Combined analyses of mitochondrial DNA (mtDNA) and microsatellite loci were performed to assess the genetic differentiation of two spring‐spawning ciscoes from each other and from sympatric Coregonus ...albula in two German lakes. Polymorphism was screened at six microsatellite loci and mtDNA for a total of 247 and 94 ciscoes, respectively. Microsatellite data showed a weak differentiation between spring‐spawning Coregonus fontanae and sympatric C. albula in Lake Stechlin (FST = 0–0·008), whereas a significant differentiation was observed between spring‐spawning Coregonus lucinensis and sympatric C. albula in Lake Breiter Luzin (FST = 0·013–0·039). A more pronounced genetic difference was observed between both spring‐spawning species (FST = 0·05–0·128). Shared mtDNA haplotypes among sympatric species within both Lake Stechlin and Lake Breiter Luzin were observed, whereas no haplotype was shared between C. fontanae and C. lucinensis. These results suggest an independent origin for spring‐spawning ciscoes in each lake. Evidence is also provided for mtDNA introgression of Coregonus sardinella into C. lucinensis and C. albula in Lake Breiter Luzin. Postglacially, this species or at least a population which showed mtDNA introgression has colonized the Baltic Sea basin up to the glacial margin that was located between Lakes Stechlin and Breiter Luzin.
Depassivation processes known by literature detected by electrochemical methods during polarization experiments of Mg substrates in alkaline silicate PEO electrolytes below the ignition voltage were ...investigated in more detail by SEM images of the generated reaction layers. Remarkable crater structures were discovered on the Mg alloy AZ31 that do not form on the aluminum AlMgSi1 substrate under identical experimental conditions. The formations of these structures were described using the concepts of the high-field model. Through systematic comparative investigations of PEO processes on the two studied substrates by means of in situ process diagnostics, electrochemical subprocesses in the plasma electrolytic oxidation of magnesium were identified by analyzing electrical and photoelectric process signals. A negative influence on the morphology of PEO layers on magnesium was attributed to subprocesses in the context of the depassivation mechanism below the ignition voltage as well as microstructure images of the generated PEO layers.
Display omitted
•Discovery of microstructural conditions of depassivation mechanisms on magnesium.•Successful implementation of in-situ process diagnostics.•Identification of electrochemical subprocesses during PEO.
SUMMARY 1. This paper is an introduction to a special issue of Freshwater Biology containing selected papers from an international symposium on Food Web Effects of Fish in Lake Ecosystems: Research ...Progress, Water Quality and Fisheries Management held from 31 May to 3 June 2000 in Rheinsberg, Germany. The primary goal of the workshop was to enlarge the current view of fish‐induced effects on lake ecosystems. An additional goal was to promote biomanipulation as a multiple‐use tool for managing freshwater ecosystems.
2. The three main topics addressed at the workshop were: (i) mechanisms involved in biomanipulation, (ii) whole‐lake case studies and (iii) management aspects in water quality and fisheries.
3. Mortality of Daphnia, nutrient recycling, habitat selection and fish predation are reported as important mechanisms governing food‐web effects as a result of biomanipulation.
4. Whole‐lake case studies indicate that repeated fish removal can help improve water quality of shallow lakes, but successful biomanipulation of deep, thermally stratifying lakes remains difficult.
5. In many cases, biomanipulation of lakes has proved to provide benefits in addition to improving water quality. As all lake users are potentially affected when biomanipulation is used as a lake management tool, their concerns need to be clearly recognised if biomanipulation is to be successful in practice.
– Structural complexity offered by submerged macrophytes was shown to have fundamental effects on interactions between fish and their prey. However, less information is available for littoral reed ...(Phragmites spp.) stands. A previous field study found juvenile roach and perch to coexist within the reed stands. It was suggested that reed serves mainly as refuge against littoral piscivores, such that coexistence of perch and roach in the reed was externally forced. Several hypotheses were raised to explain why roach nevertheless showed good growth performance. Three of the hypotheses were tested experimentally. In particular, we were interested in how the confinement of fish to one of the reed or open water habitats alters feeding and growth patterns of juvenile age‐1 perch and roach. Fish were stocked separately into littoral enclosures for a 3‐week period in densities which had been found in the surrounding lake. Development of zooplankton and macroinvertebrate biomasses was observed by sampling the enclosures three times over the experimental period. Individual consumption of prey groups by the fish was calculated with a bioenergetics model, and was compared with prey group biomass in the enclosure treatments. The confinement of fish to one littoral habitat had clear effects on diet composition and growth rates. Roach fed less zooplankton and partially switched to macroinvertebrates in the reed enclosures when compared with the open water treatments, and consequently their growth rates were lower in the reed. Perch preferred macroinvertebrates in both habitats, without any difference in growth rates between the habitats. Effects of fish predation on both zooplankton and macroinvertebrate biomass were low in open water and reed enclosures. Daily consumption rates were only in a few cases higher than 40% of the available biomass of the respective prey group, but mainly were below 10% of available biomass. Therefore, we argue that both the diel horizontal migrations of roach and the relatively low consumption rate of fish when compared with the available resource biomass allow the coexistence of juvenile roach and perch in littoral reed stands.
