The interplay of co-occurring ecosystem engineers shapes the structure of benthic communities – a mesocosm experiment

IntroductionEcosystem engineers play a pivotal role in shaping habitats through their activities and presence. In shallow Baltic waters, seagrasses, patch-forming mussels, and infaunal clams modify soft bottom habitats, impacting benthic community structure. While the individual effects of these ecosystem engineers are well studied, interactions among co-occurring engineers are poorly understood. MethodsWe conducted a mesocosm experiment to assess the independent and combined impacts of seagrass (Zostera marina), epifaunal mussels (Mytilus spp.), and infaunal clams (Macoma balthica) on invertebrate colonization in soft sediments. ResultsOur findings reveal significant engineer-driven alterations in macrofaunal community structure. Combined engineer effects diverged from individual impacts, indicating potential synergies or antagonisms in sediment (re)colonization. Notably, a higher number of engineer species positively affected the diversity of settled macrofauna, with the lowest macrofaunal abundance and biomass but the highest Shannon diversity found in the presence of all three engineers. DiscussionResults suggest that seagrass, mussels, and clams influence benthos through larval settlement and sediment biogeochemistry, providing insights into the distinct roles of habitat-forming organisms in shaping the benthic communities in coastal ecosystems of the Baltic Sea

Coastal ecosystem engineers and their impact on sediment dynamics: Eelgrass–bivalve interactions under wave exposure

Habitat forming ecosystem engineers play critical roles in structuring coastal seascapes. Many ecosystem engineers, such as seagrasses and epifaunal bivalves, are known to have positive effects on sediment stability and increase coastal protection and ecosystem resilience. Others, such as bioturbating infaunal bivalves, may instead destabilize sediment. However, despite the common co-occurrence of seagrasses and bivalves in coastal seascapes, little is known of their combined effects on sediment dynamics. Here, we used wave flumes to compare sediment dynamics in monospecific and multispecific treatments of eelgrass, Zostera marina, and associated bivalves (infaunal Limecola balthica, infaunal Cerastoderma edule, epifaunal Magellana gigas) under a range of wave exposures. Eelgrass reduced bedload erosion rates by 25–50%, with digital elevation models indicating that eelgrass affected the sediment micro-bathymetry by decreasing surface roughness and ripple sizes. Effects of bivalves on sediment mobilization were species-specific; L. balthica reduced erosion by 25%, C. edule increased erosion by 40%, while M. gigas had little effect. Importantly, eelgrass modified the impacts of bivalves: the destabilizing effects of C. edule vanished in the presence of eelgrass, while we found positive additive effects of eelgrass and L. balthica on sediment stabilization and potential for mutual anchoring. Such interspecific interactions are likely relevant for habitat patch emergence and resilience to extreme wave conditions. In light of future climate scenarios where increasing storm frequency and wave exposure threaten coastal ecosystems, our results add a mechanistic understanding of sediment dynamics and interactions between ecosystem engineers, with relevance for management and conservation.publishedVersio

A trait-based framework for seagrass ecology: Trends and prospects

In the last three decades, quantitative approaches that rely on organism traits instead of taxonomy have advanced different fields of ecological research through establishing the mechanistic links between environmental drivers, functional traits, and ecosystem functions. A research subfield where trait-based approaches have been frequently used but poorly synthesized is the ecology of seagrasses; marine angiosperms that colonized the ocean 100M YA and today make up productive yet threatened coastal ecosystems globally. Here, we compiled a comprehensive trait-based response-effect framework (TBF) which builds on previous concepts and ideas, including the use of traits for the study of community assembly processes, from dispersal and response to abiotic and biotic factors, to ecosystem function and service provision. We then apply this framework to the global seagrass literature, using a systematic review to identify the strengths, gaps, and opportunities of the field. Seagrass trait research has mostly focused on the effect of environmental drivers on traits, i.e., “environmental filtering” (72%), whereas links between traits and functions are less common (26.9%). Despite the richness of trait-based data available, concepts related to TBFs are rare in the seagrass literature (15% of studies), including the relative importance of neutral and niche assembly processes, or the influence of trait dominance or complementarity in ecosystem function provision. These knowledge gaps indicate ample potential for further research, highlighting the need to understand the links between the unique traits of seagrasses and the ecosystem services they provide

Context-dependent community facilitation in seagrass meadows along a hydrodynamic stress gradient

