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Because silver nanoparticles (Ag NPs) are broadly applied in consumer products, their leaching will result in the continuous release of Ag NPs into the natural aquatic environment. Therefore, bacterial biofilms, as the prominent life form of microorganisms in the aquatic environment, are most likely confronted with Ag NPs as a pollutant stressor. Notwithstanding the significant ecological relevance of bacterial biofilms in aquatic systems, and though Ag NPs are expected to accumulate within these biofilms in the environment, the knowledge on the environmental and ecological impact of Ag NPs, is still lagging behind the industrial growth of nanotechnology. Consequently, aim of this thesis was to perform effect assessment of Ag NP exposure on bacterial biofilms with ambient Ag NPs concentrations and under environmentally relevant conditions. Therefore, a comprehensive set of methods was applied in this work to study if and how Ag NPs of two different sizes (30 and 70 nm) affect bacterial biofilms i.e. both monospecies biofilms and freshwater biofilms in environmentally relevant concentrations (600 - 2400 µg l-1). Within the first part of this work, a newly developed assay to test the mechanical stability of
monospecies biofilms of the freshwater model bacterium Aquabacterium citratiphilum was validated. In the first study, to investigate the impact of Ag NPs on the mechanical stability of bacterial biofilms, sublethal effects on the mechanical stability of the biofilms were observed with negative implications for biostabilization. Furthermore, as it is still challenging to monitor the ecotoxicity of Ag NPs in natural freshwater environments, a mesocosm study was performed in this work to provide the possibility for the detailed investigation of effects of Ag NPs on freshwater biofilms under realistic environmental conditions. By applying several approaches to analyze biofilms as a whole in response to Ag NP treatment, insights into the resilience of bacterial freshwater biofilms were obtained. However, as revealed by t-RFLP fingerprinting combined with phylogenetic studies based on the 16S gene, a shift in the bacterial community composition, where Ag NP-sensitive bacteria were replaced by more Ag NP-tolerant species with enhanced adaptability towards Ag NP stress was determined. This shift within the bacterial community may be associated with potential detrimental effects on the functioning of these biofilms with respect to nutrient loads, transformation and/or degradation of pollutants, and biostabilization. Overall, bringing together the key findings of this thesis, 4 general effect mechanisms of Ag NP treatment have been identified, which can be extrapolated to natural freshwater biofilms i.e. (i) the identification of Comamonadaceae as Ag NP-tolerant, (ii) a particular resilient behaviour of the biofilms, (iii) the two applied size fractions of Ag NPs exhibited similar effects independent of their sizes and their synthesis method, and (iv) bacterial biofilms show a high uptake capacity for Ag NPs, which indicates cumulative enrichment.
Cultural eutrophication due to excessive inputs of nutrients seriously threatens aquatic ecosystems worldwide and is one of the major anthropogenic stressors on aquatic biota in European rivers. In streams and shallow rivers, its effects include excessive periphyton growth, which causes biological clogging and thereby oxygen depletion in the hyporheic zone. The result is a serious degradation of habitat quality for benthic invertebrates as well as for the eggs and larvae of gravel-spawning fish. Unlike in standing waters, efficient tools for controlling eutrophication in rivers are lacking. However, top-down control of the food-web by manipulating fish stocks, similar to the biomanipulation successfully applied in lakes, offers a promising approach to mitigating the effects of eutrophication in shallow rivers, especially those in which major reductions in nutrient inputs are not feasible. The overall aim of this thesis was to assess the potential for top-down control by two large cypriniform fish, the common nase (Chondrostoma nasus), the only obligate herbivorous fish species in European rivers, and the omnivorous European chub (Squalius cephalus), to mitigate the effects of eutrophication in medium-sized rivers. I therefore conducted field experiments on different spatial and temporal scales in the hyporhithral zone of a eutrophic gravel-bed river. Generally, the results of those experiments revealed the crucial role of fish-mediated top-down effects in river food webs. In a 4-year reach-scale experiment, the key contribution of my thesis, the enhancement of fish densities significantly increased both oxygen availability and water exchange in the upper layer of the hyporheic zone, even though the top-down effects of the fish on periphyton biomass were relatively small. These findings were supported by those of a 4-week mesocosm experiment, which also provided insights into the mechanisms underlying the mitigation of eutrophication effects by nase and chub. The top-down effects of both fish species reduced hyporheic oxygen depletion, suggesting a reduction of biological clogging. The positive effects of herbivorous nase on hyporheic oxygen availability could be attributed to benthic grazing, whereas the reduction of hyporheic oxygen depletion in the presence of omnivorous chub was best explained by the enhanced bioturbation induced by the fish’s benthic foraging. Overall, the results of my thesis demonstrate that biomanipulation achieved by enhancing herbivorous and omnivorous fish stocks can mitigate the effects of eutrophication in medium-sized European rivers. The results may be the first step towards the establishment of biomanipulation as a supportive management tool for eutrophication control in running waters and therefore as a strategy to preserve aquatic biodiversity.
