Backshore nourishment of a beach degraded by off-road vehicles: Ecological impacts and benefits

Worldwide, spoil from maintenance dredging of navigation channels is increasingly used to opportunistically nourish beaches. This is often justified on the presumption that nourishment will improve public beach amenity and restore sandy beach habitat. However, this is not necessarily the case, especially for beaches that do not have an immediate threat of significant erosion. We addressed the ecological impacts and benefits of a backshore sand nourishment project conducted along an off-road vehicle (ORV) damaged section of Blacksmiths Beach, New South Wales, Australia. Sediment, sourced from dredging the inlet of nearby Lake Macquarie, was placed on the foredune, ORVs were excluded and low-density vegetation was planted. Sampling before and after the management interventions, at the Impact (nourished) site, two Control sites (with ORVs), and two Reference sites (without ORVs), assessed ecological impacts of nourishment and the efficacy of the interventions in rehabilitating vegetation and invertebrate communities degraded by ORVs. Nourishment initially had large negative impacts on vegetation cover, as well as on invertebrate abundance and richness. Recovery to a pre-nourished state was, however, observed for vegetation cover after 9 months and invertebrate communities after 21 months. Nevertheless, by the end of our study that extended 21 months post-nourishment and ORV exclusion, there was no evidence of change in the nourished site towards the state of Reference sites. Overall, our study suggests that small-scale backshore sand nourishments of ocean beaches may have only short-term negative impacts on foredune ecosystems when accompanied with some replanting. Nevertheless, where the frequency of sand disposals is greater than the required recovery time, or cumulative effects amass, longer-term or sustained impacts may occur. Our study does not support the efficacy of sand nourishment as a tool for ecological restoration, at least in the short term, without sustained replanting and weeding efforts aimed at reinstating the vegetation community.

P321Subthreshold delayed afterdepolarizations form a substrate for conduction block in the infarcted heart

Funding Acknowledgements

National Institute for Health Research; British Heart Foundation; and The Wellcome Trust and Engineering and Physical Sciences Research Council.

Background

Delayed afterdepolarizations (DADs) due to spontaneous calcium (Ca) release (SCR) events from the sarcoplasmic reticulum have been implicated with a variety of arrhythmias. Such SCR events have also been reported in cells that survive in the infarct border zone (BZ). While the potential of Ca-mediated DADs to become suprathreshold and propagate in the form of ectopic beats has been well characterized, the role of subthreshold DADs in arrhythmia formation in the infarcted heart remains to be elucidated.

Purpose

To use computational modelling to investigate whether subthreshold Ca-mediated DADs may form a substrate for conduction block and reentry in the BZ. Our hypothesis is that subthreshold DADs can hinder local tissue excitability in critical infarct BZ regions by inactivating the fast sodium current (INa), leading to temporary unidirectional conduction block providing a trigger for arrhythmogenesis.

Methods

We developed an idealized infarct model of the left ventricle. The infarct region consisted of a non-conducting scar transcended by an isthmus of cells that survived myocardial infarction (border zone). These cells were made prone to Ca-mediated DADs described by a phenomenological model of SCR events. The model was pre-paced at the apex followed by a 1500ms-pacing pause to see whether DADs would emerge. An extra beat with a longer coupling interval (CI) was then applied. The following electrophysiological changes resulting from remodeling processes in the isthmus were simulated to assess their contribution to the arrhythmogenic potential of subthreshold DADs: INa loss-of-function due to a (2.5mV and 5mV) negative-shift in the steady-state channel inactivation; 50% reduction in tissue conductivity; and increased levels of fibrosis (up to 50%).

Results

On average, Ca-mediated DADs reached their maximum value 1065ms after the last paced beat (Fig. A). Despite this, in the default electrophysiological setup, simulations with extra beats with 1000ms > CI > 1100ms did not result in conduction block in any of the experiments. When repeated with combined changes of reduced tissue conductivity and fibrosis, subthreshold DADs were still unable to create a substrate for block. However, when combined with a 5mV-shift in INa inactivation, block at isthmus’ mouth proximal to the stimulus site was detected for extra beats 1010 ms ≥ CI ≥ 1070ms (see Fig. B). The cause of block was due to a subthreshold DAD occurring just prior to the arrival of the extra beat. All blocked beats degenerated into reentry.

