Short introduction on coastal biogeomorphology with examples for water systems of the Netherlands

Author: M.J. Baptist

1. INTRODUCTION

Biogeomorphology is a discipline that combines ecology and geomorphology. Geomorphology is the study of landforms and their formation. Ecology is the study of the relationships between biota and their environment. The environment is defined as factors that affect biota. These factors can be abiotic (physical, chemical), biotic (other organisms) or anthropogenic (humans). Abiotic geomorphological processes may affect biota and biota may in turn affect geomorphological processes. The interaction between both defines the discipline of biogeomorphology. Biogeomorphology is the study of the interaction between geomorphological processes and biota.

2. ESSENTIAL CONCEPTS

The term biogeomorphology was first used in the eighties (Viles, 1988), although earlier studies have been conducted that were focused on biogeomorphology without using this term. Biogeomorphology is studied in terrestrial as wells as in aquatic systems. In coastal systems biogeomorphological interactions are clearly demonstrated in the shallow, productive waters and various sedimentary environments. Examples of biogeomorphological interrelationships include sand dune development, tidal flats, saltmarshes, mangrove systems and coral reefs.

Relevant geomorphological factors in coastal systems are bathymetry, bed composition (rock, gravel, sand, silt), and the transport of sediment. It also includes factors that drive morphological processes, such as water flow and wave energy. The biota involved in coastal biogeomorphology include plants and animals, ranging from very small (algae) to very large (whales).

The geomorphological influence on biota is in its most direct form the influence on habitats (living environments) of flora and fauna. The coastal morphology and geomorphological processes define the gradients between high and low, between wet and dry and between sedimentation and erosion. These gradients and the processes that cause them are determinative for gradients in grain size of the sediment, nutrient levels, organic matter levels and moisture. Plants and animals are tuned to specific conditions and will therefore be abundant in specific locations.

The biological influence on geomorphological processes is the influence of biota to create, maintain or transform their own geomorphological surroundings. This is demonstrated by the influence of vegetation on the hydraulic resistance, erodibility and sedimentation, or by the influence of fauna on sediment characteristics through bioturbation and biostabilization.

In some cases morphological processes are dominant over biological processes and therefore the biota have to adjust to their environment. In other cases biological processes are dominant. The most interesting are those cases where there is a mutual interaction that leads to feedback coupling of processes. When looking for these cases, it is important to examine the temporal and spatial scales of the mutually interacting processes. Biogeomorphological interrelationships can be found in several coastal environments, for both hard and soft substrates.

Biogeomorphology for hard substrates

On rocky shores and coral reefs a typical community of organisms thrives that affects the erosion rates of its substrate. Influenced by abiotic factors such as wave energy, splash water, inundation frequency and -period, depth, desiccation and substrate type, a clear zonation can be found of various cyanobacteria, (macro-)algae, fungi, lychens, molluscs, sponges, worms, sea urchins, fish, etc. Some of these organisms dwell on the surface of the substrate, while others live within the substrate. Their effect on erosion of the substrate is divided in ‘biological corrosion’, processes that modify the substrate but provides no erosion product, and ‘biological abrasion’ (see bioerosion), processes that do generate an erosion product. Grazing, burrowing and boring on or in the substrate carries out biological abrasion, and is most significantly found in coral reef systems.

Biogeomorphology for soft substrates

In soft coastal systems, the interrelationships between geomorphological factors and biota can mainly be noticed for benthic fauna and flora. The presence of benthic species is affected by hydraulic and morphologic conditions, such as depth, current velocity, salinity and grain size. The effect of soft substrate communities on geomorphology is divided in biostabilization and biodestabilization. Biostabilization leads to an increase in soil resistance, preventing erosion, while biodestabilization leads to an increased erodibility.

Biostabilization by plants

On tidal flats, small algae (diatoms) are capable of affecting the geomorphology. These diatoms can form extensive algal mats and excrete EPS mucus, which is a sticky substance made of polysaccharides that glues the sediment together and therefore protects the sediment against erosion.
Seagrass is dependent on clear water, it needs sunlight to grow. A seagrass meadow slows down the current velocity near the bed and therefore sand and silt will not resuspend in the water, which otherwise would lead to turbid water. Furthermore, their root system binds the substrate. Ultimately, deposition of suspended sediment is encouraged in a seagrass meadow, which leads to the supply of organic material with nutrients, needed for growth.
Seaweeds are also capable of adjusting their physical environment by damping down wave energy and also salt marshes play an important role in stabilizing sediments. Salt marsh vegetation makes fine sediment settle down resulting in a continuous heightening of the marsh. The higher the marsh gets, the more vegetation can grow and the better the marsh is protected against erosion.
Other stabilizing effects result from cementation of beachrock by cyanobacteria and stromatolite formation by algae.

