Rebecca J. Aspden

The pervasive role of biological cohesion in bedform development

Sediment fluxes in aquatic environments are crucially dependent on bedform dynamics. However, sediment-flux predictions rely almost completely on clean-sand studies, despite most environments being composed of mixtures of non-cohesive sands, physically cohesive muds and biologically cohesive extracellular polymeric substances (EPS) generated by microorganisms. EPS associated with surficial biofilms are known to stabilize sediment and increase erosion thresholds. Here we present experimental data showing that the pervasive distribution of low levels of EPS throughout the sediment, rather than the high surficial levels of EPS in biofilms, is the key control on bedform dynamics. The development time for bedforms increases by up to two orders of magnitude for extremely small quantities of pervasively distributed EPS. This effect is far stronger than for physical cohesion, because EPS inhibit sand grains from moving independently. The results highlight that present bedform predictors are overly simplistic, and the associated sediment transport processes require re-assessment for the influence of EPS.

Sticky stuff: Redefining bedform prediction in modern and ancient environments

The dimensions and dynamics of subaqueous bedforms are well known for cohesionless sediments. However, the effect of physical cohesion imparted by cohesive clay within mixed sand-mud substrates has not been examined, despite its recognized influence on sediment stability. Here we present a series of controlled laboratory experiments to establish the influence of substrate clay content on subaqueous bedform dynamics within mixtures of sand and clay exposed to unidirectional flow. The results show that bedform dimensions and steepness decrease linearly with clay content, and comparison with existing predictors of bedform dimensions, established within cohesionless sediments, reveals significant over-prediction of bedform size for all but the lowermost clay contents examined. The profound effect substrate clay content has on bedform dimensions has a number of important implications for interpretation in a range of modern and ancient environments, including reduced roughness and bedform heights in estuarine systems and the often cited lack of large dune cross-sets in turbidites. The results therefore offer a step change in our understanding of bedform formation and dynamics in these, and many other, sedimentary environments.

Light-Dependant Biostabilisation of Sediments by Stromatolite Assemblages

For the first time we have investigated the natural ecosystem engineering capacity of stromatolitic microbial assemblages. Stromatolites are laminated sedimentary structures formed by microbial activity and are considered to have dominated the shallows of the Precambrian oceans. Their fossilised remains are the most ancient unambiguous record of early life on earth. Stromatolites can therefore be considered as the first recognisable ecosystems on the planet. However, while many discussions have taken place over their structure and form, we have very little information on their functional ecology and how such assemblages persisted despite strong eternal forcing from wind and waves. The capture and binding of sediment is clearly a critical feature for the formation and persistence of stromatolite assemblages. Here, we investigated the ecosystem engineering capacity of stromatolitic microbial assemblages with respect to their ability to stabilise sediment using material from one of the few remaining living stromatolite systems (Highborne Cay, Bahamas). It was shown that the most effective assemblages could produce a rapid (12–24 h) and significant increase in sediment stability that continued in a linear fashion over the period of the experimentation (228 h). Importantly, it was also found that light was required for the assemblages to produce this stabilisation effect and that removal of assemblage into darkness could lead to a partial reversal of the stabilisation. This was attributed to the breakdown of extracellular polymeric substances under anaerobic conditions. These data were supported by microelectrode profiling of oxygen and calcium. The structure of the assemblages as they formed was visualised by low-temperature scanning electron microscopy and confocal laser microscopy. These results have implications for the understanding of early stromatolite development and highlight the potential importance of the evolution of photosynthesis in the mat forming process. The evolution of photosynthesis may have provided an important advance for the niche construction activity of microbial systems and the formation and persistence of the stromatolites which came to dominate shallow coastal environments for 80% of the biotic history of the earth.

