Environmental and spatial patterns of freshwater fish diversity in Flanders

Riverine biodiversity is deteriorating rapidly as a result of multiple interacting anthropogenic pressures. Freshwater species are subject to physico-chemical pollution originating from various sources (e.g. agriculture, industries, households), hydro-morphological alterations (e.g. construction of dams, river calibration), and the introduction of invasive species (e.g. topmouth gudgeon, Chinese mitten crab, round goby in Flanders). Conservation and restoration actions generally focus on protecting freshwater habitats and species, and the assessment of ecological quality is based on the diversity and abundance of species and specific taxa. However, other metrics, such as genetic diversity and the composition of parasite communities, provide important information about environmental and ecological patterns (Chapter 1). In this PhD study, I investigated environmental and spatial patterns of fish communities, fish parasite communities, and genetic structure with a focus on responses to environmental pollution and degradation in Flanders (Belgium). Assessment of parasite communities and genetic patterns focused on three riverine fish species (three-spined stickleback, stone loach, topmouth gudgeon) with contrasting ecology, environmental tolerance, life history traits, dispersal capacity, and invasion history. In Chapter 2, I studied the response of fish communities to environmental pollution and degradation, using a community model based approach. Physico-chemical, hydro-morphological, and ecological quality of freshwater systems in Flanders and many other member states of the European Union remains inadequate. Ecological quality based on fish community structure (e.g. Fish Index in Flanders), in particular, lags behind and only 1% of all waterbodies in Belgium reached sufficient quality by the end of 2015. Hence, successful restoration and conservation practices should be identified and prioritized. I aimed at identifying responses of fish communities to environmental policy targets based on the Flemish implementation of the Water Framework Directive. Responses to both physico-chemical (oxygen, conductivity, chemical oxygen demand, and nitrogen) and hydromorphological (riverbed, profile, flow, longitudinal and lateral continuity) targets were modelled using a joint species distribution model. Species richness responded most strongly to a decrease in conductivity and nitrogen levels, and an increase in riverbed quality. However, responses to current quality standards were weak. Although not directly included in the Water Framework Directive, analysis of parasite communities is important in the assessment of environmental status of rivers, partly because parasites provide a direct indication of biodiversity, and partly because parasite communities can be used as indicators of environmental quality. Nevertheless, processes and factors influencing parasite infection in freshwater fish and the link with pollution are not well understood. To better understand anthropogenic and natural factors driving variation in parasite infection, I studied parasite communities of three-spined stickleback, stone loach, and topmouth gudgeon in the Demer basin. Ectoparasites of three-spined stickleback were positively correlated with nutrient levels (Chapter 3), while the endoparasitic species, depending on trophic ingestion for their transmission, did not appear affected by pollution. Moreover, pollution did not influence body condition of three-spined stickleback, suggesting that other processes are responsible for the increase in ectoparasite abundance. In Chapter 4, I explored variation in parasite communities of the three focal species. Topmouth gudgeon harboured fewer parasites, a pattern often observed in invasive species, while parasite communities of the native species reflected (physico-chemical and host-related) environmental and spatial heterogeneity. The importance of genetic diversity for the persistence of species has been widely recognized, but genetic patterns have not often been included in conservation planning of riverine species. Analysis of genetic diversity highlighted different population genetic patterns (Chapter 5). We observed strong population genetic structure in three-spined stickleback, accompanied by low levels of genetic diversity. Population structure was less pronounced in stone loach and diversity was higher compared to three-spined stickleback. Populations of topmouth gudgeon, however, were less differentiated compared to the native species. Population structure was strongly driven by waterway distances and the presence of migration barriers, while other environmental and spatial factors are likely responsible for the observed population structure in both native species. When comparing patterns of genetic and species diversity (i.e. species-genetic diversity correlations, SGDCs), results show that α- species and genetic diversity are non-congruent in Flemish riverine systems (Chapter 6). Community dissimilarity (β-diversity), on the other hand, correlated to population differentiation in topmouth gudgeon, and to a lesser extent in stone loach. This is likely a result of the response of both species and genetic composition to the presence of migration barriers and other spatial processes. Overall, the results of this PhD indicate that prioritizing locations and areas for conservation and restoration will be challenging as levels of species, genetic, and parasite diversity do not strongly overlap. Although some general drivers could be identified, it is clear that an interplay of complex processes shapes different aspects of riverine diversity and ecosystems in a different manner.

Jaar van publicatie:2021

Toegankelijkheid:Open