Foundation species host diverse associated communities by ameliorating environmental stress. The strength of this facilitative effect can be highly dependent on the underlying biotic and abiotic context. We investigated community level patterns of macrofauna associated with and adjacent to the marine foundation species eelgrass (Zostera marina) along a hydrodynamic stress gradient. We could demonstrate that the relative importance of this foundation species for its infaunal community increases with environmental variables associated with increasing hydrodynamic stress (depth, sand ripples formation, sediment grain size and organic content). Faunal assemblages in proximity to the Zostera patch edges, however, showed no (infauna) or negative (epifauna) response to hydrodynamic stress. Our study highlights that the facilitative outcome of a foundation species is conditional to the faunal assemblage in question and can be highly variable even between positions within the habitat.Peer reviewe

Facilitation and Feedbacks in Seagrass Communities : Testing the Role and Context-Dependency of Ecosystem Engineers

In physically unstable habitats such as coastal soft sediments, facilitative interactions are essential for structuring associated ecological communities. Here, habitat amelioration by ecosystem engineers such as seagrasses, salt marshes or bivalves can often increase the realized niche of associated species by controlling resource availability and/or mitigating environmental stressors. Multiple ecosystem engineers often co-occur, but to date we lack a mechanistic understanding of how their co-existence and interactions affect habitat structure and ecosystem processes. Positive interactions between ecosystem engineers may lead to mutualistic associations that affect ecosystem properties synergistically, while antagonistic ecosystem engineering may lead to competitive exclusion, potentially limiting co-occurrence to habitat transition zones. In this thesis, I explore the role of eelgrass, Zostera marina, and commonly associated bivalves Macoma balthica, Mytilus edulis, Crassostrea gigas, Cerastoderma edulis) as ecosystem engineers and the relevance of eelgrass-bivalve interactions. Based on four chapters, I specifically address: (I) the importance of eelgrass as habitat for associated species along a hydrodynamic gradient, (II) the influence of M. balthica on eelgrass survival and growth, (III) eelgrass seed dispersal and burial in eelgrass and bivalve patches, and (IV) effects of eelgrass and bivalves on sediment dynamics and stability under wave exposure. Therefore, I conducted a series of field surveys, manipulative field experiments and mesocosm experiments in the Finnish Archipelago Sea and at the Swedish west coast. My findings substantiate the role of ecosystem engineers for modifying ecosystem processes in coastal soft sediments. Eelgrass and bivalves strongly affected hydro- and sediment dynamics, thereby controlling sediment stability and transport of propagules. I further show that the relevance of ecosystem engineering may depend on underlying environmental conditions. Relative importance of eelgrass for associated macrofauna changed along a wave exposure gradient. Under benign conditions, total macrofauna abundance was ~50 % higher in eelgrass than adjacent bare sediments, but was ~ 300 % higher at the exposed end of the gradient where habitat amelioration by eelgrass was likely more important. Similarly, field manipulations indicated that M. balthica can facilitate eelgrass growth, when in situ porewater nutrient concentrations are low, potentially by stimulating nutrient uptake through nutrient release. In contrast, I found inverse negative effects under high nutrient concentrations, where nutrient release through M. balthica might have promoted algal overgrowth and nutrient toxicity. Overall, findings from this thesis underpin the complexity of interactions between ecosystem engineers, thus environmental context and density-dependence may be critical for determining the outcome of co-occurrence between eelgrass and bivalves.---------- I fysiskt instabila habitat, som kustnära mjukbottnar, är faciliterande artinteraktioner centrala för att strukturera associerade ekologiska samhällen. I dessa miljöer kan ekosystemingenjörer såsom sjögräs, marskväxter och musslor öka den realiserade nischen för associerade arter genom att reglera tillgängligheten på resurser och/eller lindra miljöstress. Vi saknar dock en mekanistisk förståelse av interaktionerna mellan ekosystemingenjörer och vilka konsekvenser dessa har för habitatets struktur och ekosystemprocesser. Ekosystemingenjörer samexisterar ofta mutualistiskt genom positiva interaktioner vilket kan påverka ekosystemets egenskaper, men i gränszonen mellan habitat kan antagonistiska effekter leda till att två konkurrerande arter inte kan samexistera. I denna avhandling undersöker jag ålgräsets (Zostera marina) och associerade musslors (Macoma balthica, Mytilus edulis, Crassostrea gigas, Cerastoderma edulis) roll som ekosystemingenjörer, samt betydelsen av interaktionen mellan ålgräs och musslor. I fyra kapitel behandlar jag specifikt (I) betydelsen av ålgräs som habitat för associerade arter längs en hydrodynamisk gradient, (II) betydelsen av östersjömusslan M. balthica för ålgräsets överlevnad och tillväxt, III) hur ålgräsfrön sprids och blir begravda i ålgräs- och musselbestånd, och (IV) effekterna av ålgräs och bivalver på sedimentets dynamik och stabilitet under olika grad av vågexponering. Jag gjorde fältundersökningar samt fält- och mesokosmexperiment i Skärgårdshavet och på den svenska västkusten. Min forskning bekräftar ekosystemingenjörernas roll som modifierare av ekosystemprocesser i kustnära mjuka sediment. Ålgräs och musslor påverkade både sediment- och hydrodynamiken, vilket i sin tur påverkade sedimentets stabilitet och transporten av ålgräsfrön. Jag visar vidare att ekosystemingenjörernas betydelse kan bero på underliggande miljöförhållanden. Exempelvis ändrades den relativa betydelsen av ålgräs för associerad makrofauna längs en vågexponeringsgradient. Under gynnsamma miljöförhållanden var den totala mängden makrofauna ca 50% högre i ålgräs än i omgivande bara sediment, men ca 300% högre i den starkast exponerade delen av gradienten där ålgräsets gynnsamma effekt var relativt sett viktigare. Fältexperiment indikerade att M. balthica kan stimulera ålgräsets tillväxt genom att frigöra näringsämnen då det omgivande sedimentets näringshalt är låg. När sedimentets näringshalt var hög var effekten däremot negativ, eftersom frigöring av näringsämnen från M. balthica kan ha orsakat algtillväxt och näringstoxicitet. Sammantaget bekräftar denna undersökning att interaktionerna mellan ekosystemingenjörer är komplexa, och att miljön och täthetsberoende faktorer sannolikt är avgörande för utfallet av interaktionen mellan ålgräs och musslor

