Jost Borcherding

Modelling the Chlorophyll a Content of the River Rhine - Interrelation between Riverine Algal Production and Population Biomass of Grazers, Rotifers and the Zebra Mussel, Dreissena polymorpha

The chlorophyll a content of the German part of the River Rhine is simulated with the aid of a deterministic water quality model (QSIM). At the upper boundary of the investigated river stretch at km 359 (Karlsruhe) low chlorophyll a concentrations ( 50-130 μg/l) occurred in spring time and late summer. The model-based analysis of the algal biomass shows that beside inoculation by the main tributaries Neckar, Main, Lahn and Moselle, algal production in the Rhine itself is of major concern. The calculated losses of phytoplankton showed seasonal dependency with a maximum grazing rate of about 0.04 d -1 for rotifers and 0.2-0.3 d -1 for D. polymorpha. Depending on the algal food content, the biomass of rotifers and Dreissena polymorpha population is simulated and compared with observed rotifer abundances and estimated zebra mussel biomass derived from measured densities and size structures of local populations. In agreement with observations modelled rotifer and D. polymorpha population biomass is increasing downstream, explained by modelling as a matter of improved food supply. Nevertheless, both groups were unable to control algal development in River Rhine between km 359 and 860, due to favourable growth conditions for phytoplankton.

Valve movement response of the mussel Dreissena polymorpha -- the influence of pH and turbidity on the acute toxicity of pentachlorophenol under laboratory and field conditions

The Dreissena-Monitor is a biological early warning system for the continuous monitoring of water quality, based on the computer assessment of valve movements in two groups of 42 zebra mussels (Dreissena polymorpha). In the laboratory, two series of toxicity tests were conducted with PCP. (1) At neutral pH conditions only the concentration of PCP was altered. The dose--response relationship revealed ∼15 μg l−1 PCP for the number of valve movements and ∼20 μg l−1 PCP for the percentage of open mussels. (2) At 50 μg l−1 PCP the pH value was altered from 6.5 to 8.4, demonstrating an inverse relationship between the toxicity of PCP and the pH. The detection limits evaluated from a series of toxicity tests under the field conditions of the River Rhine at Koblenz were nearly ten times higher than those from the laboratory. During a second series of toxicity tests under the field conditions of the River Rhine at Bad Honnef, the toxicity of PCP seemed to be reduced in relation to the increasing turbidity. The differences between the results obtained under laboratory and field conditions are discussed with respect to the influence of the pH and turbidity at the toxicity of PCP on D. polymorpha

Strong body mass decrease of the invasive clam Corbicula fluminea during summer

The filter-feeding clam Corbicula fluminea has widely spread from its Asiatic origin into freshwater habitats on several continents, where it often has a considerable impact on ecosystem processes. The present study was stimulated by the observation that C. fluminea can experience mass mortality during warm summers, even when temperatures are still far below the lethal level. We hypothesized that starvation due to low food quantities during summer is a main factor in this context. In order to test nutritional conditions in an environment where summer mortality occurred, the clams’ body mass was tracked in river bypass systems installed at the Lower River Rhine (Germany and The Netherlands). Two food levels were adjusted in the bypass channels: one corresponding to the original chlorophyll level in the river (ambient food) and one with a chlorophyll a level reduced by about 50% (low food). The clams kept at the ambient food level increased their shell length during summer, although growth rates decreased at low food levels in the River Rhine in late summer. In contrast to shell length, body mass decrease in late summer cumulated in 94% reduction from August until October. This trend was enhanced by the experimental food reduction, i.e., clams kept in the low food level treatments weighed 60% less than the clams in the ambient food treatment at the end of summer. However, mortality was low in both treatments. The data demonstrate a high plasticity in the body mass of C. fluminea. The corresponding possibility to respire body mass can be seen as one strategy to survive longer starvation periods.