Large-scale variation in combined impacts of canopy loss and disturbance on community structure and ecosystem functioning

Ecosystems are under pressure from multiple human disturbances whose impact may vary depending on environmental context. We experimentally evaluated variation in the separate and combined effects of the loss of a key functional group (canopy algae) and physical disturbance on rocky shore ecosystems at nine locations across Europe. Multivariate community structure was initially affected (during the first three to six months) at six locations but after 18 months, effects were apparent at only three. Loss of canopy caused increases in cover of non-canopy algae in the three locations in southern Europe and decreases in some northern locations. Measures of ecosystem functioning (community respiration, gross primary productivity, net primary productivity) were affected by loss of canopy at five of the six locations for which data were available. Short-term effects on community respiration were widespread, but effects were rare after 18 months. Functional changes corresponded with changes in community structure and/or species richness at most locations and times sampled, but no single aspect of biodiversity was an effective predictor of longer-term functional changes. Most ecosystems studied were able to compensate in functional terms for impacts caused by indiscriminate physical disturbance. The only consistent effect of disturbance was to increase cover of non-canopy species. Loss of canopy algae temporarily reduced community resistance to disturbance at only two locations and at two locations actually increased resistance. Resistance to disturbance-induced changes in gross primary productivity was reduced by loss of canopy algae at four locations. Location-specific variation in the effects of the same stressors argues for flexible frameworks for the management of marine environments. These results also highlight the need to analyse how species loss and other stressors combine and interact in different environmental contexts.

The role of biophysical cohesion on subaqueous bed form size

Abstract Biologically active, fine‐grained sediment forms abundant sedimentary deposits on Earths surface, and mixed mud‐sand dominates many coasts, deltas, and estuaries. Our predictions of sediment transport and bed roughness in these environments presently rely on empirically based bed form predictors that are based exclusively on biologically inactive cohesionless silt, sand, and gravel. This approach underpins many paleoenvironmental reconstructions of sedimentary successions, which rely on analysis of cross‐stratification and bounding surfaces produced by migrating bed forms. Here we present controlled laboratory experiments that identify and quantify the influence of physical and biological cohesion on equilibrium bed form morphology. The results show the profound influence of biological cohesion on bed form size and identify how cohesive bonding mechanisms in different sediment mixtures govern the relationships. The findings highlight that existing bed form predictors require reformulation for combined biophysical cohesive effects in order to improve morphodynamic model predictions and to enhance the interpretations of these environments in the geological record.

Marine Biodiversity and Ecosystem Functioning: Frameworks, methodologies, and integration

1. Marine biodiversity: its history, present status and future threats 2. Biodiversity in the context of ecosystem function 3. Ecosystem function and co-evolution of terminology in marine science and management 4. Ecological consequences of declining biodiversity: A biodiversity-ecosystem function (BEF) framework for marine systems 5. Lessons from the fossil record: the Ediacaran radiation, the Cambrian radiation, and the end-Permian mass extinction 6. The analysis of biodiversity-ecosystem function experiments: partitioning richness and density-dependent effects 7. The importance of body size, abundance, and food web structure for ecosystem functioning 8. Effects of biodiversity-environment conditions on the interpretation of biodiversity-function relations 9. Extending the approaches of biodiversity and ecosystem functioning to the deep ocean 10. Incorporating extinction risk and realistic biodiversity futures: Implementation of trait based extinction scenarios 11. Biodiversity and ecosystem functioning: an ecosystem-level approach 12. Multitrophic biodiversity and the responses of marine ecosystems to global change 13. Reality check: issues of scale and abstraction in biodiversity research, and potential solutions 14. Why bother going outside: the role of observational studies in understanding biodiversity-ecosystem function relationships 15. Implementing an ecosystem approach: predicting and safeguarding marine biodiversity futures Index

Wave and sediment dynamics along a shallow subtidal sandy beach inhabited by modern stromatolites

To help define the habitat of modern marine stromatolites, wave-dominated flow and sediment transport were studied in the shallow subtidal region (1-2 m depth) along the slightly concave, windward face of Highborne Cay, Exuma, Bahamas - the only face of the cay that includes a population of stromatolites concentrated near the region of highest curvature of the beach. Wave energy impacting this islands most exposed beach was driven by local wind forcing which increases largely in response to the passage of atmospheric disturbances that typically affect the region for periods of a few days. Although some wave energy is almost always noted (maximum horizontal orbital speeds at the bottom are rarely <10 cm s(-1)), wave conditions remain comparatively calm until local winds increase above speeds of approximately 3-4 m s(-1) at which point maximum wave speeds rapidly increase to 50-80 cm s(-1). Stromatolites, which are largely restricted to the shoreward side of a shallow platform reef, are sheltered by the reef beyond which wave speeds are one to four times higher (depending on tidal stage). Moreover, stromatolite populations are predominantly found along a region of this wave-exposed beach that experiences comparatively reduced wave energy because of the curved morphology of the islands face. Maximum wave speeds are 1.4 to 2 times higher along more northern sections of the beach just beyond the locus of stromatolite populations. A quantitative model of sediment transport was developed that accurately predicted accumulation of suspended sediment in sediment traps deployed in the shallow subtidal zone along this beach. This model, coupled with in situ wave records, indicates that gross rates of suspended sediment deposition should be two to three times higher northward of the main stromatolite populations. Regions of the beach containing stromatolites nevertheless should experience significant rates of gross suspended sediment deposition averaging 7-10 g cm(-2) day(-1) ( approximately 4-6 cm day(-1)). Results suggest that one axis of the habitat of modern marine stromatolites may be defined by a comparatively narrow range of flow energy and sediment transport conditions.