Conclusions

Under most physiological conditions, subthreshold DADs are unlikely to provide a substrate for unidirectional block. However, under conditions of decreased excitability, subthreshold DADs can hinder tissue excitability in the infarcted region leading to conduction block and reentry.

Abstract Figure. DAD-mediated conduction block in the BZ

221Evaluating the ability of different substrate mapping techniques to identify scar-related ventricular tachycardia circuits using computational modelling

Funding Acknowledgements

National Institute for Health Research; British Heart Foundation; and The Wellcome Trust and Engineering and Physical Sciences Research Council.

Background

Accurate identification of targets for catheter ablation therapy of ventricular tachycardias (VTs) in the postinfarction heart remains a significant challenge. Identification of such targets often requires VT-induction to delineate the entry/exit points of the reentrant circuit sustaining the VT. However, inducibility may not be possible due to hemodynamic instability. In this scenario, substrate ablation strategies can still be performed to uncover the arrhythmogenic substrate during sinus or paced rhythm. However, substrate mapping may fail to accurately delineate the reentrant circuit resulting in VT recurrence after the procedure.

Purpose

To use computer simulations to compare the ability of different electroanatomical maps constructed following typical substrate ablation strategies to identify the VT exit site.

Methods

An image-based computational model of the porcine post-infarction left ventricle was constructed to simulate VT and paced rhythm. Electroanatomical maps were constructed based on the following features extracted from electrograms computed on the endocardial surface: activation time (AT), bipolar electrogram amplitude, signal fractionation and the reentry vulnerability index (RVI - a metric combining activation and repolarization timings to identify tissue susceptibility to reentry). Potential ablation targets during substrate mapping were compared for: highest 5% AT gradient; lowest 5% bipolar signal amplitudes; areas with fragmented signals (more than one peak); and lowest 5% RVI. The minimum distance, d, between the manually identified VT exit site and the targets was measured.

Results

The RVI performed better than the other metrics at detecting the VT exit site (see Figure). The minimum distance between sites of lowest RVI and the exit site was 3.2mm compared to 13.1mm and 15.9mm in traditional AT and voltage maps, respectively. As the scar was not transmural, parameters derived from all electrograms (including those located on dense scar regions) were used to construct the electroanatomical maps. This improved the performance of the RVI significantly, making it more specific than the other metrics as can be seen in the Figure.

Conclusions

Among all metrics investigated here, the RVI identified the vulnerable region closest to VT exit site. This finding suggests that activation-repolarization metrics may improve the detection of pro-arrhythmic regions without having to induce VT. Moreover, the RVI may be particularly well suited for detecting vulnerable regions within non-transmural scars.

Abstract Figure. VT and Substrate Mapping

P532Endocardial pacing is less arrhythmogenic than conventional epicardial pacing when pacing in proximity to scar in patients with ischemic heart failure

Funding Acknowledgements

WT 203148/Z/16/Z; MR/N011007/1; RE/08/003; PG/15/91/31812; PG/16/81/32441

Background

Endocardial pacing has been shown to improve response to cardiac

resynchronization therapy (CRT) in comparison to conventional epicardial pacing and the

physiological activation, endocardium to epicardium, is proposed to make it less arrhythmogenic.

However, the relative arrhythmic risk of endocardial and epicardial pacing has not been

systematically investigated. Pacing in proximity to scar increases susceptibility to arrhythmogenesis

during epicardial pacing. Whether this is also the case during endocardial pacing is currently

unknown.

Purpose

We investigate 1) whether endocardial pacing is less arrhythmogenic than epicardial

pacing, 2) whether pacing location relative to scar plays a role in arrhythmogenesis during

endocardial pacing, and 3) whether these findings could be explained by the direction of the

transmural action potential duration (APD) gradient.

Methods

We used computational models of ischemic heart failure and patient-specific (n = 24) left ventricular anatomy and scar morphology to simulate repolarization during endocardial and

epicardial pacing. Pacing locations were selected 0.2-3.5cm from a scar. We ran simulations with a

20ms transmural APD gradient, as found in heart failure, from the epicardium to endocardium

(physiological) and with this gradient inverted. We computed the volume of high

(>3ms/mm) repolarization gradients (HRG) within 1cm around a scar, as a surrogate for arrhythmia

risk, and analysed these with ANOVA and Tukey-Kramer post-hoc tests.

