Flow-plant interaction at a leaf scale: effects of leaf shape and flexural rigidity

River hydrodynamicsBed roughness and flow resistanc

Efficient sampling methodologies for lake littoral invertebrates in compliance with the European Water Framework Directive

Lake shores are characterised by a high natural variability, which is increasingly threatened by a multitude of anthropogenic disturbances including morphological alterations to the littoral zone. The European Water Framework Directive (EU WFD) calls for the assessment of lake ecological status by monitoring biological quality elements including benthic macroinvertebrates. To identify cost- and time-efficient sampling strategies for routine lake monitoring, we sampled littoral invertebrates in 32 lakes located in different geographical regions in Europe. We compared the efficiency of two sampling methodologies, defined as habitat-specific and pooled composite sampling protocols. Benthic samples were collected from unmodified and morphologically altered shorelines. Variability within macroinvertebrate communities did not differ significantly between sampling protocols across alteration types, lake types and geographical regions. Community composition showed no significant differences between field composite samples and artificially generated composite samples, and correlation coefficients between macroinvertebrate metrics calculated with both methods and a predefined morphological stressor index were similar. We conclude that proportional composite sampling represents a time- and cost-efficient method for routine lake monitoring as requested under the EU WFD, and may be applied across various European geographical regions

Biomechanics of aquatic plants and its role in flow-vegetation interactions

River hydrodynamicsBed roughness and flow resistanc

Flow-plant interactions at leaf, stem and shoot scales: drag, turbulence, and biomechanics

Flow-plant interactions are experimentally investigated at leaf, stem, and shoot scales in an open-channel flume at a range of Reynolds numbers. The experiments included measurements of instantaneous drag forces acting on leaves, stems, and shoots of the common freshwater plant species Glyceria fluitans, complemented with velocity measurements, high-resolution video recordings, and biomechanical tests of leaf and stem properties. The analyses of bulk statistics, power spectral densities, transfer functions, and cross-correlations of measured velocities and drag forces revealed that flow characteristics, drag force, and plant biomechanical and morphological properties are strongly interconnected and scale-dependent. The plant element-flow interactions can be subdivided into two classes: (I) passive interactions when the drag variability is due to the time variability of the wetted and frontal areas and squared approach velocity (due to the large-scale turbulence); and (II) active interactions representing a range of element-specific instabilities that depend on the element flexural rigidity and morphology. Implications of experimental findings for plant biophysics and ecology are briefly discussed

Distorted Communication, Unequal Representation: Constituents Communicate Less to Representatives Not of Their Race

Communications from constituents strongly shape the representation politicians provide. However, if politicians hear less from some constituents than others, this unequal communication may lead to unequal representation. In this article, I present a field experiment demonstrating that constituents are less likely to communicate to representatives not of their race. The experiment exploited electoral rules in Maryland, where several multimember districts have both black and white representatives. I provided 8,829 residents of such districts an opportunity to communicate to one of their actual representatives, whose race I randomized. Both blacks and whites were markedly less likely to communicate to their representatives not of their race. These results imply that politicians receive racially distorted communication, hearing disproportionately infrequently from constituents unlike them. The fact that most racial minorities have white representatives may thus help explain both minorities' less frequent communication to their representatives and the diminished substantive representation minorities typically receive. © 2013, Midwest Political Science Association

The aquatic moth Acentria ephemerella as a key species in submerged aquatic vegetation : direct and trait-mediated interactions with predators and food plants

