11 resultados para Trade unions and union structure
em Helda - Digital Repository of University of Helsinki
Resumo:
The coherence of the Soviet bloc was seriously tested at the turn of the 1970s, as the Soviet Union and its allies engaged in intensive negotiations over their relations with the European Communities (EC). In an effort to secure their own national economic interests many East European countries began independent manoeuvres against the wishes of their bloc leader. However, much of the intra-bloc controversy was kept out of the public eye, as the battle largely took place behind the scenes, within the organisation for economic cooperation, the Council for Mutual Economic Assistance (CMEA). The CMEA policy-making process vis-à-vis the EC is described in this study with reference to primary archival materials. This study investigates the negotiating positions and powers of the CMEA member states in their efforts to deal with the economic challenge created by the progress of the EC, as it advanced towards the customs union. This entails an analysis of the functioning principles and performance of the CMEA machinery. The study traces the CMEA negotiations that began in 1970 over its policy toward the EC. The policy was finally adopted in 1974, and was followed by the first official meeting between the two organisations in early 1975. The story ends in 1976, when the CMEA s efforts to enter into working relations with the EC were seemingly frustrated by the latter. The first major finding of the study is that, contrary to much of the prior research, the Soviet Union was not in a hegemonic position vis-à-vis its allies. It had to use a lot of its resources to tame the independent manoeuvring of its smaller allies. Thus, the USSR was not the kind of bloc leader that the totalitarian literature has described. Because the Soviet Union had to spend so much attention on its own bloc-politics, it was not able to concentrate on formulating a policy vis-à-vis the EC. Thus, the Soviet leadership was dependent on its allies in those instances when the socialist countries needed to act as a bloc. This consequently opened up the possibility for the USSR s allies to manoeuvre. This study also argues that when the CMEA did manage to find a united position, it was a force that the EC had to reckon with in its policy-making. This was particularly the case in the implementation of the EC Common Commercial Policy. The other main finding of the study is that, although it has been largely neglected in the previous literature on the history of West European integration, the CMEA did in fact have an effect on EC decision-making. This study shows how for political and ideological reasons the CMEA members did not acknowledge the EC s supranational authority. Therefore the EC had no choice but to refrain from implementing its Common Commercial Policy in full.
Resumo:
When genome sections of wild Solanum species are bred into the cultivated potato (S. tuberosum L.) to obtain improved potato cultivars, the new cultivars must be evaluated for their beneficial and undesirable traits. Glycoalkaloids present in Solanum species are known for their toxic as well as for beneficial effects on mammals. On the other hand, glycoalkaloids in potato leaves provide natural protection against pests. Due to breeding, glycoalkaloid profile of the plant is affected. In addition, the starch properties in potato tubers can be affected as a result of breeding, because the crystalline properties are determined by the botanical source of the starch. Starch content and composition affect the texture of cooked and processed potatoes. In order to determine glycoalkaloid contents in Solanum species, simultaneous separation of glycoalkaloids and aglycones using reversed-phase high-performance liquid chromatography (HPLC) was developed. Clean-up of foliage samples was improved using a silica-based strong cation exchanger instead of octadecyl phases in solid-phase extraction. Glycoalkaloids alpha-solanine and alpha-chaconine were detected in potato tubers of cvs. Satu and Sini. The total glycoalkaloid concentration of non-peeled and immature tubers was at an acceptable level (under 20 mg/100 g of FW) in the cv. Satu, whereas concentration in cv. Sini was 23 mg/100 g FW. Solanum species (S. tuberosum, S. brevidens, S. acaule, and S. commersonii) and interspecific somatic hybrids (brd + tbr, acl + tbr, cmm + tbr) were analyzed for their glycoalkaloid contents using liquid chromatography-electrospray ionization-mass spectrometry (LC-ESI-MS). The concentrations in the tubers of the brd + tbr and acl + tbr hybrids remained under 20 mg/100 g FW. Glycoalkaloid concentration in the foliage of the Solanum species was between 110 mg and 890 mg/100 g FW. However, the concentration in the foliage of S. acaule was as low as 26 mg/100 g FW. The total concentrations of brd + tbr, acl + tbr, and cmm + tbr hybrid foliages were 88 mg, 180 mg, and 685 mg/100 g FW, respectively. Glycoalkaloids of both parental plants as well as new combinations of aglycones and saccharides were detected in somatic hybrids. The hybrids contained mainly spirosolanes, and glycoalkaloid structures having no 5,6-double bond in the aglycone. Based on these results, the glycoalkaloid profiles of the hybrids may represent a safer and more beneficial spectrum of glycoalkaloids than that found in currently cultivated varieties. Starch nanostructure of three different cultivars (Satu, Saturna, and Lady Rosetta), a wild species S. acaule, and interspecific somatic hybrids were examined by wide-angle and small-angle X-ray scattering (WAXS, SAXS). For the first time, the measurements were conducted on fresh potato tuber samples. Crystallinity of starch, average crystallite size, and lamellar distance were determined from the X-ray patterns. No differences in the starch nanostructure between the three different cultivars were detected. However, tuber immaturity was detected by X-ray scattering methods when large numbers of immature and mature samples were measured and the results were compared. The present study shows that no significant changes occurred in the nanostructures of starches resulting from hybridizations of potato cultivars.