Results

Simulations with a physiological APD gradient predict that endocardial pacing creates a

smaller (34%) volume of HRG around (1cm) a scar compared to epicardial pacing when

pacing 0.2cm from scar (Figure 1-A). The volume of HRG decreases (P < 0.05) with distance

from scar for epicardial pacing but not endocardial pacing (Figure 1-A). Inverting the

transmural APD gradient, inverts the trend observed with a physiological gradient. In this case, the

volume of HRG is unaffected by pacing location during epicardial pacing, whereas it decreases (19%)

with the distance from scar for endocardial pacing. This is illustrated

in the regions highlighted in yellow in Figure 1 for endocardial pacing at 0.2 and 3.5cm from a scar

with a physiological (B) and an inverted (C) gradient.

Conclusions

Endocardial pacing is less arrhythmogenic (purpose 1) than conventional epicardial

pacing when pacing in proximity to scar and is also less susceptible to pacing location relative to scar

(purpose 2). The direction of the transmural APD gradient offers a mechanistic explanation for

reduced susceptibility to arrhythmogenesis during endocardial pacing compared to epicardial pacing

(purpose 3). Endocardial pacing is an attractive alternative to conventional epicardial pacing in

patients with scar, as it allows pacing in proximity to scar while avoiding increasing arrhythmogenic

risk in patients with ischemic heart failure.

Abstract Figure.

Urban blue: A global analysis of the factors shaping people's perceptions of the marine environment and ecological engineering in harbours

Marine harbours are the focus of a diverse range of activities and subject to multiple anthropogenically induced pressures. Support for environmental management options aimed at improving degraded harbours depends on understanding the factors which influence people's perceptions of harbour environments. We used an online survey, across 12 harbours, to assess sources of variation people's perceptions of harbour health and ecological engineering. We tested the hypotheses: 1) people living near impacted harbours would consider their environment to be more unhealthy and degraded, be more concerned about the environment and supportive of and willing to pay for ecological engineering relative to those living by less impacted harbours, and 2) people with greater connectedness to the harbour would be more concerned about and have greater perceived knowledge of the environment, and be more supportive of, knowledgeable about and willing to pay for ecological engineering, than those with less connectedness. Across twelve locations, the levels of degradation and modification by artificial structures were lower and the concern and knowledge about the environment and ecological engineering were greater in the six Australasian and American than the six European and Asian harbours surveyed. We found that people's perception of harbours as healthy or degraded, but not their concern for the environment, reflected the degree to which harbours were impacted. There was a positive relationship between the percentage of shoreline modified and the extent of support for and people's willingness to pay indirect costs for ecological engineering. At the individual level, measures of connectedness to the harbour environment were good predictors of concern for and perceived knowledge about the environment but not support for and perceived knowledge about ecological engineering. To make informed decisions, it is important that people are empowered with sufficient knowledge of the environmental issues facing their harbour and ecological engineering options.

Building blue infrastructure: Assessing the key environmental issues and priority areas for ecological engineering initiatives in Australia's metropolitan embayments

Ecological engineering principles are increasingly being applied to develop multifunctional artificial structures or rehabilitated habitats in coastal areas. Ecological engineering initiatives are primarily driven by marine scientists and coastal managers, but often the views of key user groups, which can strongly influence the success of projects, are not considered. We used an online survey and participatory mapping exercise to investigate differences in priority goals, sites and attitudes towards ecological engineering between marine scientists and coastal managers as compared to other stakeholders. The surveys were conducted across three Australian cities that varied in their level of urbanisation and environmental pressures. We tested the hypotheses that, relative to other stakeholders, marine scientists and coastal managers will: 1) be more supportive of ecological engineering; 2) be more likely to agree that enhancement of biodiversity and remediation of pollution are key priorities for ecological engineering; and 3) identify different priority areas and infrastructure or degraded habitats for ecological engineering. We also tested the hypothesis that 4) perceptions of ecological engineering would vary among locations, due to environmental and socio-economic differences. In all three harbours, marine scientists and coastal managers were more supportive of ecological engineering than other users. There was also greater support for ecological engineering in Sydney and Melbourne than Hobart. Most people identified transport infrastructure, in busy transport hubs (i.e. Circular Quay in Sydney, the Port in Melbourne and the Waterfront in Hobart) as priorities for ecological engineering, irrespective of their stakeholder group or location. There were, however, significant differences among locations in what people perceive as the key priorities for ecological engineering (i.e. biodiversity in Sydney and Melbourne vs. pollution in Hobart). Greater consideration of these location-specific differences is essential for effective management of artificial structures and rehabilitated habitats in urban embayments.