Biostabilization by animals

Some macrozoobenthos can actively catch sediment particles from the water column and bring it to the bed. The presence of a mussel bank for example will alter the bed in different ways. Mussels slow down the water flow and they protect the bed against erosion. Mussels also actively catch small particles from the water column by filterfeeding and subsequently excrete these as pseudofaeces. This results in a change in soil composition to finer sediments.
Animal tube fields are also believed to stabilize the sediment, because there is a clear accumulation of fine particles and organic matter between the tubes. The tubes itself may affect small-scale turbulence and therefore have a stabilizing effect, however, a great deal may be attributed to the community of microorganisms between the tubes that excrete mucus.
Other stabilizing effects result from large banks of dead shells and mucus binding by meio- and macrofauna.
 
Biodestabilization

Benthic fauna may destabilize the substrate by their digging and feeding activities (bioturbation). The constant mixing and recycling of sediment in the top centimeters of the bed results in a characteristic vertical particle size profile. The selective uptake and excretion of preferred particle sizes results in sorting and pelletizing sediments. Together with the digging of burrows and the constant movement within the substrate, these activities lead to the generation of a surface micro-relief that has a higher hydraulic roughness and is more prone to erosion. Furthermore, bioturbation also affects the sediment water content, porosity and sediment cohesion.

Scale interactions in biogeomorphology

Different physical and biological processes can have dynamic interactions when they operate on the same spatial and temporal scales. Processes that act on a very small scale may appear as noise in the interactions with processes on larger scales. Their effect can be accounted for by proper averaging procedures (e.g. for turbulence). Processes that act on a large scale may be treated as slowly varying or even constant boundary conditions when studying their effects on processes on smaller scales (e.g. sea level rise due to climate change). Techniques for scale interactions are well established in geomorphology (De Vriend, 1991) and are based on scale linkage via sediment transport. In biology however, population and community dynamics give rise to spatial and temporal structures that are not easily linked. In recent years the importance of scale has been increasingly recognized (Legendre et al., 1997) as an essential aspect of understanding the biotic and abiotic processes that affect the biogeomorphology of coastal systems.

3. BIOGEOMORPHOLOGY IN WATER SYSTEMS OF THE NETHERLANDS

In many water systems in the Netherlands, biogeomorphological interactions can be found. In some cases morphological processes are dominant over biological processes and therefore organisms have to adjust to their environment. In other cases biological processes are dominant. The most interesting are those cases where there is a mutual interaction that leads to feedback coupling of processes. When looking for these cases, it is important to examine the temporal and spatial scales of the mutually interacting processes.

This short paper describes biogeomorphological interactions for three main types of water systems in the Netherlands. These are sea & coast, estuaries & Wadden Sea and rivers & streams.

Sea & coast

The North Sea is not a homogeneous pool of water. It can be classified in distinct regions, characterised by differences in water temperature, salinity, nutrient levels, turbidity, and other factors. The geomorphology of the North Sea shows many features as sand dunes, shore-face connected ridges, gravel banks and deep silty pits.

The abundance of organisms in the North Sea is to a large extent defined by the local abiotic conditions, such as depth, temperature, currents and bed composition. The influence of geomorphological factors on the species abundance and composition can mainly be noticed at the bottom organisms, (benthic organisms). Examples of benthic organisms are shell fish and worms that live in the bottom and crabs and sea stars that live on the bottom. Especially in the young phases of their lives (eggs and larvae) the geomorphology provides the conditions for failure or success.

The influence of organisms on the geomorphological conditions in the North Sea is not very large. Some organisms are capable of changing the local bed characteristics, for example by their digging activities (bioturbation), because they are building structures where they live in or because they are actively catching sediment particles from the water column and bring it to the bottom (filter feeding). Only when these organism are distributed over a large area, they can affect a large area, but they are not able to change the large-scale patterns in the North Sea.

