The plants and animals that inhabit river channels may act as zoogeomorphic agents affecting the nature and rates of sediment recruitment, transport and deposition. The impact of benthic-feeding fish, which disturb bed material sediments during their search for food, has received little attention, even though benthic feeding species are widespread in rivers and may collectively expend significant amounts of energy foraging across the bed. A series of experiments were conducted to investigate the impacts of benthic feeding fish on the structure and composition of gravel-bed river sediments, and the implications for bed material transport. An ex-situ experiment was conducted to investigate the impact of a benthic feeding fish (European Barbel Barbus barbus) on particle displacements, bed sediment structures, gravel entrainment and transport fluxes. In a laboratory flume, changes in bed surface topography were measured and grain displacements examined when an imbricated, water-worked bed of 5.6-16 mm gravels was exposed to feeding juvenile Barbel. For substrates that had been exposed to feeding fish and control substrates which had not, grain entrainment rates and bedload fluxes were measured under a moderate transport regime. On average, approximately 37% of the substrate, by area, was modified by foraging fish during a four-hour treatment period, resulting in increased microtopographic roughness and reduced particle imbrication. Structural changes caused by fish increased bed load flux by 60% under entrainment flows, whilst on average the total number of grains transported during the entrainment phase was 82% higher from substrates that had been disturbed by Barbel. An ex-situ experiment utilising Barbel and Chub Leuciscus cephalus extended this initial study by considering the role of fish size and species as controls of sediment disturbance by foraging. Increasing the size of Barbel had a significant effect on measured disturbance and bedload transport. Specifically, the area of disturbed substrate, foraging depth, microtopographic roughness and sediment structure all increased as functions of fish size, as did bedload flux and total transported mass. In a comparison of the foraging effects of like-sized Barbel and Chub 8-10 in length, Barbel foraged a larger area of the riverbed and had a greater impact on microtopographic roughness and sediment structure. Foraging by both species was associated with increased sediment transport, but the bed load flux after foraging by Barbel was 150% higher than that following foraging by Chub and the total transported mass of sediment was 98% greater. An in-situ experiment quantified the effects of foraging fish, primarily Cyprinids (specifically Barbel and Chub), on gravel-river bed sediment structures, surface grain-size distributions, sediment transport fluxes and grain entrainment in the River Idle, Nottinghamshire, UK. This was achieved by installing large experimental sediment trays seeded with food at typical densities. The experiments yielded data about 1) topographic and structural differences between pre- and post-feeding substrates using DEMs interpolated from laser scans, 2) modifications to surface and sub-surface grain-size distributions as a function of fish foraging and 3) differences in sediment entrainment from water-worked substrates exposed to feeding fish and control substrates, without fish. Small sections of the substrate trays were recovered in tact from the field and for substrates that had been exposed to feeding fish and control substrates which had not, grain entrainment rates and bedload fluxes were measured under a moderate transport regime in the laboratory. On average, approximately 74% of the substrate, by area, was modified by foraging fish during a twelve-hour period, resulting in increased microtopographic roughness and substrate coarsening which had significant implications for bed material transport during the steady entrainment flow. Together, results from these experiments indicate that by increasing surface microtopography, modifying the composition of fluvial substrates and undoing the naturally stable structures produced by water working, foraging can influence sediment transport dynamics, predominately by increasing the mobility of river bed materials. The implication of this result is that by influencing the quantity of available, transportable sediment and entrainment thresholds, benthic feeding may affect sediment transport fluxes in gravel-bed rivers. In addition, three discrete studies were performed alongside the core experiments described above. A quantitative examination of habitat conditions favoured by feeding Barbel was conducted in the River Idle (Nottinghamshire, UK) which served to supplement existing literature pertaining to Barbel ecology, and inform experimental design during the core experiments. Two further studies considered the potential importance of foraging as a zoogeomorphic activity in terms of spatial extent, at a variety of scales, thereby extending core experiments to larger spatial scales in-situ.

