Interaction networks in coastal soft-sediments highlight the potential for change in ecological resilience

Reconstruction of the Maërl habitat to better understand its ecological integrity

Maërl habitats are composed of coralline red algae species that can live freely rolling on the seabed and forming nodules, the so-called rhodoliths, or incrusted forming coralligenous habitats. Maërl habitats are generally distributed in the Mediterranean at a depth of between 30 m and 70 m and are considered one of the most emblematic Mediterranean seabeds. In the present study, the complex structure of maërl habitats was investigated to i) characterise the relief features and classify the different sediments, ii) to estimate the abundance of the coralline red algae (both rhodoliths and encrusting ones) and iii) to analyse the biodiversity of the species inhabiting the habitat. Data were obtained from an approximately 11 km-long transect, using non-intrusive sampling methods, integrating information from video images collected using the Remotely Operated Vehicle LIROPUS (IEO_CSIC), and multibeam bathymetry and backscatter data. Video images were used to reconstruct (using GIS) the habitat structure and characteristics. Throughout the transect, a strong relationship between habitat characteristics and the effect of trawling activity and the geomorphology of the studied area was observed. The closed area to fishing activity showed a high abundance of rhodoliths in well-structured megaripples reliefs. Contrarily, the areas affected by fishing showed an important destructuring of the relief with a low density of rhodoliths. Last, the muddy bottoms showed areas with no characteristic features and no rhodoliths. All this information has allowed to reconstruct the maërl habitat in the Blanes continental shelf (NW Mediterranean) and analyse the fragmentation of the assemblages seen in the video to assess its good environmental status (GES), and finally to identify the level of ecological integrity of this vulnerable habitat.

Scaling up benthic primary productivity estimates in a large intertidal estuary using remote sensing

As two main primary producers in temperate intertidal regions, seagrass and microphytobenthos (MPB) support estuarine ecosystem functions in multiple ways including stabilizing food webs and regulating sediment resuspension among others. Monitoring estuary productivity at large scales can inform ecosystem scale responses to environmental stressors (climate change, pollution and habitat degradation). Here we use a case study to show how Sentinel-2 data can be used to estimate estuary-wide emerged and submerged gross primary productivity (GPP) on intertidal flats by coupling a new machine learning model to map seagrass and unvegetated habitats with literature-derived photosynthesis-irradiance (P - I) relationships. The model consisted of (1) supervised classification with random forest to delineate seagrass and unvegetated areas and (2) artificial neural network (ANN) regression to predict % seagrass coverage. Our seagrass delineation by supervised classification had an overall accuracy of 0.96, while the ANN regression on seagrass coverage provided high predictive accuracy (R2 = 0.71 and RMSE = 0.11). The estimated GPP showed seagrass contributed slightly more to intertidal benthic productivity than MPB in the case-study estuary over the 3-year study period. This model can be used to predict the response of seagrass and MPB GPP to sea level rise, which shows that the future state may be very sensitive to increased turbidity. For example, by the year 2100, the model shows a sharp decline in productivity with sea level rise, assuming current turbidity trends, (loss of up to 52-53 % for seagrass and 23-45 % for MPB, a function of whether shoreward migration of seagrass is incorporated). However, GPP under conditions of unchanging turbidity (and no seagrass migration), exhibits minimal negative impact of sea level rise (loss of 3 % for seagrass and increase of 29 % for MPB). Therefore, controlling water turbidity might be an efficient solution to maintaining the current GPP as sea level rises.

Toward a network perspective in coastal ecosystem management

Environmental management in coastal ecosystems has been challenged by the complex cumulative effects that occur when many small issues result in large ecological shifts. Current environmental management of these spaces focuses on identifying and limiting problematic stressors via a series of assessment techniques. Whilst there is a strong desire among managers to consider complexity in ecological responses to cumulative effects, current approaches for assessing risk focus on breaking down the issues into multiple cause and effect relationships. However, uncertainty arises when data and information for a place are limited, as is commonly the case, and this creates decision paralysis while more information is generated. Here, we discuss how ecological understanding of network interactions in coastal marine ecosystems can be used as a lens to bring together multiple lines of evidence and create actions. We list and describe four characteristics of marine ecosystem interaction networks including the possibility for; 1) indirect effects, 2) effects that emerge as stressor magnitude increases the number of network components implicated, 3) network interactions that amplify these indirect effects, and 4) feedbacks that reinforce or stabilise against indirect effects. We then link these four characteristics to three case studies of common coastal environmental issues to demonstrate how a general understanding of ecological interaction networks can enhance priorities for stressor management that can be applied even when specific data is limited.

