Spatial patterns and ecological determinants of the diatom communities in an alpine flow-regime river (Adige River, North-Eastern Italy). Implications for the ecological status' assessment

My PhD work draws from a project called PlanAdige that has been funded by the Basin Authority of River Adige to the Foundation E. Mach of San Michele all’Adige in the early 2007 for a period of three years. The main objective of this project was the study of the watercourse following an integrated approach that would encompass both its physical, chemical and biological features aiming at assessing its ecological status. I believe we are facing a particular moment in Ecological Sciences since we have to deal with “hot topics” such as climate change, water scarcity and resources depletion: meeting today’s environmental challenges requires gaining reliable knowledge about ecosystems in their deeper details (Schmitz, 2007). It is therefore vital to gather all the knowledge and the science that we know in order to cope with these big issues at our best: every little study or survey that has been made can give a contribution to tackle these concerns. For my PhD Internship I have focused my attention on diatoms and rivers, whose links have been widely used to assess watercourses’ general water quality. Diatoms are very important aquatic organisms: some researchers have estimated that there are 10 million diatom species worldwide but only 11.000 have been identified up to date (Poulin & Williams, 1998) and that they account for 25% of the living matter (Werner, 1977). Because of their abundance in marine plankton, especially in nutrient-rich areas of the world’s oceans, diatoms probably account for as much as 20% of global photosynthetic fixation of carbon (ca. 20 Pg carbon fixed per year: Mann, 1999). Diatoms are essential links for energy transfer to upper tropic levels as they are a preferred high quality food source for primary consumers (Brett & Mullen-Navarra, 1997) and are the principal source of biologically induced carbon export from surface to deep waters, therefore playing a central role in nutrient cycling. Some studies have recently showed that it is expected that climate change will affect diatom abundance and community structure (Henson et al., 2010; Widdicombe et al., 2010) as well as increase the density gradient which in turn will suppress the upward flux of nutrients (Behrenfeld et al., 2006). It is expected that warmer climate will preferably favor small-sized diatom cells that show a high surface area to volume ratio (Winder et al., 2009) turning in cell size to be a powerful predictor of optimum dynamic performance (Reynolds et al., 2002). In earlier times, ecological research on rivers focused on the descriptive research of biological communities in small streams (Minshall, 1988; Cummins et al., 1995). Research on large rivers was limited partly by tradition and partly by methodological problems, considering the large geographical scale of these ecosystems. Nowadays, it is clearer that river ecologists are developing a more integrated view of rivers: this reflects the need to increase the knowledge about these systems in all its components. As for diatoms, unfortunately, we yet do not have a full comprehension of the temporal factors shaping the diatom communities of a watercourse (Passy, 2007) also because only few studies have examined both physical and chemical parameters effects on the communities concurrently (Duncan & Blinn, 1989). Little is known about factors driving diatom species diversity and geographic distribution: studies focusing on the mechanisms generating species diversity are needed (Vanormelingen et al., 2008; Larned, 2010) since force fitting has severe consequences both on ecological and management sides (e.g. the Water Framework Directive 2000/60/CE requires ecological lumping of watercourses for ecoregions’ definition). With these premises, the specific objectives of my PhD thesis, are: 1- Assessment of the main biotic and abiotic drivers affecting the composition and temporal dynamics of the diatom communities in the River Adige. In the view that the abiotic environment has probably set the main stage for evolutionary development of specific traits and associated life-history strategies in the ecosystem (Lytle & Poff, 2004) and that these factors drive a large proportion of the subsequent biotic interactions (Biggs et al., 2005), there is a need to clarify the direct measures of these driving forces together with the associated biotic response variables, at different scales. This ambitious objective rises from the scarcity of studies that look at multiple samples per sites over times and which relate results to contemporary management problems (Kelly, 2002). Benthic diatom communities have a spatial and temporal variation:it follows that we cannot conclude much from such studies (Kelly et al., 2009). In addition, most researches up to date have focused only on a limited component of the diatom community (e.g. the epilithon) almost completely neglecting the other ones (epipelon, epiphyton and epipsammon). In addition, only sporadic comparisons were made between benthic and planktonic algae in a same watercourse (Werner & Köhler, 2005). 2- Evaluation of diatom assemblage structures and its drivers at the light of biomonitoring techniques. The need to use diatom indices for water quality is universally claimed: given that diatom communities vary with abiotic and biotic factors, great potentials exist for refining our assessment of biological and pollution condition by accounting for natural variation. These refinements will turn out to be essential for increasing accuracy, precision and fairness of ecological assessments. Much has been said about this topic but there are still some uncertainties to be discussed: there are few studies which have properly addressed the small-scale (e.g. within-riffle scale) community variations in streams (Soininen, 2007) and even fewer that have studied impaired water courses, such as Adige River. 3- Provide a framework of knowledge on diatoms’ autoecology which is the first step to further deal with these organisms. Periphyton communities are solar-powered biogeochemical reactors, biogenic habitats, hydraulic roughness elements, early warning systems of environmental degradation and troves of biodiversity (Larned, 2010) and therefore it is essential to study the rules that govern their components, both individually and among taxa. There is a scarcity of such studies and this turns in consequently hampering of the prediction of changes in resource requirements and endangering of the tout-court application of diatom indices. Little is known about diatom ecology, biology and the factors driving diatom species diversity and geographic distribution (Mann, 1999; Chepurnov et al.,2004): especially in impacted environments and this could lead to ecological biases. For instance, in the case of Adige River which is highly influenced by discharge regimes (Centis et al., 2010), it is essential to know the impact of this variable on diatom communities . It has to be kept in mind that the relationship between diatom species composition and prevailing hydraulic conditions is one of the original research problems in periphyton ecology (Butcher, 1940; Patrick, 1948). 4- These results will be worked out also as contributions to the exploration of diatom index based on phytoplankton that could help figuring the potential of this community in water quality assessment. Even if this topic has been developed for lakes (Thunmark, 1945; Nygaard, 1949; Stockner, 1971; Catalan, 2003) little has been done for rivers, if we exclude the works by Mischke (Mischke, 2007; Mischke & Behrendt, 2006; Mischke & Behrendt, in prep.), and Borics et al. (2007). Overall, the aim of this study is to give a contribution to the evolving legacy of stream ecology: at the past two meetings of the North American Benthological Society (Grand Rapids Michigan, 2009; Santa Fè, New Mexico, 2010) has emerged the need to push towards an holistic perspective that would consider global changes occurring in these riverine ecosystems. A number of conceptual models have been proposed to synthesize empirical information (to cite some: Ward et al., 2002; Benda et al., 2004; Thorp et al., 2006) and they are indeed very valuable in organizing what otherwise might be a collection of seemingly unique case studies. This is especially worthwhile since stream ecology is moving towards the disturbance issue (Stanley et al., 2010) and the multiple stressors perspective (Thorp, pers. comm.).