Plants (sea weeds) can also be found in the North Sea. Their distribution is limited to the zone where light can penetrate the water. Plants usually grow on hard substrates, such as wrecks or dikes. Sea weeds are capable of adjusting their physical environment. It is known that sea weeds on a dike can damp down the wave energy so the dike is better protected against waves.

Estuaries & Wadden Sea

In the Netherlands, only two estuaries are left, that of the Western Scheldt and that of the Ems-Dollard and the Wadden Sea. Estuaries are characterised by a river flowing out to sea in a long transitional area. Estuaries therefore know a large variance in salinity, silt and nutrients and have morphological features such as mud-flats, shoals and salt marshes.

A noticeable difference in comparison to the North Sea is that the energy of the physical factors is smaller. This gives more opportunities for benthic organisms to grow and therefore the density and biomass of benthos is higher. There are also more possibilities for plants, not only for sea weeds, but also for sea grass and vegetation of salt marshes. Recursively, the influence organisms can have on their physical environment is larger.

Mussels for example, are growing in places where the current velocities are high enough to bring food (algae). The presence of a mussel bank will alter the bed in different ways. They slow down the water flow and they protect the bed against erosion. Mussels also actively catch small particles from the water column and subsequently excrete these. This results in a change in soil composition to finer sediments.

On tidal flats even very small algae (diatoms) are capable of affecting the geomorphology. These diatoms excrete a sticky substance that glues the sediment together and protect it against erosion.

Sea grass is dependent on clear water, because it needs sunlight to grow. A field of sea grass will slow down the current velocity near the bottom and therefore sand and silt is not resuspended in the water, which otherwise would lead to turbid water.

As a last example, salt marshes play an important role in biogeomorphology. The vegetation of salt marshes make fine sediment settle down resulting in a continuous heightening of the marsh. The higher it gets, the more plants can grow and the better the marsh is protected against erosion.

Rivers & streams

Generally speaking, in rivers and streams the role of biology is increasing in comparison to the previously mentioned water systems. Plants and animals are larger and the environment is less dynamic. Of course the power of water during a period of high discharge cannot be underestimated, but this is more an extreme event.

Especially in (small) streams, the influence of organisms on the water flow and the transport of sediment can be fairly large. Fallen trees can block the water and the roots and leaves of grass and other vegetation can resist erosion very well. Animal live also plays a role. Rats can dig holes in dikes and cows are known to trample down stream banks.

The large rivers in the Netherlands show a gradient in physical environments. The distribution of organism is very much depending on these gradients in depth, flow and inundation frequency. In the middle of the main channel flow velocities are too high for most organisms. But in the inner bends sand is deposited and plants may establish. Close to the water line, a barren sediment can be found, but slightly higher upshore, pioneer plants can grow, keeping the sediment together and protecting it against erosion of wind and water.

The largest influence of organisms can be found in the floodplains. Here, a wide variety of grasses, shrubs and woodlands can be found. During winter floods, the area is inundated, but the vegetation is highly effective in slowing down the water. Close to the river the water is already slowed down so much that a thick layer of sand is deposited. Further down the floodplain the water will almost be stopped completely, leading to the deposition of fine silt carrying important nutrients. 

4. CONCLUSION

There are numerous examples of interrelationships between organisms and geomorphological processes. Research in the field of biogeomorphology tries to unravel the complex interactions in order to support decisions in integrated water management.

5. BIBLIOGRAPHY

De Vriend, H.J., 1991. Mathematical modelling and large-scale coastal behaviour, Part 1: Physical processes. Journal of Hydraulic Research, Vol. 29, No. 6, pp. 727-740.

Legendre, P., S.F. Thrush, V.J. Cummings, P.K. Dayton, J. Grant, J.E. Hewitt, A.H. Hines, B.H. McArdle, R.D. Pridmore, D.C. Schneider, S.J. Turner, R.B. Whitlatch & M.R. Wilkinson, 1997. Spatial structure of bivalves in a sand flat: Scale and generating processes. Journal of Experimental Marine Biology and Ecology, Vol. 216, pp. 99-128.

Viles, H.A. (ed.), 1988. Biogeomorphology. Oxford: Basil Blackwell Ltd.