Real-world impacts of microplastic pollution on seafloor ecosystem function

Emerging research shows that microplastic pollution could be impacting seafloor ecosystem function, but this has been primarily demonstrated without environmental and ecological context. This causes uncertainty in the real-world effects of microplastic pollution and leaves out essential information guiding policy and mitigation. In this study, we take a well-supported sampling design and statistical approach commonly employed in benthic ecology to evaluate real-world effects of microplastic pollution on coastal, benthic ecosystem function. We utilised environmental gradients in the Waitemata Harbour of Auckland, New Zealand to evaluate the importance of commonly assessed biological, chemical, and geological sediment variables and the characteristics of microplastic contaminants in driving essential ecosystem functions. Our results showed that models including microplastic terms were more accurate and explained more variability than those without microplastic terms, highlighting that microplastics impact real-world seafloor ecosystem function. Specifically, microplastic fibers significantly influenced oxygen flux (p < 0.03) and the diverse forms of microplastics (i.e., richness) significantly influenced ammonium flux (p < 0.02). Additionally, interactions between microplastic fiber concentrations and mollusc abundances significantly contributed to oxygen flux (p < 0.02). These results provide the first evaluation of in situ relationships between microplastics and ecosystem function. Even more importantly, this study suggests the value of environmental and ecological context for addressing microplastic impacts on benthic ecosystems and argues for further field examination.

Effects of sediment on spore performance as a potential constraint on kelp distribution

Kelp habitats contribute to marine productivity and diversity, making understanding the constraints on their distribution important. In the Gulf of St. Lawrence, Alaria esculenta occupies a subset of Saccharina latissima's range. Since tolerance to sedimentation by early life stages was suggested to cause this contrasting distribution, we tested the influence of sediment levels on spore attachment and development. For both species, the proportion of attached spores that developed decreased with increasing sediment. However, spore attachment and gametophyte density increased with sediment concentration but only for Saccharina. At the maximum sediment level examined, spore attachment and gametophyte densities of the two species were similar, contrary to the idea that sediment effects on early life stages explain differences in adult distribution. Further investigation, particularly with higher sediment loads, is required to confirm this conclusion. As turbidity is increasing globally, understanding the mechanisms underpinning changes in seaweed distribution will facilitate appropriate local-scale management.

Increased river flow enhances the resilience of spatially patterned mudflats to erosion

Estuarine mudflats are profoundly affected by increased coastal erosion and reduced sediment delivery from major rivers. Although managers are having difficulties to control the cause of increased coastal erosion, they can help to manage the resilience of mudflat ecosystems to erosion through river flow regulation. In this study, we associated the resilience of a mudflat ecosystem to erosion with various magnitudes of river flow using a mechanism-based eco-morphodynamic model. Ecosystem resilience was reported in terms of i) what range of erosion rate the system can withstand before function collapse (persistence), ii) at which point function can be recovered (recovery), and iii) the uncertainty of system response to disturbances (response uncertainty). Specifically, the function of intertidal mudflat was characterized by landscape heterogeneity, primary productivity, and sediment stabilization. In a case study of the Yellow River Estuary (YRE) of China, it is found that increased erosion induced a collapse of the functioning state. Once collapsed, the erosion rate at which mudflat could recovered was lower than the erosion rate at which mudflat collapsed. Increased river flow enhanced the resilience of the mudflat ecosystem to erosion by increasing sediment deposition rate, which was an important attribute in the interaction process driving ecosystem resilience. Furthermore, given the same river flow allocation, the system with dynamic grazer population was more resilient than the system with a constant grazer number, highlighting the importance of controlling mudflat aquaculture to optimize the performance of river flow regulation. Our modeling results are dependent on the environment with several assumptions, however, as a preliminary, we believe our work represents a fundamental shift to modeling ecosystem resilience based on the mechanism of bio-physical interactions rather than relying on just quantifying the vital rates of particular species to compare river flow scenarios.

