Climate cascades affect coastal Antarctic seafloor ecosystem functioning.

Polar seafloor ecosystems are changing rapidly and dramatically, challenging previously held paradigms of extreme dynamical stability. Warming-related declines in polar sea ice are expected to alter fluxes of phytoplankton and under-ice algae to the seafloor. Yet, how changes in food flux cascade through to seafloor communities and functions remains unclear. We leveraged natural spatial and temporal gradients in summertime sea ice extent to better understand the trajectories and implications of climate-related change in McMurdo Sound, Antarctica. McMurdo Sound was expected to be one of the last coastal marine environments on Earth to be affected by planetary warming, but the situation may be changing. Comparing satellite observations of selected coastal sites in McMurdo Sound between 2010-2017 and 2002-2009 revealed more ice-free days per year, and shorter distances to open water during the warmest months each year, in the more recent period. Interdecadal Pacific Oscillation (IPO), Oceanic Niño Index (ONI) and Antarctic Oscillation (AAO) climate indices peaked concurrently between 2014 and 2017 when sea ice breakouts in McMurdo Sound were most spatially and temporally extensive. Increases in sediment chlorophyll a and phaeophytin content (indicating increased deposition of detrital algal food material) were recorded during 2014-2017 at three coastal study sites in McMurdo Sound following the major sea ice breakouts. Soft-sediment seafloor ecosystem metabolism (measured in benthic incubation chambers as dissolved oxygen and inorganic nutrient fluxes) was correlated with sediment algal pigment concentration. Epifaunal invertebrate density, particularly opportunistic sessile suspension feeders, and infaunal community composition also shifted with increased food supply. The ecological characteristics and functions measured at the food-poor sites shifted towards those observed at richer sites at a surprisingly fast pace. These results indicate the sensitivity of the benthos and shed light on Antarctic marine trophic cascades and trajectories of response of iconic high-latitude seafloor habitats to a warming climate.

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.

Mapping the estuarine ecosystem service of pollutant removal using empirically validated boosted regression tree models.

Humans rely on the natural environment and benefit from the goods and services provided by natural ecosystems. Quantification and mapping of ecosystem services (ES) is required to better protect valued ES benefits under pressure from anthropogenic activities. The removal of excess nitrogen, a recognized catchment-derived pollutant, by biota in estuarine soft sediments is an important ES that potentially ameliorates the development of eutrophication symptoms. Here, we quantified estuarine benthic sediment characteristics and denitrification enzyme activity (DEA), a proxy of inorganic N removal, at 109 sites in four estuaries to develop a general ("global") model for predicting DEA. Our initial global model for linking DEA and environmental characteristics had good explanatory power, with sediment mud content having the strongest influence on DEA (60%), followed by sediment organic matter content (≈35%) and sediment chlorophyll a content (≈5%). Predicted and empirically evaluated DEA values in a fifth estuary (Whitford, n = 90 validation sites) were positively correlated (r = 0.77), and the fit and certainty of the model (based on two types of uncertainty measures) increased further after the validation sites were incorporated into it. The model tended to underpredict DEA at the upper end of its range (at the muddier, more organically enriched sites), and the relative roles of the three environmental predictors differed in Whitford relative to the four previously sampled estuaries (reducing the explained deviance relative to the initial global model). Our detailed quantification of DEA and methodological description for producing empirically validated maps, complete with uncertainty information, represents an important first step in the construction of nutrient pollution removal ES maps for use in coastal marine spatial management. This technique can likely be adapted to map other ecosystem functions and ES proxies worldwide.

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.

Integrating rapid habitat mapping with community metrics and functional traits to assess estuarine ecological conditions: A New Zealand case study.

