Short introduction on coastal biogeomorphology
with examples for water systems of the Netherlands
Author: M.J. Baptist
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.
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
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
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
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.
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.
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.
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.
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.
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
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
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 &
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
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
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.
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,
Viles, H.A. (ed.), 1988. Biogeomorphology. Oxford:
Basil Blackwell Ltd.