Informing the management of multiple stressors on estuarine ecosystems using an expert-based Bayesian Network model

The approach of applying stressor load limits or thresholds to aid estuarine management is being explored in many global case studies. However, there is growing concern regarding the influence of multiple stressors and their cumulative effects on the functioning of estuarine ecosystems due to the considerable uncertainty around stressor interactions. Recognising that empirical data limitations hinder parameterisation of detailed models of estuarine ecosystem responses to multiple stressors (suspended sediment, sediment mud and metal content, and nitrogen inputs), an expert driven Bayesian network (BN) was developed and validated. Overall, trends in estuarine condition predicted by the BN model were well supported by field observations, including results that were markedly higher than random (71-84% concordance), providing confidence in the overall model dynamics. The general BN framework was then applied to a case study estuary to demonstrate the model's utility for informing management decisions. Results indicated that reductions in suspended sediment loading were likely to result in improvements in estuarine condition, which was further improved by reductions in sediment mud and metal content, with an increased likelihood of high abundance of ecological communities relative to baseline conditions. Notably, reductions in suspended sediment were also associated with an increased probability of high nuisance macroalgae and phytoplankton if nutrient loading was not also reduced (associated with increased water column light penetration). Our results highlight that if stressor limit setting is to be implemented, limits must incorporate ecosystem responses to cumulative stressors, consider the present and desired future condition of the estuary of interest, and account for the likelihood of unexpected ecological outcomes regardless of whether the experts (or empirical data) suggest a threshold has or has not been triggered.

Density-Dependent and Species-Specific Effects on Self-Organization Modulate the Resistance of Mussel Bed Ecosystems to Hydrodynamic Stress

AbstractSelf-organized, regular spatial patterns emerging from local interactions among individuals enhance the ability of ecosystems to respond to environmental disturbances. Mussels self-organize to form large, regularly patterned biogenic structures that modify the biotic and abiotic environment and provide numerous ecosystem functions and services. We used two mussel species that form monospecific and mixed beds to investigate how species-specific behavior affects self-organization and resistance to wave stress. Perna perna has strong attachment but low motility, while Mytilus galloprovincialis shows the reverse. At low density, the less motile P. perna has limited spatial self-organization compared with M. galloprovincialis, while when coexisting, the two species formed random spatial patterns. At high density, the two species self-organized in similar ways, while when coexisting, patterns were less strong. Spatial pattern formations significantly shaped resistance to hydrodynamic stress. At low density, P. perna beds with strong attachment and M. galloprovincialis beds with strong spatial organization showed higher retention rates than mixed beds. At high density, the presence of strongly attached P. perna significantly increased retention in mixed and P. perna beds compared with M. galloprovincialis beds. Our study emphasizes the importance of the interplay of species-specific behaviors to spatial self-organization and stress tolerance in natural communities.

A call to evaluate Plastic's impacts on marine benthic ecosystem interaction networks

Plastic pollution continues to seep into natural and pristine habitats. Emerging laboratory-based research has evoked concern regarding plastic's impact on ecosystem structure and function, the essence of the ecosystem services that supports our life, wellbeing, and economy. These impacts have yet to be observed in nature where complex ecosystem interaction networks are enveloped in environmental physical and chemical dynamics. Specifically, there is concern that environmental impacts of plastics reach beyond toxicity and into ecosystem processes such as primary production, respiration, carbon and nutrient cycling, filtration, bioturbation, and bioirrigation. Plastics are popularly regarded as recalcitrant carbon molecules, although they have not been fully assessed as such. We hypothesize that plastics can take on similar roles as natural recalcitrant carbon (i.e., lignin and humic substances) in carbon cycling and associated biogeochemistry. In this paper, we review the current knowledge of the impacts of plastic pollution on marine, benthic ecosystem function. We argue for research advancement through (1) employing field experiments, (2) evaluating ecological network disturbances by plastic, and (3) assessing the role of plastics (i.e., a carbon-based molecule) in carbon cycling at local and global scales.

