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The present study analyzes climate change effects on the water temperature of the Rhine, one of the largest rivers in Central Europe. Simulation calculations were performed based on a range of climate and river flow projections for the near (2021–2050) and for the far future (2071–2100) compared to a reference period (1961–1990). Changes in mean annual water temperature in the near future range between +0.6 and +1.4 °C and between +1.9 and +2.2 °C in the far future (average of nine stations). Monthly mean values of the far future change in a more differentiated way by +0.4 to +1.3 °C in spring and +2.7 to +3.4 °C in late summer. The length of periods of high water temperature, expressed as successive days with water temperatures over 27 °C, increases by a factor of four until 2100. These prolonged durations of periods with unusually high water temperatures may provoke changes in the food web and in the rates of biological processes in the Rhine.

Fate and occurrence of 4 selective serotonin reuptake inhibitors, one serotonin-noradrenergic reuptake inhibitor and one noradrenergic-dopamineric reuptake inhibitor and their human metabolites were determined in a German municipal wastewater treatment plant as well as in the Rhine River and selected tributaries. The enantiomeric fractions of venlafaxine and its metabolites were not altered during wastewater treatment and were similar in all river samples underlining that no appreciable biodegradation occurs. In the Rhine catchment area highest concentrations were detected for venlafaxine, citalopram and their human metabolites. Projected future climate change would lead to an increased portion of treated wastewater in rivers due to reduced discharges during low flow situations by the end of the 21st century. However, the effect of climate change on the pattern and concentrations of antidepressants is predicted to be of minor importance in comparison to altered consumption quantities caused by demographic developments and changes in life styles.

We measured the clearance rates of the filter-feeding bivalve Corbicula fluminea over a period of 2 years. Strong seasonal variations, such as a 50-fold increase from February to July, were observed. These variations were only poorly linked to temperature, as they could be found at both the ambient field temperature and a constant temperature of 15°C. Instead, peaks in the filtration activity corresponded to the spawning activity. Additionally, a strong interannual variability with much lower clearance rates in 2009 than in 2008 was identified. The low clearance rates were linked to a preceding period of low winter temperatures close to the lethal temperature of 2°C and the associated reduced reproduction rates. Our findings demonstrate that other factors besides temperature and body mass can strongly affect clearance rates. These results should be considered when predicting the effects of changing temperatures on the ecosystem impact of filter-feeding bivalves.

We investigated the role of a macrograzer (the filter feeding mussel Dreissena polymorpha) in mediating effects of high summer temperatures on the dominant components of natural river plankton (i.e., bacteria, algae, and heterotrophic flagellates) in flow channel experiments. Effects of adaptation (by comparing mussels from a southern and a northern population) and thermal acclimation of the mussels were considered. Both heterotrophic flagellates and algae are released from grazing pressure and increase in abundance at temperatures above 20°C. Bacterial abundance, however, decreased with increasing temperature, suggesting a trophic cascade (mussel-flagellates-bacteria) that is altered by the temperature response of the mussel ingestion rate. Warm acclimation of the mussels did not change the outcome of the experiments. The dreissenids from the southern population showed a significantly higher ingestion rate than those from the northern population only in July. The general pattern (i.e., decreasing ingestion rates at high temperatures) was found in both populations. Microbial communities controlled by macrofauna can experience substantial changes in warm summers because of differential development of direct and indirect grazing effects with increasing temperature.

KLIWAS-Schriftenreihe / BfG – KLIWAS-10/2013

Climate models predict an increasing frequency of extremely hot summer events in the northern hemisphere for the near future. We hypothesised that microbial grazing by the metazoan macrofauna is an interaction that becomes unbalanced at high temperatures due to the different development of the grazing rates of the metazoans and the growth rates of the microbial community with increasing temperature. In order to test this hypothesis, we performed grazing experiments in which we measured the impact of increasing temperatures on the development of the grazing rates of riverine mussels in relation to the growth rates of a unicellular prey community (a natural heterotrophic flagellate community from a large river). In a first experimental series using Corbicula fluminea as a grazer and under the addition of a carbon source (yeast extract), the increase of the prey's growth rates was considerably stronger than that of the predator's grazing rates when temperatures were increased from 19 to over 25 degrees C. This was also the outcome when the mussels had been acclimatized to warm temperatures. Hereafter, specific experiments with natural river water at temperatures of 25 and 30 degrees C were performed. Again, a strong decrease of the mussels' grazing rates in relation to the flagellate growth rates with increasing temperature occurred for two mussel species (C. fluminea and Dreissena polymorpha). When performing the same experiment using a benthic microbial predator community (biofilms dominated by ciliates) instead of the benthic mussels, an increase of the grazing rates relative to the growth rates with temperature could be observed. Our data suggest that predator-prey interactions (between metazoans and microbes) that are balanced at moderate temperatures could become unbalanced at high temperatures. This could have significant effects on the structure and function of microbial communities in light of the predicted increasing frequency of summer heat waves.