Boldness in two perch populations – long‐term differences and the effect of predation pressure

1. Populations of the same species often display different behaviours, for example, in their response to predators. The question is whether this difference is developed as part of a divergent selection caused by differences in predation pressure, or as a result of phenotypic responses to current environmental conditions. 2. Two populations of Eurasian perch were investigated over a time span of 6 years to see whether risk-taking behaviour in young-of-the-year perch were consistent across cohorts, or if behaviour varied over time with changes in predation regime. 3. Boldness was estimated in aquarium studies by looking at how the fish made trade-offs between foraging in a risky area and staying in shelter. Predation risk of each year and lake was estimated from fishing surveys, using an individual-based model calculating attack rates for cannibalistic perch. 4. The average boldness scores were consistently lower in perch from Fisksjön compared with those in Ängersjön, although the magnitude of the difference varied among years. Variance component analyses showed that differences between lakes in boldness scores only explained 12 per cent of the total variation. Differences between years were contributing at least similarly or more to the total variance, and the variation was higher in Fisksjön than in Ängersjön. 5. The observed risk-taking behaviour of young-of-the-year perch, compared across cohorts, was significantly correlated with the year-specific estimates of cannibalistic attack rates, with lower boldness scores in years with higher predation pressure. In Fisksjön, with significant changes over the years in population structure, the range of both predation risk and boldness scores was wider than in Ängersjön. 6. By following the two perch populations over several years, we have been able to show that the differences in risk-taking behaviour mainly are due to direct phenotypic responses to recent experience of predation risk. Long-term differences in behaviour among perch populations thus reflect consistent differences in predation regime rather than diverging inherent traits.

Match or mismatch: the influence of phenology on size‐dependent life history and divergence in population structure

1. In gape-limited predators, body size asymmetries determine the outcome of predator-prey interactions. Due to ontogenetic changes in body size, the intensity of intra- and interspecific interactions may change rapidly between the match situation of a predator-prey system and the mismatch situation in which competition, including competition with the prey, dominates. 2. Based on a physiologically structured population model using the European perch (Perca fluviatilis), analysis was performed on how prey density (bream, Abramis brama), initial size differences in the young-of-the-year (YOY) age cohort of the predator, and phenology (time-gap in hatching of predator and prey) influence the size structure of the predator cohort. 3. In relation to the seasonality of reproduction, the match situation of the predator-prey system occurred when perch hatched earlier than bream and when no gape-size limitations existed, leading to decreased size divergence in the predator age cohort. Decreased size divergence was also found when bream hatched much earlier than perch, preventing perch predation on bream occurring, which, in turn, increased the competitive interaction of the perch with bream for the common prey, zooplankton; i.e. the mismatch situation in which also the mean size of the age cohort of the predator decreased. 4. In between the total match and the mismatch, however, only the largest individuals of the perch age cohort were able to prey on the bream, while smaller conspecifics got trapped in competition with each other and with bream for zooplankton, leading to enlarged differences in growth that increased size divergence. 5. The modelling results were combined with 7 years of field data in a lake, where large differences in the length-frequency distribution of YOY perch were observed after their first summer. These field data corroborate that phenology and prey density per predator are important mechanisms in determining size differences within the YOY age cohort of the predator. 6. The results demonstrate that the switch between competitive interactions and a predator-prey relationship depended on phenology. This resulted in pronounced size differences in the YOY age cohort, which had far-reaching consequences for the entire predator population.

Prey or predator: 0+ perch (Perca fluviatilis) in the trade-off between food and shelter

The 0+ cohort of perch can split into a slow-growing planktivorous and a fast-growing piscivorous cohort during their first months of life. Both cohorts are, however, vulnerable to predation by piscivorous fish. Laboratory experiments were performed to test the behavior of 0+ perch as a predator of cyprinids, and in the trade-off between food and shelter from the threat of predators. In the foraging trials, 0+ perch attacked bream faster than they did carp, and vegetation hampered the aggression against bream. In the second experiment, the habitat selection of two size classes of 0+ perch under the threat of predation was monitored. Overall, vegetation structures were preferred by both size classes of 0+ perch. When small fish were offered to the 0+ perch as food, the open water becomes more attractive. The results of the habitat use trials further show that the two size cohorts of 0+ perch may also differ in their behavior, in that the availability of fish as food becomes more important than the shelter of vegetation structures for the larger perch.

Anthropogenic hybridization between endangered migratory and commercially harvested stationary whitefish taxa (Coregonus spp.)

