Carola Winkelmann

Physiological indicators of fitness in benthic invertebrates: a useful measure for ecological health assessment and experimental ecology

Physiological indicators of fitness present a measure of an organism’s response to a changing environment. An analysis of how these organisms allocate and store their energy resources provides an understanding of how they cope with such environmental changes. Each individual has to balance the investment necessary to acquire a certain resource with the energy gained by it. This trade-off can be monitored by measuring several physiological indicators of fitness such as energy storage components, metabolic state or RNA/DNA ratio. Because environmental adaptations and ecological strategies of survival are best examined within the natural environment, our research has to rely on the physiological indicators that are easily accessible in the field. The physiological indicators presented here are significant for an individual’s fitness and in turn lead to reliable values in field-collected samples. Based on our own expertise and on a literature survey, the physiological relevance of the presented indicators is explained. Furthermore, some consideration to the analytical methods used to obtain the physiological indicators is given, and possible errors introduced at the sampling site and during the laboratory procedures are discussed. This work demonstrates that the integration of ecological and physiological expertise facilitates the identification of future ecological problems much earlier than separate approaches of both disciplines alone.

The management of metabolic energy storage during the life cycle of mayflies: a comparative field investigation of the collector-gatherer Ephemera danica and the scraper Rhithrogena semicolorata

The concentration and seasonal dynamics of the major energy storage components, triglycerides and glycogen, were measured in two species of mayfly (Rhithrogena semicolorata and Ephemera danica) with contrasting life cycle strategies living in a small mountain stream. E. danica is a burrowing, semivoltine collector-gatherer; R. semicolorata is univoltine and scrapes periphyton from stones. This is the first publication which focuses on the role of metabolic energy sources during the larval life span of two mayfly species until the larvae emerge. Although triglycerides are the major energy reserve in both species (>84% of total energy storage) throughout the whole larval development their seasonal dynamic differed considerably. In R. semicolorata the triglyceride concentration declined during the last weeks prior to emergence in both sexes. The same pattern was found in female larvae of E. danica, but not in male E. danica. It is suggested that females use triglycerides in the last larval stages for egg maturation, which is completed in the last larval instar. In male E. danica the triglyceride concentrations remained high until emergence, presumably due to their high energy demands as adults for their swarming flights. Glycogen concentrations did not show such a difference between species and sexes. Its significance as a storage substrate for energy is rather low; however, concentrations decreased in both species and sexes prior to emergence.

Omnivores as seasonally important predators in a stream food web1

Abstract. Omnivory is prevalent in terrestrial and aquatic food webs. However, the extent and seasonality of predatory feeding by omnivores in stream food webs is largely unknown. To understand better these aspects of omnivory in stream food webs, we investigated seasonal changes in the trophic positions of 2 omnivores in a small forested stream. We selected the amphipod, Gammarus pulex, and the caddisfly larvae, Hydropsyche spp., as key organisms because both taxa are common, reach high biomasses in many stream ecosystems, and have broad food spectra. We used stable-isotope analysis of the most prevalent taxa in the benthic macroinvertebrate community to assess the trophic positions of the 2 omnivorous taxa in different seasons. We estimated the degree of predatory feeding by quantifying the importance of different food resources with a stable-isotope mixing model (IsoSource). The predation capacity of omnivores, defined as the fraction of omnivore biomass associated with predation, was compared to the biomass of strictly predatory invertebrates. Our analysis indicated a predation capacity of Hydropsyche spp. similar to that of strict invertebrate predators, whereas the predatory biomass of G. pulex was even higher than those of the other invertebrate predators because of a combination of high biomass and large proportions of animal prey (50–90%) in the diets of both omnivores. The trophic position of omnivores in winter was comparable to that of invertebrate predators (trophic level 3), whereas in summer, their trophic position was comparable to primary consumers (level 2). These shifts might be caused by seasonally varying prey availability for Hydropsyche spp. and life-cycle patterns for G. pulex. Predation by omnivores was generally high in our study stream and probably has been underestimated in other stream ecosystems where omnivores, such as Hydropsyche spp. and G. pulex, are common.

Abiotic features and macroinvertebrate colonization of the hyporheic zones of two tributaries of the river Elbe (Germany)

Abstract Whole-ecosystem experiments may reach statistical pertinence when reference and treatment sites are compared. Therefore at least two similar systems must be found. To this aim we tried to answer the question if the hyporheic zones of both streams have similar features. We postulated that both streams would be unsuitable for a comparison in an ecosystem experiment if the structure of their hyporheic invertebrate communities differs significantly. Although pH, electrical conductivity and nitrate concentration differed in the surface water, variability of all chemical criteria studied in the hyporheic zones was much lower between the two streams compared to the variability within each stream. Thus the structure of both hyporheic invertebrate communities did not differ significantly. Because of this uniformity both streams are regarded to be suitable for a comparison in a whole-ecosystem experiment.