Resumen
1. La presencia de hábitats estructuralmente complejos en las zonas litorales de los lagos puede tener efectos fundamentales sobre las interacciones entre los peces y sus presas. Varios estudios han mostrado que las macrófitas sumergidas pueden servir de refugio tanto para los invertebrados contra la predación de los peces como para los peces‐presa contra la de los peces piscívoros. Sin embargo, hay menos información para las manchas de cañas litorales (Phragmites spp.) donde los patrones de interacción pueden diferir de las de las macrófitas debido a la potencialmente menor complejidad ofrecida por las cañas. Dado que el éxito de forrajeo de los predadores decrece al incrementar la complejidad estructural, la supervivencia de las presas es normalmente mas alta en las estructuras mas densas.
2. Un estudio previo encontró que los juveniles de Rutilus rutilus y Percafluviatilis co‐existen de manchas de Phragmites. La abundancia numérica de R. rutilus en las cañas y su presencia preferida en estructuras litorales complejas no reflejaron la menor eficiencia de forrajeo de R. rutilus por presas de invertebrados en hábitats estructurados que habían sido mostrados ser menores que para P. fluviatilis. Ello sugirió que las cañas sirven esencialmente como refugio contra los piscívoros litorales de manera que la co‐existencia de P. fluviatilis y R. rutilus en las cañas fue forzada externamente. Planteamos varias hipótesis sobre por quéR. rutilus mostró un buen desarrollo de crecimiento. Tres de estas hipótesis fueron analizadas experimentalmente. En particular estuvimos interesados en saber como el confinamiento de los peces en hábitats de caña contra hábitats de aguas abiertas altera los patrones alimenticios y de crecimiento de los juveniles de edad 1+ de P. fluviatilis y R. rutilus.
3. Los peces fueron repoblados separadamente en cerrados litorales por un período de tres semanas a densidades similares a las observadas en algunos lagos de los alrededores. Los cerrados fueron colocados tanto en manchas densas de Phragmites como en zonas de agua abiertas libres de vegetación. Las biomasas de zooplancton y de macro‐invertebrados fueron observados muestreando los cerrados tres veces durante el período experimental. El consumo individual de grupos de presas por los peces fue calculado con un modelo bio‐energético y comparado con las biomasas de los grupos de presas de los tratamientos.
4. El confinamiento de los peces en un hábitat litoral tuvo claros efectos sobre la composición de la dieta y sobre las tasas de crecimiento. R. rutilus se alimentó menos de zooplancton y parcialmente cambió a macro‐invertebrados en los cerrados de Phragmites comparado con los tratamientos en aguas abiertas. Como consecuencia, sus tasas de crecimiento fueron menores en las cañas. P. fluviatilis prefirió macro‐invertebrados en ambos hábitats sin que aparecieran diferencias en las tasas de crecimiento. Los efectos de la predación sobre las biomasas de zooplancton y de macro‐invertebrados fueron bajas tanto en los cerrados de aguas abiertas como en los de cañas. Las tasas de consumo alimenticio fueron en unos pocos casos mayores del 40% de la biomasa disponible de los respectivos grupos de presas pero básicamente estuvieron por debajo del 10% de la biomasa disponible. Por ello, argumentamos que tanto las migraciones circadianas horizontales de R. rutilus y el relativamente bajo consumo alimenticio de los peces comparado con la biomasas de los recursos disponibles permite la coexistencia de los juveniles de R. rutilus y P. fluviatilis en las manchas de Phragmites litorales.
