Tributary streams create spatial discontinuities in habitat, biological productivity, and diversity in mainstem rivers

Abstract

Investigations utilizing a one-dimensional sediment routing model demonstrate that moderate inputs of water and sediment at tributary junctions greatly increase physical heterogeneity in the recipient channel. Simulated physical heterogeneity is most sensitive to the ratios of tributary to mainstream bed load flux and bed load grain size and is less sensitive to relative discharge. Within the model, aggradation drives the processes that augment habitat variability, and in general, any aggradational confluence will be associated with elevated physical diversity. Model output reveals elevated physical diversity at two scales: between distinctive upstream and downstream zones separated by a confluence step and within each zone as a function of local environmental gradients. Total diversity increases as tributary sediment load and caliber increase relative to the mainstream. The ecological implications of the patterns and magnitude of tributary-induced physical heterogeneity are considered, and testable hypotheses are presented. Results highlight the need to accurately characterise patterns of sediment production, delivery, and routing in order to predict local tributary impacts and thereby understand patterns of habitat diversity and biodiversity at network scales. Résumé :Des recherches utilisant un modèle unidimensionnel de transport des sédiments montrent que des apports modérés d'eau et de sédiments aux points de jonction des tributaires augmentent considérablement l'hétérogénéité du chenal récepteur. L'hétérogénéité physique simulée est très sensible au rapport des flux des charges de fond du tributaire sur ceux du cours principal ainsi qu'au rapport des tailles des particules; il est moins sensible aux débits relatifs. Dans le modèle, l'alluvionnement entraîne les processus qui augmentent la variabilité de l'habitat et, en général, toute confluence d'alluvionnement est associée à une augmentation de la diversité physique. Les résultats du modèle montrent des augmentations de la diversité physique à deux échelles, soit entre des zones distinctes d'amont et d'aval séparées par un seuil de confluence, soit dans un même zone en fonction des gradients environnementaux locaux. La diversité totale augmente en fonction de l'accroissement de la charge et du calibre des sédiments du tributaire par rapport à ceux du cours principal. Nous examinons les conséquences écologiques des patterns et de l'amplitude de l'hétérogénéité physique générés par les tributaires et nous présentons des hypothèses testables. Nos résultats soulignent la nécessité de caractériser avec précision les patrons de production, de livraison et de cheminement des sédiments afin de pouvoir prédire les impacts locaux des tributaires et ainsi comprendre les patrons de diversité d'habitat et de biodiversité à l'échelle des réseaux.[Traduit par la Rédaction] Rice et al. 2566

Evaluating macroinvertebrate community shifts in the confluence of freestone and limestone streams

“…We explored the influence of geology on community structure where streams merge and water from two distinct geological backgrounds (freestone and limestone) flow together to create a unique microhabitat that amalgamates properties from both streams. Areas immediately downstream of the confluence between two streams receive water, sediments, and organic material from both the main stem and the tributary draining into it, which have been shown to influence both the composition and the diversity of the macroinvertebrate community below the confluence (Rice et al, 2001;Kiffney et al, 2006;Grant et al, 2007;Collier and Lill, 2008;Mac Nally et al, 2011;Milesi and Melo, 2014). A potential change in macroinvertebrate community structure should be dependent on the characteristics of the streams that are converging (Rice et al, 2001).…”

Periphytic community structure of Ostracoda (Crustacea) in the river-floodplain system of the Upper Paraná River

“…For example, the increase of taxonomic richness and density of macroinvertebrates along a longitudinal gradient in the Dam-regulated River in Japan was related to the confluence of a tributary, owing to the increase of environmental heterogeneity, food resources and recruitment of colonizers (Katano et al, 2009). In addition, an increase of nutrient concentrations in the main channel by the confluence of the tributaries showed a positive relation with density of macroinvertebrates downstream of the dam (Kiffney et al, 2006).…”

Bidirectional connectivity in rivers and implications for watershed stability and management

“…Moving beyond the linear perspective of rivers, Benda et al (2004) described how tributaries can deposit sediments and create areas of physical heterogeneity of sediments in the main river channel. These tributary junctions represent hotspots of both physical, chemical, and biological variability (Kiffney et al 2006). Similarly, there is increasing appreciation of the importance of both upstream and downstream connections in the dynamics of rivers and their biota (Ward 1989;Gomi et al 2002).…”