Same pattern, different mechanism: Locking onto the role of key species in seafloor ecosystem process

Modelled broad-scale shifts on seafloor ecosystem functioning due to microplastic impacts on bioturbation

Bioturbating species play an essential role in regulating nutrient cycling in marine sediments, but their interaction with microplastics (MP) remains poorly understood. Here we investigated the linkage between MP and ecosystem functioning using experimental observations of luminophore distribution in the sediment to parametrize bioturbation coefficients (Db). this information as fed into a simplified transport-reaction model, allowing us to upscale our experimental results. We found that the composition of bioturbators modulated shifts in the ecosystem functioning under microplastic stress. Maldanid worms (Macroclymenella stewartensis), functionally deep burrowing and upward-conveyor belt feeders, became less active. The Db of M. stewartensis reduced by 25% with the addition of 0.002 g MP cm-2 at surface sediment, causing accumulation of organic matter in the oxic sediment zone and stimulating aerobic respiration by 18%. In contract, the tellinid bivalve Macomona liliana, functionally a surface -deposit feeder that excretes at depth, maintained particle mixing behaviour in MP-contaminated systems. This study provides a mechanistic insight into the impacts of MP and indicates that the functional role of bioturbating species should be involved in assessing the global impact of MP. The model allowed us to understand the broad-scale impact of MP on seafloor habitat.

Estimating Effects of Sea Level Rise on Benthic Biodiversity and Ecosystem Functioning in a Large Meso-Tidal Coastal Lagoon

Estuaries are among the world's most productive ecosystems, but due to their geographic location, they are at the forefront of anthropogenic pressures. Sea level rise (SLR) is one major consequence of climate change that poses a threat to estuaries with extensive intertidal habitats. The ecological implications of intertidal habitat loss have been largely overlooked despite their likely significance. We aimed to address this knowledge gap by investigating how benthic macroinvertebrate communities and their contributions to ecosystem function are likely to respond to SLR. Based on a spatially extensive dataset (119 sites) from a large coastal lagoon, depth, sediment chlorophyll concentrations, mud content, and average current speed were identified as the main drivers of community compositional turnover. Shifts in benthic community structure and associated functional implications were then evaluated using depth as a proxy for SLR. Three main macrofaunal groups representing intertidal, shallow subtidal, and deep subtidal habitats were identified. Functional trait analysis indicated low functional redundancy for a key intertidal suspension-feeding bivalve (Austrovenus stutchburyi) and the lack of a shallow subtidal functional replacement should intertidal habitats become inundated. These findings strongly suggest SLR and the associated environmental changes will alter estuarine macroinvertebrate communities, with implications for future ecosystem function and resilience.

Stressors Increase the Impacts of Coastal Macrofauna Biodiversity Loss on Ecosystem Multifunctionality

There is substantial evidence that biodiversity underpins ecosystem functioning, but it is unclear how these relationships change with multiple stressors in complex real-world settings. Coastal zones are affected by numerous stressors (for example, sediment input and nutrient runoff from land) and the cumulative effects of these stressors may result in pronounced and unexpected changes in the functioning of ecosystems. To investigate the cumulative effects of turbidity and elevated nutrients on coastal biodiversity-ecosystem functioning relationships, we performed a large-scale field experiment manipulating in situ sediment porewater ammonium concentrations and measured multiple ecosystem functions related to carbon fixation and mineralisation in 15 estuaries with varying levels of turbidity. The results indicated that the benthic macrofauna diversity (species richness, abundance, and functional richness) declined with increased porewater ammonium concentrations and there were clear thresholds in light levels at the seafloor in relation to the biodiversity-ecosystem function relationships. Multifunctionality indices (an integrated index of all measured functions) in moderately turbid and turbid estuaries (daily mean seafloor PAR < 420 µmol m−2 s−1) decreased with the loss of macrofauna biodiversity. Functioning in low-turbidity estuaries (daily mean PAR > 420 µmol m−2 s−1) however remained relatively constant, suggesting that they were more resilient against the nutrient-induced biodiversity loss. Our results demonstrate that ecosystems already stressed by stressors that alter functional performance (turbidity) may be more prone to loss of overall functioning if biodiversity is reduced by another stressor (nutrient enrichment), highlighting the potential snowballing effects of cumulative change.