Resumo:
The life cycle and genetic diversity of the red alga Furcellaria lumbricalis (Hudson) Lamouroux were investigated in 15 populations in northern Europe. The occurrence of different life cycle phases and seasonality of reproduction were studied in four brackish populations in the northern Baltic Sea. Furthermore, a new method, based on genome screening with ISSR markers combined with a restriction-ligation method, was developed to discover microsatellite markers for population genetic analyses. The mitochondrial DNA cox2-3 spacer sequence and four microsatellite markers were used to examine the genetic diversity and differentiation of red algal populations in northern Europe. In addition, clonality and small-scale genetic structure of one Irish and four Baltic Sea populations were studied with microsatellite markers. It was discovered that at the low salinities of the northern Baltic Sea, only tetrasporophytes and males were present in the populations of F. lumbricalis and that winter was the main season for tetrasporangial production. Furthermore, the population occurring at the lowest salinity (3.6 practical salinity units, psu) did not produce spores. The size of the tetraspores was smaller in the Baltic Sea populations than that in the Irish population, and there were more deformed spores in the Baltic Sea populations than in the Irish populations. Studies with microsatellite markers indicated that clonality is a common phenomenon in the Baltic Sea populations of F. lumbricalis, although the proportion of clonal individuals varied among populations. Some genetic divergence occurred within locations both in Ireland and in the northern Baltic Sea. Even though no carpogonia were detected in the field samples during the study, the microsatellite data indicated that sexual reproduction occurs at least occasionally in the northern Baltic Sea. The genetic diversity of F. lumbricalis was highest in Brittany, France. Since no variation was discovered in the mtDNA cox2-3 spacer sequence, which is generally regarded as an informative phylogeographic marker in red algae, it can be assumed that the studied populations probably share the same origin.
Resumo:
Intensified agricultural practises introduced after the Second World War are identified as a major cause of global biodiversity declines. In several European countries agri-environment support schemes have been introduced to counteract the ongoing biodiversity declines. Farmers participating in agri-environment schemes are financially compensated for decreasing the intensity of farming practises leading to smaller yields and lower income. The Finnish agri-environment support scheme is composed of a set of measures, such as widened field margins along main ditches (obligatory measure), management of features increasing landscape diversity, management of semi-natural grasslands, and organic farming (special agreement measures). The magnitude of the benefits for biodiversity depends on landscape context and the properties of individual schemes. In this thesis I studied whether one agri-environment scheme, organic farming, is beneficial for species diversity and abundance of diurnal lepidopterans, bumblebees, carabid beetles and arable weeds. I found that organic farming did not enhance species richness of selected insect taxa, although bumblebee species richness tended to be higher in organic farms. Abundance of lepidopterans and bumblebees was not enhanced by organic farming, but carabid beetle abundance was higher in mixed farms with both cereal crop production and animal husbandry. Both species richness and abundance of arable weeds were higher in organic farms. My second objective was to study how landscape structure shapes farmland butterfly communities. I found that the percentage of habitat specialists and species with poor dispersal abilities in butterfly assemblages decreased with increasing arable field cover, leading to a dramatic decrease in butterfly beta diversity. In field boundaries local species richness of butterflies was linearly related to landscape species richness in geographic regions with high arable field cover, indicating that butterfly species richness in field boundaries is more limited by landscape factors than local habitat factors. In study landscapes containing semi-natural grasslands the relationship decelerated at high landscape species richness, suggesting that local species richness of butterflies in field boundaries is limited by habitat factors (demanding habitat specialists that occurred in semi-natural grasslands were absent in field margins). My results suggest that management options in field margins will affect mainly generalists, and species with good dispersal abilities, in landscapes with high arable field cover. Habitat specialists and species with poor dispersal abilities may benefit of management options if these are applied in the vicinity of source populations.