Estuarine ecosystems are under pressure from anthropogenic stressors in many parts of the world. In a New Zealand estuary, we integrated rapid habitat mapping, univariate and multivariate benthic community metrics and functional traits-based metrics to provide a holistic assessment of its ecological condition. Our integrative approach identified 11 different habitat classes across the study estuary and revealed how univariate, multivariate, and functional traits-based metrics varied spatially. Variation was driven mainly by sedimentary conditions, particularly, the sediment's organic matter and silt-clay content. Degraded conditions were apparent in several parts of the estuary, which overlapped with areas showing high concentrations of decaying algae, anoxic conditions, and low values of univariate and functional metrics. Our holistic approach, integrating mapping and quantitative community assessments, provided a more complete picture of estuarine conditions and enabled us to identify locations across the estuary with the greatest potential for restoration success.

Climatic, oceanic, freshwater, and local environmental drivers of New Zealand estuarine macroinvertebrates.

Understanding the responses of organisms to different environmental drivers is critical for improving ecosystem management and conservation. Estuarine ecosystems are under pressure from multiple anthropogenic stressors (e.g. increasing sediment and nutrient loads, pollution, climate change) that are affecting the functions and services these ecosystems provide. Here, we used long-term estuarine benthic invertebrate monitoring data (∼30 year time-series) to evaluate the responses of macrobenthic invertebrate communities and indicator species to climatic, oceanic, freshwater, and local environmental drivers in New Zealand estuaries. We aimed to improve our ability to predict ecosystem change and understand the effects of multiple environment drivers on benthic communities. Our analyses showed that the abundance and richness of macrobenthic fauna and four indicator taxa (bivalves known to have differing tolerances to sediment mud content: Austrovenus stutchburyi, Macomona liliana, Theora lubrica, and Arthritica bifurca) responded to unique combinations of multiple environmental drivers across sites and times. Macrobenthic responses were highly mixed (i.e., positive and negative) and site-dependent. We also show that responses of macrobenthic fauna were lagged and most strongly related to climatic and oceanic drivers. The way the macrobenthos responded has implications for predicting and understanding the ecological consequences of a rapidly changing environment and how we conserve and manage coastal ecosystems.

Biogenic habitat transitions influence facilitation in a marine soft-sediment ecosystem.

Habitats are often defined by the presence of key species and biogenic features. However, the ecological consequences of interactions among distinct habitat-forming species in transition zones where their habitats overlap remain poorly understood. We investigated transition zone interactions by conducting experiments at three locations in Mahurangi Harbour, New Zealand, where the abundance of two habitat-forming marine species naturally varied. The two key species differed in form and function: One was a sessile suspension-feeding bivalve that protruded from the sediment (Atrina zelandica; Pinnidae); the other was a mobile infaunal urchin that bioturbated sediment (Echinocardium cordatum; Spatangoida). The experimental treatments established at each site reflected the natural densities of the species across sites (Atrina only, Echinocardium only, Atrina and Echinocardium together, and plots with neither species present). We identified the individual and combined effects of the two key species on sediment characteristics and co-occurring macrofauna. After five months, we documented significant treatment effects, including the highest abundance of co-occurring macrofauna in the Atrina-only treatments. However, the facilitation of macrofauna by Atrina (relative to removal treatments) was entirely negated in the presence of Echinocardium at densities >10 individuals/m2. The transitional areas in Mahurangi Harbour composed of co-occurring Atrina and Echinocardium are currently widespread and are probably more common now than monospecific patches of either individual species, due to the thinning of dense Atrina patches into sparser mixed zones during the last 10-15 years. Thus, although some ecologists avoid ecotones and habitat edges when designing experiments, suspecting that it will skew the extrapolation of results, this study increased our understanding of benthic community dynamics across larger proportions of the seascape and provided insights into temporal changes in community structure associated with patch dynamics. Particularly in situations where non-abrupt habitat transitions are commonplace, documentation of community dynamics in individual biogenic habitats and in mixed transition zones is required in order to scale-up and generalize results.

When small changes matter: the role of cross-scale interactions between habitat and ecological connectivity in recovery.