Cumulative stressors reduce the self-regulating capacity of coastal ecosystems

Marine ecosystems are prone to tipping points, particularly in coastal zones where dramatic changes are associated with interactions between cumulative stressors (e.g., shellfish harvesting, eutrophication and sediment inputs) and ecosystem functions. A common feature of many degraded estuaries is elevated turbidity that reduces incident light to the seafloor, resulting from multiple factors including changes in sediment loading, sea-level rise and increased water column algal biomass. To determine whether cumulative effects of elevated turbidity may result in marked changes in the interactions between ecosystem components driving nutrient processing, we conducted a large-scale experiment manipulating sediment nitrogen concentrations in 15 estuaries across a national-scale gradient in incident light at the seafloor. We identified a threshold in incident light that was related to distinct changes in the ecosystem interaction networks (EIN) that drive nutrient processing. Above this threshold, network connectivity was high with clear mechanistic links to denitrification and the role of large shellfish in nitrogen processing. The EIN analyses revealed interacting stressors resulting in a decoupling of ecosystem processes in turbid estuaries with a lower capacity to denitrify and process nitrogen. This suggests that, as turbidity increases with sediment load, coastal areas can be more vulnerable to eutrophication. The identified interactions between light, nutrient processing and the abundance of large shellfish emphasizes the importance of actions that seek to manage multiple stressors and conserve or enhance shellfish abundance, rather than actions focusing on limiting a single stressor.

Seawater Acidification Affects Beta-Diversity of Benthic Communities at a Shallow Hydrothermal Vent in a Mediterranean Marine Protected Area (Underwater Archaeological Park of Baia, Naples, Italy)

One of the most important pieces of climate change evidence is ocean acidification. Acidification effects on marine organisms are widely studied, while very little is known regarding its effects on assemblages’ β-diversity. In this framework, shallow hydrothermal vents within a Marine Protected Area (MPA) represent natural ecosystems acting as laboratory set-ups where the continuous carbon dioxide emissions affect assemblages with consequences that can be reasonably comparable to the effects of global water acidification. The aim of the present study is to test the impact of seawater acidification on the β-diversity of soft-bottom assemblages in a shallow vent field located in the Underwater Archeological Park of Baia MPA (Gulf of Naples, Mediterranean Sea). We investigated macro- and meiofauna communities of the ‘Secca delle fumose’ vent system in sites characterized by sulfurous (G) and carbon dioxide emissions (H) that are compared with control/inactive sites (CN and CS). Statistical analyses were performed on the most represented macrobenthic (Mollusca, Polychaeta, and Crustacea), and meiobenthic (Nematoda) taxa. Results show that the lowest synecological values are detected at H and, to a lesser extent, at G. Multivariate analyses show significant differences between hydrothermal vents (G, H) and control/inactive sites; the highest small-scale heterogeneities (measure of β-diversity) are detected at sites H and G and are mainly affected by pH, TOC (Total Organic Carbon), and cations concentrations. Such findings are probably related to acidification effects, since MPA excludes anthropic impacts. In particular, acidification markedly affects β-diversity and an increase in heterogeneity among sample replicates coupled to a decrease in number of taxa is an indicator of redundancy loss and, thus, of resilience capacity. The survival is assured to either tolerant species or those opportunistic taxa that can find good environmental conditions among gravels of sand.

Evidence for better microphytobenthos dynamics in mixed sand/mud zones than in pure sand or mud intertidal flats (Seine estuary, Normandy, France)

Understanding the dynamics of microphytobenthos biomass and photosynthetic performances in intertidal ecosystems will help advance our understanding of how trophic networks function in order to optimize ecological management and restoration projects. The main objective of this study was to investigate microphytobenthic biomass and photosynthetic performances as a function of the sedimentary and environmental variabilities in the range of intertidal habitats in the downstream Seine estuary (Normandy, France). Our results highlight higher biomass associated with more stratified biofilms and better photosynthetic performances in areas characterized by a sand/mud mixture (40–60% of mud) compared to pure sand or pure mud environments. This type of sediment probably offers an efficient trade-off between the favorable characteristics of the two types of sediments (sand and mud) with respect to light penetration and nutrient accessibility. Moreover, the large quantities of exopolysaccharides produced in sand/mud mixtures emphasizes the functional role played by microphytobenthos in promoting sediment stability against erosion. This allows us to show that despite the strong increase in sand content of the downstream Seine estuary, intertidal flats are still productive since microphytobenthic biomass, photosynthetic performances and exopolysaccharides secretion are highest in sand-mud mixtures. This study also underlines the impact of ecosystem modifications due to human disturbance and climate change on the dynamics of key primary producers in estuaries.