Signal crayfish are an internationally widespread invasive species that can have important detrimental ecological impacts. This thesis aims to determine whether signal crayfish have the potential to also impact the physical environment in rivers. A series of experiments were undertaken in purpose-built still-water aquaria using a laser scanner to obtain Digital Elevation Models (DEMs) of narrowly-graded gravel surfaces before and after exposure to crayfish. The difference between DEMs was used to quantify volumetric changes in surface topography due to crayfish activity. Two distinct types of topographic change were identified. The first was the construction of pits and mounds which resulted in an increase in surface roughness and grain exposure. The second was the rearrangement of surface material caused by crayfish brushing past grains when walking and foraging, reorientating grains and altering friction angles. A series of 80 flume runs were undertaken to quantify alterations made by crayfish to water-worked, as well as loose, gravel substrates at low velocity flows. Crayfish significantly altered the structure of water-worked substrates, reversing the imbrication of surface grains to a more random arrangement. Surfaces were entrained at a relatively high velocity flow subsequent to crayfish activity in order to directly link topographic and structural alterations to substrate stability. Nearly twice as many grains were mobilised from surfaces which had been disturbed by crayfish in comparison to control surfaces that were not exposed to crayfish. A field investigation aimed to determine the potential significance of the geomorphic impact of crayfish in rivers. Signal crayfish were tracked through a 20 m reach of a small, lowland alluvial river for 150 days using a Passive Integrated Transponder (PIT) system. Crayfish were active throughout the channel, although their activity became limited as water temperature dropped and flow stage increased. Substrate was not an important determinant of crayfish activity at this scale. Instead, crayfish tended to be found along the inner bank of a meander bend where there was a substantial cover of macrophytes. Consequently, signal crayfish were active for extended periods on substrates of a similar size to those that they could disturb in flume experiments. These results suggest that signal crayfish could have important geomorphic effects in rivers, disturbing bed structures and increasing the mobility of coarse material. This may have important implications for both the management of some rivers and benthic organisms that reside on the river bed.

Engenharia de ecossistemas refere-se à habilidade de certos organismos de modificar ativamente o ambiente que os cerca. No contexto ecológico, engenheiros de ecossistemas são espécies-chave que modificam ou criam habitats por meios mecânicos ou usando suas próprias estruturas corporais. Ao criarem novos nichos, castores, recifes de corais e sociedades humanas primitivas garantem tanto a própria existência, quanto a de outros organismos no mesmo ecossistema. Devido a seu caráter de longa duração, algumas destas modificações podem persistir até mais do que a duração de uma população de engenheiros, implicando em consequências evolutivas. O estudo teórico de tal fenômeno ecológico é relativamente recente se comparado com a descrição de interações tipo predador-presa ou de competição. Apenas em 1996 Gurney & Lawton introduziram um modelo descrevendo a dinâmica populacional de engenheiros de ecossistemas, mas a partir de lá poucas modificações apareceram. Aqui nós complementamos tal modelo ao permitir que engenheiros se movam difusivamente através dos sítios de um mapa acoplado, uma formulação discreta no espaço e no tempo. A análise de estabilidade local revela a existência dos regimes estável, cíclico e caótico, com uma cascata de bifurcações levando a órbitas caóticas. Obtemos que apenas para altas taxas de crescimento, onde ocorre comportamento caótico, a dispersão influencia na dinâmica das metapopulações. Neste regime, o caos é suprimido e a extinção pode ser evitada. / Ecosystem engineering refers to the ability of certain organisms to actively modify their surrounding environment. In an ecological context, ecosystem engineers are keystone species that modify or create habitats via mechanical means or by using their own physical structures. By creating new niches, beavers, coral reefs and primitive human societies would guarantee both their and other species survival in a shared ecosystem. Due to its long-lasting character, some of this changes might outlive the engineers populations, leading to evolutionary consequences. The theoretical study of such ecological phenomena is relatively recent when compared to the description of predator-prey or competition interactions. Only in 1996 Gurney & Lawton introduced a model to describe the population dynamics of ecosystem engineers, yet since then few modifications appeared. Here we build on this model by allowing the engineers to move diffusively through the patches of a coupled map lattice, a framework discrete both in time and space. The local stability analysis reveals the existence of stable, cyclic and chaotic regimes, with period-doubling bifurcations leading to chaotic orbits. We find that only for large intrinsic growth rates, where chaotic behavior occurs, dispersal influences the metapopulation dynamics. In this regime, chaos is suppressed and extinction can be avoided.