Resumo:
The European aspen (Populus tremula) is a keystone species for biodiversity in boreal forests. However, the future of aspen may be threatened, because large aspens have mostly been removed from managed forests, whereas regeneration and the long-term persistence of mature trees are subjects of concern in protected areas. Aspen is a pioneer tree, and it can reproduce both sexually by seed and asexually by root suckers. Through asexual reproduction aspen forms clones, groups of genetically identical trees (ramets). In my thesis, I have studied the structure of aspen populations in terms of number, size, clonal and demographic properties. Additionally, I have investigated the emergence and survival of seedlings as well as the seed quantity and quality in crosses between the European and hybrid aspen. To study the regeneration and population structure, mature aspens were recorded in old-growth and managed forests in eastern Finland based on a large-scale inventory (11 400 ha). In addition, small aspen trees were surveyed on sample plots. Clonal structure was investigated both by morphological characters and by DNA-based markers (microsatellites). Seedling emergence and survival was studied with two sowing experiments. With crosses between European and hybrid aspens we wanted to study whether elevated temperatures due to climate change would benefit the different crosses of European and hybrid aspen unequally and thus affect the gene flow between the two species. The average volumes of mature aspen were 5.3 m3/ha in continuous old-growth, and 0.8 m3/ha in managed forests. Results indicate also that large aspen trees in managed forests are a legacy of the past less intensively managed forest landscapes. Long-term persistence of aspen in protected areas can only be secured by restoration measures creating sufficiently large gaps for regeneration. More emphasis should be given to sparing aspens in thinnings and to retaining of mature aspens in regeneration cutting in managed forests. Aspen was found to be spatially aggregated in the landscape. This could be explained by site type, disturbance history and / or limitations in seed dispersal. Clonal structure does not explain the spatial aggregation, since average size of the clones was only 2.3 ramets, and most clones (70 %) consisted of just one ramet. The small size of the clones suggests that most of them are relatively young. Therefore, sexual reproduction may be more common than has previously been thought. Seedling emergence was most successful in mineral soil especially, when the site had been burned. Only few seedlings occurred on humus. Survival of the seedlings was low, and strongly dependent on moisture, but also on seedbed conditions. The seeds were found to maintain their germinability longer than has earlier been thought to be possible. Interspecific crosses produced more seeds with higher quality than intraspecific crosses. When temperature was elevated, germination of hybrid aspen seeds increased more than seeds from P. tremula x P. tremula crosses. These results suggest that hybrid aspen may have a significant genetic impact on the European aspen, and this effect may become strengthened by climate warming.