Cockle as Second Intermediate Host of Trematode Parasites: Consequences for Sediment Bioturbation and Nutrient Fluxes across the Benthic Interface

Trematode parasites are distributed worldwide and can severely impact host populations. However, their influence on ecosystem functioning through the alteration of host engineering behaviours remains largely unexplored. This study focuses on a common host parasite system in marine coastal environments, i.e., the trematode Himasthla elongata, infecting the edible cockle Cerastoderma edule as second intermediate host. A laboratory experiment was conducted to investigate the indirect effects of metacercarial infection on sediment bioturbation and biogeochemical fluxes at the sediment water interface. Our results revealed that, despite high parasite intensity, the sediment reworking and bioirrigation rates, as well as nutrient fluxes, were not impacted. This finding was unexpected since previous studies showed that metacercarial infection impairs the physiological condition of cockles and induces a mechanical obstruction of their feet, thus altering their burrowing capacity. There are several explanations for such contrasting results. Firstly, the alteration of cockle behavior could arise over a longer time period following parasite infection. Secondly, the modulation of cockle bioturbation by parasites could be more pronounced in older specimens burying deeper. Thirdly, the intensity of the deleterious impacts of metacercariae could strongly vary across parasite species. Lastly, metacercarial infection alters cockle fitness through an interaction with other biotic and abiotic environmental stressors.

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.

The impacts of polyethylene terephthalate microplastics (mPETs) on ecosystem functionality in marine sediment

The effects of microplastics (MPs) on the ecological functioning in marine sediments is largely unknown. However, coastal marine sediments and their resident communities play critical roles in the transformation of organic matter and the cycling of nutrients that influence both local and global processes. To investigate how microplastics influence ecosystem functions associated with sediment biogeochemistry, large bivalves and microphytobenthos, we conducted a 31-day laboratory experiment. The experiment tested the role of micro-polyethylene terephthalate (mPETs) at five concentrations (0%, 1%, 3%, 6%, and 8% based on wet weight of top 1 cm sediment). Canonical principle of coordinate analysis (CAP) was applied to assess change on the ecosystem functionality with increasing concentrations of mPETs. Our results highlight that stress effects on ecosystem function are the product of the interaction between Macomona liliana, microphytobenthos and mPETs.

Does the use of biological traits predict a smooth landscape of ecosystem functioning?

The biodiversity crisis has increased interest in understanding the role of biodiversity for ecosystem functioning. Functional traits are often used to infer ecosystem functions to increase our understanding of these relationships over larger spatial scales. The links between specific traits and ecosystem functioning are, however, not always well established. We investigated how the choice of analyzing either individual species, selected modalities, or trait combinations affected the spatial patterns observed on a sandflat and how this was related to the natural variability in ecosystem functioning. A large dataset of 400 benthic macrofauna samples was used to explore distribution patterns. We hypothesized that (1) if multiple species (redundancy) represent a trait combination or a modality their spatial patterns would be smoothed out, and (2) the lost spatial variability within a trait combination or modality, due to the smoothing effect, would potentially affect their utility for predicting ecosystem functioning (tested on a dataset of 24 samples). We predicted that species would show heterogeneous small spatial patterns, while modalities and trait combinations would show larger and more homogeneous patterns because they would represent a collection of many distributions. If modalities and trait combinations are better predictors of ecosystem functioning than species, then the smoother spatial patterns of modalities and trait combinations would result in a more homogeneous landscape of ecosystem function and the number of species exhibiting specific traits would provide functional redundancy. Our results showed some smoothing of spatial patterns progressing from species through modalities to trait combinations, but generally spatial patterns reflected a few dominant key species. Moreover, some individual modalities and species explained more or equal proportions of the variance in the ecosystem functioning than the combined traits. The findings thus suggest that only some spatial variability is lost when species are combined into modalities and trait combinations and that a homogeneous landscape of ecosystem function is not likely.

Water Column Turbidity Not Sediment Nutrient Enrichment Moderates Microphytobenthic Primary Production

Soft sediment intertidal habitats are under intense anthropogenic pressure resulting from increased land derived sediment and nutrient delivery. Long term, this can cause high water column turbidity and nutrient enrichment of sediment porewaters, which has cascading effects on coastal ecosystem functionality. However, how these stressors may interact and influence benthic productivity over alternating periods of submergence and emergence is largely unknown. This study investigates the effects of sediment nutrient enrichment (at three levels for 20 months) on benthic primary production at six sites in four New Zealand estuaries that spanned a gradient in water column turbidity. While nutrient enrichment had no detectable effect on microphytobenthic primary production, water column turbidity had a significant influence, explaining up to 40% of variability during tidal submergence, followed by temperature and sediment characteristics. In addition, negative net primary production (NPP) estimates and therefore net heterotrophy for the most turbid estuaries during tidal submergence resulted in an increased reliance on production during emerged periods, where NPP was positive across all sites. This study highlights the prominent role of water column turbidity over nutrient enrichment in moderating microphytobenthic production, and the increasing importance of emerged periods to maintain the health and functioning of coastal habitats.