Morphotypes for 67 lakes in the German lowlands were derived, based on maximum depth and mixis type. A threshold of 11 m maximum depth was identified to be the best level to discriminate shallow from ...deep lake morphotypes. The fish communities in these two morphotypes were significantly different. Indicator species analyses based on fish biomasses found vendace Coregonus albula in deep lakes and ruffe Gymnocephalus cernuus, bream Abramis brama, white bream Abramis bjoerkna, roach Rutilus rutilus, pikeperch Sander lucioperca and small perch Perca fluviatilis in shallow lakes to be the most representative species of their communities. Lake productivity was closely related to biomass and in part abundance of the type‐indicator species, with vendace declining with increasing chlorophyll a concentration in the deep lakes, whereas biomass of pikeperch, bream, white bream and ruffe increased and biomass of small perch decreased with increasing chlorophyll a. These results indicate that assessment of ecological integrity of lakes by their fish fauna is generally possible, if lakes are initially separated according to a depth‐related morphotype before the assessment, and if eutrophication is considered to be the main anthropogenic degradation.
A carp angler survey indicated that specialised carp anglers used on average 215 kg groundbaits (mostly cereals, nuts, beans, boilies) per year which corresponded to a mean annual gross P‐input of ...1018 g P angler−1. A feeding experiment was conducted with juvenile carp fed with a random sample of the main bait types used in coarse angling to estimate the nutritional quality and P‐retention efficiency. The investigated groundbaits had a rather low nutritional quality. Based on the examined P‐retention efficiencies, the net annual P‐input was calculated to be 839 g P per angler. To counterbalance this P‐input, an annual harvest rate of 175–225 kg carp biomass per angler would be necessary. Thus, under certain conditions, a substantial contribution by carp and other coarse angling to the P‐loading is likely. Further studies on the issue of groundbaiting are needed to develop guidelines for more effective management of coarse angling.
In this study gillnet catches of large roach Rutilus rutilus and their potential predators, large perch Perca fluviatilis, pikeperch Sander lucioperca and pike Esox lucius, in combination with data ...on food resources and abiotic variables were used to reveal the variables influencing the habitat use of large roach in Lake Großer Vätersee. The occurrence of large roach was negatively coupled with the occurrence of potential predators. Although almost all roach studied were larger than those actually fed on by the predators, high roach catches never coincided with high predator catches. Since the overall habitat‐specific distribution of roach and its predators was nevertheless highly similar, this indicated that large roach obviously avoided the immediate vicinity of the predators without being forced to a full habitat shift. The behavioural response to predators of roach in Lake Großer Vätersee was clearly length dependent, since the patterns of diel habitat use differed between large and small roach. For lake‐wide estimates of trophic interactions, size groups of prey should be separated, since size‐dependent trait changes may impact habitat‐specific resources in a contrasting way.
Biomanipulation, the reduction of planktivorous fish to enhance filter‐feeding zooplankton, has been used to rehabilitate eutrophied lakes. However, efficacy and long‐term success were dependent on ...nutrient load, lake morphometry and biomanipulation measures. The ongoing focus on sustainable use of aquatic resources offers the chance to perform lake rehabilitation using a combined strategy of nutrient load reduction and traditional inland fisheries management techniques. Particularly in Central and Western Europe where piscivorous fish are the target species of most commercial and recreational fisheries, an enhancement of the piscivores by stocking and harvest regulations may act successfully in the co‐management of ecosystem and fisheries. Guidelines are presented on how biomanipulation can be used as in lake rehabilitation by considering the objectives and constraints of traditional fisheries management. Alternatives in the decision tree are elucidated by examples from biomanipulations and lake management programmes in the temperate zone of Europe and North America. It is suggested that biomanipulation may support many lake rehabilitation programmes where fisheries’ stakeholders are the principal user groups.
Eutectic high-entropy alloys (EHEAs) with a fine-lamellar structure and homogenous property profile are of particular interest for wear and corrosion protection coatings. High cooling rates in the ...laser metal deposition (LMD) process can induce microstructure refinement and allow the formation of a supersaturated solid solution in EHEAs. A subsequent solution annealing can create the equilibrium state. In the present study, LMD coatings with an ultrafine-grained Widmanstätten structure were produced from the EHEA Al
0.3
CoCrFeNiMo
0.75
gas atomized powder. High cooling rates during deposition led to a supersaturated solid solution with face-centered cubic (FCC) structure. The LMD coating exhibits the highest average hardness of 734 HV0.5, which drops to approx. 200 HV0.5 due to an increased microstructural domain size after heat treatment. Under oscillating wear, the formation of oxidized wear debris promotes material removal in the heat-treated condition. Corrosion tests reveal a deterioration of the passivation behavior. LMD processes exhibit great potential to create supersaturated solid solutions with refined structure in EHEAs to enhance the property profile.
PDF
