Turbulence-mediated facilitation of resource uptake in patchy stream macrophytes

Many landscapes are characterized by a patchy, rather than homogeneous, distribution of vegetation. Often this patchiness is composed of single-species patches with contrasting traits, interacting with each other. To date, it is unknown whether patches of different species affect each other’s uptake of resources by altering hydrodynamic conditions, and how this depends on their spatial patch configuration. Patches of two contrasting aquatic macrophyte species (i.e., dense canopy-forming Callitriche and sparse canopy-forming Groenlandia) were grown together in a racetrack flume and placed in different patch configurations. We measured 15NH4+ uptake rates and hydrodynamic properties along the centerline and the lateral edge of both patches. When the species with a taller, denser canopy (Callitriche) was located upstream of the shorter, sparser species (Groenlandia), it generated turbulence in its wake that enhanced nutrient uptake for the sparser Groenlandia. At the same time, Callitriche benefited from being located at a leading edge where it was exposed to higher mean velocity, as its canopy was too dense for turbulence to penetrate from upstream. Consistent with this, we found that ammonium uptake rates depended on turbulence level for the sparse Groenlandia and on mean flow velocity for the dense Callitriche, but Total Kinetic Energy was the best descriptor of uptake rates for both species. By influencing turbulence, macrophyte species interact with each other through facilitation of resource uptake. Hence, heterogeneity due to multi-specific spatial patchiness has crucial implications for both species interactions and aquatic ecosystem functions, such as nitrogen retention

Modifications physiques de l'habitat par les végétaux aquatiques : conséquences pour les processus biogéochimiques et rétroactions pour les plantes

Submerged aquatic vegetation often grows in lotic systems in patches generated by scale-dependent feedbacks. As ecosystem engineers, plants modify the physical environment triggering positive feedbacks within the patch and negative feedbacks alongside the patch, resulting in regular pattern formation. These scale-dependent feedbacks enable to explain only the lateral expansion of patches, but not their longitudinal development. The objective was to study the processes that trigger positive and negative feedbacks for plants along patches and the consequences for patch dynamics. In situ coupled measurements of hydrodynamics, sediment characteristics, and plant morphology were performed along patches of increasing length. The results demonstrated that a minimum patch length was needed to induce in-patch velocity reduction and fine sediment accumulation. As a consequence of these modifications, patch length influenced the nutrient concentrations in interstitial water of the in-patch sediment, this effect being observed only over a certain threshold length. Over this threshold length, the sediment presented an accumulation of ammonium and depletion of nitrates. Plant height was related to patch length by a quadratic relationship, suggesting that negative feedbacks occur over a certain patch length, probably due to the high ammonium concentration that can be toxic for plants in the range measured. The threshold lengths over which patches influence the biogeochemical processes and negative feedbacks occur were reduced in the ecosystem presenting the highest nutrient level. The results also demonstrated that the physical habitat modifications induced by patches depend on the plant traits and patch characteristics. The plant-induced modifications of the physical habitat have cascading effects on the biogeochemical processes and plant growth, which depended on the environmental conditions, with consequences for patch dynamics and ecosystem functioningDans les systèmes lotiques, la végétation aquatique se développe en formant des taches générées par des rétroactions échelle-dépendantes. Les plantes modifient l'environnement physique (i.e. organismes ingénieurs), induisant des rétroactions positives dans les taches et négatives à côté, ce qui conduit à la formation de patrons réguliers. Ces rétroactions échelle-dépendantes ne permettent d'expliquer que l'expansion latérale des taches, mais pas leur développement longitudinal. L'objectif était d'étudier les processus qui induisent des rétroactions pour les plantes et les conséquences pour la dynamique des taches. Des mesures de l'hydrodynamique, des caractéristiques des sédiments et de la morphologie des plantes ont été faites in situ le long de taches de longueur croissante. Les résultats ont démontré qu'une longueur minimale est nécessaire pour induire une réduction de la vitesse du courant et une accumulation de sédiments fins dans les taches. L’ensemble conduit à des changements des concentrations en nutriments dans l'eau interstitielle au delà d’une certaine longueur de tache, consistant en une accumulation d'ammonium et une diminution des nitrates. La hauteur des plantes est liée à la longueur de la tache selon un modèle quadratique, suggérant l’existence d’une rétroaction négative au delà d’une longueur seuil, probablement due à la concentration élevée en ammonium qui peut être toxique pour les plantes. Les longueurs au delà desquelles ont lieu des changements des processus biogéochimiques et des rétroactions négatives sont plus faibles dans l’écosystème avec le niveau de nutriments le plus élevé. Enfin, les modifications de l'habitat induites par les taches dépendent des caractéristiques des plantes et des taches. Ces modifications induites par les plantes ont des effets en cascade sur les processus biogéochimiques et la croissance des plantes, avec des conséquences pour la dynamique des taches et le fonctionnement de l'écosystèm