Biodiversity breakpoints along stress gradients in estuaries and associated shifts in ecosystem interactions

Denitrification in coastal sediments can provide resilience to eutrophication in estuarine ecosystems, but this key ecosystem function is impacted directly and indirectly by increasing stressors. The erosion and loading of fine sediments from land, resulting in sedimentation and elevated sediment muddiness, presents a significant threat to coastal ecosystems worldwide. Impacts on biodiversity with increasing sediment mud content are relatively well understood, but corresponding impacts on denitrification are uncharacterised. Soft sediment ecosystems have a network of interrelated biotic and abiotic ecosystem components that contribute to microbial nitrogen cycling, but these components (especially biodiversity measures) and their relationships with ecosystem functions are sensitive to stress. With a large dataset spanning broad environmental gradients this study uses interaction network analysis to present a mechanistic view of the ecological interactions that contribute to microbial nitrogen cycling, showing significant changes above and below a stressor (mud) threshold. Our models demonstrate that positive biodiversity effects become more critical with a higher level of sedimentation stress, and show that effective ecosystem management for resilience requires different action under different scenarios.

Translating Ecological Integrity terms into operational language to inform societies

It is crucial that societies are informed on the risks of impoverished ecosystem health for their well-being. For this purpose, Ecological Integrity (EI) is a useful concept that seeks to capture the complex nature of ecosystems and their interaction with social welfare. But the challenge remains to measure EI and translate scientific terminology into operational language to inform society. We propose an approach that simplifies marine ecosystem complexity by applying scientific knowledge to identify which components reflect the state or state change of ecosystems. It follows a bottom-up structure that identifies, based on expert knowledge, biological components related with past and present changing conditions. It is structured in 5 stages that interact in an adaptive way: stage 1, in situ observations suggest changes could be happening; stage 2 explores available data that represent EI; stage 3, experts' workshops target the identification of the minimum set of variables needed to define EI, or the risk of losing EI; an optative stage 4, where deviance from EI, or risk of deviance, is statistically assessed; stage 5, findings are communicated to society. We demonstrate the framework effectiveness in three case studies, including a data poor situation, an area where lack of reference sites hampers the identification of historical changes, and an area where diffuse sources of stress make it difficult to identify simple relationships with of ecological responses. The future challenge is to operationalise the approach and trigger desirable society actions to strengthen a social-nature link.

Rising tides, cumulative impacts and cascading changes to estuarine ecosystem functions

In coastal ecosystems, climate change affects multiple environmental factors, yet most predictive models are based on simple cause-and-effect relationships. Multiple stressor scenarios are difficult to predict because they can create a ripple effect through networked ecosystem functions. Estuarine ecosystem function relies on an interconnected network of physical and biological processes. Estuarine habitats play critical roles in service provision and represent global hotspots for organic matter processing, nutrient cycling and primary production. Within these systems, we predicted functional changes in the impacts of land-based stressors, mediated by changing light climate and sediment permeability. Our in-situ field experiment manipulated sea level, nutrient supply, and mud content. We used these stressors to determine how interacting environmental stressors influence ecosystem function and compared results with data collected along elevation gradients to substitute space for time. We show non-linear, multi-stressor effects deconstruct networks governing ecosystem function. Sea level rise altered nutrient processing and impacted broader estuarine services ameliorating nutrient and sediment pollution. Our experiment demonstrates how the relationships between nutrient processing and biological/physical controls degrade with environmental stress. Our results emphasise the importance of moving beyond simple physically-forced relationships to assess consequences of climate change in the context of ecosystem interactions and multiple stressors.

