Azra Meadows

Biogeomorphological implications of microscale interactions between sediment geotechnics and marine benthos: a review

Abstract At the foundations of biogeomorphological processes in the sea lie interactions between the activities of marine benthic animals and the geotechnical properties of their sedimentary environments. The potential significance of these interactions, which take place at a microscale level of millimetres to metres, for the large-scale geomorphology of the seabed has rarely been appreciated. In the context of this review, large-scale is defined as greater than 50 m to hundreds of kilometres. The present review addresses this link, drawing examples from a wide range of marine environments, including estuaries, the intertidal zone, continental shelves and slopes, and the deep sea. It firstly considers sediment stabilisation, slope failure, sediment mixing, biodeposition, sediment compaction, and hydrodynamic effects. This is followed by a consideration of two extremes of the ecological pyramid—the effects of marine meiofauna and marine vertebrates. The final section draws attention to the central role of faunal mucus and extracellular polymeric material (ECPM) in many of the microscale interactions that we describe. The implications of these microscale biological processes and features are discussed in terms of their influence on and control of the large-scale geomorphology of the seabed.

Mussels and mussel beds (Mytilus edulis) as stabilizers of sedimentary environments in the intertidal zone

Abstract Coastal zone protection of the intertidal zone is of great importance in view of the threats of sea level rise around the globe. Environmentally-friendly methods involving biological control may offer a solution. This paper studies mussels (Mytilus edulis) as stabilizers of the intertidal zone because they form beds that armour some intertidal sediments, hence reducing erosion. In field experiments we seeded high energy lower intertidal and low energy upper intertidal environments with isolated mussels and preformed clumps of mussels. More animals were lost from the higher energy site. Those that remained were transported landward for significant distances. Clumping protected animal loss at the low energy but not at the high energy site. Gravel placed under mussels improved their stability because mussels attached threads to them. Laboratory experiments show that mussels form clumps by attaching to other mussels, and when available to gravel which has to be at the sediment surface. Flume studies show single mussels decrease the critical erosion velocity of sediment near them. Similar effects are seen in the field at the edge of small mussel beds.

Microscale biogeotechnical differences in intertidal sedimentary ecosystems

Abstract Intertidal sediments are inhabited by organisms that can modify the geotechnical and sedimentological properties of the sediment. We have analysed small scale differences in these properties at four closely adjacent sites on Ardmore Bay, Clyde Estuary, Scotland. The sites were an Enteromorpha algal mat site (EAM), a Corophium volutator site (CV), the head shafts of Arenicola marina (AMHS) and the tail shafts of Arenicola marina (AMTS) burrows. Three replicate cores were taken from each site. We measured load resistance, particle size parameters of mean particle size, sorting, skewness and kurtosis, and total organic matter (TOM) and carbonate. Load resistance was measured with a newly developed microscale load resistance penetrometer which measured load resistance of the sediment at 1 mm intervals through the sediment core from the surface to 100 mm. Sediment cores were then sectioned every 10 mm and particle size, total organic matter and carbonate content measured. In situ shear strength measurements were also taken at the four sites. The data were analysed by bivariate correlation analysis and multivariate cluster analysis. There was a number of significant positive and negative correlations between the parameters at the four sites. Significant down-core changes in the sediment parameters and their correlations were noted. These differed between the four sites. The CV site had the largest number of significant correlations, the EAM site had the least. Cluster analyses of the sites showed that in general the sites clustered separately, although there were a number of overlaps. The EAM site showed distinct clusters for its three separate replicate cores, while the CV site clustered into a top part and a bottom part of the core. Cluster analyses of the depths across all the sites identified a break in the data at depths of between 40 and 70 mm. There was a linear relationship between field shear strength and laboratory penetration resistance. The results are discussed in relation to the fine-scale geotechnical and sedimentological heterogeneity of intertidal sediments, and the effects of biological activity.

Biological control of avalanching and slope stability in the intertidal zone

Abstract Sediment structure and stability are important features of soils and sedimentary environments, and are of great importance for the maintenance of estuarine, and intertidal banks and channels. Avalanching and slope stability are critical factors controlling the shape of intertidal dunes. The role of biological activity in these processes, either in the form of vegetation and roots, animal burrows, algal mats or microbial glues, is now a forefront area of research. The objective of our research has been to quantify the effects of biological activity on the avalanching of intertidal sediments. Natural microbial and meiofaunal communities and particularly burrowing infauna such as Corophium volutator and Nereis diversicolor dramatically increase angles of avalanche and factors of safety. They do not significantly alter angles of repose. Factors of safety, duration of avalanches, and percentage increase in sediment volume, all increase with increasing angles of avalanche. Video analysis of the avalanching process shows that different slope failure mechanisms are associated with the different types of biological activity, and probably reflect specific chemical and mechanical stabilizing effects produced by the different types of biological activity. These latter include tubes, extracellular polymeric materials, and a small element of compaction. These mechanisms include rotational failure, block and wedge failure and translational failure. The significance of our results for the stability of slopes under field conditions is discussed, and it is concluded that field seeding experiments in the intertidal zone with the species that we have used would be very productive.

Spatial heterogeneity in an intertidal sedimentary environment and its macrobenthic community

Abstract The intertidal sedimentary environment is an extremely varied one in which marked differences in habitats and animal communities occur over very short distances. This study reports small scale heterogeneity of an intertidal macrobenthic community and its sedimentary environment in large sand waves in a bay in the Clyde Estuary, Scotland. Three 50 m transects ran across sand waves along a peak (P), along a trough (T) and across a peak and trough (P/T). Five sediment parameters [redox potential (Eh), organic carbon, particle size, shear strength and water table] and abundance of 14 macrobenthic species were measured in contiguous 1 m2 quadrats along each transect. There were more positive and negative significant correlations along the P/T transect than the P and T transects. Thirteen correlations were common between pairs of transects. Multivariate cluster and principal component analyses were conducted on the data. The results of the two techniques were broadly in agreement. Distinct clusters of macrobenthic species and sediment parameters were identified. Some species clustered with sediment parameters. Others clustered only with species. These analyses may imply important causal relationships some of which are physically controlled and some of which are biologically controlled. Some clusters contained solely species that were deposit feeders. Clusters of quadrats along the P/T transect grouped quadrats on the peaks, in the troughs and on the slopes of the sand waves. These show that the peaks, troughs and slopes are significantly different sedimentary microenvironments. Clusters of quadrats along the P and along the T transect grouped quadrats that were spatially adjacent. These clusters indicate significant small scale spatial variability along the peaks and along the troughs of the sand waves.