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Wagner A.,TU Dresden | Hulsmann S.,TU Dresden | Horn W.,Saxon Academy of science at Leipzig | Schiller T.,TU Dresden | And 4 more authors.
Freshwater Biology | Year: 2013

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 (14days 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 4weeks, 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. © 2012 Blackwell Publishing Ltd. Source

Brauer K.,Helmholtz Center for Environmental Research | Kampf H.,Helmholtz Center Potsdam | Koch U.,Saxon Academy of science at Leipzig | Strauch G.,Helmholtz Center for Environmental Research
Chemical Geology | Year: 2011

We report new data on gas and isotope compositions of mantle-derived exhalations from five locations in the Vogtland (Germany)/NW Bohemia (Czech Republic) area, close to the Nový Kostel focal zone, recorded by monthly sampling of gases over a three year period. This region is the locus typicus for the term "earthquake swarm"; since 1985/86 the largest numbers of earthquake swarms in the Vogtland/NW Bohemia have occurred in the Nový Kostel focal zone. Mantle-derived degassing has been studied at four locations within the Cheb Basin degassing centre, and at the Wettinquelle spring to the north on the edge of the Cheb Basin. The Bublák and U Mostku locations are on the Počatky Plesná Fault Zone (PPZ) while the Kopanina and Dolni Častkov locations are on the Mariánské Lázně Fault (MLF). The mantle-derived helium content at locations along the PPZ covers the range of the sub-continental lithospheric mantle (SCLM). Along the MLF the 3He/4He ratios are with up to 5.4Ra a little bit lower as at the PPZ locations. The CO2/3He ratios point to a predominantly magmatic source. Depending on the magnitude of gas flux the monthly sampling results indicated various major influences on the fluid signatures at the monitored locations due to seasonal cycles based on the different strong influence of CO2/water interaction. Micro-seismicity occurred repeatedly during the monitoring period and seismically triggered geochemical anomalies were repeatedly observed at the locations close to the focal zone. The 3He/4He ratios drop from 5.9Ra (mean) up to 5.6Ra at Bublák and from 5.6Ra up to 5.3Ra at U Mostku. A decrease in 3He/4He ratios was observed prior to seismic events due to stress accumulation before rupturing, and also after the events due to the release of crustal-derived components within the area of the focal zone. Due to the higher 'helium baseline content' of the Kopanina gas the 3He/4He ratios decreased there only from 4.6 to 4.5Ra. At the Wettinquelle, the most distant from the focal zone, the 3He/4He ratios ranged between 2.3 and 2.5Ra altogether. Superimposed to the seismically induced anomalies a clear three month-lasting increase of mantle-derived helium was observed at the locations on the PPZ. The 3He/4He ratios increased to 6.3Ra at Bublák and to 6.0Ra at U Mostku. An increase in the 3He/4He ratios was also noted at Kopanina (to 4.7Ra) and Dolni Častkov (to 5.4Ra), interpreted as indicating an ascent of less degassed magma (dyke intrusions) from the deeper lithospheric mantle reservoir. Superimposed geochemical effects initiated by the temporal and spatial evolution of a hidden magmatic-driven geodynamic process have therefore been observed in a continental rift system for the first time. © 2011 Elsevier B.V. Source

Paul R.J.,University of Munster | Mertenskotter A.,University of Munster | Pinkhaus O.,University of Munster | Pirow R.,University of Munster | And 5 more authors.
Limnology and Oceanography | Year: 2012

A field and laboratory study was carried out over 3 yr to determine relationships between seasonal and interannual changes in temperature (year-specific temperature courses, presence or absence of ice in winter) and the genetic structure (composition of multilocus genotypes [MLGs]) of a Daphnia longispina assemblage. Field studies on temperature and genetic structures were linked with laboratory analyses to evaluate the thermal tolerance of long-term 12°C-, 18°C-, and 24°C-acclimated clonal lineages (CLs) derived from abundant MLGs sampled in the field (surface water and thermocline). The tolerance to warm temperatures (heat tolerance) was lowest in CLs derived from MLGs that were dominant directly after or before winter (winter-CLs), higher in "spring-autumn-CLs," and highest in "summer-CLs." Winter-CLs also showed the highest degree of physiological plasticity. The differences in heat tolerance were mainly related to the different genotypes of the phosphoglucomutase (PGM) locus. Temperature conditions during winter and early spring affected the heat tolerance of all CLs as well as the success of different winter survival strategies (overwintering, resting eggs). Heat tolerance was lowest in CLs derived from MLGs sampled in 2006 (after the coldest winter and spring period), higher in CLs from 2005 (after a less cold winter and spring period), and highest in CLs from 2007 (after a warm, ice-free winter). In addition to other environmental factors (predation, parasitism, food), seasonal and interannual changes in temperature affect Daphnia genetic structure through genetic differences in thermal responses, thermal tolerance, and physiological plasticity. © 2012, by the Association for the Sciences of Limnology and Oceanography, Inc. Source