The influence of hydrodynamics and ecosystem engineers on eelgrass seed trapping.

Propagule dispersal is an integral part of the life cycle of seagrasses; important for colonising unvegetated areas and increasing their spatial distribution. However, to understand recruitment success, seed dispersal and survival in habitats of different complexity remains to be quantified. We tested the single and synergistic effects of three commonly distributed ecosystem engineers-eelgrass (Zostera marina), oysters (Magellana gigas) and blue mussels (Mytilus edulis)-on trapping of Z. marina seeds in a hydraulic flume under currents. Our results suggest that seed retention increases with habitat complexity and further reveal insights into the underlying mechanisms. In eelgrass canopy, trapping occurred mostly through direct blocking of a seed's pathway, while trapping in bivalve patches was mainly related to altered hydrodynamics in the lee side, i.e. behind each specimen. With increasing flow velocity (24-30 cm s-1 in eelgrass canopy, 18-30 cm s-1 in bivalve patches), modifications of the sediment surface through increased turbulence and erosive processes became more important and resulted in high seed trapping rates. Furthermore, we show that while monospecific patches of seagrass and bivalves had different trapping optima depending on flow velocities, intermixing resulted in consistently high trapping rates throughout the investigated hydrodynamic gradient. Our results highlight the importance of positive interactions among ecosystem engineers for seed retention and patch emergence in eelgrass

The interplay of co-occurring ecosystem engineers shapes the structure of benthic communities – a mesocosm experiment

Introduction Ecosystem engineers play a pivotal role in shaping habitats through their activities and presence. In shallow Baltic waters, seagrasses, patch-forming mussels, and infaunal clams modify soft bottom habitats, impacting benthic community structure. While the individual effects of these ecosystem engineers are well studied, interactions among co-occurring engineers are poorly understood.Methods We conducted a mesocosm experiment to assess the independent and combined impacts of seagrass (Zostera marina), epifaunal mussels (Mytilus spp.), and infaunal clams (Macoma balthica) on invertebrate colonization in soft sediments.Results Our findings reveal significant engineer-driven alterations in macrofaunal community structure. Combined engineer effects diverged from individual impacts, indicating potential synergies or antagonisms in sediment (re)colonization. Notably, a higher number of engineer species positively affected the diversity of settled macrofauna, with the lowest macrofaunal abundance and biomass but the highest Shannon diversity found in the presence of all three engineers.Discussion Results suggest that seagrass, mussels, and clams influence benthos through larval settlement and sediment biogeochemistry, providing insights into the distinct roles of habitat-forming organisms in shaping the benthic communities in coastal ecosystems of the Baltic Sea

Context-dependency of eelgrass-clam interactions: Implications for coastal restoration

