Physical modelling of water, fauna and flora: knowledge gaps, avenues for future research and infrastructural needs

peer reviewedPhysical modelling is a key tool for generating understanding of the complex interactions between aquatic organisms and hydraulics, which is important for management of aquatic environments under environmental change and our ability to exploit ecosystem services. Many aspects of this field remain poorly understood and the use of physical models within eco-hydraulics requires advancement in methodological application and substantive understanding. This paper presents a review of the emergent themes from a workshop tasked with identifying the future infrastructure requirements of the next generation of eco-hydraulics researchers. The identified themes are: abiotic factors, adaptation, complexity and feedback, variation, and scale and scaling. The paper examines these themes and identifies how progress on each of them is key to existing and future eðorts to progress our knowledge of eco-hydraulic interactions. Examples are drawn from studies on biofilms, plants, and sessile and mobile fauna in shallow water fluvial and marine environments. Examples of research gaps and directions for educational, infrastructural and technological advance are also presented.PISCES work package of HYDRALAB FP

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

Plants in aquatic ecosystems: current trends and future directions

Aquatic plants fulfil a wide range of ecological roles, and make a substantial contribution to the structure, function and service provision of aquatic ecosystems. Given their well-documented importance in aquatic ecosystems, research into aquatic plants continues to blossom. The 14th International Symposium on Aquatic Plants, held in Edinburgh in September 2015, brought together 120 delegates from 28 countries and six continents. This special issue of Hydrobiologia includes a select number of papers on aspects of aquatic plants, covering a wide range of species, systems and issues. In this paper we present an overview of current trends and future directions in aquatic plant research in the early 21st century. Our understanding of aquatic plant biology, the range of scientific issues being addressed and the range of techniques available to researchers have all arguably never been greater; however, substantial challenges exist to the conservation and management of both aquatic plants and the ecosystems in which they are found. The range of countries and continents represented by conference delegates and authors of papers in the special issue illustrate the global relevance of aquatic plant research in the early 21st century but also the many challenges that this burgeoning scientific discipline must address

Adaptations to increasing hydraulic stress: morphology, hydrodynamics and fitness of two higher aquatic plant species.

10 pagesSessile organisms often exhibit morphological changes in response to permanent exposure to mechanical stimulation (wind or water movements). The adaptive value of these morphological changes (hydrodynamic performance and consequences on fitness) has not been studied extensively, particularly for higher plants submitted to flow stress. The aim was to determine the adaptive value of morphological patterns observed within two higher aquatic plant species, Berula erecta and Mentha aquatica, growing along a natural flow stress gradient. The hydrodynamic ability of each ramet was investigated through quantitative variables (drag coefficient and E-value). Fitness-related traits based on vegetative growth and clonal multiplication were assessed for each individual. For both species, the drag coefficient and the E-value were explained only to a limited extent by the morphological traits used. B. erecta exhibited a reduction in size and low overall plant drag at higher flow velocities, despite high drag values relative to leaf area, due to a low flexibility. The plants maintained their fitness, at least in part, through biomass reallocation: one tall ramet at low velocity, but shorter individuals with many interconnected stolons when flow velocity increased. For M. aquatica, morphological differences along the velocity gradient did not lead to greater hydrodynamic performance. Plant size increased with increasing velocities, suggesting the indirect effects of current favouring growth in high velocities. The fitness-related traits did not demonstrate lower plant fitness for high velocities. Different developmental constraints linked to plant morphology and trade-offs between major plant functions probably lead to different plant responses to flow stress

Resprouting Response of Aquatic Clonal Plants to Cutting May Explain Their Resistance to Spate Flooding

International audienceResprouting ability may increase a plant's resistance to recurrent disturbances in aquatic ecosystems. We investigated the effect of mechanical disturbances on survival and regrowth patterns in three clonal aquatic species of similar growth form but with different ecological ranges in terms of flooding (Myriophyllum verticillatum, Myriophyllum spicatum and Potamogeton coloratus). P. coloratus prefers to colonize stable habitats, whereas M. verticillatum occurs in intermediately flooded habitats and M. spicatum is tolerant to a high flooding frequency. Two cutting treatments (single cuts or repeated cuts) were applied under controlled conditions. We hypothesized that M. verticillatum and M. spicatum would be resistant to cutting displaying either a tolerant or an escape strategy whereas P. coloratus would be sensitive to cutting. Our hypothesis was validated, as the three species displayed contrasting responses to disturbance. M. verticillatum displayed efficient clonal propagation following breakage (escape strategy), but its growth rate decreased after recurrent disturbances. P. coloratus displayed a close response but was unable to compensate biomass loss even after one cut. M. spicatum maintained a similar growth rate by developing a densely branched form despite recurrent disturbances but with a low investment in clonal growth (tolerance strategy). Both biomass compensation and clonal propagation influence plant fitness, but their relative advantage differs depending on the flooding frequency experienced by plants in their natural habitats. Clonal propagation may promote recolonization after disturbances in infrequently flooded sites, but seems less efficient than a tolerance strategy for survival in frequently flooded sites