Unraveling ecosystem functioning in intertidal soft sediments: the role of density-driven interactions

Although they only occupy a relatively small portion of the surface of the planet, coastal habitats are some of the most productive and valued ecosystems in the world. Among these habitats, tidal flats are an important component of many harbours and estuaries, but their deterioration due to human activities poses a serious threat to biodiversity and ecosystem function. Benthic communities are usually arranged in patches dominated by key species with overlapping distributions. Understanding the ecological consequences of interactions between these species in transition zones where their habitats overlap is necessary in order to quantify their contribution to overall ecosystem functioning and to scale-up and generalize results. Spatial transition in abundance and the interaction of multiple factors that drive ecosystem function are complex processes that require real-world research. Through a multi-site mensurative experiment, we show that transition areas drive non-linear effects on biogeochemical fluxes that have important implications for quantifying overall functioning. In our study the main drivers of ecosystem function were the abundance of two large but functionally very different species rather than biodiversity per se. Furthermore, we demonstrate that the use of the biogenic features created by specific infaunal species at the sediment–water interface is a better predictor of ecosystem functioning than the density of the species per se, making this approach particularly appealing for large scale, mapping and monitoring studies.

Sea-level rise and the emergence of a keystone grazer alter the geomorphic evolution and ecology of southeast US salt marshes

Human disturbances, climate change, and their combined effects on species distributions and environmental conditions are increasingly modifying the organization of our world’s oceans, forests, grasslands, wetlands, tundras, and reefs. Here, we reveal that these contemporary conditions can trigger the emergence of novel keystone species. Across the southeastern US coastal plain, sea-level rise is outpacing salt marsh vertical accretion, causing these grasslands to be tidally inundated for longer and softening marsh substrates to levels optimal for crab burrowing. Using field experiments, measurements, surveys, and models, we show that these conditions amplify the burrowing and grazing effects of a previously inconspicuous crab, enabling it to redefine predator–prey interactions, eco-geomorphic feedbacks, and the mechanisms by which salt marshes are responding to climate change.

Keystone species have large ecological effects relative to their abundance and have been identified in many ecosystems. However, global change is pervasively altering environmental conditions, potentially elevating new species to keystone roles. Here, we reveal that a historically innocuous grazer—the marsh crab Sesarma reticulatum—is rapidly reshaping the geomorphic evolution and ecological organization of southeastern US salt marshes now burdened by rising sea levels. Our analyses indicate that sea-level rise in recent decades has widely outpaced marsh vertical accretion, increasing tidal submergence of marsh surfaces, particularly where creeks exhibit morphologies that are unable to efficiently drain adjacent marsh platforms. In these increasingly submerged areas, cordgrass decreases belowground root:rhizome ratios, causing substrate hardness to decrease to within the optimal range for Sesarma burrowing. Together, these bio-physical changes provoke Sesarma to aggregate in high-density grazing and burrowing fronts at the heads of tidal creeks (hereafter, creekheads). Aerial-image analyses reveal that resulting “Sesarma-grazed” creekheads increased in prevalence from 10 ± 2% to 29 ± 5% over the past <25 y and, by tripling creek-incision rates relative to nongrazed creekheads, have increased marsh-landscape drainage density by 8 to 35% across the region. Field experiments further demonstrate that Sesarma-grazed creekheads, through their removal of vegetation that otherwise obstructs predator access, enhance the vulnerability of macrobenthic invertebrates to predation and strongly reduce secondary production across adjacent marsh platforms. Thus, sea-level rise is creating conditions within which Sesarma functions as a keystone species that is driving dynamic, landscape-scale changes in salt-marsh geomorphic evolution, spatial organization, and species interactions.