Makrophytenbestände sind bedeutende Habitate für Fische und Makroinvertebrate und wichtige, strukturgebende Bestandteile des Uferbereichs von Stillgewässern und marinen Ökosystemen. Die Wassermotte Acentria ephemerella ist eine wichtige herbivore Insektenart, die starke Fraßschäden an submersen Wasserpflanzen verursacht und häufig in Flüssen, Seen und Küstenregionen der nördlichen Hemisphäre vorkommt. In dieser Arbeit wurden die trophischen Interaktionen zwischen Stichlingen (Gasterosteus aculeatus), der herbivoren, aquatischen Lepidoptere Acentria ephemerella und der submersen Wasserpflanzenart Potamogeton perfoliatus mit Mesokosmosexperimenten und Freilandstudien untersucht, um Einblick in den Einfluss von Acentria auf Potamogeton durch Herbivorie und die Populationsregulation von Acentria durch Prädation zu bekommen.Acentria übte einen starken Frassdruck auf Potamogeton perfoliatus aus: Mesokosmosexperimente zeigten, dass Herbivorie durch Acentria (1) die Biomasse von Potamogeton perfoliatus reduzierte und (2) die Seneszenz von Potamogeton perfoliatus Sprossen verstärkte und (3), dass Potamogeton perfoliatus Nährstoffe (Phosphor und Stickstoff) in Überwinterungsknollen im Boden verlagerte, die vor Herbivorie geschützt sind. Daher sind eine erhöhte Sprossseneszenz und eine Verlagerung der Nährstoffe Teil des tolerance/escape syndromes als Toleranzstrategie von Potamogeton perfoliatus gegen Acentria-Frass als Folge des Fehlens morphologischer und chemischer Resistenzstrategien. Einem schnellen Pflanzenwachstum und dem Aufbau einer ausreichend großen unterirdischen Überwinterungsbiomasse folgt eine verstärkte Seneszenz der oberirdischen Biomasse. Diese Reaktionen von Potamogeton dienen zusammen mit einer Verlagerung von Nährstoffen in die Überwinterungsorgane als Verteidigungsstrategie gegen Herbivorie.Da Acentria einen starken Frassdruck auf Potamogeton perfoliatus Bestände ausübt und saisonale Massenvermehrungen stattfinden, kann Acentria als ecosystem engineer angesehen werden, der die Struktur und Qualität von Wasserpflanzenbeständen beeinflusst, die als Habitat für andere Organismen dienen. Freilandprobenahmen haben gezeigt, dass Frass und Seneszenz die Sprossdichte und somit die strukturelle Komplexität von submerser aquatischer Vegetation verringerten: Benthivore Fische, insbesondere Stichlinge und juvenile Flussbarsche (Perca fluviatilis), bevorzugten Makrophytenbestände mit einem mittleren Anteil seneszenter Sprosse und damit einer optimalen, mittleren strukturellen Komplexität, da eine hohe Sprossdichte/geringe Seneszenz die Bewegungsmöglichkeiten der Fische einschränkt, wohingegen eine niedrige Sprossdichte/starke Seneszenz die verfügbare Menge an wirbellosen Futterorganismen und die Funktionsfähigkeit des Wasserpflanzenbestandes als Schutz vor piscivoren Räubern verringert.Die Rolle als ecosystem engineer wird direkt von den Mortalitäts- und Wachstumsraten von Acentria beeinflusst. Unter dem Prädationsdruck von Stichlingen verpuppten sich die Acentria Larven bei einer geringeren Größe und eine Veränderung der Entwicklungsstrategien fand statt: gegen Ende der Wachstumssaison zogen weibliche Larven die Überwinterung der Verpuppung vor, wohingegen männliche Larven zu gleichen Teilen überwinterten und sich verpuppten. Dies führte zu einem stark männchen-dominierten Geschlechterverhältnis der Puppen. Die Reduktion der Puppengröße und die Präferenz der Weibchen zu überwintern sind wahrscheinlich Life-History Strategien, um der Fischprädation zu entkommen, insbesondere für die flügellosen adulten Weibchen, die den aquatischen Lebensraum im Gegensatz zu den geflügelten Männchen nicht verlassen können. Zusammenfassend zeigten Freiland- und Mesokosmosexperimente, dass sowohl lethale (d.h. eine Reduktion der Populationsdichten) als auch nicht-lethale Prädationseffekte (d.h. ein stark männchen-dominiertes Geschlechterverhältnis der Puppen und eine verringerte Puppengröße) zu einer Verringerung des Populationswachstums von Acentria führen, indem sich (1) die Gesamtanzahl der adulten Individuen verringerte, (2) die Anzahl der für die Reproduktion verfügbaren adulten Weibchen durch ein stark männchen-dominiertes Geschlechterverhältnis verringerte und (3) die mittlere Gelegegröße durch eine geringere Größe der adulten Weibchen verkleinerte.Die starken Interaktionen zwischen Stichlingen und Acentria und zwischen Acentria und Potamogeton legen schließlich die mögliche Existenz einer trophischen Kaskade in submersen Pflanzenbeständen des Littorals von Seen und Teichen nahe, bestehend aus den trophischen Ebenen Stichlinge - Acentria - Potamogeton perfoliatus. Durch die weite Verbreitung der beteiligten Organismen ist diese trophische Kaskade wahrscheinlich von großer Bedeutung bei der Strukturierung der Uferzonen vieler Süsswasserseen und Brackwasserökosysteme der nördlichen Breiten

How to cope with a superior enemy? Plant defencestrategies in response to annual herbivore outbreaks