Fish predation can induce mesohabitat-specific differences in food web structures in small stream ecosystems

Diverse benthic communities in streams include a wide variety of predators with different habitat preferences, e.g. for pools or riffles. We hypothesised that these preferences result in mesohabitat-specific predator community structures with quantitative differences concerning predation intensity by vertebrate and invertebrate predators, importance of intraguild predation, or top–down pressure. This hypothesis was evaluated for a small submontane stream by means of mesohabitat-specific quantification of prey consumption by two benthivorous fish species (Gobio gobio and Barbatula barbatula) and several invertebrate predators. The estimation was based on daily food rations and diet composition of predators and mesohabitat-specific predator biomass. We found clear differences between the two mesohabitat types. Predator food webs were less complex in pools than in riffles. Fish predation was more important than invertebrate predation in pools, and intraguild predation had a higher relative importance in these mesohabitats. These differences were probably caused by the mesohabitat use of G. gobio, the largest top predator, which preferred pools. Consequently, the predator food webs were more similar between the mesohabitats when fish were absent. Top–down pressure on primary consumers by all predators together was lowest in pools without fish, but the effect was not significant. Omnivory (including cannibalism) was intense, but its potentially destabilising effects were probably counterbalanced by mesohabitat connectivity. From the results of our experimental study, we conclude that even in small stream ecosystems, food web structures and predation pathways can differ between mesohabitats and that a mesohabitat-specific consideration will help to explain the variety of top–down effects on benthic communities.

River-specific effects of the invasive amphipod Dikerogammarus villosus (Crustacea: Amphipoda) on benthic communities

The invasive amphipod Dikerogammarus villosus is assumed to threaten native biodiversity in rivers. In spite of small-scale experiments and field observations, its impact on natural communities is largely unknown because it seems to be variable and long-term analyses are rare. We analysed long-term data from the Upper Elbe and Middle Rhine (Germany) for invasion patterns and changes in the community structure. In addition, mesocosm experiments were performed in both rivers to identify density effects of D. villosus on the communities. We assumed that D. villosus is a driver of changes in the macroinvertebrate community and that effects are river-specific due to differing benthic communities. We found two invasion patterns for D. villosus with fast invasion in the River Elbe and slower invasion in the River Rhine. The impact of D. villosus on the species composition was weak in both river communities. Invasion seems to have reduced taxa number and individuals and increased Shannon diversity in the River Rhine, but not in the River Elbe. The correlations between the densities of the invader and other taxa in the long-term data were mostly positive with the exception of two native taxa in the River Rhine, indicating a lack of strong negative species interactions. Also in the mesocosm experiments, the biomass gradient of D. villosus adults did not cause significant changes in the communities. The community in the River Rhine seemed to be more vulnerable to the D. villosus invasion than that in the River Elbe. This might be caused by a dominance of invasive species interacting positively with one another, as suggested by the ‘invasional meltdown’ theory. The study suggests that community-level effects of invasion may differ between rivers, probably due to differences in the community composition.

Physiological and behavioral responses of the invasive amphipod, Dikerogammarus villosus, to ammonia.

We studied the physiological and behavioral responses of the Ponto-Caspian amphipod Dikerogammarus villosus during exposure to four total ammonia concentrations (NH3+NH4(+); TotAmm): 0.003 (control), 0.06, 1.6, and 7.0 mmol L(-1) (0.042, 0.84, 22.4, and 98.0 mg L(-1)) for a period of up to 12 h at 21 °C. During the transition period from the control to treatment concentration as well as during the first hour of exposure to 0.06 and 1.6 mmol L(-1), gammarids increased their locomotor activity, which was manifested in significantly higher routine metabolic rates compared to control conditions. At control conditions, the resting metabolic rate amounted to 0.98±0.26 mW g(-1) and significantly increased by 19 and 37% at 0.06 and 1.6 mmol L(-1), respectively. The highest examined [TotAmm] caused a rapid and significant increase in resting metabolic rate by 37% within the first 4 h of exposure before gammarids died. The exposure to elevated [TotAmm] also resulted in a significant decreased RNA:DNA ratio and significantly higher glycogen concentrations compared to the control. We conclude that even a short exposure to TotAmm of 0.06 mmol L(-1), which may occur in natural habitats, disturbs the physiology and behavior of D. villosus and leads to increased metabolic costs of the maintenance and reduced protein synthesis.

Larval growth and metabolic energy storage of Micropterna lateralis (Trichoptera: Limnephilidae) in an intermittent stream: glycogen dominates in final instars

The caddisfly species Micropterna lateralis is an abundant representative of limnephilids in intermittent streams. Yet, its basic life history characteristics and adaptations related to environmental factors, such as stream drying, are comparatively understudied. Here, we investigated larval growth and metabolic energy reserves (glycogen, triglycerides) through development in their natural habitat. We concentrated on the larval development because this period represents the important phase of energy accumulation necessary for growth, metamorphosis and embryogenesis. Besides larval physiology, female adults were studied in terms of ovarian maturation. Our results indicate that adult females lack an imaginal diapause, which is otherwise often observed in intermittent stream-inhabiting Limnephilidae. Further, M. lateralis is univoltine and exhibits a relatively fast larval development with five distinct instars, of which four are characterised here (instars II–V). Accrual of biomass occurs in final instars, where a high amount of glycogen is accumulated. Lipid concentrations, on the other hand, are kept constant in final stages and slightly lower than in preceding instars. This dominance of glycogen in final instars found in M. lateralis is highly unusual in insects and of potential adaptive significance for the species’ ability to exploit intermittent habitats.