Effects of Polyester Microfibers on Microphytobenthos and Sediment-Dwelling Infauna

Microfibers often dominate sediment microplastic samples, but little is known about their ecological effects on benthic organisms and functions. Polyethylene terephthalate) (PET) microfibers were added to 36 sediment chambers at six concentrations (0-0.5 g kg^-1 sediment) to assess the effects on microphytobenthos (MPB), a key deposit-feeding bivalve, Macomona liliana, and sediment nutrient pools. MPB photosynthesis was promoted in 18 chambers through a 12 h light/dark cycle. Another 18 chambers were maintained under dark conditions to inhibit photosynthesis. After 35 days of MPB growth and stabilization, four M. liliana were added to each chamber for a further 40 days. MPB biomass and composition were examined alongside M. liliana biochemical and behavioral properties and porewater dissolved inorganic nutrient concentrations. Increasing microfibers resulted in lower MPB biomass, fewer diatom-associated fatty acids (FAs), and an increase in cyanobacteria. The changes in MPB coincided with up to 75% lower energy reserves and reduced burrowing activity in M. liliana. In the light, nitrate + nitrate (NO_x) was significantly elevated and related to M. liliana and MPB biochemical properties. Ammoniu (NH₄⁺) concentrations increased but were variable in both the light and the dark. Our results suggest that increasing microfiber concentrations influence the interactions between M. liliana and MPB and affect biogeochemical processing in coastal marine sediments.

Large Infaunal Bivalves Determine Community Uptake of Macroalgal Detritus and Food Web Pathways

Abstract

Human activities alter biodiversity, influencing bottom-up and top-down control on food webs which can affect ecosystem functioning. In marine ecosystems, large bivalves play a critical role in benthic–pelagic coupling including nutrient cycling; however, their influence on the uptake of detrital organic matter by benthic communities is less understood. In a replicated factorial field experiment, we examined how the presence or absence (overharvesting scenario) of a large suspension-feeding clam on an intertidal sandflat and the addition of isotopically enriched macroalgal (Ulva sp.) detritus (eutrophication scenario) influenced infaunal biodiversity, and how changes in trophic interactions influenced key ecosystem functions (nutrient cycling and benthic metabolism and primary production). Both clams and Ulva increased community metabolism, but only clams had an effect on nutrient regeneration. We used the 13C- and 15N-enriched Ulva to quantify the effect of clams on detritus uptake in fauna and recovery in sediment. Due to their large biomass, nitrogen incorporation by clams constituted one-third of the infaunal community uptake after 14 days. Clam uptake likely resulted from ingestion of resuspended microphytobentos which had utilized 15N leaking out from decomposing Ulva. In plots without Ulva addition, the effect of clams on the overall resource utilization by the benthic community using natural abundance isotope niche metrics were tested. In plots without clams, the isotope niche of the community was reduced, and less carbon of pelagic origin was channelled into the infaunal food web. Our results imply that the loss of clams changes trophic pathways and reduces community uptake of macroalgal detritus, potentially exacerbating eutrophication.

Graphic Abstract

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.

Community bioirrigation potential (BIPc), an index to quantify the potential for solute exchange at the sediment-water interface

BIOIRRIGATION: the animal-induced exchange of solutes between pore water and overlying water - is a key process in sediments with profound implications for biogeochemical processes such as nutrient cycling and organic matter regeneration at the sediment water interface. There is an urgent need to understand how a changing environment will affect the irrigation activity of macrofauna and vice versa. A shift in species composition (e.g. from deep burrowing species to smaller, more opportunistic and shallow burrowing species) will have large effects on bioirrigation and thus on ecosystem function (such as benthic pelagic coupling). Considering the difficulties to determine area-covering rates of bioirrigation (e.g. in terms tracer-based fluxes) and the complexity of interactions of multiple species in the community that prohibit a direct measure of bioirrigation attributable to each species, a mechanistically-based approach is needed to predict relative intensities of bioirrigation activity based on the fundamental functional traits. We propose a conceptual framework to develop an index of bioirrigation that takes into account the biological mechanisms of bioirrigation and provides a simplified, yet functionally based approach to quantify the bioirrigation potential of benthic communities. We developed the community bioirrigation potential (BIP_c) that provides a biomass- and abundance-weighted scoring system considering functional traits related to pore water and solute exchange. It may be used as a supplement to established methods to assess the function of marine soft sediments related bioirrigation. In analogy to the particle-related community bioturbation potential of Solan et al. (2004), context dependent organismal traits that affect ventilation and bioirrigation (feeding type, morphology of burrows, and burrowing depth) are combined with the data on abundance and biomass of the respective species. These are subsequently summed up to a community bioirrigation potential (BIP_c). This review considers ecological traits relevant for bioirrigation and their classification into a bioirrigation index. Furthermore the necessary simplifications in the index (e.g. limiting its applicability to interfacial nutrient fluxes) are discussed. We also provide a working example from the southwestern Baltic Sea to illustrate the practical application of the index and a compilation of key species related to this area containing their classification into the considered bioirrigation traits.