Ooid Accreting Diatom Communities from the Modern Marine Stromatolites at Highborne Cay, Bahamas

The modern marine stromatolites at Highborne Cay, Bahamas are inhabited by diverse surface microbial communities. Although these communities are most often dominated by cyanobacteria (e.g., Schizothrix gebeleinii, Solentia sp., Oscillatoria sp. etc.), diatoms can be abundant and have been implicated in stromatolite biogenesis. We have identified two distinct surface diatom communities involved in sediment deposition: (1) a thick (0.5–1 cm) yellow surface layer (yellow fur) of the stalked diatoms Licmophora remulus, L. paradoxa, and Striatella unipunctata, and (2) a colorless cohesive surface layer (“pustular blanket”) formed by a tube-forming diatom with individual cells resembling Navicula. The stalks could be labeled with a lectin–FITC conjugate and observed with confocal scanning laser microscopy (CSLM). Ooids were seen trapped within a well-developed network of stalks at the surface and clusters of stalks oriented vertical to the surface were found to penetrate the subsurface to a depth of several centimeters. Interestingly, the tubular structures formed by the naviculid species did not stain with the lectin–FITC conjugate. Nevertheless, manual manipulations of the sediment indicated that this diatom community also trapped ooids. Rapid sediment accretion could be attributed to these two diatom surface communities. Observations over the past decade as well as intensive monitoring over a 3-year period (2005–2007) indicate, however, that these surface diatom communities occur only at particular times of the year and do not survive burial. In addition, the surface layer of diatom-trapped ooids readily erodes unless stabilized by subsequent infiltration of extracellular polymeric substances (EPS) secreting cyanobacteria (e.g., S. gebeleinii). These observations suggest a limited contribution of diatoms to stromatolite biogenesis.

Biodynamics of Modern Marine Stromatolites

Microbial assemblages provide important ecosystem services within coastal systems such as the production of oxygen, sequestration of carbon, and contributing resilience and stability to coastlines. This is particularly the case for assemblages that have adapted to the highly dynamic and oligotrophic environment, in which Bahamian stromatolites occur. Research into the potential of modern stromatolitic microbial mats to biostabilize sediments has been carried out in the Exuma Cays, as part of the RIBS program (Research Initiative on Bahamian Stromatolites). The engineering capacity of these microbial assemblages was studied under ambient conditions. Surface stromatolitic assemblages were sampled, reconstituted, and the time taken to produce significant restabilization of the surface sediment determined. Samples placed in darkness exhibited lower biostabilization as determined by cohesive strength meter than those kept under ambient light, suggesting that, at least the initial stabilization of surface layers was carried out by the assemblage of autotrophic organisms including diatoms and their associated products, such as extracellular polymeric substances. These studies were supported by further research determining the influence of relative microbial biomass on the capacity of microbial assemblages to stabilize natural carbonate sediments. A significant reduction in the biostabilization process was observed in all samples where the natural biomass was diluted, suggesting that a set threshold of biomass was required to initiate significant stabilization. These studies show that once the stromatolitic microbial assemblages exceed a critical biomass, they become highly effective ecosystem engineers, capable of mediating the regeneration and long-term stability of stromatolites within the Exuma Cays. However, a reduction in microbial biomass, as experimentally represented here but caused by natural factors such as competition with eukaryotes, pollution, grazing, or other factors, may limit the capacity of stromatolites to retain sediments and threaten their long-term future.