Resumo:
Soy-derived phytoestrogen genistein and 17β-estradiol (E2), the principal endogenous estrogen in women, are also potent antioxidants protecting LDL and HDL lipoproteins against oxidation. This protection is enhanced by esterification with fatty acids, resulting in lipophilic molecules that accumulate in lipoproteins or fatty tissues. The aims were to investigate, whether genistein becomes esterified with fatty acids in human plasma accumulating in lipoproteins, and to develop a method for their quantitation; to study the antioxidant activity of different natural and synthetic estrogens in LDL and HDL; and to determine the E2 esters in visceral and subcutaneous fat in late pregnancy and in pre- and postmenopause. Human plasma was incubated with [3H]genistein and its esters were analyzed from lipoprotein fractions. Time-resolved fluoroimmunoassay (TR-FIA) was used to quantitate genistein esters in monkey plasma after subcutaneous and oral administration. The E2 esters in women s serum and adipose tissue were also quantitated using TR-FIA. The antioxidant activity of estrogen derivatives (n=43) on LDL and HDL was assessed by monitoring the copper induced formation of conjugated dienes. Human plasma was shown to produce lipoprotein-bound genistein fatty acid esters, providing a possible explanation for the previously reported increased oxidation resistance of LDL particles during intake of soybean phytoestrogens. Genistein esters were introduced into blood by subcutaneous administration. The antioxidant effect of estrogens on lipoproteins is highly structure-dependent. LDL and HDL were protected against oxidation by many unesterified, yet lipophilic derivatives. The strongest antioxidants had an unsubstituted A-ring phenolic hydroxyl group with one or two adjacent methoxy groups. E2 ester levels were high during late pregnancy. The median concentration of E2 esters in pregnancy serum was 0.42 nmol/l (n=13) and in pre- (n=8) and postmenopause (n=6) 0.07 and 0.06 nmol/l, respectively. In pregnancy visceral fat the concentration of E2 esters was 4.24 nmol/l and in pre- and postmenopause 0.82 and 0.74 nmol/l. The results from subcutaneous fat were similar. In serum and fat during pregnancy, E2 esters constituted about 0.5 and 10% of the free E2. In non-pregnant women most of the E2 in fat was esterified (the ester/free ratio 150 - 490%). In postmenopause, E2 levels in fat highly exceeded those in serum, the majority being esterified. The pathways for fatty acid esterification of steroid hormones are found in organisms ranging from invertebrates to vertebrates. The evolutionary preservation and relative abundance of E2 esters, especially in fat tissue, suggest a biological function, most likely in providing a readily available source of E2. The body s own estrogen reservoir could be used as a source of E2 by pharmacologically regulating the E2 esterification or hydrolysis.
Resumo:
This study analyzes the forming of the occupational identity of the well-educated fixed-term employees. Fixed-term employment contracts amongst the well-educated labour force are exceptionally common in Finland as compared to other European countries. Two groups of modern fixed-term employees are distinguished. The first comprises well-educated women employed in the public sector whose fixed-term employment often consists of successive periods as temporary substitutes. The other group comprises well-educated, upper white-collar men aged over 40, whose fixed-term employment careers often consist of jobs of project nature or posts that are filled for a fixed period only. Method of the study For the empirical data I interviewed 35 persons (26 women and 9 men) in 33 interviews, one of which was conducted by e-mail and one was a group interview. All the interviews were electronically recorded and coded. All the interviewees have two things in common: fixed-term employment and formal high education. Thirteen (13) of them are researchers, four nurses, four midwives, four journalists, and ten project experts. I used the snowball method to get in touch the interviewees. The first interviewees were those who were recommended by the trade unions and by my personal acquaintances. These interviewees, in turn, recommended other potential interviewees. In addition, announcements on the internet pages of the trade unions were used to reach other interviewees. In analysing process I read the research material several times to find the turning points in the narrative the interviewees told. I also searched for the most meaningful stories told and the meaning the interviewees gave to these stories and to the whole narrative. In addition to that I paid attention to co-production of the narrative with the interviewees and analyzed the narrative as performance to be able to search for the preferred identities the interviewees perform. (Riesman 2001, 698-701). I do not pay much attention to the question of truth of a narrative in the sense of its correspondence with facts; rather I think a working life narrative has two tasks: On the one hand one has to tell the facts and on the other hand, he/she has to describe the meaning of these facts to herself/himself. To emphasize the double nature of the narrative about one’s working life I analyzed the empirical data both by categorizing it according to the cultural models of storytelling (heroic story, comedy, irony and tragedy) and by studying the themes most of the interviewees talked about. Ethics of the study I chose to use narrative within qualitative interviews on the grounds that in my opinion is more ethical and more empowering than the more traditional structured interview methods. During the research process I carefully followed the ethical rules of a qualitative research. The purpose of the interviews and the research was told to the interviewees by giving them a written description of the study. Oral permission to use the interview in this research was obtained from the interviewees. The names and places, which are mentioned in the study, are changed to conceal the actual identity of the interviewees. I shared the analysis with the