Species contributions to ecosystem process and function can be population dependent and modified by biotic and abiotic setting

There is unequivocal evidence that altered biodiversity, through changes in the expression and distribution of functional traits, can have large impacts on ecosystem properties. However, trait-based summaries of how organisms affect ecosystem properties often assume that traits show constancy within and between populations and that species contributions to ecosystem functioning are not overly affected by the presence of other species or variations in abiotic conditions. Here, we evaluate the validity of these assumptions using an experiment in which three geographically distinct populations of intertidal sediment-dwelling invertebrates are reciprocally substituted. We find that the mediation of macronutrient generation by these species can vary between different populations and show that changes in biotic and/or abiotic conditions can further modify functionally important aspects of the behaviour of individuals within a population. Our results demonstrate the importance of knowing how, when, and why traits are expressed and suggest that these dimensions of species functionality are not sufficiently well-constrained to facilitate the accurate projection of the functional consequences of change. Information regarding the ecological role of key species and assumptions about the form of species–environment interactions needs urgent refinement.

Changes in the location of biodiversity-ecosystem function hot spots across the seafloor landscape with increasing sediment nutrient loading

Declining biodiversity and loss of ecosystem function threatens the ability of habitats to contribute ecosystem services. However, the form of the relationship between biodiversity and ecosystem function (BEF) and how relationships change with environmental change is poorly understood. This limits our ability to predict the consequences of biodiversity loss on ecosystem function, particularly in real-world marine ecosystems that are species rich, and where multiple ecosystem functions are represented by multiple indicators. We investigated spatial variation in BEF relationships across a 300 000 m² intertidal sandflat by nesting experimental manipulations of sediment pore water nitrogen concentration into sites with contrasting macrobenthic community composition. Our results highlight the significance of many different elements of biodiversity associated with environmental characteristics, community structure, functional diversity, ecological traits or particular species (ecosystem engineers) to important functions of coastal marine sediments (benthic oxygen consumption, ammonium pore water concentrations and flux across the sediment-water interface). Using the BEF relationships developed from our experiment, we demonstrate patchiness across a landscape in functional performance and the potential for changes in the location of functional hot and cold spots with increasing nutrient loading that have important implications for mapping and predicating change in functionality and the concomitant delivery of ecosystem services.

Consequences of biodiversity loss diverge from expectation due to post-extinction compensatory responses

Consensus has been reached that global biodiversity loss impairs ecosystem functioning and the sustainability of services beneficial to humanity. However, the ecosystem consequences of extinction in natural communities are moderated by compensatory species dynamics, yet these processes are rarely accounted for in impact assessments and seldom considered in conservation programmes. Here, we use marine invertebrate communities to parameterise numerical models of sediment bioturbation – a key mediator of biogeochemical cycling – to determine whether post-extinction compensatory mechanisms alter biodiversity-ecosystem function relations following non-random extinctions. We find that compensatory dynamics lead to trajectories of sediment mixing that diverge from those without compensation, and that the form, magnitude and variance of each probabilistic distribution is highly influenced by the type of compensation and the functional composition of surviving species. Our findings indicate that the generalized biodiversity-function relation curve, as derived from multiple empirical investigations of random species loss, is unlikely to yield representative predictions for ecosystem properties in natural systems because the influence of post-extinction community dynamics are under-represented. Recognition of this problem is fundamental to management and conservation efforts, and will be necessary to ensure future plans and adaptation strategies minimize the adverse impacts of the biodiversity crisis.

Specific arrangements of species dominance can be more influential than evenness in maintaining ecosystem process and function

The ecological consequences of species loss are widely studied, but represent an end point of environmental forcing that is not always realised. Changes in species evenness and the rank order of dominant species are more widespread responses to directional forcing. However, despite the repercussions for ecosystem functioning such changes have received little attention. Here, we experimentally assess how the rearrangement of species dominance structure within specific levels of evenness, rather than changes in species richness and composition, affect invertebrate particle reworking and burrow ventilation behaviour - important moderators of microbial-mediated remineralisation processes in benthic environments - and associated levels of sediment nutrient release. We find that the most dominant species exert a disproportionate influence on functioning at low levels of evenness, but that changes in biomass distribution and a change in emphasis in species-environmental interactions become more important in governing system functionality as evenness increases. Our study highlights the need to consider the functional significance of alterations to community attributes, rather than to solely focus on the attainment of particular levels of diversity when safeguarding biodiversity and ecosystems that provide essential services to society.