1. In a combined field and laboratory study, seasonal relationships between water temperature and oxygen content, genetic structure (composition of MultiLocus Genotypes, MLGs) of a Daphnia assemblage (D. galeata-hyalina hybrid species complex), and the physiological properties of clones of frequent MLGs were studied. In accordance with the oxygen-limited thermal tolerance hypothesis, essential physiological variables of oxygen transport and supply were measured within the tolerable temperature range. 2. A few MLGs (types T1-T4) were frequent during early spring and late autumn at surface temperatures below 10°C. Clones of T1-T4 showed a low tolerance towards higher temperatures (above 20°C) and a high phenotypic plasticity under thermal acclimation in comparison to clones derived from frequent MLGs from later seasons, and stored highmedium quantities of carbohydrates at 12 and 18°C. 3. Another MLG (T6) succeeded the MLGs T1-T4. T6 was frequent over most of the year at temperatures above 10°C and below 20°C. A clone derived from T6 exhibited a high tolerance towards warm temperatures and a more restricted phenotypic plasticity. It stored high-medium quantities of carbohydrates at 12, 18 and 24°C and showed a high capacity for acclimatory adjustments based on haemoglobin expression. 4. During the summer period at temperatures ‡20°C, the MLG T6 was found mainly near to the thermocline, where temperature and oxygen content were distinctly lower, and to a lesser extent in surface water. At the surface, another MLG (T19) was predominant during this period. A clone of this MLG showed a very high tolerance towards warm temperatures, minimal phenotypic plasticity, low carbohydrate stores and a high capacity for circulatory adjustments to improve oxygen transport at higher temperatures. 5. This study provides evidence for connections between the spatio-temporal genetic heterogeneity of a Daphnia assemblage and the seasonal changes of water temperature and oxygen content. The data also suggest that not only the actual temperature but also the dynamics of temperature change may influence the genetic structure of Daphnia populations and assemblages.

Summary 1. Climate warming may cause disruption of trophic linkages in aquatic ecosystems and lead to changes in abundance and genetic structure of zooplankton populations. We monitored the community of the Daphnia galeata‐hyalina hybrid complex in the Saidenbach Reservoir (Saxony, Germany) using allozyme electrophoresis for three consecutive years (2005–07), including one (2007) following an unusually warm winter that prevented the formation of ice cover for the first time in the history of the reservoir. 2. Genetic composition during the 2007 season differed substantially from the two preceding years that experienced the usual 3‐month ice period. Three abundance peaks in June, July and October 2007 were dominated by hybrids of Daphnia galeata x hyalina, whereas in the 2005 and 2006 seasons two peaks in June and September were dominated by Daphnia hyalina genotypes. 3. The genetic composition of the pool of diapausing eggs produced in autumn and the rate of change of genotype abundance during the following spring indicate recruitment of the D. hyalina subpopulation from ex‐ephippial animals during the spring population increase. 4. The differing potential to contribute to the overwintering animal pool or to the inoculum from diapausing eggs was confirmed by results from laboratory life‐table experiments. Daphnia galeata clones survived longer and produced parthenogenetic offspring under winter conditions, whereas D. hyalina clones showed a shorter lifespan and produced resting eggs. 5. Our results indicate a profound role of recruitment strategy in the observed shift in genetic composition. Increasing winter temperatures predicted in the context of climate change may thus favour overwintering animals, leading to an increase in the contribution of these genotypes to the population. Such microevolutionary processes may dampen possible seasonal mismatches between daphnid populations and their food or predator populations.

Summary 1. Long‐term data on the meteorology, hydrology, physicochemistry and plankton of a reservoir and its tributaries in SE Germany run from 1976 until now. This dimictic reservoir changed from mesotrophic to eutrophic in the 1970s, remained eutrophic in the 1980s and returned to the mesotrophic state after a sharp reduction in P loading in 1990. 2. Phytoplankton biomass reaches an annual maximum in spring and consists almost entirely of diatoms. While Asterionella formosa was dominant until 1990, Aulacoseira subarctica became more frequent at the end of the 1990s and was particularly abundant in years with short winters. 3. Statistical analyses suggested that these changes were triggered primarily by the mild winters that were frequent after 1988. Climate‐related hydrophysical variables and the initial biomass of the diatoms at the beginning of the year, considered as an ‘inoculum’, were identified as most important. These variables explained 39% of the total variance of the relative abundance, whereas the change in trophic conditions was responsible for about 20%. 4. The absolute and relative abundance of A. subarctica was positively related to short ice cover, early ice‐out and a long‐lasting spring circulation. Owing to its physiological traits, and particularly its ability to survive under low‐light conditions, A. subarctica benefitted from short, mild winters. Under such conditions, it could sustain or establish a high initial biomass, whereas the concentrations of the other diatoms decreased over winter. However, this advantage may be lost if further warming causes an early onset of summer stratification. Because of its low population growth rate and requirement for high turbulence, A. subarctica needs long, cold springs to exploit the improved starting conditions and to become abundant. 5. In contrast to A. subarctica, A. formosa required a substantial soluble reactive phosphorus supply to compete successfully. The eutrophic conditions until 1990 were the prerequisite for its mass growth under low‐light and low‐temperature conditions during the spring. After reduction in P concentration from 1990, A. formosa declined and other diatom species became more abundant. 6. These other diatoms may be viewed as ‘stopgaps’ when conditions were not favourable for A. subarctica or A. formosa. Diatoma elongatum exploited brief circulation periods in years with low P loading. Synedra acus and Fragilaria crotonensis, because of their poor competitive ability at low light intensity, reached high density in the upper water column in the transitional period between spring circulation and summer stratification. 7. Our study suggests that climate‐related variables have crucial impacts on the spring phytoplankton dynamics of deep stratified waterbodies. They can mask the consequences of changes in the trophic conditions and, corresponding to the functional traits of the different phytoplankton species, also decisively control their relative abundances. In this reservoir, the warmer winters and prolonged spring circulation...