Interaction between the diversity of local communities and the degree of connectivity between them has the potential to influence local recovery rates and thus profoundly affect community dynamics in the face of the cumulative impacts that occur across regions. Although such complex interactions have been modeled, field experiments in natural ecosystems to investigate the importance of interactions between local and regional processes are rare, especially so in coastal marine seafloor habitats subjected to many types of disturbance. We conducted a defaunation experiment at eight subtidal sites, incorporating manipulation of habitat structure, to test the relative importance of local habitat features and colonist supply in influencing macrobenthic community recovery rate. Our sites varied in community composition, habitat characteristics, and hydrodynamic conditions, and we conducted the experiment in two phases, exposing defaunated plots to colonists during periods of either high or low larval colonist supply. In both phases of the experiment, five months after disturbance, we were able to develop models that explained a large proportion of variation in community recovery rate between sites. Our results emphasize that the connectivity to the regional species pool influences recovery rate, and although local habitat effects were important, the strength of these effects was affected by broader-scale site characteristics and connectivity. Empirical evidence that cross-scale interactions are important in disturbance-recovery dynamics emphasizes the complex dynamics underlying seafloor community responses to cumulative disturbance.

NZTD - The New Zealand Trait Database for shallow-water marine benthic invertebrates.

Macrobenthic traits, for example feeding mode, life history, morphology, are increasingly used for determining responses of macrobenthic fauna to environmental change and influences on ecosystem functioning. Yet, trait information is scarce or non-existent in several parts of the world, such as New Zealand. This deficit makes collecting trait data a difficult and time-consuming task, limiting its potential use in trait-based assessments. Here, we present the New Zealand Trait Database (NZTD) for marine benthic invertebrates, the first comprehensive assessment of macrobenthic traits in New Zealand. The NZTD provides trait information for more than 700 macrobenthic taxa, categorised by 18 traits and 77 trait modalities. The NZTD includes five freely downloadable datasets, (1) the macrobenthic trait dataset, with outcomes from a fuzzy coding procedure, (2) the trait source information, (3) the references by taxa, (4) the full references list, and (5) the full taxa list used in the NZTD. Establishing the NZTD closes the trait knowledge gap in New Zealand and facilitates future research applying trait-based approaches to New Zealand's coastal macrofauna.

Positive contribution of macrofaunal biodiversity to secondary production and seagrass carbon metabolism.

Coastal vegetated habitats such as seagrasses are known to play a critical role in carbon cycling and the potential to mitigate climate change, as blue carbon habitats have been repeatedly highlighted. However, little information is known about the role of associated macrofauna communities on the dynamics of critical processes of seagrass carbon metabolism (e.g., respiration, turnover, and production). We conducted a field study across a spatial gradient of seagrass meadows involving variable environmental conditions and macrobenthic diversity to investigate (1) the relationship between macrofauna biodiversity and secondary production (i.e., consumer incorporation of organic matter per time unit), and (2) the role of macrofauna communities in seagrass organic carbon metabolism (i.e., respiration and primary production). We show that, although several environmental factors influence secondary production, macrofauna biodiversity controls the range of local seagrass secondary production. We demonstrate that macrofauna respiration rates were responsible for almost 40% of the overall seafloor community respiration. Macrofauna represented on average >25% of the total benthic organic C stocks, high secondary production that is likely to become available to upper trophic levels of the coastal food web. Our findings support the role of macrofauna biodiversity in maintaining productive ecosystems, implying that biodiversity loss due to ongoing environmental change yields less productive seagrass ecosystems. Therefore, the assessment of carbon dynamics in coastal habitats should include associated macrofauna biodiversity elements if we aim to obtain robust estimates of global carbon budgets required to implement management actions for the sustainable functioning of the world's coasts.

Who is contributing where? Predicting ecosystem service multifunctionality for shellfish species through ecological principles.