Natural hybridization plays a key role in the process of speciation. However, anthropogenic (human induced) hybridization of historically isolated taxa raises conservation issues. Due to weak barriers to gene flow and the presence of endangered taxa, the whitefish species complex is an excellent study system to investigate the consequences of hybridization in conservation. We focused on three naturally reproductively isolated whitefish taxa in Germany: the endangered, anadromous North Sea houting (NSH) and Baltic houting (BH), which were reintroduced after local extinction, and the commercially stocked European whitefish (EW). To evaluate the genetic integrity of each taxon, source and reintroduced populations of NSH and BH, and EW populations were characterized based on two mitochondrial and 17 microsatellite loci. Additionally, we investigated gill raker counts as an adaptive phenotypic trait. Even though clear genetic and phenotypic differentiation confirmed the houtings as separate evolutionarily significant units, admixture analyses revealed an extensive hybrid zone. Hybridizations were introgressive, positively correlated with genetic diversity, and were reflected in the gill raker counts. The BH distribution range showed higher heterogeneity and stronger admixture than the NSH range. Erroneous stocking with non‐native genotypes best explained these patterns, which pose challenges for the conservation of the endangered NSH and BH.

Migration history of North Sea houting (Coregonus oxyrinchus L.) caught in Lake IJsselmeer (The Netherlands) inferred from scale transects of 88 Sr: 44 Ca ratios

Abstract.North Sea houting, Coregonus oxyrinchus, became extinct in the River Rhine in the 1940 s and was reintroduced in the 1990 s. To study the migration history of individuals, the 88Sr:44Ca ratio of scales of 39 houting (10–44 cm TL) caught in Lake IJsselmeer was analysed using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Scales of houting inhabiting freshwater ponds and two Danish rivers containing the last original populations in the North Sea basin were used as controls. Fish that lived exclusively in freshwater had 88Sr:44Ca ratios of around 0.2 from the nucleus to the edge; 29 of the analysed houting from Lake IJsselmeer were of this type. Most of these were small, but some were mature and up to 42 cm in length. Seven houting had 88Sr:44Ca ratios over 0.27 from the nucleus to the scale maximum values, indicating migration to the sea at early life stages. Three houting with low 88Sr:44Ca ratios at the scale nucleus and increased 88Sr:44Ca ratios towards the scale edge probably lived in freshwater for a longer period after hatching and then moved to brackish/marine environments. The scale analysis indicates different migration patterns for houting in Lake IJsselmeer and provides evidence that this species (1) is sometimes able to pass the migratory barriers between the Wadden Sea and Lake IJsselmeer, and (2) does not need to migrate to sea to reach maturity.

The seasonal succession of macroinvertebrates, in particular the zebra mussel (Dreissena polymorpha), in the River Rhine and two neighbouring Gravel-Pit lakes monitored using artificial substrates

Artificial substrate samplers were used to study the seasonal succession of macroinvertebrates in the River Rhine and two neighbouring gravel-pit lakes. In two experimental series nearly all epilithic macroinvertebrates normally found at each site settled on the artificial substrates. The analysis of the results showed that the use of artificial substrates is an adequate tool for studying (1) different aspects of the life cycles of species which frequently settle on the plates (e.g. Bryozoa, Corophium curvispinum), (2) seasonal successions of macroinvertebrate communities, and (3) aspects of competition and support within a macroinvertebrate community. The total biomass of macroinvertebrates, in particular the growth of juvenile Dreissena polymorpha, accurately reflected the productivity of the different waters, with highest values for the lake connected to the river, somewhat lower ones for the Rhine, and very low production values for the isolated lake. In other words, a few easily-made measurements of the productivity of primary consumers may be very useful for biological monitoring studies of the floodplains of rivers.

Temporal effects on host-parasite associations in four naturalized goby species living in sympatry

Introduced host species are often characterised by reduced parasite numbers compared to their native populations. Any such advantage gained from parasite release following introduction into a new area may often diminish over a short period as the new host gradually acquires local parasites. In this study, the metazoan parasite communities of four goby species (Proterorhinus semilunaris, Ponticola kessleri, Neogobius melanostomus, and Neogobius fluviatilis) recently introduced into the lower River Rhine were investigated. Mean parasite abundance and infracommunity richness were positively associated with time since host introduction, both parasite variables being the highest in P. semilunaris. In Ponticola and Neogobius species, parasite species richness and the dominance of larval parasites in the Lower Rhine were similar to that for non-native populations in the middle Danube. Sporadic local parasite acquisition and infection, predominantly by species commonly found in the native range, led to a relatively high qualitative similarity in parasite communities between hosts. The relationship between parasite abundance and fish size reflected size-dependant food selectivity and/or parasite accumulation throughout the host’s life. Data from this study emphasise the importance of duration of co-occurrence, host habitat and foraging preference, as well as the co-introduction of suitable intermediate hosts, for parasite community composition in related species.