Biogeomorphology of aquatic systems
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This article considers the biogeomorphology of aquatic systems. The biogeomorpholgy of both hard and soft substratum including the role in which plants and animals play in stabilizing and de-stabilizing these aquatic systems is also explored.
Biogeomorphology considers the interactions between the ecology and geomorphology of a system. Ecology is the study of relationships between the biota and their environment, and geomorphology examines landforms and how they are formed. Biogeomorphology can be studied in terrestrial as well as aquatic systems. Within aquatic systems biogeomorphological relationships can be found for both hard substrates (rocky shores / coral reefs) and soft substrates (muddy / sandy coastal sediments). Biota can alter geomorphology by creating hard substrates (e.g. coral reefs) or by modifying the stability or erodability of soft substrates. Key species in the benthic communities of sediment shores can influence geomorphology by acting as biostabilizers or biodestabilizers of sediment. The impact of organisms on coastal morphology can be both dramatic (e.g. coral reefs, saltmarshes, mussel beds) and more subtle by modifying rates of sediment erosion and accretion. Biota are not only influenced by various environmental factors (abiotic – physical & chemical; biotic – other organisms; anthropogenic – humans), but organisms also act as ecosystem engineers and modify their environment (physical, chemical and biological aspects). Ecosystem engineers create valuable habitats (e.g. coral reefs and saltmarshes) that are important for fish and birds and serve as a vital part of our natural coastal defence against storm surges and sea level rise.
The term biogeomorphology was first used in the eighties (Viles, 1988), although earlier studies focused on the topic without using this term. In coastal systems biogeomorphological interactions are clearly demonstrated in shallow, productive waters and various sedimentary environments. Examples of biogeomorphological interrelationships include sand dune development, tidal flats, salt marshes, 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 biogeomorphologyinclude plants and animals, ranging from very small (microorganisms) to very large (feeding pits of whales).
The geomorphological directly affects biota through its influence on habitats. 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 determine the gradients in grain size, nutrient levels, organic matter levels and water content. Plants and animals are adapted to specific conditions and will therefore be found and abundant at specific locations (often forming habitats).
The biological influence on geomorphological processes is through the ability of biota to create, maintain or transform their geomorphological surroundings. This is demonstrated by the influence of vegetation and macrofauna on water flow, sediment erosion and deposition, or by the influence of fauna on sediment characteristcs through bioturbation and biostabilization.
In some cases morphological processes are dominant over biological processes and therefore it is necessary for biota to adjust to their environment. In other cases biological processes are dominant (e.g. coral reefs, salt marshes). 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 spatial and temporal scales of the mutually interacting processes.
Biogeomorphology for hard substrates
On rocky shores and coral reefs the typical community of organisms modifies the erosion of substrate. Influenced by abiotic factors such as wave energy, splash water, inundation frequency and -period, depth, desiccation and substrate type, a clear zonation of various cyanobacteria, (macro-)algae, fungi, lichens, molluscs, sponges, worms, sea urchins, fish, etc. can be found. 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 into ‘biological corrosion’ processes that modify the substrate but produce no erosion product, and ‘biological abrasion’ processes that generate an erosion product. Grazing, burrowing and boring on or in the substrate involves biological abrasion, and is most apparent in coral reef systems.
Biogeomorphology for soft substrates
In soft coastal systems, the interrelationships between geomorphological factors and biota mainly apply to benthic fauna and flora. The presence of benthic species is affected by hydraulic and morphologic conditions, such as water depth or elevation on the shore, current velocity, wave action, salinity and grain size. The biota responsible for changes in geomorphology of soft substrates can be divided into two functional groups, namely biostabilizers and biodestabilizers. Biostabilization leads to increased sediment stability and a reduction in erosion potential, whereas biodestabilization leads to reduced sediment stability and an increase in 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 extracellular polymeric substances, which is a sticky mucus-like 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 (Fig. 2) slows down the current velocity near the bed, therefore reducing the resuspension of sand and silt, and this in turn helps to maintain clear water. Their root system also promotes binding of the substrate. In addition, seagrass meadows encourage deposition of suspended sediment, which leads to the supply of organic material and nutrients needed for growth.
Seaweeds and salt marsh plants are capable of modifying their physical environments by damping down wave energy and tidal currents. Salt marsh vegetation can also promote sediment deposition resulting in a gradual elevation of the marsh on the upper shore. The higher the marsh gets, the more vegetation can grow and the better the protection against erosion. Other stabilizing effects result from cementation of beach-rock by cyanobacteria and stromatolite formation by algae. Recent research of Temmerman et al. (2005, 2007) points out that the interaction of vegetation growth, tides and sedimentation is crucial in the formation of the typical levee-basin configuration of the saltmarsh landscape.
Biostabilization by animals
Some macrozoobenthos can actively filter sediment particles from the water column and deposit them on the bed. The presence of a mussel bank, for example, will alter the bed in different ways. Mussels at high densities will protect the underlying sediment from erosion by waves and currents (armouring effect). In addition, mussels are filter feeders and actively remove small suspended particles from the water column and any inorganic material (e.g. silt) is rejected as faeces and pseudofaeces. Some of this material will get deposited and incorporated into the bed, causing a shift in the composition to finer sediments and an increase in bed-level. Animal tube fields at high densities can also stabilize the sediment, because there is an accumulation of fine particles and organic matter between the tubes (Fig. 4). The tubes may also reduce near-bed flow and create skimming flow above thus creating a stabilizing effect. Mucus production by the community of microorganisms, meiofauna and macrofauna between the tubes may also contribute to sediment stability.
Benthic fauna may destabilize the substrate by their burrowing and surface deposit feeding activity (e.g. bivalves, snails and crustaceans; Fig. 5). They also destabilize the sediment as a result of grazing on biostabilizers such as microalgae. The constant mixing and recycling of sediment in the top centimeters of the bed, known as bioturbation, results in a change in the vertical particle size profile. Selective uptake of preferred particle sizes and their subsequent defaecation results in sorting and pelletizing of sediments. Together, the burrowing and the constant movement within the substrate, results in the generation of a surface micro-relief that has a higher hydraulic roughness, which increases turbulence and increases the potential for erosion. Furthermore, bioturbation also affects the sediment water content, porosity and sediment cohesion. Research of Paarlberg et al. (2005) has shown that impacts of (de)stabilizing biota on mudflat morphology and bed composition can be significant and can be quantified by adding a limited number of algorithms to existing hydro-morphological models. Borsje et al. (2007, submitted to ECSS) show that the influence of (de)stabilizers might extend beyond the mudflat scale to the estuary scale in cases of meso-tidal highly productive systems, such as the Wadden Sea.
Examples of biota acting as ecosystem engineers in the intertidal (Fig. 6). Key biota are divided into two functional groups, the bio-stabilisers and bio-destabilisers, with varying spatial distribution along estuarine gradients (both axial and vertical).
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