| Summary: | As fundamental components of chalk stream ecosystems, aquatic macrophytes are intrinsically linked to flow regime and physicochemical stability. Assessment of the River Itchen, Hampshire, a classic lowland chalk stream faced with ecosystem degradation, indicates the significance of the discharge regime for controlling both water quality and the spatiotemporal distribution of macrophyte assemblages. Experimental studies using outdoor artificial stream mesocosms signify their effectiveness for macrophyte growth studies and in identifying causality attributed to environmental stressors. In such experiments, the keystone chalk stream macrophyte Ranunculus pseudofluitans was identified as having preferences to moderate water velocities, with morphological and physiological trait responses causing distinct morphotypes depending on development in optimal or sub-optimal conditions. Furthermore, when subjected to flow, nutrient and periphytic competitive stressors, main trait responses were categorised as developmental, functional and confounded, respectively, with most traits linked to healthy development associated with flow. In addition, significant filamentous algal growth under low-nutrient conditions, but removal in increased velocities, highlights the importance of flow as a control mechanism. Examination of ontogenetic effects suggest trait variation with age, and overall developmental stage linked to a combination of environmental and plant age effects. This study demonstrates the necessity for good, consistent flow regimes in chalk streams, which enhances macrophyte community diversity, promoting development of keystone taxa, which in turn encourage beneficial heterogeneous flow patterns.
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