Congruence of intraspecific variability in leaf traits for two co-occurring estuarine angiosperms

Studies seeking to identify sources of variability and trade-offs in leaf traits have done so by assembling large databases of traits, across species and time points. It is unclear to what extent interspecific patterns derived in such a manner apply to intraspecific variation, particularly at regional scales, and the extent to which interspecific patterns vary temporally. We tested the hypothesis that the leaf traits of two foundation species, the mangrove Avicennia marina and the eelgrass Zostera muelleri, would display similar patterns of intraspecific variability across gradients of latitude and estuarine condition, that match previously reported interspecific patterns, and that persist through time. We found intraspecific patterns of decreasing carbon to nitrogen ratio and mechanical elasticity, and increasing nitrogen content with latitude that were consistent between the two plant species, and with previously reported interspecific patterns for other groups of species. Specific leaf area, leaf toughness and total phenolics, by contrast, displayed species-specific patterns that varied markedly through time. Relationships between estuarine condition and leaf traits were highly variable temporally, and also displayed markedly different patterns of intraspecific variability between the two species. Our study highlights the considerable within-species variation in leaf traits that should be accounted for in regional to biome scale analyses. Although some intraspecific patterns mirrored those found across species, at global scales, the considerable variability in other leaf traits between species and through time highlights the need to better understand the drivers and constraints of this intraspecific variation.

Image-Based Personalization of Cardiac Anatomy for Coupled Electromechanical Modeling

Computational models of cardiac electromechanics (EM) are increasingly being applied to clinical problems, with patient-specific models being generated from high fidelity imaging and used to simulate patient physiology, pathophysiology and response to treatment. Current structured meshes are limited in their ability to fully represent the detailed anatomical data available from clinical images and capture complex and varied anatomy with limited geometric accuracy. In this paper, we review the state of the art in image-based personalization of cardiac anatomy for biophysically detailed, strongly coupled EM modeling, and present our own tools for the automatic building of anatomically and structurally accurate patient-specific models. Our method relies on using high resolution unstructured meshes for discretizing both physics, electrophysiology and mechanics, in combination with efficient, strongly scalable solvers necessary to deal with the computational load imposed by the large number of degrees of freedom of these meshes. These tools permit automated anatomical model generation and strongly coupled EM simulations at an unprecedented level of anatomical and biophysical detail.

Image-Based Personalization of Cardiac Anatomy for Coupled Electromechanical Modeling

Computational models of cardiac electromechanics (EM) are increasingly being applied to clinical problems, with patient-specific models being generated from high fidelity imaging and used to simulate patient physiology, pathophysiology and response to treatment. Current structured meshes are limited in their ability to fully represent the detailed anatomical data available from clinical images and capture complex and varied anatomy with limited geometric accuracy. In this paper, we review the state of the art in image-based personalization of cardiac anatomy for biophysically detailed, strongly coupled EM modeling, and present our own tools for the automatic building of anatomically and structurally accurate patient-specific models. Our method relies on using high resolution unstructured meshes for discretizing both physics, electrophysiology and mechanics, in combination with efficient, strongly scalable solvers necessary to deal with the computational load imposed by the large number of degrees of freedom of these meshes. These tools permit automated anatomical model generation and strongly coupled EM simulations at an unprecedented level of anatomical and biophysical detail.