Sediment type rules the response of aquatic plant communities to dewatering in wetlands

International audienceQuestionsThe effect of dewatering on aquatic plant communities may vary with sediment properties, such as particle size and organic matter content, as both control water retention in the sediment during dewatering. No study has tested how sediment type affects the short-term response of plant communities to dewatering. We hypothesized that for the same dewatering event: (1) organic, silt and coarse sediments rank along a gradient of water deficit, with which community resistance and resilience decrease; (2) species survival during the event depend on their known ecological affinity for water; and (3) a peak in species richness associated with an intermediate water deficit occurs in silty habitats.LocationRiverine wetlands in the floodplain of the Ain River, France.MethodsEighteen sampling units were defined, set over three sediment types: gravel-dominated coarse sediment, silt and organic matter-dominated sediment. For each sediment type, three sampling units were permanently aquatic, and three sampling units underwent summer dewatering. A survey of species cover was conducted in each sampling unit at four times: before summer dewatering, at the beginning of the event, at the end of the event and 2months after rewetting. Community resistance and resilience were assessed, as were changes over time in the proportions of species according their water affinity (hydrophytes, amphiphytes and helophytes, documented from the floras), and the effect of dewatering on species renewal and richness.ResultsThe sediment type affected aquatic plant community resistance and resilience, with increasing disturbance intensity for silty, followed by coarse compared with organic sediment. Organic sediment retained water efficiently during dewatering, supporting high community resistance, with the maintenance of amphiphytes and more tolerant hydrophytes. On silty sediment, disturbance was sufficiently high to cause the disappearance of hydrophyte vegetative parts, but propagules rapidly sprouted after rewetting, suggesting their preservation in the sediment and enabling good community resilience. On coarse sediment, a decrease in resident amphiphyte abundance, together with helophyte colonization and maintenance after rewetting were observed. Coarse sediment is not favourable to propagule survival, explaining the low community resilience. Contrary to our hypothesis, a linear positive relationship between disturbance intensity and species richness was observed after dewatering.ConclusionThe present study demonstrates that a simple description of sediment type allows prediction of dewatering impact on aquatic plant communities: organic, silt and coarse sediments were ranked along a gradient of water deficit, along which resistance and resilience decreased

Functional importance of freshwater amphipods in the leaf litter recycling process: the role of leaf litter characteristics

International audienceImpact of leaf characteristics on the keystone amphipod species for litter decomposition process

Functional importance of freshwater amphipods in the leaf litter recycling process : the role of leaf litter characteristics

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Effect of climate-related change in vegetation on leaf litter consumption and energy storage by Gammarus pulex from Continental or Mediterranean populations.

As a consequence of global warming, it is important to characterise the potential changes occurring for some functional processes through the intra-specific study of key species. Changes in species distribution, particularly when key or engineer species are affected, should contribute to global changes in ecosystem functioning. In this study, we examined the potential consequences induced by global warming on ecosystem functioning in term of organic matter recycling. We compared consumption of leaf litter by some shredder populations (Gammarus pulex) between five tree species inhabiting continental (i.e., the northern region of the Rhône River Valley) and/or Mediterranean (i.e., the southern region of the Rhône River Valley) conditions. To consider any potential adaptation of the gammarid population to vegetation in the same climate conditions, three populations of the key shredder Gammarus pulex from the northern region and three from the southern region of the Rhône River Valley were used. We experimentally compared the effects of the geographical origin of both the gammarid populations and the leaf litter species on the shredding activity and the physiological state of animals (through body triglyceride content). This study demonstrated that leaf toughness is more important than geographical origin for determining shredder leaf litter consumption. The overall consumption rate of the gammarid populations from the southern region of Rhône Valley was much higher than that of the populations from the northern region, but no clear differences between the origins of the leaf litter (i.e., continental vs. Mediterranean) were observed. The northwards shift of G. pulex populations adapted to warmer conditions might significantly modify organic matter recycling in continental streams. As gammarid populations can demonstrate local adaptations to certain leaf species as a trophic resource, changes in riparian vegetation associated with climate change might locally affect the leaf litter degradation process by this shredder

Effects of rising temperature on a functional process: consumption and digestion of leaf litter by a freshwater shredder

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