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71.
The dynamic properties of a flexible net sheet exposed to waves and current were investigated by using a numerical model. The net was modelled by dividing it into super elements, and the structural and hydrodynamic forces were calculated for each element. The movement of the net was found from time integrating the equation of motion at each node. The model was used to study the influence of different parameters on the behaviour of the net. Through six different cases the impact of the following five parameters were investigated: (i) floater movement, (ii) wave period/height, (iii) current velocity, (iv) net solidity and (v) bottom weight. 相似文献
72.
In two separate experiments, juvenile Arctic charr (Salvelinus alpinus) were held at three different water depths (12, 24 and 37 cm) and at three different stocking densities (10, 50 and 100 kg/m3). Mortalities from swim bladder stress syndrome (SBSS) were noted over 2 months. Low water depth caused high mortality while stocking density did not induce the stress syndrome to any notable degree. An analysis of catecholamines in the blood plasma of a small number of fish indicated elevated dopamine levels in stressed fish. 相似文献
73.
Heidi Moe Tim Dempster Leif Magne Sunde Ulf Winther & Arne Fredheim 《Aquaculture Research》2007,38(1):91-99
Escapes of cod (Gadus morhua) from sea cages represent an economic problem for farmers and a potential environmental problem. We estimate that 0–6% of cod held in sea‐cage farms in Norway were reported to have escaped each year from 2000 to 2005, which is a high proportion compared with salmon. We interviewed employees at 19 coastal sea‐cage cod farms in Norway to investigate both how and why cultured cod escape and to document cage handling and management strategies that were effective in minimizing escapes. Based on the interviews, we describe five working hypotheses that may explain why a greater proportion of cod than salmon escape: (1) cod are more willing to escape than salmon; (2) cod bite the net cage and create wear and tear; (3) net cages have insufficient technical standards for cod culture; (4) cod are placed in sea cages at considerably smaller sizes than salmon; and (5) cod are more popular feed for predators. Preliminary testing of the hypothesis that cod bite netting and create holes was done by placing pre‐damaged net panels with cut twines and control panels inside sea cages. Holes in the pre‐damaged net panels increased in size over a period of 3 months. The type of damage indicated that biting of netting twines was the likely cause. 相似文献
74.
Kjell Johansson Mats Aastrup Arne Andersson Lage Bringmark Ake Iverfeldt 《Water, air, and soil pollution》1991,56(1):267-281
As a result of air pollution, the content of Hg in fish has significantly increased in a large part of Scandinavia and North
America. In this paper, the occurence and fluxes of Hg in Swedish forest soils and waters are reviewed and synthesized. The
main objective is to describe and evaluate the present transport of anthropogenic Hg from atmospheric deposition, through
the terrestrial compartment and running waters to lake basins and also to comprehend the main factors influencing these fluxes.
The transportation and distribution. of Hg in forest soils and waters is closely related to the flow of organic matter. The
content of Hg in humic matter is higher in southern and central areas compared to the north of the country. Compared to background
concentrations, the Hg content has increased in the southern and central part by about a factor of 4–7, while the overall
increase in the north is by about a factor of 2 to 3. The increased content of Hg in forest soils may have an effect on organisms
and biological processes in the soil. Regarding budget calculations for whole catchment areas and for the mor layer of the
soil, a reduction of about 80% from present atmospheric wet deposition must be obtained to reach “critical load” with respect
to conditions in Scandinavia. 相似文献
75.
The composition of the soil solution in unfertilized and well-fertilized plots of three long-term field experiments has been determined at the beginning and at the end of the growing season. All the plots were manured and the K and P fertilizers given in the autumn before the growth of sugar beet. N was applied in the spring, 3–4 weeks before the first soil sampling.The soil solution was removed at a pressure of 5 atm. The composition and the corresponding osmotic pressure were calculated for the moisture contents at field capacity and wilting point.In spring the salt concentrations, cations + anions, at the field capacity were in the range 91–97 mmoles/l in the fertilized plots. In autumn the concentration had dropped to 13.9–20.3, a decrease of 78–85%. At the wilting point, the concentrations in spring were as high as 211–307 mmoles/l and in autumn 31.5–69 mmoles/l. The corresponding osmotic pressures at field capacity in the spring ranged 2.2–2.3 atm. and at wilting point 5.1–7.4 atm.In spring the unfertilized plots showed concentrations of 16–21 mmoles/l and osmotic pressures of 0.38–0.51 atm., the values decreasing 52–65% during the growing season.It was pointed out that the high soil-solution concentration and osmotic pressure at low moisture contents may lead to an unfavourable effect on root metabolism. Further, the obscuring effect of the varying soil-solution concentration on the relationships between root ion exchange and nutrient uptake by plants has been discussed. 相似文献
76.