Phptoplankton losses due to zooplankton grazing in the Saidenbach drinking wnter reservoir were ralrulated on the basis of laboratory experiments and field studies. The dependence of grazing rate and filtering rate on phytoplankton concentration, temperature and other factors is described.The amount of wnter filtered daily by the zooplankton population is 100-200 ml/l. d. Thegrazing performance of the zooplankton thus sometimes exceeds the growth rate of the phytoplankton population and leads to its decimation, this being shown especially clearly by phytoplankton loss rates up to 0.2 d-1 in the early summer.

Weigel, U., Horn, W. and Hock, B. 1984. Endogenous auxin levels in stem cuttings of Chrysatuhemum tnorifoUum during adventitious rooting. -Physiol. Plant 61 • 422-428.Free and ester-bound IAA were determined in Chtysattthemum tnorifoliutn Ratnat cv. 'Yellow Galaxy' by means of a solid phase enzyme immunoassay. In shoots, free auxin decteases basipetally whereas ester IAA teaches a maximum in the middle part. After making the cuttings, a strong increase in both free and ester iAA (or total IAA, respectively) is found up to the time when the first adventitious roots become visible. Only prolonged irradiance of stock plants at high light intensities (40 W tn--) delays this increase in the cuttings, concomitantly with a lower number of roots compared to the controls (4.5 W m-), although root growth as determined by measuring root length or fresh weight is not affected. A distinct relation is found between IAA content of stock plants at the time when cuttings are taken and the number of adventitious roots formed by the cuttings 20 days later.Additional key words -^nzyrac immunoassay, light.U. Weigel and B. Hock (reprint requests), Lehrstuhl fiir Botanik, Fakidtdt Landwirtschaft und

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Summary 1. In an extensive field study, we tested the hypothesis that warming during a sensitive period alters trophic interactions in pelagic food webs of dimictic lakes: Matching of predation on the key herbivore Daphnia by vertebrate (Perca fluviatilis) and invertebrate (Leptodora kindtii) predators would destabilise Daphnia populations during summer. To predict food‐web‐mediated effects of climate warming, we relied on phase‐specific warming trends (during winter, early thermal stratification, summer stagnation) of an 11‐year (1999–2009) period to study instantaneous or time‐delayed ecological responses. 2. Warming during the period of early thermal stratification (here corresponding to May) rather than during winter or summer was found to generate complex, time‐delayed cascading effects on Daphnia via timing and rates of predation. In contrast to our hypothesis, warming exceeding a critical mean temperature (14 °C) during May increased the stability of the Daphnia population during summer. 3. The termination of predation on Daphnia by older perch and L. kindtii was advanced even stronger (14 days per degree Celsius warming during May) than the start of predation. Warming during May thus shortened the period of synchronised predation on Daphnia by older perch, young‐of‐the‐year perch and L. kindtii (match index) by up to 4 weeks, thereby decreasing total rates of predation on Daphnia during July by up to 40%. 4. The timing of the top‐down control of Leptodora dynamics by perch proved to be a key process for population stability of Daphnia during summer compared to the less important direct cascading effect of fish on Daphnia and temperature effects on Leptodora growth and consumption or on Daphnia reproduction. 5. We conclude that predicting the consequences of climate warming for plankton dynamics requires a mechanistic understanding of lake‐specific temperature‐driven changes in trophic cascades, especially when an invertebrate predator is dynamically coupled both to the key herbivore and to their predators.

In two reservoirs of different trophic state, the abundance of autotrophic picoplankton was continuously assessed for 7 and 11 years, respectively. Simultaneously, other important environmental and biotic factors (nutrients, temperature, phyto-and zooplankton) were analysed. The mesotrophic Saidenbach Reservoir receives five-to sevenfold higher phosphorus imports, and exhibits twice higher phosphorus contents, and tenfold higher phytoplankton concentrations than the oligotrophic Neunzehnhain Reservoir. Despite these differences, the quantities and dynamics of autotrophic picoplankton (APP) showed remarkable similarities in both reservoirs. The APP consisted almost exclusively of Phycoerythrin-rich cyanobacteria. A notable growth of cyanobacteria never started before the onset of stratification and finished no later than the beginning of the autumnal turnover. Although the soluble reactive phosphorus (SRP) concentrations in the epilimnion were always near the limit of detection, there was no evidence for growth restriction of APP by nutrients in either of the reservoirs. However, the data reveal a strong grazing impact of all potential grazers, especially of the daphnids. This zooplankton group always showed a strict behaviour opposite of the APP during its growth periods. The grazing influence of rotifers and protozoans is less pronounced but also obvious, notable by the frequently observed inverse relationships between their quantities and those of APP. It is assumed that during the stratification period in the summer the APP dynamics is mainly top-down controlled.