| 251. |
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| 252. |
- Persson, Anders, 1956-, et al.
(författare)
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Effects of benthivorous fish on biogeochemical processes in lake sediments
- 2006
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Ingår i: Freshwater Biology. - West Sussex, UK : Wiley-Blackwell. - 0046-5070 .- 1365-2427. ; 51:7, s. 1298-1309
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Tidskriftsartikel (refereegranskat)abstract
- 1. Studies of aquatic environments have shown that community organisation may strongly affect ecosystem functioning. One common phenomenon is a change in nutrient level following a shift in the fish community composition. Although several hypotheses have been suggested, there is no consensus on which mechanisms are involved. Our study evaluated indirect effects of benthivorous fish on the biogeochemical processes at the sediment-water interface separately from direct effects caused by nutrient excretion or sediment resuspension.2. We assigned field enclosures to three treatments representing typical pond communities; without fish, addition of approximately 10 small tench or addition of one large bream. After one summer, we monitored the water chemistry, benthic invertebrates and periphyton in the enclosures and sampled sediment cores for laboratory analysis of biochemical process rates (oxygen, phosphorus and nitrogen exchange between sediment and water, and denitrification rate).3. Fish had strong negative effects on benthic invertebrates, but weaker effects on periphyton, organic content and porosity of the sediment. Moreover, there were significant positive fish effects on both phosphorus and nitrogen concentrations in the water. However, there were no general treatment effects on sediment processes that could explain the treatment effects on water chemistry in the enclosures.4. Hence, overall treatment effects attenuated along the chain of interactions. We conclude that the observed effect of benthic fish on water chemistry was probably because of direct effects on nutrient excretion or resuspension of sediment. The similarity between bream and tench treatments suggests large niche complementarity despite their different habitat preferences.
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| 253. |
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| 254. |
- Persson, Anders, et al.
(författare)
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Effects of enrichment on simple aquatic food webs
- 2001
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Ingår i: American Naturalist. - : University of Chicago Press. - 0003-0147 .- 1537-5323. ; 157:6, s. 654-669
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Tidskriftsartikel (refereegranskat)abstract
- Simple models, based on Lotka-Volterra types of interactions between predator and prey, predict that enrichment will have a destabilizing effect on populations and that equilibrium population densities will change at the top trophic level and every second level below. We experimentally tested these predictions in three aquatic food web configurations subjected to either high or low nutrient additions. The results were structured by viewing the systems as either food chains or webs and showed that trophic level biomass increased with enrichment, which contradicts food chain theory. However, within each trophic level, food web configuration affected the extent to which different functional groups responded to enrichment. By dividing trophic levels into functional groups, based on vulnerability to consumption, we were able to identify significant effects that were obscured when systems were viewed as food chains. The results support the prediction that invulnerable prey may stabilize trophic-level dynamics by replacing other, more vulnerable prey. Furthermore, the vulnerable prey, such as Daphnia and edible algae, responded as predicted by the paradox of enrichment hypothesis; that is, variability in population density increased with enrichment. Hence, by describing ecosystems as a matrix of food web interactions, and by recognizing the interplay between interspecific competition and predation, a more complete description of the ecosystem function was obtained compared to when species were placed into distinct trophic levels.
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| 255. |
- Persson, Anders, et al.
(författare)
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Effects of enrichment on simple aquatic food webs
- 2001
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Ingår i: The American Naturalist. ; 157:6, s. 654-669
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Tidskriftsartikel (refereegranskat)abstract
- Simple models, based on Lotka-Volterra types of interactions between predator and prey, predict that enrichment will have a destabilizing effect on
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| 256. |
- Persson, Anders
(författare)
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Effects of Fish Predation and Excretion on the Configuration of Aquatic Food Webs
- 1997
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Ingår i: Oikos. - 1600-0706. ; 79:1, s. 137-146
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Tidskriftsartikel (refereegranskat)abstract
- The relative importance for algae of fish excretion and fish predation on zooplankton was studied in experimental pools. Even- (zoo- and phytoplankton) and odd-link systems (planktivorous fish, zoo- and phytoplankton) were established, in which the standing stock of primary producers was expected to follow the predictions from food chain theory sensu Oksanen. Two intermediate treatments (fish predation and nutrient regeneration by fish alone) were used to determine the relative power of fish in affecting the transition from an even-link system to an odd-link system, i.e. their relative power in the trophic cascade. Algal density and production were found to only be enhanced when both fish predation and fish excretion acted together. A reduction in the grazer biomass alone did not increase the algal biomass. Fish excretion alone increased the algal biomass somewhat, but not significantly so compared with the control. The results suggest that both fish predation on zooplankton and the regeneration of nutrients by fish may be necessary to create the difference in algal biomass between an even- and an odd-link system predicted by food chain models, at least in the absence of an external input of nutrients. In addition, the effects of increased nutrient regeneration due to use of non-zooplankton resources by the fish, nutrient maintenance in living biota, and decreased grazing pressure seem to reinforce the cascading effect of planktivorous fish on algae. Consequently, use of a fixed carrying capacity in simple predatorprey models may lead to loss of important information on the dynamics of aquatic food webs, preventing an adequate understanding of cascading trophic interactions from being achieved.
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| 257. |
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| 258. |
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| 259. |
- Persson, Anders
(författare)
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Ers majestät Lunds universitet
- 2001
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Ingår i: Lunds Universitet Meddelar. - 0345-7354. ; :3/2001
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Tidskriftsartikel (populärvet., debatt m.m.)
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| 260. |
- Persson, Anders, et al.
(författare)
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Etch-stop technique for patterning of tunnel junctions for a magnetic field sensor
- 2011
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Ingår i: Journal of Micromechanics and Microengineering. - : IOP Publishing. - 0960-1317 .- 1361-6439. ; 21:4, s. 045014-045022
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Tidskriftsartikel (refereegranskat)abstract
- Spin-dependent tunnelling devices, e. g. magnetic random access memories and highly sensitive tunnelling magnetoresistance (TMR) sensors, often consist of a large number of magnetic tunnel junctions (MTJs) of uniform quality over the whole device. The uniformity and yield of the fabrication of such a device are therefore very important. A major source of yield loss is the short-circuiting of junctions by redeposition of etch residues. This can be prevented by terminating of the etch in the typically 1 nm thick tunnelling barrier. Here, electron spectroscopy for chemical analysis for monitoring the etching semi-continuously is proposed. The fabrication scheme employs Ar ion milling for etching the MTJs, and photoelectron spectroscopy for analysing the composition of the etched surface in situ. Junctions etched either to or through the barrier were used for this. The quality of the etch stop was investigated using transmission electron microscopy (TEM), and it was confirmed that the etch could be stopped in the MgO barrier. The TEM imaging also showed clear signs of redeposition. Such redeposition was attributed to being partly caused by the reduction of the TMR ratio of the junctions etched through the barrier, which was only 15% as compared with 150% for junctions etched to the barrier. Also, the latter junctions exhibited 2.7 times less noise in the low-frequency regime, resulting in a 27 times improvement of the signal-to-noise ratio with the etch stop. The barrier also proved effective in protecting the bottom contact from oxidation during the capping and contacting of the junctions.
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