Martina I. Gocke Axel Don Arne Heidkamp Florian Schneider Wulf Amelung 《植物养料与土壤学杂志》2021,184(1):51-64
Background : In search for more sustainable crop production, the subsoil has recently come into focus as considerable reservoir of nutrients and water. Aims : Dimensions of subsoil phosphorus (P) reserves are yet largely unknown but crucial for identifying regions suitable to include subsoil into sustainable management strategies. Methods : We analyzed stocks of total and plant‐available (calcium acetate lactate‐extractable) P in 96 representative soil profiles of German arable land down to 1 m depth. Results : We found that the German arable soils stored, on average, 8 t ha?1 of total P, of which nearly 500 kg ha?1 were readily plant‐available. Notably, one third of plant‐available P was located below the plow layer and one fifth even at depths below 0.5 m. The depth gradients of plant‐available P stocks were affected more by major reference soil group than by texture. Generally, Chernozem but also Anthrosol, Gleysol and Fluvisol exhibited the largest P stocks in German cropland. The contribution of plant‐available P to total P stocks was larger in sandy and extremely acidic (pH < 4.5) soils compared with more fine‐grained and slightly acidic to alkaline soils, possibly because fertilization compensated for overall lower total P stocks at these sites. Generally, the more P was stored in topsoils, the more P was stored also in subsoils. Conclusions : A hypothetical crop utilization of 10% from plant‐available P stocks and 0.1% from total P stocks from shallow subsoil could compensate for P fertilization by ca. 8 kg ha?1, but the rate of plant‐available P replenishment in subsoil likely remains the crucial factor for the role of subsoil P stocks in crop nutrition. Generally, the large P reserves found in subsoil could act as an ‘insurance' system for crops. 相似文献
77.
Zhang Nina Xiaoning Andringa Joke Brouwer Jitske Alba Juan M. Kortbeek Ruy W. J. Messelink Gerben J. Janssen Arne 《Journal of pest science》2022,95(3):1343-1355
Journal of Pest Science - It has become clear that omnivorous predators can induce plant defences that affect the performance and host plant choice of herbivores. They are also known to induce the... 相似文献
78.
79.
Peer?B.?MortensenEmail author Louise?Schlüter Arne?Jensen Henning?F.?Aaser Kjeld?S.?Hansen 《Journal of Soils and Sediments》2004,4(2):101-106
Background and Scope
Lake sediment contains information on the historical development and former conditions of lakes, which is very useful when information about the undisturbed (reference) conditions of a lake is needed. Phytoplankton contains a range of different pigments, some of which are specific for individual algal groups and can be used as diagnostic markers. Phytoplankton pigments in lake sediments are indicators of prototrophic changes in lakes, since the pigments document changes in algae biomass and the composition of phytoplankton communities. Lake sediments can be dated along with pigment analysis by use of the naturally occurring radioactive isotope210Pb. 相似文献80.
Arne Helweg 《Water, air, and soil pollution》1979,12(3):275-281
Degradation of14C-labeled 2-aminobenzimidazole was measured in sandy loam soils by means of the evolution of14C into CO2. The correlation between soil temperature and 2-AB-degradation was investigated utilizing a temperature gradient incubator for temperatures between 1 and 40°C with a water content of 100% of field capacity. Within the temperature interval 1 to 20°C, the evolution of14C was exponentially related to the inverse of the absolute temperature, in accordance with the Arrhenius' equation. Maximum evolution of14C was at 22°C while between 25 and 35°C the evolution remained almost constant and at 40°C it was almost nil. The correlation between soil water content and 2-AB-degradation was measured at 25°C in air dry soil and with water contents varying from 5 to 41% (equivalent to 28 to 227% of field capacity (FC)). From 28 to 94% of FC an exponential increase in the evolution of14C was observed while the evolution was slightly decreasing in soil with water contents above this level. Degradation of 2-AB in soil was greatly enhanced when the soil was inoculated with liquid or with soil aliquots from a soil perfusion apparatus through which 2-AB had been percolating for 6 mo. This indicates the presence of organisms able to decompose 2-AB in the pre-treated soil and in the perfusing water. 相似文献