Cross-Scale Variation in Biodiversity-Environment Links Illustrated by Coastal Sandflat Communities

Spatial variation in the composition of communities is the product of many biotic and environmental interactions. A neglected factor in the analysis of community distribution patterns is the multi-scale nature of the data, which has implications for understanding ecological processes and the development of conservation and environmental management practice. Drawing on recently established multivariate spatial analyses, we investigate whether including relationships between spatial structure and abiotic variables enable us to better discern patterns of species and communities across scales. Data comprised 1200 macrozoobenthic samples collected over an array of distances (30 cm to 1 km) in three New Zealand harbours, as well as commonly used abiotic variables, such as sediment characteristics and chlorophyll a concentrations, measured at the same scales. Moran’s eigenvector mapping was used to extract spatial scales at which communities were structured. Benthic communities, representing primarily bivalves, polychaetes and crustaceans, were spatially structured at four spatial scales, i.e. >100 m, 50–100 m, 50–15 m, and < 15 m. A broad selection of abiotic variables contributed to the large-scale variation, whereas a more limited set explained part of the fine-scale community structure. Across all scales, less than 30% of the variation in spatial structure was captured by our analysis. The large number of species (48) making up the 10 highest species scores based on redundancy analyses illustrate the variability of species-scale associations. Our results emphasise that abiotic variables and biodiversity are related at all scales investigated and stress the importance of assessing the relationship between environmental variables and the abundance and distribution of biological assemblages across a range of different scales.

Detecting Subtle Shifts in Ecosystem Functioning in a Dynamic Estuarine Environment

Identifying the effects of stressors before they impact ecosystem functioning can be challenging in dynamic, heterogeneous 'real-world' ecosystems. In aquatic systems, for example, reductions in water clarity can limit the light available for photosynthesis, with knock-on consequences for secondary consumers, though in naturally turbid wave-swept estuaries, detecting the effects of elevated turbidity can be difficult. The objective of this study was to investigate the effects of shading on ecosystem functions mediated by sandflat primary producers (microphytobenthos) and deep-dwelling surface-feeding macrofauna (Macomona liliana; Bivalvia, Veneroida, Tellinidae). Shade cloths (which reduced incident light intensity by ~80%) were deployed on an exposed, intertidal sandflat to experimentally stress the microphytobenthic community associated with the sediment surface. After 13 weeks, sediment properties, macrofauna and fluxes of oxygen and inorganic nutrients across the sediment-water interface were measured. A multivariate metric of ecosystem function (MF) was generated by combining flux-based response variables, and distance-based linear models were used to determine shifts in the drivers of ecosystem function between non-shaded and shaded plots. No significant differences in MF or in the constituent ecosystem function variables were detected between the shaded and non-shaded plots. However, shading reduced the total explained variation in MF (from 64% in non-shaded plots to 15% in shaded plots) and affected the relative influence of M. liliana and other explanatory variables on MF. This suggests that although shade stress may shift the drivers of ecosystem functioning (consistent with earlier investigations of shading effects on sandflat interaction networks), ecosystem functions appear to have a degree of resilience to those changes.

Experimenting with ecosystem interaction networks in search of threshold potentials in real-world marine ecosystems

Thresholds profoundly affect our understanding and management of ecosystem dynamics, but we have yet to develop practical techniques to assess the risk that thresholds will be crossed. Combining ecological knowledge of critical system interdependencies with a large-scale experiment, we tested for breaks in the ecosystem interaction network to identify threshold potential in real-world ecosystem dynamics. Our experiment with the bivalves Macomona liliana and Austrovenus stutchburyi on marine sandflats in New Zealand demonstrated that reductions in incident sunlight changed the interaction network between sediment biogeochemical fluxes, productivity, and macrofauna. By demonstrating loss of positive feedbacks and changes in the architecture of the network, we provide mechanistic evidence that stressors lead to break points in dynamics, which theory predicts predispose a system to a critical transition.

The role of recurrent disturbances for ecosystem multifunctionality

Ecosystem functioning is threatened by an increasing number of anthropogenic stressors, creating a legacy of disturbance that undermines ecosystem resilience. However, few empirical studies have assessed to what extent an ecosystem can tolerate repeated disturbances and sustain its multiple functions. By inducing increasingly recurring hypoxic disturbances to a sedimentary ecosystem, we show that the majority of individual ecosystem functions experience gradual degradation patterns in response to repetitive pulse disturbances. The degradation in overall ecosystem functioning was, however, evident at an earlier stage than for single ecosystem functions and was induced after a short pulse of hypoxia (i.e., three days), which likely reduced ecosystem resistance to further hypoxic perturbations. The increasing number of repeated pulse disturbances gradually moved the system closer to a press response. In addition to the disturbance regime, the changes in benthic trait composition as well as habitat heterogeneity were important for explaining the variability in overall ecosystem functioning. Our results suggest that disturbance-induced responses across multiple ecosystem functions can serve as a warning signal for losses of the adaptive capacity of an ecosystem, and might at an early stage provide information to managers and policy makers when remediation efforts should be initiated.

