Rivers exhibit hierarchical spatial structures of habitat and physical attributes, providing directed pathways for biological population and community dynamics and thus shaping the meta-ecosystems therein. While the River Continuum Concept has generalized the spatial patterns of abiotic and biotic components along rivers, a mechanistic understanding of how river networks' shape may constrain the attributes of riverine meta-ecosystems remains lacking. Here, we address this gap with an in silico study. We integrate the Optimal Channel Network concept (and the well-established scaling of river geomorphological and hydrological attributes) with a meta-ecosystem model (with trait-based food-web dynamics and spatial dynamics of species and inorganic resources) to explore how distinct river network shapes (elongated vs. compact) may drive the spatial patterns of biodiversity and community trophic structure. We analyse metrics of biomass distribution, trophic structure and composition of locally realized food webs and show that elongated and compact networks foster very different upstream-downstream biological patterns, and even more contrasting patterns are often observed between the long and short paths of elongated networks. Overall, we observe a transition from prevailing detritus-based to nutrient-based trophic channels moving downstream, leading to peaks in alpha diversity at intermediate river size, where both channels are more balanced. Higher spatial heterogeneity in community composition and lower biomass levels are observed in elongated than in compact networks, driven by higher variability in nutrient input loads and higher water volumes, respectively. Together, our findings associate river shapes to the emergent riverine meta-ecosystems properties and help reveal the underpinning physical attributes-driven mechanisms.