interviewees by sending each of them the first analysis of their personal interview. This way I asked them to make sure that the identity was hidden well enough and hoped to give interviewees a chance to look at their narratives, to instigate new actions and sustain the present one (Smith 2001, 721). Also I hoped to enjoy a new possibility of joint authorship. Main results As a result of the study I introduce six models of telling a story. The four typical western cultural models that guide the telling are: heroic story, comedy, tragedy and satirical story (Hänninen 1999). In addition to these models I found two ways of telling a career filled with fixed-term employments that differ significantly from traditional career story telling. However, the story models in which the interviewees pour their experience locates the fixed term employers work career in an imagined life trajectory and reveals the meaning they give to it. I analyze the many sided heroic story that Liisa tells as an example of the strength of the fear of failing or losing the job the fixed term employee feels. By this structure it is also possible to show that success is felt to be entirely a matter of chance. Tragedy, the failure in one’s trial to get something, is a model I introduce with the help of Vilppu’s story. This narrative gets its meaning both from the sorrow of the failure in the past and the rise of something new the teller has found. Aino tells her story as a comedy. By introducing her narrative, I suggest that the purpose of the comedy, a stronger social consensus, gets deeper and darker shade by fixed-term employment: one who works as a fixed term employee has to take his/her place in his/her work community by him/herself without the support the community gives to those in permanent position. By studying the satiric model Rauno uses, I argue that using irony both turns the power structures to a carnival and builds free space to the teller of the story and to the listener. Irony also helps in building a consensus, mutual understanding, between the teller and the listener and it shows the distance the teller tells to exist between him and others. Irony, however, demands some kind of success in one’s occupational career but also at least a minor disappointment in the progress of it. Helmi tells her story merely as a detective story. By introducing Helmi’s narrative, I argue that this story model strengthens the trust in fairness of the society the teller and the listener share. The analysis also emphasizes the central position of identity work, which is caused by fixed-term employment. Most of the interviewees talked about getting along in working life. I introduced Sari’s narrative as an example of this. In both of these latter narratives one’s personal character and habits are lifted as permanent parts of the actual professional expertise, which in turn varies according to different situations. By introducing these models, I reveal that the fixed-term employees have different strategies to cope with their job situations and these strategies vary according to their personal motives and situations and the actual purpose of the interview. However, I argue that they feel the space between their hopes and fears narrow and unsecure. In the research report I also introduce pieces of the stories – themes – that the interviewees use to build these survival strategies. They use their personal curriculum vitae or portfolio, their position in work community and their work morals to build their professional identity. Professional identity is flexible and varies in time and place, but even then it offers a tool to fix one’s identity work into something. It offers a viewpoint to society and a tool to measure one’s position in surrounding social nets. As one result of the study I analyze the position the fixed-term employees share on the edge of their job communities. I summarize the hopes and fears the interviewees have concerning employers, trade unions, educational institutions and the whole society. In their opinion, the solidarity between people has been weakened by the short-sighted power of the economy. The impact the fixed-term employment has on one’s professional identity and social capital is a many-sided and versatile process. Fixed-term employment both strengthens and weakens the professional identity, social capital and the building of trust. Fixed-term employment also affects one’s day-to-day life by excluding her/him from the norm and by one’s difficulty in making long-term plans (Jokinen 2005). Regardless of the nature of the job contract, the workers themselves are experts in making the best of their sometimes less than satisfying work life and they also build their professional identity by using creatively their education, work experiences and interpersonal relations. However, a long career of short fixed-term employments may seriously change the perception of employee about his/her role. He/she may start concentrating only in coping in his/her unsatisfactory situation and leaves the active improvement of the lousy working conditions to other people. Keywords: narrative, fixed-tem employment, occupational identity, work, story model, social capital, career
Resumo:
Many languages exploit suprasegmental devices in signaling word meaning. Tone languages exploit fundamental frequency whereas quantity languages rely on segmental durations to distinguish otherwise similar words. Traditionally, duration and tone have been taken as mutually exclusive. However, some evidence suggests that, in addition to durational cues, phonological quantity is associated with and co-signaled by changes in fundamental frequency in quantity languages such as Finnish, Estonian, and Serbo-Croat. The results from the present experiment show that the structure of disyllabic word stems in Finnish are indeed signaled tonally and that the phonological length of the stressed syllable is further tonally distinguished within the disyllabic sequence. The results further indicate that the observed association of tone and duration in perception is systematically exploited in speech production in Finnish.