A key challenge in environmental management is determining how to manage multiple ecosystem services (ES) simultaneously, to ensure efficient and sustainable use of the environment and its resources. In marine environments, the spatial assessment of ES is lagging as a result of data-scarcity and modelling complexity. Applying mechanistic models to link ecological processes with ecosystem functions and services to assess areas of high ES potential can bridge this gap and accommodate assessments of functional differences between service providers. Here, we applied an ecosystem principles approach to assess ES potential for food provision, water quality regulation, nitrogen removal, and sediment stabilisation, provided by two estuarine bivalves (Austrovenus stutchburyi and Paphies australis) that differ in habitat association (broad and narrow distributions), to gain insight into the utility of these models for local-scale management. Maps of individual ES displayed differing patterns related to habitat associations of the species providing them, with variation in the quantities of services being delivered and locations of importance. Areas of importance for the provision of multiple services (number of services provided and their combined intensity per species) were assessed using hotspot analyses, which suggested that areas of high shellfish density at the harbour entrances were important for ES multifunctionality. A targeted management approach that includes environmental context, rather than a focus solely on the protection of high-density shellfish areas, is required to sustain the provision of individual ES.

Altered nitrogen transformation pathways and a legacy of sediment organic matter enrichment.

Estuaries are ecologically valuable ecosystems that process nutrients through complex biogeochemical processes. Here we identify drivers and inhibitors of nitrogen removal in unvegetated intertidal sandflats at six sites in Manukau Harbour (37° 2.00'S 174° 42.00'E) to quantify the exchange of solutes across the sediment-water interface, with nitrogen removal rates (NRR) measured at two contrasting sites (PI and CB) near and far, respectively, from an historical wastewater treatment plant. Solute fluxes were paired with sediment and macrofauna community data to identify drivers of ecosystem function. Fluxes of oxygen, nitrogen and phosphorous were found to vary among sites, with differences attributed to variation in sediment characteristics (grain size, chlorophyll a, organic content) and macrofauna community structure. Mean NRR was found to vary between sites (PI = 32.2 vs CB = 217.9 µmol N2/m2/h), with bioturbating macrofauna (bivalves Austrovenus stutchburyi and Macomona liliana), microphytobenthic biomass, and exposure to nutrients likely key contributing drivers.

Disturbance of sandflats by thin terrigenous sediment deposits: consequences for primary production and nutrient cycling.

The contributions of biodiversity to ecosystem functioning are increasingly recognized by ecologists, with biodiversity loss considered a significant threat to the maintenance of life-supporting ecosystem goods and services. Although ecologists have increased the amount of realism in biodiversity-ecosystem functioning (BEF) experiments, effects on functioning are rarely investigated in the field in conjunction with disturbances that affect biodiversity. Here, effects on functioning were investigated in situ following experimental disturbance of a realistic type and magnitude. Experimental deposits of terrigenous sediment (5 mm thickness) were applied at three intertidal sites in Whangapoua Harbour (Coromandel Peninsula, New Zealand), where soil erosion associated with logging is a serious problem. Experimental disturbance by terrigenous sediment impacted macrofaunal abundance and community composition (suspension and deposit feeders), and there were coincident shifts in ecosystem functioning at all three sites. Relative to controls, sediment-treated plots had higher rates of ammonium efflux (despite 50% fewer macrofaunal individuals) and lower rates of gross primary production and photosynthetic efficiency (despite similar sediment chlorophyll a concentrations). Judging from nutrient ratios in sediment pore water, microphyte primary producers living on the sediment surface in control plots were likely nitrogen limited (the normal situation for marine waters), whereas microphytes in sediment-treated plots were likely phosphate limited. Gross primary production and photosynthetic efficiency were significantly and positively correlated with ammonium uptake in control plots (r2 = 0.463 and 0.856, respectively) but not in treated plots (P > 0.05, r2 < 0.3). We suggest that the higher content of charged particles (clay) in the experimental deposits bound up phosphate, limiting its bio-availability, and shifted functional relationships between sandflat macrofauna and sediment primary productivity.

Sea Ice Dynamics Drive Benthic Microbial Communities in McMurdo Sound, Antarctica.

