The impact of water temperature on the transmission of aquatic parasites

The present study evaluated and compared the response of benthic macroinvertebrate, fish and parasite communities to changes in water temperature across two mid-sized mountain rivers in the south-eastern part of the German federal state North Rhine-Westphalia (NRW). In 2009 and 2010, the two rivers Ruhr and Lenne have been investigated at comparable river sections up- and downstream of thermal pollution by cold hypolimnetic water from reservoirs as well as by cooling water released from a power station. The main focus was to extend the knowledge about the use of metazoan fish parasites as bioindicators for changes in freshwater biodiversity in relation to water temperature. The research focus was aimed at the faunistical and ecological aspects of parasite communities of Brown trout (S. trutta fario) in relation to temperature variations. At 6 river sections (3 cold-warm site pairs) benthic macroinvertebrate, fish and parasite communities were examined in autumn 2009 and in spring and autumn 2010. Additionally, thermal characteristics were monitored over one and a half year. The annual mean water temperature differences of this study ranged within 0.7 K (Ruhr) to 1.9 K (Lenne1). The analysis of degree-days confirmed the separation of sampling sites by temperature. Looking at the influence of water temperature on benthic macroinvertebrate communities (chapter 3), BMI communities showed highest average dissimilarity at Lenne1 (ΔWT: 1.9 K). A similar but more differentiated pattern was observed when looking at the EPT community. An increase of average abundance of H. siltalai (Trichoptera) has been detected at all warm sites as well as rising average abundance of C. rivulorum (Ephemeroptera) for the Lenne1 and the Ruhr warm sites. As the majority of BMI were present at both cold and warm sites, the limit of tolerance range of the majority of taxa was not exceeded by the temperature differences found in the present study. The results indicated that temperature changes have an impact on BMI species, in particular EPT-taxa, and as a consequence BMI communities are affected. The results of chapter 4 – the effect of water temperature on the levels of hepatic glycogen and hsp70 in brown trout (Salmo trutta fario L.) – showed a general increase for hepatic glycogen levels at warmer sections and a clear seasonal pattern with higher concentrations in autumn. Besides, increasing temperature had effects on the condition of examined brown trout. Mean condition factors were lower at warm sites compared to respective cold sites. Additionally, condition factors were equal or slightly higher in spring samples than in autumn samples. Gonadosomatic indices showed a general decrease at warmer sections, while hepatosomatic indices showed no clear pattern. Hepatic hsp70 levels showed no clear pattern at warm sites, while the concentrations in the liver tissues remained more or less similar in spring and autumn. At Ruhr and Lenne2 cold sites, hepatic hsp70 levels had their maximum in spring. The results obtained indicated the ability of brown trout to respond to increased energy demands as part of an increase in water temperature without losing their regulatory capability. In light of the temperature differences found in this study, one should bear in mind that the limit of tolerance range might not have been reached yet. The impact of water temperature on the parasite communities in brown trout (Salmo trutta fario L.) is the content matter of chapter 5. Between autumn 2009 and autumn 2010, twelve metazoan parasite species including five nematodes, three acanthocephalans, three trematodes and one cestode were found in 365 S. trutta fario examined from six sites. The results revealed significant, temperature dependent changes in abundances for most of the parasite species found. According to these results, two opposing trends were observed: • an increase in calculated infection indices (prevalence, abundance and mean intensity) which concerns both, specific parasites of salmonids (E. truttae and P. salvelini) as well as rather generalist parasites (Diplostomum sp., R. acus, C. truncatus and N. rutili) • a decrease in calculated infection indices (prevalence, abundance and mean intensity) which only concerns the two salmon-specific species Crepidostomum sp. and C. ephemeridarum. The results also showed that changes in water temperature cause both qualitative and quantitative changes in parasite communities of brown trout. Since the difference in parasite infracommunity composition and structure reflect the difference of the community composition of free living animals, the study shows an effect of water temperature on free living animals such as BMI in both rivers Ruhr and Lenne. This is related to differential occurrence and abundance of intermediate hosts of a few species which were identified as key discriminating species in chapter 3. Likewise, a mean annual temperature difference of 0.7 K (Ruhr) and 1.9 K (Lenne1) caused a two- to fourfold increase of diversity parasite component community in autumn. The present study provides evidence that metazoan fish parasites will benefit from water temperature increase following climate change and associated shifts of host communities. Finally, the present study delivered a detailed parasite monitoring on Brown trout in the river Ruhr and Lenne. The results of the meta-analysis in chapter 6 – Effects of elevated water temperature on aquatic organisms: a meta-analysis – confirmed the findings off the previous chapters. The quantitative analysis based on a modified response ratio provided information on both the typical magnitude and direction of effect. The effects on infracommunity level are barely recognizable, whereas the greatest impacts are likely to occur in metapopulations of parasites. The results of the present study revealed the parasite´s susceptibility to increasing water temperature and showed that changes in temperature due to global climate change can already affect the transmission of parasites. The findings provide evidence that the parasite community patterns of brown trout may differ essentially in relation to changes in water temperature. All these facts highlight the usefulness of parasite communities for aquatic monitoring and research and therefore should be considered in future studies. However there is also need for further research regarding autecological characteristics of parasite species. In many rivers it is still impossible to monitor and quantify the status of biodiversity due to a dearth of species occurrence data at relevant scales and compatible formats and a lack of knowledge on parasite life cycles and its linked intermediate hosts. For these reasons, considering parasites, will improve the validity of biological water quality assessments. Future research as well as success of tackling climate change can profit from increasing the awareness of aquatic parasites and their hosts as a functional unity. Therefore, available data on freshwater parasites and their hosts groups should be collected as a basis for analyses of distribution patterns in different river types. Additionally, aquatic parasite communities and freshwater organism groups should be monitored over longer time spans. The improvement and conservation measures on biodiversity of freshwater ecosystems should also broaden the focus on aquatic parasite communities. Finally, a standardized assessment method should be developed to quantify temperature effects on aquatic parasite communities including the identification of indicator species specific for different fish types. More research is needed to understand how temperature-related changes in freshwater community compositions, affect life cycles of aquatic parasites.