Resumo:
A comparison of microsite occupancy and the spatial structure of regeneration in three areas of late-successional Norway spruce dominated forest. Pallas-Ylläs is understood to have been influenced only by small-scale disturbance; Dvina-Pinega has had sporadic larger-scale disturbances; Kazkim has been affected by fire. All spruce and birch trees with diameter at breast height (DBH) ?10 cm were mapped in five stands on 40 m x 400 m transects, and those with DBH < 10 cm on 2 or 4 m x 400 m subplots. Microsite type was inventoried at 1m intervals along the centre line and for each tree with DBH < 10 cm. At all study areas small seedlings (h < 0.3 m, DBH < 10 cm) preferentially occupied disturbed microsites. In contrast, spruce saplings (h ? 1.3 m, DBH <10 cm) at all study areas showed less, or no, preference. At Pallas-Ylläs spruce seedlings (h < 1.3 m, DBH < 10 cm) and saplings (h ? 1.3 m, DBH < 10 cm) exhibited spatial correlation at scales from 32-52 m. At Dvina-Pinega saplings of both spruce and birch exhibited spatial correlation at scales from 32-81 m. At Kazkim spatial correlation of seedlings and saplings of both species was exhibited over variable distances. No spatial cross-correlation was found between overstorey basal area (DBH ? 10 cm) and regeneration (h ? 1.3 m, DBH < 10 cm) at any study area. The results confirm the importance of disturbed microsites for seedling establishment, but suggest that undisturbed microsites may sometimes be more advantageous for long-term tree survival. The regeneration gap concept may not be useful in describing the regeneration dynamics of late-successional boreal forests.
Resumo:
To protect and restore lake ecosystems under threats posed by the increasing human population, information on their ecological quality is needed. Lake sediments provide a data rich archive that allows identification of various biological components present prior to anthropogenic alterations as well as a constant record of changes. By providing a longer dimension of time than any ongoing monitoring programme, palaeolimnological methods can help in understanding natural variability and long-term ecological changes in lakes. As zooplankton have a central role in the lake food web, their remains can potentially provide versatile information on past trophic structure. However, various taphonomic processes operating in the lakes still raise questions concerning how subfossil assemblages reflect living communities. This thesis work aimed at improving the use of sedimentary zooplankton remains in the reconstruction of past zooplankton communities and the trophic structure in lakes. To quantify interspecific differences in the accumulation of remains, the subfossils of nine pelagic zooplankton taxa in annually laminated sediments were compared with monitoring results for live zooplankton in Lake Vesijärvi. This lake has a known history of eutrophication and recovery, which resulted from reduced external loading and effective fishing of plankti-benthivorous fish. The response of zooplankton assemblages to these known changes was resolved using annually laminated sediments. The generality of the responses observed in Lake Vesijärvi were further tested with a set of 31 lakes in Southern Finland, relating subfossils in surface sediments to contemporary water quality and fish density, as well as to lake morphometry. The results demonstrated differential preservation and retention of cladoceran species in the sediment. Daphnia, Diaphanosoma and Ceriodaphnia were clearly underrepresented in the sediment samples in comparison to well-preserved Bosmina species, Chydorus, Limnosida and Leptodora. For well-preserved species, the annual net accumulation rate was similar to or above the expected values, reflecting effective sediment focusing and accumulation in the deepest part of the lake. The decreased fish density and improved water quality led to subtle changes in zooplankton community composition. The abundance of Diaphanosoma and Limnosida increased after the reduction in fish density, while Ceriodaphnia and rotifers decreased. The most sensitive indicator of fish density was the mean size of Daphnia ephippia and Bosmina (E.) crassicornis ephippia and carapaces. The concentration of plant-associated species increased, reflecting expanding littoral vegetation along with increasing transparency. Several of the patterns observed in Lake Vesijärvi could also be found within the set of 31 lakes. According to this thesis work, the most useful cladoceran-based indices for nutrient status and planktivorous fish density in Finnish lakes were the relative abundances of certain pelagic taxa, and the mean size of Bosmina spp. carapaces, especially those of Bosmina (E.) cf. coregoni. The abundance of plant-associated species reflected the potential area for aquatic plants. Lake morphometry and sediment organic content, however, explained a relatively high proportion of the variance in the species data, and more studies are needed to quantify lake-specific differences in the accumulation and preservation of remains. Commonly occurring multicollinearity between environmental variables obstructs the cladoceran-based reconstruction of single environmental variables. As taphonomic factors and several direct and indirect structuring forces in lake ecosystems simultaneously affect zooplankton, the subfossil assemblages should be studied in a holistic way before making final conclusions about the trophic structure and the change in lake ecological quality.