The implementation of physiological indicators reflecting the response of organisms to changes in their environment is assumed to provide potential benefits for ecological studies. By analysing the physiological condition of organisms in freshwater ecological studies rather than their ultimate effects, physiological indicators can contribute to a faster assessment of effects than using traditional ecological indicators, such as the evaluation of the benthic community structure or the determination of the reproductive success of organisms. This can increase the effectiveness of environmental health assessment and experimental ecology. In this respect the thesis focuses on physiological measures characterizing the energetic condition and energy consumption (the concentration of energy storage compounds, the adenylate energy charge, the energy consumption in vivo), as well as individual growth (RNA:DNA ratio) of organisms. Although these sub-individual indicators are commonly applied in marine ecology and more recently in ecotoxicology, they have been rarely applied in freshwater ecology to date. With respect to an increased use of physiological indicators in freshwater ecological studies, the objectives of the present thesis are twofold. First, it highlights the potential of assessing the individual fitness by means of physiological indicators in freshwater ecological studies. For that reason, Chapter 2 provides the basic assumptions as well as the theoretical and methodological fundamentals necessary for the application of physiological indicators within freshwater ecology and, furthermore, points out their applicability by several case studies. As second objective, the thesis addresses selected ecophysiological aspects of native and non-native freshwater amphipods, which are considered suitable candidates for the determination of physiological indicators in ecological studies due to their function as keystone species within aquatic habitats. The studies presented in Chapters 3−5 of the thesis provide information on (i) species- and sex-specific seasonal variations within the energetic condition of natural Gammarus populations (G. fossarum, G. pulex), (ii) differences in metabolic activity and behaviour between different amphipod species (G fossarum, G. roeselii and D. villosus), as well as (iii) the direct effects of ambient ammonia on the physiology and behaviour of D. villosus. The fundamental conclusions drawn from the conducted field and laboratory studies, as well as their relevance and general implications for the application of physiological indicators in freshwater ecological research are discussed in Chapter 6.
Estuaries are characterized by a longitudinal salinity gradient. This gradient is one of the main environmental factors responsible for the distribution of organisms. Distinguishing salinity zones is of crucial importance, e.g., for the development of tools for the assessment of ecological quality. The methods most often applied for classifying water according to salinity are the Venice System and the method of Bulger et al. (1993), both of which determine zone boundaries using species occurrences relative to mean salinity. However, although these methods were developed for homoiohaline waters, they have also been routinely applied to poikilohaline systems. I tested the applicability of both methods using salinity and macroinvertebrate data for the poikilohaline Elbe Estuary (Germany). My results showed that the mid-estuary distribution of macro-invertebrates is determined by variation in salinity rather than by mean salinity. Consequently, neither of the two methods is applicable for defining salinity zones in the Elbe Estuary. Cluster analysis combined with a significance test, by contrast, was a better tool for identifying the boundaries of salinity zones in poikilohaline systems.
In many estuaries, such as the Elbe Estuary, a maximum turbidity zone (MTZ) develops, where suspended matter accumulates owing to circulation processes. It is assumed that the MTZ is a stressful environment with an excess of organic matter, high deposition rates, large variations in salinity, and dredging activities. Under such harsh conditions, populations might remain below the carrying capacity, and it is assumed that competition is of little importance, as predicted by the stress gradient hypothesis. I tested whether competition for food is important in the MTZ of the Elbe Estuary using stable isotope analysis of the macroinvertebrate community. The isotopic niches of no two taxa within a feeding group overlapped, which indicated different resource use and the absence of competition. The main reasons for the lack of overlap of isotopic niches were differences in habitat, feeding behavior, and migration behavior.
The Elbe Estuary is nowadays highly industrialized and has long been subjected to a plethora of human-caused alterations. However, it is largely unknown what changes occurred in benthic communities in the last century. Hence, I considered taxonomic and functional aspects of macrobenthic invertebrates of the Elbe Estuary given in data from 1889 (most natural state), 1985 and 1986 (highly polluted state), and 2006 (recent state) to assess benthic community shifts. Beta-diversity analysis showed that taxonomic differences between the sampling dates were mainly due to species turnover, whereas functional differences were predominantly a result of functional nestedness. Species number (S), functional richness (FRic), and functional redundancy reached minimum values in 1985 and 1986 and were highest and rather similar in 1889 and 2006. The decline in FRic from 1889 to 1985/1986 was non-random, consistent with habitat filtering. FRic, functional beta diversity, and S data suggested that the state of the estuary from 1889 was almost re-established in 2006. However, the community in 1889 significantly differed from that in 2006 owing to species replacement. My results indicate that FRic and FR in 1889 could have promoted ecosystem resilience and stability.