How to build vegetation patches in hydraulic studies: a hydrodynamic-ecological perspective on a biological object

Vegetation in freshwater and coastal ecosystems modifies flows, retains sediment, protects banks and shorelines from erosion. Hydraulic laboratory studies with live vegetation or artificial plant mimics, or numerical models with abstracted patches, are often used to quantify the effects of vegetation on water flow and sedimentation. However, the choice of plant and patch characteristics is often not supported by field observations of patch dimensions, density or spacing between consecutive patches. The discrepancy between plants in natural conditions and in flume experiments or numerical studies may affect the relevance of these findings for natural ecosystems. In this study, we provide guidelines for building realistic vegetation patches in hydraulic studies. We collected data on four species of fully submerged freshwater aquatic macrophytes that can grow into well-defined patches. We considered three relevant levels: individual plants (inside patches), isolated patches and multiple neighbouring patches. At the plant level, we observed significant differences in biomechanical traits (Young’s modulus, flexural stiffness), resulting in stem Cauchy numbers ranging from 85.25 to 325.84, and leaf Cauchy numbers from 163.81 to 2003.97. At the patch level, we found significant relationships between patch length, width and height, showing covariation among different patch characteristics. The relationships among patch dimensions differed significantly among sampling sites for three of the four species, suggesting high intraspecific variability in patch sizes. By providing a first set of guidelines for choosing correct and ecologically relevant plant characteristics, this dataset aims to improve our understanding of the complex processes occurring inside and around submerged vegetated patches

The role of patch size in ecosystem engineering capacity: a case study of aquatic vegetation

International audienceSubmerged aquatic plants are ecosystem engineers that are able to modify their habitat. However, the role of patch size in the engineering capacity of aquatic plants has not yet been fully investigated, while it could be essential for elucidating the consequences of plant presence. Our objectives were to investigate the effects of patch size on plant-flow-sediment interac- tions in lotic ecosystems and to determine whether these effects differed according to environmental characteristics. We performed in situ measurements of velocity and grain size along natural patches of increasing length (L) at two sites pre- senting different flow and sediment characteristics. Our results indicated that a minimum patch size was needed to induce in-patch reduction of the time averaged velocity component in the flow direction (i.e. streamwise velocity) and fine sediment accumulation. Streamwise velocity decreased linearly with L independently of the site conditions. The sediment texture was instead dependent on site conditions: for the site characterized by higher velocity and coarser sediment, the sediment grain size exponentially decreased with L, reaching a minimum value at L ≥ 1.0 m, while for the site characterized by lower velocity and finer sediment, it reached a minimum value already at L > 0.3 m. This study demonstrated that a minimal patch size is required to trigger the ecosystem engineering capacity of aquatic plant patches in lotic environments and that this capacity increases with patch length. Small patches induce little to no modification of the physical habitat, with possible negative feedbacks for plants. With increasing patch size, the habitat modifications induced by plants become more important,

The role of patch size in ecosystem engineering capacity: a case study of aquatic vegetation

Submerged aquatic plants are ecosystem engineers that are able to modify their habitat. However, the role of patch size in the engineering capacity of aquatic plants has not yet been fully investigated, while it could be essential for elucidating the consequences of plant presence. Our objectives were to investigate the effects of patch size on plant-flow-sediment interactions in lotic ecosystems and to determine whether these effects differed according to environmental characteristics. We performed in situ measurements of velocity and grain size along natural patches of increasing length (L) at two sites presenting different flow and sediment characteristics. Our results indicated that a minimum patch size was needed to induce in-patch reduction of the time averaged velocity component in the flow direction (i.e. streamwise velocity) and fine sediment accumulation. Streamwise velocity decreased linearly with L independently of the site conditions. The sediment texture was instead dependent on site conditions: for the site characterized by higher velocity and coarser sediment, the sediment grain size exponentially decreased with L, reaching a minimum value at L ≥ 1.0 m, while for the site characterized by lower velocity and finer sediment, it reached a minimum value already at L > 0.3 m. This study demonstrated that a minimal patch size is required to trigger the ecosystem engineering capacity of aquatic plant patches in lotic environments and that this capacity increases with patch length. Small patches induce little to no modification of the physical habitat, with possible negative feedbacks for plants. With increasing patch size, the habitat modifications induced by plants become more important, potentially triggering positive feedbacks for plants