Facilitative interactions between co-occurring species sustain diverse communities and constitute a vital functional component of coastal marine ecosystems. In seagrass ecosystems, facilitation ensures the survival and resilience of this important habitat. As seagrass meadows are in decline, innovative restoration strategies incorporating facilitative interactions could open new avenues in marine restoration. Here, we investigated the interactions between eelgrass Zostera marina and the Baltic clam Macoma balthica, and tested whether clams could enhance early survival and biomass increase of transplanted eelgrass shoots in the northern Baltic Sea. We measured eelgrass responses to differing densities of clams, as well as porewater ammonium (NH4+) and phosphate (PO43-) concentrations in field and aquarium experiments. Overall, survival of transplanted plots was high, independent of clam density. Specifically, we found that clams facilitated eelgrass above- and below-ground biomass in low porewater nutrient conditions, potentially through nutrient release, but inhibited growth in high-nutrient conditions, particularly where clams were added at high densities. Our results show the important role of infaunal bivalves for nutrient fluxes within seagrass meadows. Most notably, we highlight the importance of considering and testing context- and density-dependency when studying interspecific interactions, as clams could both benefit and hamper Zostera biomass increase. This becomes particularly crucial when incorporating such interactions in a restoration context

Coastal ecosystem engineers and their impact on sediment dynamics: Eelgrass–bivalve interactions under wave exposure

Habitat forming ecosystem engineers play critical roles in structuring coastal seascapes. Many ecosystem engineers, such as seagrasses and epifaunal bivalves, are known to have positive effects on sediment stability and increase coastal protection and ecosystem resilience. Others, such as bioturbating infaunal bivalves, may instead destabilize sediment. However, despite the common co-occurrence of seagrasses and bivalves in coastal seascapes, little is known of their combined effects on sediment dynamics. Here, we used wave flumes to compare sediment dynamics in monospecific and multispecific treatments of eelgrass, Zostera marina, and associated bivalves (infaunal Limecola balthica, infaunal Cerastoderma edule, epifaunal Magellana gigas) under a range of wave exposures. Eelgrass reduced bedload erosion rates by 25–50%, with digital elevation models indicating that eelgrass affected the sediment micro-bathymetry by decreasing surface roughness and ripple sizes. Effects of bivalves on sediment mobilization were species-specific; L. balthica reduced erosion by 25%, C. edule increased erosion by 40%, while M. gigas had little effect. Importantly, eelgrass modified the impacts of bivalves: the destabilizing effects of C. edule vanished in the presence of eelgrass, while we found positive additive effects of eelgrass and L. balthica on sediment stabilization and potential for mutual anchoring. Such interspecific interactions are likely relevant for habitat patch emergence and resilience to extreme wave conditions. In light of future climate scenarios where increasing storm frequency and wave exposure threaten coastal ecosystems, our results add a mechanistic understanding of sediment dynamics and interactions between ecosystem engineers, with relevance for management and conservation

A facultative mutualism facilitates European seagrass meadows

Coastal ecosystem functioning often hinges on habitat-forming foundation species that engage in positive interactions (e.g. facilitation and mutualism) to reduce environmental stress. Seagrasses are important foundation species in coastal zones but are rapidly declining with losses typically linked to intensifying global change-related environmental stress. There is growing evidence that loss or disruption of positive interactions can amplify coastal ecosystem degradation as it compromises its stress mitigating capacity. Multiple recent studies highlight that seagrass can engage in a facultative mutualistic relationship with lucinid bivalves that alleviate sulphide toxicity. So far, however, the generality of this mutualism, and how its strength and relative importance depend on environmental conditions, remains to be investigated. Here we study the importance of the seagrass-lucinid mutualistic interaction on a continental-scale using a field survey across Europe. We found that the lucinid bivalve Loripes orbiculatus is associated with the seagrasses Zostera noltii and Zostera marina across a large latitudinal range. At locations where the average minimum temperature was above 1 °C, L. orbiculatus was present in 79% of the Zostera meadows; whereas, it was absent below this temperature. At locations above this minimum temperature threshold, mud content was the second most important determinant explaining the presence or absence of L. orbiculatus. Further analyses suggest that the presence of the lucinids have a positive effect on seagrass biomass by mitigating sulphide stress. Finally, results of a structural equation model (SEM) support the existence of a mutualistic feedback between L. orbiculatus and Z. noltii. We argue that this seagrass-lucinid mutualism should be more solidly integrated into management practices to improve seagrass ecosystem resilience to global change as well as the success of restoration efforts.</p