Engenharia de ecossistemas refere-se à habilidade de certos organismos de modificar ativamente o ambiente que os cerca. No contexto ecológico, engenheiros de ecossistemas são espécies-chave que modificam ou criam habitats por meios mecânicos ou usando suas próprias estruturas corporais. Ao criarem novos nichos, castores, recifes de corais e sociedades humanas primitivas garantem tanto a própria existência, quanto a de outros organismos no mesmo ecossistema. Devido a seu caráter de longa duração, algumas destas modificações podem persistir até mais do que a duração de uma população de engenheiros, implicando em consequências evolutivas. O estudo teórico de tal fenômeno ecológico é relativamente recente se comparado com a descrição de interações tipo predador-presa ou de competição. Apenas em 1996 Gurney & Lawton introduziram um modelo descrevendo a dinâmica populacional de engenheiros de ecossistemas, mas a partir de lá poucas modificações apareceram. Aqui nós complementamos tal modelo ao permitir que engenheiros se movam difusivamente através dos sítios de um mapa acoplado, uma formulação discreta no espaço e no tempo. A análise de estabilidade local revela a existência dos regimes estável, cíclico e caótico, com uma cascata de bifurcações levando a órbitas caóticas. Obtemos que apenas para altas taxas de crescimento, onde ocorre comportamento caótico, a dispersão influencia na dinâmica das metapopulações. Neste regime, o caos é suprimido e a extinção pode ser evitada. / Ecosystem engineering refers to the ability of certain organisms to actively modify their surrounding environment. In an ecological context, ecosystem engineers are keystone species that modify or create habitats via mechanical means or by using their own physical structures. By creating new niches, beavers, coral reefs and primitive human societies would guarantee both their and other species survival in a shared ecosystem. Due to its long-lasting character, some of this changes might outlive the engineers populations, leading to evolutionary consequences. The theoretical study of such ecological phenomena is relatively recent when compared to the description of predator-prey or competition interactions. Only in 1996 Gurney & Lawton introduced a model to describe the population dynamics of ecosystem engineers, yet since then few modifications appeared. Here we build on this model by allowing the engineers to move diffusively through the patches of a coupled map lattice, a framework discrete both in time and space. The local stability analysis reveals the existence of stable, cyclic and chaotic regimes, with period-doubling bifurcations leading to chaotic orbits. We find that only for large intrinsic growth rates, where chaotic behavior occurs, dispersal influences the metapopulation dynamics. In this regime, chaos is suppressed and extinction can be avoided.

No description available.

Marine bioeroders, borers, and burrowers can have drastic effects to marine habitats and facilities. By physically altering the structure of marine habitats, these organisms may elicit ecosystem-level effects that cascade through the community. While borer damage is typically restricted to a few substratum types, burrowing isopods in the genus Sphaeroma attack a diversity of substrata in tropical and temperate systems. My dissertation examined how boring sphaeromatid isopods affect coastal habitats (saltmarshes, mangroves) and other estuarine substrata as well as marine structures. I used a combination of lab and mensurative field experiments to quantify the effects of boring by isopods and examine how select factors affect the colonization, hence burrowing damage by isopods. I explored these questions primarily using the temperate boring sphaeromatid, Sphaeroma quoianum, as a model organism. My initial lab experiments quantified the per capita erosion rates of S. quoianum in four commonly attacked estuarine substrata. I found marsh banks and Styrofoam substrata were the most affected per capita. I supplemented this lab experiment with a year-long mensurative field experiment examining how erosion rates differ between marshes infested and uninfested by boring isopods. Marshes infested with isopods eroded 300% faster than uninfested marshes. I further examined the boring effects on Styrofoam floats. I compiled surveys and observations and conducted a short experiment to describe how isopods affect Styrofoam floats used in floating docks. I observed dense colonies of isopods attacking floats and expelling millions of plastic particles in the ocean. The boring effects to simulated Styrofoam floats were also affected by seawater temperature. Burrowing effects in Styrofoam floats exhibited a curvilinear relationship with temperature and peaked around 18°C. These results suggest a 1-2°C increase in water temperature could increase boring effects 5-17% of populations of isopods in Oregon and California bays. To examine the small-scale factors that mediate colonization and boring, I conducted a series of binary choice experiments. I found the presence of conspecifics, biofilm, and shade were important factors influencing colonization. These small scale factors likely explain why isopod attack is focused in some substrata. Finally, to examine the boring effects of tropical isopods in mangroves, I examined the associations between burrowing by S. terebrans and mangrove performance and fecundity. I found negative relationships between boring effects and performance and fecundity in two mangrove species in a restored mangrove stand in Taiwan. Together, these studies elucidate the effects of bioerosive isopods on saltmarshes, mangroves, and marine structures. However, the similar mechanisms involved in bioerosion in other boring species suggest that these results can be used to infer similar effects of other borers. In addition, since many species of sphaeromatid isopods have been introduced, this research shows how the effects of a non-native bioeroder can damage marine facilities and degrade and alter marine habitats. Through biological erosion and thus changing the physical structure of a marine habitat these non-native species can have ecosystem-level effects that cascade throughout the local community.