Ecological assessment approaches based on benthic invertebrates in Euphrates tributaries in Turkey
(2019)
Sustainable water management requires methods for assessing ecological stream quality. Many years of limnological research are needed to provide a basis for developing such methods. However, research of this kind is still lacking in Turkey. Therefore, the aim of this doctoral thesis was to provide basic research in the field of aquatic ecology and to present methods for the assessment of ecological stream quality based on benthic invertebrates. For this purpose, I selected 17 tributaries of the Euphrates with a similar typology/water order and varying levels of pollution or not affected by pollution at all. The characterisation of the natural mountain streams was the first important step in the analysis of ecological quality. Based on community indices, I found that the five selected streams had a very good ecological status. I also compared the different biological indications, collected on two occasions ¬– once in spring (May) and once in autumn (September) – to determine the optimal sampling time. The macroinvertebrate composition differed considerably between the two seasons, with the number of taxa and Shannon index being significantly higher in autumn than in spring. In the final step, I examined the basal resources of the macroinvertebrates in the reference streams with an isotope analysis. I found that FPOM and biofilm were the most relevant basal resources of benthic invertebrates. Subsequently, based on the similarity of their community structures, I divided the 17 streams into three quality classes, supported by four community indices (EPT [%], EPTCBO [%], number of individuals, evenness). In this process, 23 taxa were identified as indicators for the three quality classes. In the next step, I presented two new or adapted indices for the assessment of quality class. Firstly, I adapted the Hindu Kush-Himalaya biotic index to the catchment area of the Euphrates and created a new, ecoregion-specific score list (Euph-Scores) for 93 taxa. The weighted ASPT values, which were renamed the Euphrates Biotic Score (EUPHbios) in this study, showed sharper differentiations of quality classes compared to the other considered ASPT values. Thus, this modified index has proved to be very effective and easy to implement in practical applications. As a second biological index, I suggested the proportion of habitat specialists. To calculate this index, the habitat preferences of the 20 most common benthic invertebrates were identified using the new habitat score. The proportion of habitat specialists differed significantly among the three quality classes with higher values in natural streams than in polluted streams. The methods and results presented in this doctoral thesis can be used in a multi-metric index for a Turkish assessment programme.
Small headwater streams comprise most of the total channel length and catchment area in fluvial networks. They are tightly connected to their catchments and, thus, are highly vulnerable to changes in catchment hydrologic budgets and land use. Although these small, often fishless streams are of little economic interest, they are vital for the ecological and chemical state of larger water bodies. Although numerous studies investigate the impact of various anthropogenic stressors or altered catchment conditions, we lack an in-depth understanding of the natural conditions and processes in headwater streams. This natural state, however, largely affects how a headwater stream responds to anthropogenic or climatic changes. One of the major threats to aquatic ecosystems is the excessive anthropogenic input of nutrients leading to eutrophication. Nutrients exert a bottom-up effect in the food web, foremost affecting primary producers and their consumers, i.e. periphyton and benthic grazers in headwater streams. The periphyton-grazer link is the main path of autochthonous (in-stream) production into the stream food web and the strength of this link largely determines the effectiveness of this pathway. Therefore, this thesis aims at elucidating important biological processes with the explicit focus on periphyton-grazer interactions. I assessed different aspects of periphyton-grazer interactions using laboratory experiments to solve methodological problems, and using a field study to compare the benthic communities of three morphologically similar, phosphorus-limited, near-natural headwater streams. With the results of the laboratory experiments, I was able to show that periphyton RNA/DNA ratios can be used as proxy for periphyton growth rates in controlled experiments and that the fatty acid composition of grazing mayfly nymphs responds to changes in fatty acids provided by the diet after only two weeks. The use of the RNA/DNA ratio as a proxy for periphyton growth rate allows a comparison of these growth rates even in simple experimental set-ups and thereby permits the inclusion of this important process in ecotoxicological or ecological experiments. The observed fast turnover rates of fatty acids in consumer tissues show that even short-term changes in available primary producers can alter the fatty acid composition of primary consumers with important implications for the supply of higher trophic levels with physiologically important polyunsaturated fatty acids. With the results of the field study, I revealed gaps in the understanding of the linkages between catchment and in-stream phosphorus availability under near-natural conditions and demonstrated that seemingly comparable headwater streams had significantly different benthic communities. These differences most likely affect stream responses to environmental changes.