Conditional Responses of Benthic Communities to Interference from an Intertidal Bivalve

Habitat-modifying organisms that impact other organisms and local functioning are important in determining ecosystem resilience. However, it is often unclear how the outcome of interactions performed by key species varies depending on the spatial and temporal disturbance context which makes the prediction of disturbance-driven regime shifts difficult. We investigated the strength and generality of effects of the filter feeding cockle Cerastoderma edule on its ambient intertidal benthic physical and biological environment. By comparing the magnitude of the effect of experimental cockle removal between a non-cohesive and a sheltered cohesive sediment in two different periods of the year, we show that the outcome of cockle interference effects relates to differences in physical disturbance, and to temporal changes in suspended sediment load and ontogenetic changes in organism traits. Interference effects were only present in the cohesive sediments, though the effects varied seasonally. Cockle presence decreased only the density of surface-dwelling species suggesting that interference effects were particularly mediated by bioturbation of the surface sediments. Furthermore, density reductions in the presence of cockles were most pronounced during the season when larvae and juveniles were present, suggesting that these life history stages are most vulnerable to interference competition. We further illustrate that cockles may enhance benthic microalgal biomass, most likely through the reduction of surface-dwelling grazing species, especially in periods with high sediment load and supposedly also high bioturbation rates. Our results emphasize that the physical disturbance of the sediment may obliterate biotic interactions, and that temporal changes in environmental stressors, such as suspended sediments, may affect the outcome of key species interference effects at the local scale. Consequently, natural processes and anthropogenic activities that change bed shear stress and sediment dynamics in coastal soft-sediment systems will affect cockle-mediated influences on ecosystem properties and therefore the resilience of these systems to environmental change.

Counting on β-diversity to safeguard the resilience of estuaries

Coastal ecosystems are often stressed by non-point source and cumulative effects that can lead to local-scale community homogenisation and a concomitant loss of large-scale ecological connectivity. Here we investigate the use of β-diversity as a measure of both community heterogeneity and ecological connectivity. To understand the consequences of different environmental scenarios on heterogeneity and connectivity, it is necessary to understand the scale at which different environmental factors affect β-diversity. We sampled macrofauna from intertidal sites in nine estuaries from New Zealand's North Island that represented different degrees of stress derived from land-use. We used multiple regression models to identify relationships between β-diversity and local sediment variables, factors related to the estuarine and catchment hydrodynamics and morphology and land-based stressors. At local scales, we found higher β-diversity at sites with a relatively high total richness. At larger scales, β-diversity was positively related to γ-diversity, suggesting that a large regional species pool was linked with large-scale heterogeneity in these systems. Local environmental heterogeneity influenced β-diversity at both local and regional scales, although variables at the estuarine and catchment scales were both needed to explain large scale connectivity. The estuaries expected a priori to be the most stressed exhibited higher variance in community dissimilarity between sites and connectivity to the estuary species pool. This suggests that connectivity and heterogeneity metrics could be used to generate early warning signals of cumulative stress.

Detecting shifts in ecosystem functioning: the decoupling of fundamental relationships with increased pollutant stress on sandflats

In this study, we investigated the influence of low level contamination by copper, lead and zinc on the functioning of estuarine sandflat ecosystems by comparing the strength and variability of relationships between benthic macrofauna and fluxes (oxygen and nutrients) at three clean and three mildly contaminated sites. Specifically, as indicators of ecosystem functioning, we examined relationships between bivalve biomass, total benthic respiration and ammonium release, and ammonium uptake and benthic primary production. Furthermore, a small amount of organic matter was added to experimental plots at all sites (35 g/0.2 m²) to evaluate stress-on-stress responses relative to controls. Relationships were strongest at the clean sites (steepest slopes, highest r² values, lowest p-values) and weakest at the mildly contaminated sites and in organically enriched plots. Our results suggest that changes in ecosystem functioning may be occurring at mild (<ERL) levels of pollution that are unlikely to cause widespread mortality of macrofauna.