Climate change is driving dramatic variability in sea ice dynamics, a key driver in polar marine ecosystems. Projected changes in Antarctica suggest that regional warming will force dramatic shifts in sea ice thickness and persistence, altering sea ice-associated primary production and deposition to the seafloor. To improve our understanding of the impacts of sea ice change on benthic ecosystems, we directly compared the benthic microbial communities underlying first-year sea ice (FYI) and multi-year sea ice (MYI). Using two tractable coastal habitats in McMurdo Sound, Antarctica, where FYI (Cape Evans) and MYI (New Harbour) prevail, we show that the structure and composition of the benthic microbial communities reflect the legacy of sea ice dynamics. At Cape Evans, an enrichment of known heterotrophic algal polysaccharide degrading taxa (e.g., Flavobacteriaceae, unclassified Gammaproteobacteria, and Rubritaleaceae) and sulfate-reducing bacteria (e.g., Desulfocapsaceae) correlated with comparatively higher chlorophyll a (14.2±0.8µgg-1) and total organic carbon content (0.33%±0.04), reflecting increased productivity and seafloor deposition beneath FYI. Conversely, at New Harbour, an enrichment of known archaeal (e.g., Nitrosopumilaceae) and bacterial (e.g., Woeseiaceae and Nitrospiraceae) chemoautotrophs was common in sediments with considerably lower chlorophyll a (1.0±0.24µgg-1) and total organic carbon content (0.17%±0.01), reflecting restricted productivity beneath MYI. We also report evidence of a submarine discharge of sub-permafrost brine from Taylor Valley into New Harbour. By comparing our two study sites, we show that under current climate-warming scenarios, changes to sea ice productivity and seafloor deposition are likely to initiate major shifts in benthic microbial communities, with heterotrophic organic matter degradation processes becoming increasingly important. This study provides the first assessment of how legacy sea ice conditions influence benthic microbial communities in Antarctica, contributing insight into sea ice-benthic coupling and ecosystem functioning in a polar environment.

Bioturbators enhance ecosystem function through complex biogeochemical interactions.

Predicting the consequences of species loss is critically important, given present threats to biological diversity such as habitat destruction, overharvesting and climate change. Several empirical studies have reported decreased ecosystem performance (for example, primary productivity) coincident with decreased biodiversity, although the relative influence of biotic effects and confounding abiotic factors has been vigorously debated. Whereas several investigations focused on single trophic levels (for example, grassland plants), studies of whole systems have revealed multiple layers of feedbacks, hidden drivers and emergent properties, making the consequences of species loss more difficult to predict. Here we report functionally important organisms and considerable biocomplexity in a sedimentary seafloor habitat, one of Earth's most widespread ecosystems. Experimental field measurements demonstrate how the abundance of spatangoid urchins--infaunal (in seafloor sediment) grazers/deposit feeders--is positively related to primary production, as their activities change nutrient fluxes and improve conditions for production by microphytobenthos (sedimentatry microbes and unicellular algae). Declines of spatangoid urchins after trawling are well documented, and our research linking these bioturbators to important benthic-pelagic fluxes highlights potential ramifications for productivity in coastal oceans.

Forecasting the limits of resilience: integrating empirical research with theory.

Despite the increasing evidence of drastic and profound changes in many ecosystems, often referred to as regime shifts, we have little ability to understand the processes that provide insurance against such change (resilience). Modelling studies have suggested that increased variance may foreshadow a regime shift, but this requires long-term data and knowledge of the functional links between key processes. Field-based research and ground-truthing is an essential part of the heuristic that marries theoretical and empirical research, but experimental studies of resilience are lagging behind theory, management and policy requirements. Empirically, ecological resilience must be understood in terms of community dynamics and the potential for small shifts in environmental forcing to break the feedbacks that support resilience. Here, we integrate recent theory and empirical data to identify ways we might define and understand potential thresholds in the resilience of nature, and thus the potential for regime shifts, by focusing on the roles of strong and weak interactions, linkages in meta-communities, and positive feedbacks between these and environmental drivers. The challenge to theoretical and field ecologists is to make the shift from hindsight to a more predictive science that is able to assist in the implementation of ecosystem-based management.

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.

Calcium carbonate alters the functional response of coastal sediments to eutrophication-induced acidification.

