Macroinvertebrates in riverine systems with different degree of intermittence: influence of community dynamics and environmental variables at different spatial scales

Abstract

Macroinvertebrate communities exhibit high variability in diversity, abundance and structure at different spatial scales. Space is currently used as an explicit predictor to discriminate between environmental forcing and biotic processes at large and medium sized scales, but it is generally neglected at smaller scales, to which macroinvertebrate community organization is generally studied only within the environmental filtering framework, disregarding processes other than those based on abiotic factors. To fill this gap, in the first section of the present thesis, we considered environmental as well as spatial variables with the aim of explaining diversity, abundance and community patterns of macroinvertebrate community at small scale, by using geostatistical and multivariate spatial analysis. At first, we performed a pilot sampling campaign in May 2015 using a specific in situ sampling design along a reach of an intermittent system (Baganza stream, Northern Italy). Overall, 5493 organisms belonging to 25 taxa were collected and identified being Chironomidae, Baetidae and Naididae the most abundant taxa. Space explained a consistent fraction of abundance and taxa richness variability both singularly and jointly with depth. Moreover, the latter was a good predictor of abundance, but not for taxa richness and community structure. Our results suggest that while organisms seem to be able to occupy almost any position in the watercourse, their abundance is modulated by habitat preference. This study represents a starting point for understanding how niche-based and dispersal processes act on macroinvertebrates communities at very small scale. Findings improve the knowledge about the fine scale organization of macroinvertebrate communities in intermittent streams. Restoration ecology, habitat suitability modelling and biomonitoring sampling methods could benefit from our approach. Based on the results of the pilot study we did a second and more in-depth study, working in three rhithral sections of perennial streams located in the Po River Basin (Northern Italy) by means of specific in situ 50-points random sampling grids. Benthic Organic Matter (BOM), velocity, depth and substrate were collected, as environmental factors, together with spatial coordinates for each sample. The relationship among metrics (taxa richness, abundance and biomass) and environmental and spatial variables was checked by means of Generalized Additive Models (GAMs). Regarding the analysis of communities, coordinates were used to produce Principal Coordinates of Neighbour Matrices (PCNM) in order to detect additional spatial structures. Data were then analysed by means of variance partitioning methods, considering spatial coordinates, PCNM and environmental factors as groups of explanatory variables. Environmental factors (primarily BOM), both with and without spatial structure resulted the main drivers affecting taxa richness, abundance and community composition. On the other hand, coordinates and PCNM accounted for a minor fraction of explained variance. Nevertheless, we found that in systems with a greater riverbed structuration, PCNMs variables had higher explanatory power, highlighting the importance of space, as a proxy of small-scale community processes. Our results suggest that trophic factors are useful predictors of macroinvertebrate community organisation in rhithral sections of perennial streams. The second section of this PhD thesis concerns the organization and variability of macroinvertebrate community in braided rivers. These systems are among the most variable and dynamic riverine systems. Changes in braided rivers are sudden and frequent, driven by the high hydrological variability. They host high levels of local heterogeneity, with many different habitats in close proximity establishing a mosaic of patches. This provides the conditions for high levels of biodiversity, with strong community variability in particular among the different habitats at the stream-reach level. Nevertheless, these systems are still poorly studied and their complexity is often not taken into account in biomonitoring protocols. We applied mixed effects modelling, spatial ordination techniques and beta-diversity partitioning (into nestedness and turnover components) with the aim of improving the knowledge of braided rivers, investigating: i) the organization of macroinvertebrate communities among the different habitats of a river reach, and ii) the temporal variability of this organization (both among seasons and during summer). We predicted a differentiation of macroinvertebrate communities between distinct habitats within rivers, with this differentiation increasing during the low-flow period. We carried out our study in four braided rivers and streams of the Po River basin (Northern Italy) sampling three different kinds of mesohabitats (main channel, secondary channel and pool) in eight stations during seven campaigns from June 2015 to April 2016. We found a high variability of taxa richness, abundance and community structure among mesohabitats, with marginal ones accounting for the greater part of macroinvertebrate diversity. Secondary channels resulted as being the habitat hosting greater taxa diversity, with 10 exclusive taxa. Surprisingly the mesohabitat communities differed greatly during the seasonal phase, whereas their dissimilarity decreased during summer. This could be explained considering the summer flow reduction as a homogenizing force, leading to a general loss of the most sensitive taxa. However, the summer taxa turnover value resulted higher than nestedness, suggesting a strong environmental control on community organization, with taxa well adapted to the different conditions of mesohabitats and able to manage the effects of flow reduction. Our work represents a remarkable issue for biomonitoring protocols, highlighting the importance of taking into account the whole complexity of braided rivers for a more realistic evaluation of macroinvertebrate communities. In the third section of this thesis, we focussed the attention on the study of large-scale distribution of macroinvertebrates and on differences in community organization in watercourses with different hydrology. Flow regime and its alterations deeply affect macroinvertebrate communities, especially considering the shift in conditions which is occurring in several mediterranean and temperate rivers. This topic has been deeply explored in mediterranean systems, but the effect of regime shift is less known in temperate areas and seldom considering it in the framework of metacommunity ecology, incorporating also space in explanatory variables. With this in mind, we did our work aiming to understand the effect of flow intermittence on the large-scale distribution of benthic invertebrates and the differential importance of explanatory variables related to different spatial scales in permanent (P) versus intermittent (I) watercourses. We carried our work in 24 watercourses (11 intermittent and 13 permanent) of the Po River Basin (N Italy) before the summer dry phase. We applied mixed effect modelling and spatial ordination techniques in order to evaluate the variation of metrics and community structure between I and P streams and variance partitioning for assessing the relevance of the different spatial scales in I versus P. Communities of I streams resulted characterized by a greater randomness than in P streams, with, in general, greater levels of diversity in P ones. Moreover, we found out that both in I and P streams, local environmental variables are the most powerful predictors of community structuration. Our findings represent valuable insight in the effects of flow alterations in the perspective of best-strategy planning to face the regime-shift phenomenon.