Scale-dependent effects of vegetation on flow velocity and biogeochemical conditions in aquatic systems

In rivers, scale-dependent feedbacks resulting from physical habitat modifications control the lateral expansion of submerged plant patches, while the mechanisms that limit patch expansion on a longitudinal dimension remain unknown. Our objective was to investigate the effects of patch length on physical habitat modification (i.e., flow velocity, sediment grain size distribution), the consequences for biogeochemical conditions (i.e., accumulation/depletion of nutrients, microbial respiration), and for individual plants (i.e., shoot length). We measured all of these parameters along natural patches of increasing length. These measurements were performed at two sites that differed in mean flow velocity, sediment grain size, and trophic level. The results showed a significant effect of patch length on organic matter content and nutrient concentrations in interstitial water. For the shortest patches sampled, all of these parameters had similar values to those measured at the upstream control position. For longer patches, organic matter content and orthophosphate and ammonium concentrations increased within the patch compared to the upstream bare sediment, whereas nitrate concentrations decreased, suggesting changes in vertical water exchanges and an increase in anaerobic microbial activities. Furthermore, plant height was related to patch length by a quadratic pattern, probably due reduced hydrodynamic stress occurring for increasing patch length, combined with conditions that are less favourable for plants over a threshold length, possibly due to the light limitation or to the high concentration of ammonium that in the concentration range we measured may be toxic for plants. The threshold lengths over which patches influence the nutrient concentrations were reduced for the site with higher nutrient levels. We demonstrated that the plant-induced modifications of the physical habitat exert important effects on biogeochemical conditions, with possible consequences for patch dynamics and ecosystem functioning

The role of patch size in ecosystem engineering capacity: a case study of aquatic vegetation

Submerged aquatic plants are ecosystem engineers that are able to modify their habitat. However, the role of patch size in the engineering capacity of aquatic plants has not yet been fully investigated, while it could be essential for elucidating the consequences of plant presence. Our objectives were to investigate the effects of patch size on plant-flow-sediment interactions in lotic ecosystems and to determine whether these effects differed according to environmental characteristics. We performed in situ measurements of velocity and grain size along natural patches of increasing length (L) at two sites presenting different flow and sediment characteristics. Our results indicated that a minimum patch size was needed to induce in-patch reduction of the time averaged velocity component in the flow direction (i.e. streamwise velocity) and fine sediment accumulation. Streamwise velocity decreased linearly with L independently of the site conditions. The sediment texture was instead dependent on site conditions: for the site characterized by higher velocity and coarser sediment, the sediment grain size exponentially decreased with L, reaching a minimum value at L ≥ 1.0 m, while for the site characterized by lower velocity and finer sediment, it reached a minimum value already at L > 0.3 m. This study demonstrated that a minimal patch size is required to trigger the ecosystem engineering capacity of aquatic plant patches in lotic environments and that this capacity increases with patch length. Small patches induce little to no modification of the physical habitat, with possible negative feedbacks for plants. With increasing patch size, the habitat modifications induced by plants become more important, potentially triggering positive feedbacks for plants

Scale-dependent effects of vegetation on flow velocity and biogeochemical conditions in aquatic systems

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Data presented in the paper “Turbulence-mediated facilitation of resource uptake in patchy stream macrophytes”

This dataset contains the experimental data presented in the paper “Turbulence-mediated facilitation of resource uptake in patchy stream macrophyte”, with the above contributors as authors, submitted to Limnology and Oceanography. The dataset contains the underlying data (flume hydrodynamic and NH4+ uptake rate measurements) presented in the manuscript

Data presented in the paper “Turbulence-mediated facilitation of resource uptake in patchy stream macrophytes”

This dataset contains the experimental data presented in the paper “Turbulence-mediated facilitation of resource uptake in patchy stream macrophyte”, with the above contributors as authors, submitted to Limnology and Oceanography. The dataset contains the underlying data (flume hydrodynamic and NH4+ uptake rate measurements) presented in the manuscript