Spartina anglica est une espèce exotique hybride qui peuple les zones humides littorales. Elle s’est installée dans le Bassin d’Arcachon au cours des années 1980, envahissant fortement les prés salés et les platiers vaseux préalablement occupés par, respectivement, Spartina maritima et Zostera noltei. Face aux inquiétudes suscitées par cette installation, cette thèse vise à comprendre, par une approche pluridisciplinaire, les mécanismes d’invasion et ses conséquences sur le milieu physique et sur les espèces végétales natives. L’objectif de ce travail est d’étudier l’occupation de niche par S. anglica et ses interactions avec deux espèces intertidales natives : S. maritima et Z. noltei.L’analyse d’images aériennes et satellitales a montré que, 30 ans après l’invasion, dans une zone densément peuplée par la Spartine native, la zone haute des prés salés a peu changé : la Spartine anglaise a occupé des niches vides et n’a pas remplacé la végétation native. Une expérience de transplantation réciproque et de mesures de biomasses confirment ce résultat, en montrant que l’espèce native offre une résistance à la colonisation de l’espèce exotique. L’expansion de la Spartine anglaise vers les replats de marée de l’intérieur du Bassin serait ainsi liée à sa capacité à tolérer les perturbations physiques, à sa forte plasticité de croissance en milieu oxygéné et riche en nutriments et à son comportement auto-facilitateur. Sa forte capacité d’ingénieur d’écosystèmes semble être liée à son système racinaire très développé, qui améliore l’aération des sols fortement anoxiques.Les effets de la colonisation par l’espèce exotique des zones intertidales basses à subtidales sur la Zostère naine sont importants sur le long-terme (dizaines d’années). En tant qu’ingénieur d’écosystèmes, la Spartine exotique favorise l’élévation du sol par sédimentation, entrainant une dessiccation du sédiment, peu favorable à la Zostère. Des mesures physiques au sein de patchs de l’espèce exotique suggèrent que l’élévation du sédiment est toutefois lente, surtout liée à une allocation de biomasse spécifique aux racines ainsi qu’à des rhizomes qui permettent de résister à l’érosion.En termes de gestion et de conservation des prés salés du Bassin d’Arcachon, ces résultats indiquent l’importance de limiter les perturbations physiques et les apports nutritifs qui pourraient rompre la résistance à l’invasion de la Spartine native. Ils supportent aussi l’idée que la Spartine anglaise pourrait être un allié robuste face à l’élévation du niveau de la mer. / Spartina anglica is a hybrid exotic cordgrass that inhabits coastal salt marshes. This species arrived in the Bay of Arcachon in the 1980s and since has importantly colonized the salt marshes and tidal flats formerly only occupied by the native Spartina maritima and Zostera noltei, respectively. This work aims at understanding, with an interdisciplinary perspective, the invasion mechanisms of this exotic cordgrass and the outcoming changes of its introduction in the Bay, both to the physical environment and to the native vegetation. Different approaches were considered in order to assess the niche occupancy by the exotic Spartina and its interactions with the native intertidal species, Spartina maritima and Zostera noltei.The analysis of aerial and satellite images has shown that, in about 30 years after the invasion, within a zone densely populated by the native Spartina, the global high marsh zone did not suffer significant changes with the arrival of the invasive species. Spartina anglica did not replace the existent marsh vegetation, it occupied empty niches along the intertidal area instead. Additionally, experimental works of cross transplantation and biomass measurements have corroborated that the native Spartina maritima offers resistance to the colonization by the exotic Spartina. It was also shown that the invasive occupies the same intertidal niche along the elevation and anoxic gradient than the native. The successful extension of Spartina anglica into the mudflat towards the inner Bay was related to its likely ability to tolerate physical disturbances, its strong growth plasticity in nutrient- and oxygen- rich patches and its self-facilitator behaviour. This latter trait is related to its strong ecosystem-engineering ability due to its prominent root system and consequent ability to ameliorate the oxygenation of highly anoxic soils.The main effect of the exotic Spartina species on the seagrass is related to its stronger ecosystem-engineering ability, favouring bed accretion up to levels that are not favourable to Z. noltei through enhancement of desiccation stress. However, hydrodynamic and altimetry measurements have shown that the process of bed accretion is slow and, due to the cordgrass’ specific preferential biomass allocation to roots, its efficiency is more linked to its resistance to erosion rather than sediment trapping.The results of this study provide relevant information for the definition of appropriate action and conservation strategies of marsh zones in the Bay of Arcachon, and in particular the importance of limiting physical disturbance and nutrient pollution that could disrupt the biotic resistance of the native cord grass. They also suggest a potentially important role of the exotic species in facing increasing Sea Level Rise.

Dans 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'écosystème / 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 functioning

No description available.