Coastal ocean acidification research is dominated by laboratory-based studies that cannot necessarily predict real-world ecosystem response given its complexity. We enriched coastal sediments with increasing quantities of organic matter in the field to identify the effects of eutrophication-induced acidification on benthic structure and function, and assess whether biogenic calcium carbonate (CaCO3) would alter the response. Along the eutrophication gradient we observed declines in macrofauna biodiversity and impaired benthic net primary productivity and sediment nutrient cycling. CaCO3 addition did not alter the macrofauna community response, but significantly dampened negative effects on function (e.g. net autotrophy occurred at higher levels of organic matter enrichment in +CaCO3 treatments than -CaCO3 (1400 vs 950 g dw m-2)). By identifying the links between eutrophication, sediment biogeochemistry and benthic ecosystem structure and function in situ, our study represents a crucial step forward in understanding the ecological effects of coastal acidification and the role of biogenic CaCO3 in moderating responses.

In situ response of Antarctic under-ice primary producers to experimentally altered pH.

Elevated atmospheric CO2 concentrations are contributing to ocean acidification (reduced seawater pH and carbonate concentrations), with potentially major ramifications for marine ecosystems and their functioning. Using a novel in situ experiment we examined impacts of reduced seawater pH on Antarctic sea ice-associated microalgal communities, key primary producers and contributors to food webs. pH levels projected for the following decades-to-end of century (7.86, 7.75, 7.61), and ambient levels (7.99), were maintained for 15 d in under-ice incubation chambers. Light, temperature and dissolved oxygen within the chambers were logged to track diurnal variation, with pH, O2, salinity and nutrients assessed daily. Uptake of CO2 occurred in all treatments, with pH levels significantly elevated in the two extreme treatments. At the lowest pH, despite the utilisation of CO2 by the productive microalgae, pH did not return to ambient levels and carbonate saturation states remained low; a potential concern for organisms utilising this under-ice habitat. However, microalgal community biomass and composition were not significantly affected and only modest productivity increases were noted, suggesting subtle or slightly positive effects on under-ice algae. This in situ information enables assessment of the influence of future ocean acidification on under-ice community characteristics in a key coastal Antarctic habitat.

Biogenic disturbance determines invasion success in a subtidal soft-sediment system.

Theoretically, disturbance and diversity can influence the success of invasive colonists if (1) resource limitation is a prime determinant of invasion success and (2) disturbance and diversity affect the availability of required resources. However, resource limitation is not of overriding importance in all systems, as exemplified by marine soft sediments, one of Earth's most widespread habitat types. Here, we tested the disturbance-invasion hypothesis in a marine soft-sediment system by altering rates of biogenic disturbance and tracking the natural colonization of plots by invasive species. Levels of sediment disturbance were controlled by manipulating densities of burrowing spatangoid urchins, the dominant biogenic sediment mixers in the system. Colonization success by two invasive species (a gobiid fish and a semelid bivalve) was greatest in plots with sediment disturbance rates < 500 cm(3) x m(-2) x d(-1), at the low end of the experimental disturbance gradient (0 to > 9000 cm(3) x m(-2) x d(-1)). Invasive colonization declined with increasing levels of sediment disturbance, counter to the disturbance-invasion hypothesis. Increased sediment disturbance by the urchins also reduced the richness and diversity of native macrofauna (particularly small, sedentary, surface feeders), though there was no evidence of increased availability of resources with increased disturbance that would have facilitated invasive colonization: sediment food resources (chlorophyll a and organic matter content) did not increase, and space and access to overlying water were not limited (low invertebrate abundance). Thus, our study revealed the importance of biogenic disturbance in promoting invasion resistance in a marine soft-sediment community, providing further evidence of the valuable role of bioturbation in soft-sediment systems (bioturbation also affects carbon processing, nutrient recycling, oxygen dynamics, benthic community structure, and so on.). Bioturbation rates are influenced by the presence and abundance of large burrowing species (like spatangoid urchins). Therefore, mass mortalities of large bioturbators could inflate invasion risk and alter other aspects of ecosystem performance in marine soft-sediment habitats.