Multi-year persistence of beach habitat degradation from nourishment using coarse shelly sediments

Beach nourishment is increasingly used to protect public beach amenity and coastal property from erosion and storm damage. Where beach nourishment uses fill sediments that differ in sedimentology from native beach sands, press disturbances to sandy beach invertebrates and their ecosystem services can occur. How long impacts persist is, however, unclear because monitoring after nourishment typically only extends for several months. Here, monitoring was extended for 3-4 years following each of two spatially separated, replicate nourishment projects using unnaturally coarse sediments. Following both fill events, the contribution to beach sediments of gravel-sized particles and shell fragments was enhanced, and although diminishing through time, remained elevated as compared to control sites at the end of 3-4 years of monitoring, including in the low intertidal and swash zones, where benthic macroinvertebrates concentrate. Consequently, two infaunal invertebrates, haustoriid amphipods and Donax spp., exhibited suppressed densities over the entire post-nourishment period of 3-4 years. Emerita talpoida, by contrast, exhibited lower densities on nourished than control beaches only in the early summer of the first and second years and polychaetes exhibited little response to nourishment. The overall impact to invertebrates of nourishment was matched by multi-year reductions in abundances of their predators. Ghost crab abundances were suppressed on nourished beaches with impacts disappearing only by the fourth summer. Counts of foraging shorebirds were depressed for 4 years after the first project and 2 years after the second project. Our results challenge the view that beach nourishment is environmentally benign by demonstrating that application of unnaturally coarse and shelly sediments can serve as a press disturbance to degrade the beach habitat and its trophic services to shorebirds for 2-4 years. Recognizing that recovery following nourishment can be slow, studies that monitor impacts for only several months are inadequate.

Morphological traits and density of foundation species modulate a facilitation cascade in Australian mangroves

Facilitation cascades are critical to the maintenance of biodiversity in a variety of habitats. Through a series of two experiments, we examined how the morphological traits and density of interacting foundation species influence the establishment and persistence of a facilitation cascade in temperate Australian mangrove forests. In this system, mangrove pneumatophores trap the free-living alga, Hormosira banksii, which, in turn, supports dense and diverse assemblages of epifaunal mollusks. The first experiment, which manipulated pneumatophore height and density, revealed that these two traits each had additive negative effects on the establishment, but additive positive effects on the persistence of the cascade. High densities of tall pneumatophores initially served as a physical barrier to algal colonization of pneumatophore plots, but over the longer-term enhanced the retention of algae. The increased algal biomass, in turn, facilitating epifaunal colonization. The second experiment demonstrated that the retention of algae by pneumatophores was influenced more by algal thallus length than vesicle diameter, and this effect occurred independent of pneumatophore height. Our study has extended facilitation theory by showing that the morphological traits and density of basal and intermediary facilitators influence both the establishment and persistence of facilitation cascades. Hence, attempts to use foundation species as a tool for restoration will require an understanding not only of the interactions among these, but also of the key traits that modify interrelationships.

The density and spatial arrangement of the invasive oyster Crassostrea gigas determines its impact on settlement of native oyster larvae

Understanding how the density and spatial arrangement of invaders is critical to developing management strategies of pest species. The Pacific oyster, Crassostrea gigas, has been translocated around the world for aquaculture and in many instances has established wild populations. Relative to other species of bivalve, it displays rapid suspension feeding, which may cause mortality of pelagic invertebrate larvae. We compared the effect on settlement of Sydney rock oyster, Saccostrea glomerata, larvae of manipulating the spatial arrangement and density of native S. glomerata, and non-native C. gigas. We hypothesized that while manipulations of dead oysters would reveal the same positive relationship between attachment surface area and S. glomerata settlement between the two species, manipulations of live oysters would reveal differing density-dependent effects between the native and non-native oyster. In the field, whether oysters were live or dead, more larvae settled on C. gigas than S. glomerata when substrate was arranged in monospecific clumps. When, however, the two species were interspersed, there were no differences in larval settlement between them. By contrast, in aquaria simulating a higher effective oyster density, more larvae settled on live S. glomerata than C. gigas. When C. gigas was prevented from suspension feeding, settlement of larvae on C. gigas was enhanced. By contrast, settlement was similar between the two species when dead. While the presently low densities of the invasive oyster C. gigas may enhance S. glomerata larval settlement in east Australian estuaries, future increases in densities could produce negative impacts on native oyster settlement. Synthesis and applications: Our study has shown that both the spatial arrangement and density of invaders can influence their impact. Hence, management strategies aimed at preventing invasive populations reaching damaging sizes should not only consider the threshold density at which impacts exceed some acceptable limit, but also how patch formation modifies this.