Schwerins S.,University of Munster | Zeis B.,University of Munster | Horn W.,Saxon Academy of science at Leipzig | Horn H.,Saxon Academy of science at Leipzig | Paul R.J.,University of Munster
Limnology and Oceanography | Year: 2010

The function of hemoglobin (Hb), which is a key protein of many species, was analyzed in Daphnia from the field. Temperature and oxygen content; abundance and composition of phytoplankton; and abundance, body, and clutch sizes of the Daphnia assemblage were measured in the epilimnion and around the thermocline of a reservoir for 2 yr. In addition, carbohydrate, lactate, and hemoglobin concentrations were determined in whole Daphnia. In Daphnia from the thermocline, hypoxia-induced Hb expression was responsible for an increase in Hb concentration in parallel with a decrease in oxygen content. In Daphnia from oxygen-rich water (epilimnion), however, Hb concentration unexpectedly showed a negative relationship to temperature and to the animals' temperature-dependent metabolic rate. Furthermore, the seasonal minima of Hb concentration coincided with low food (phytoplankton) availability and maxima of Daphnia's reproductive activity. These data suggest that Hb serves a dual function as respiratory protein and protein store. Laboratory experiments supported this hypothesis: starvation caused the concentration of Hb and other hemolymph proteins to decrease during normoxia, whereas the hemolymph protein store remained stable during hypoxia. Hemolymph proteins obviously function as a buffer between food (protein) availability and protein demand, with the latter increasing with temperature mainly because of elevated growth and reproduction rates. Accordingly, the size of the hemolymph protein store can indicate the state of nutrition of Daphnia (and possibly other species) from oxygen-rich water, which can easily be assessed by spectroscopic measurement of Hb in whole animals. © 2010, by the American Society of Limnology and Oceanography, Inc. Source

Scheifhacken N.,TU Dresden | Horn H.,Saxon Academy of science at Leipzig | Paul L.,TU Dresden
Water Research | Year: 2010

In a one-year study, the multispecies assemblages of phytoplankton (picoplankton to microplankton) within a drinking water reservoir were counted, determined and evaluated in their size fractions using microscope enumeration (MC). The manual counts were compared with the size evaluation obtained by a light obscuring particle counter (PC) in order to evaluate its use for the monitoring practice of a drinking water reservoir. With this multispecies comparison we present a novel approach for the evaluation of automated counting systems.The picoplankton clearly remained uncounted by the PC even though its lower size limits imply an adequate match. The highest and most consistent count numbers of plankton (nano- and microplankton) and particles were obtained during the spring mass development. However, from the middle of the year onwards, the measured particle concentration surpassed the counted plankton abundances by two- to threefold indicating the rise of seston within the water column. This fraction would be missed if counted solely by MC. Further, the PC consistently undersized the biological counts, but not the minerogenic fraction represented by the manganese oxidising bacteria. Consequently, the rise and decline of Metallogenium bacteria was reliably monitored with the PC.The PC provides additional size information compared to other bulk optical sensors (turbidity, chlorophyll-. a). The correlation of particles with probe measurement always exceeded the plankton coefficient, but all combinations of plankton, particle and probe measurement revealed significant linear regressions. However, the redundancy of the chlorophyll-. a probes was also shown in order to explain plankton abundances.Our results indicate that background knowledge of the monitored system and cautious interpretation of data is required to allocate and understand automated particle counts. Therefore, only in combination with MC, the PC enables phytoplankton or minerogenic particle counts under frequent real-time monitoring conditions. As such it may serve as a helpful tool for example in critical situations in the management of drinking water reservoirs. © 2010 Elsevier Ltd. Source

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