and in Saturated Zone of Sandy Soils Close to Groundwater in North-West Germany
Investigations into the periodic and vertical distribution of nitrate were conducted in three geologically, pedologically and climatically comparable arable sandy soils lying close to the groundwater zone of the coastal moorlands of N.W. Germany. Periodical and vertical distribution of nitrate was observed in unsaturated and in saturated zones of the soil.
The investigation concerns podzol-brown earths. The depth of the water-unsaturated zone is 2 to 3 m. The crops in two locations consisted of maize and winter rye; in one location, of maize only. N-manuring consisted of mineral fertilizers and/or of organic liquid manure. At two locations the investigations were conducted over 3 years, at one location, over 18 months.
The results from the three locations indicate a corresponding clear variability of nitrate amounts in the soil depending on weather conditions, on the course of growth of the crops, and on the date, the rate and the type of manuring.
The dependence of nitrate content on the time span is most clearly expressed in the top soil. The peak values of NO3 in the top soil appear in May, June and in the autumn months. Nitrate peaks in the top soil are continued in the subsequent ground levels with a time shift. Underneath the root zone, the nitrate peak values are transferred, largely without losses, by the seepage water through the capillary and groundwater zones close to the surface.
In the capillary and groundwater zones the nitrate peak values are characterized by time-dependent extension.
The time-span for transferring nitrate peak values from the top soil down to 4.00 m under the surface was determined to be approximately 6-8 months. 相似文献
Investigations into the periodic and vertical distribution of nitrate were conducted in three geologically, pedologically and climatically comparable arable sandy soils lying close to the groundwater zone of the coastal moorlands of N.W. Germany. Periodical and vertical distribution of nitrate was observed in unsaturated and in saturated zones of the soil.
The investigation concerns podzol-brown earths. The depth of the water-unsaturated zone is 2 to 3 m. The crops in two locations consisted of maize and winter rye; in one location, of maize only. N-manuring consisted of mineral fertilizers and/or of organic liquid manure. At two locations the investigations were conducted over 3 years, at one location, over 18 months.
The results from the three locations indicate a corresponding clear variability of nitrate amounts in the soil depending on weather conditions, on the course of growth of the crops, and on the date, the rate and the type of manuring.
The dependence of nitrate content on the time span is most clearly expressed in the top soil. The peak values of NO
In the capillary and groundwater zones the nitrate peak values are characterized by time-dependent extension.
The time-span for transferring nitrate peak values from the top soil down to 4.00 m under the surface was determined to be approximately 6-8 months. 相似文献
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