1. The perfoliate pondweed Potamogeton perfoliatus L. constitutes large monospecific macrophyte patches in many Central European lakes. Correlative evidence from the field suggests that P. perfoliatus is under an increasing grazing pressure during its short vegetation period from May to September due to seasonal outbreaks of the aquatic moth Acentria ephemerella Denis & Schiffermüller. We used a mesocosm experiment to determine the influence of A. ephemerella herbivory on P. perfoliatus shoot development and resting bud production and to study the defence strategies of this macrophyte.2. Herbivory resulted in a reduction of the P. perfoliatus vegetation period by more than 2 months thereby reducing the average resting bud size and the overall resting bud biomass sevenfold. This suggests that besides its severe immediate effects, herbivory affects P. perfoliatus growth and dynamics also during the subsequent season.3. As a response to herbivory P. perfoliatus translocated nutrients (phosphorus (P) and nitrogen (N)) from leaves towards buds. Acentria ephemerella larvae had a high P content, implying P limitation of larval growth especially within the herbivory treatment. This suggests that at least the P translocation from leaves towards resting buds may be viewed as an anti-predator strategy rather than as a nutrient conservation strategy.4. Acentria ephemerella herbivory changed the allocation strategy of P. perfoliatus in the size versus number of resting buds: only the number, but not the size of resting buds was reduced under shoot grazing by Acentria ephemerella. This change in the number versus size trade-off might allow the plant to produce a minimumresting bud size necessary for successful sprouting in the next spring.5. Synthesis. Overall, our results suggest an escape syndrome (after Agrawal & Fishbein, Ecology, 87 (2006) S132) as a defence strategy against herbivory for P. perfoliatus, consisting of a shortening of the growth period, a translocation of nutrients and a change in allocation strategy. The increased plant senescence that was accompanied by the shortening of the growth period has further implications for the usage of macrophyte patches as a habitat for invertebrates and fishes and for the structure of littoral food webs

Decay processes in woody debris influence the taxonomic and functional composition of littoral macroinvertebrates

We examined the effects of woody debris decay state of on the densities, taxonomic and functional composition, and diversity of macroinvertebrates in littoral zones of undeveloped lakes. Our study revealed that the interacting effects of increasing roughness and softness of the wood, as well as changes in the quality of food resources with progressing decomposition of woody debris exerted the strongest impacts on macroinvertebrate communities. Structurally complex, decomposed wood supported higher densities and species richness of macroinvertebrates than undecayed wood. We also found several taxa which were typical for decayed wood, including macroinvertebrates considered as xylophages. Decaying wood underpinned greater functional richness than undecayed wood, with high densities of collector-gatherers and shredders which could benefit from organic matter originating from decomposing wooden tissue, as well as predators attracted by numerous potential prey inhabiting this complex habitat. As decaying wood enhanced abundant and diverse macroinvertebrate communities, which in turn could subsidize upper trophic levels, it provides a valuable habitat in littoral zones, particularly in lakes with already sparse macrophyte cover.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

Experimental evidence for a strong influence of stickleback predation on the population dynamics and sex ratio of an aquatic moth

The larvae of the water moth Acentria ephemerella are important herbivores in freshwater macrophyte beds. Due to the considerable damage that Acentria can inflict on the plants, the herbivore might influence the important role of macrophytes within littoral ecosystems. It is thus important to know which factors can control Acentria populations and consequently its interannual variability. Here we use a mesocosm experiment to test whether fish predation can control Acentria population development. We followed Acentria population development and size distribution using two different stickleback, Gasterosteus aculeatus, densities and one control treatment. Sticklebacks were able to reduce population growth of Acentria compared to the control treatment without fishes. Predation of sticklebacks on Acentria was confirmed by gut content analyses of sticklebacks caught at the end of the experiment. In addition to a reduction of population growth, stickleback predation resulted in malebiased sex ratios of Acentria, which are typically observed in situ in Lake Constance. A comparison of Acentria size distributions in the experiment and in stickleback guts suggests that the male bias is not due to size-selective predation of sticklebacks on presumably larger females. In summary, stickleback predation influenced Acentria population dynamics and dampened population growth (1) directly by consuming Acentria larvae and (2) indirectly by shifting the sex ratio via sex-specific mortality towards a stronger male bias. Top-down control by fishes of Acentria, a key herbivore in macrophyte beds, suggests the existence of a previously overlooked trophic cascade of major importance in aquatic ecosystems

Lake-wide mapping of littoral habitat using underwater videography

Littoral zones − referring to benthic areas above the light compensation depth − provide numerous ecosystem functions, including mediating light, temperature, and nutrient dynamics, and supporting important foraging and refuge areas for macroinvertebrates, fishes and water birds. Habitat assessments of littoral zones remain fundamental to lake and fisheries management, however traditional field surveys are time-intensive and limited in their spatial extent, whereas desktop evaluations using remote sensing and aerial imagery are cost prohibitive and require considerable data processing expertise. In light of these challenges, this study demonstrated the ability to use simple, cost-effective underwater videography to conduct lake-wide spatially-continuous assessments of littoral habitat. For lakes across a gradient of shoreline and riparian development in northwestern United States, we map the areal coverage of macrophytes, coarse woody habitat, bottom substrates, and artificial structures in littoral zones. Underwater videography represents a relevant tool for environmental monitoring because it allows for the estimation of littoral habitats at fine spatial grains across broad spatial extents. Data can also be obtained rapidly and at relatively low cost, providing a permanent record of habitat conditions that can used to monitor trends over time