Treatise on Geomorphology || 12.13 Ecogeomorphology of Tidal Flats

12.13 Ecogeomorphology of Tidal Flats S Fagherazzi, DM FitzGerald, RW Fulweiler, and Z Hughes, Boston University, Boston, MA, USA PL Wiberg and KJ McGlathery, University of Virginia, Charlottesville, VA, USA JT Morris, Belle Baruch Institute for Marine & Coastal Sciences, University of South Carolina, Columbia, SC, USA TJ Tolhurst, School of Environmental Sciences, University of East Anglia, Norwich, UK LA Deegan and DS Johnson, Marine Biological Laboratory, Woods Hole, MA, USA 12 hy 2 12 3 12 8 12 8 12 9 12 9 12 9 12 9 12 9 12 d P 1 12 2 12 2 12 2 12 2 12 2 12 rt I 3 12 3 12.13.3.2 Vegetation Density Effects 214 12.13.3.3 Feedbacks and Bistability 215 Extracellular polymeric substances (EPS) A mucilaginous extracellular carbohydrate matrix secreted by organisms in biofilms (diatoms, cyanobacteria, and green algae). The EPS forms a cohesive matrix that the ocean. Nutrient enrichment Increased supply of nutrients, such as nitrogen and phosphorus, to an ecosystem, often caused by human activities. Fagherazzi, S., FitzGerald, D.M., Fulweiler, R.W., et al., 2013. Ecogeomorphology of tidal flats. In: Shroder, J. (Editor In Chief), Butler, D.R., Hupp, C.R. (Eds.), Treatise on Geomorphology. Academic Press, San Diego, CA, vol. 12, Ecogeomorphology, pp. 201–220. forms extensive biofilms. (mostly diatoms and cyanobacteria) living at the bottom of Diatoms Unicellular algae with a silica cell wall, which Microphytobenthos The assemblage of microalgae in soil or carried by water in suspension. submergent, or floating. Detritus Dead particulate organic material, accumulating apparent to the naked eye. Macrophytes may be emerge nt, photosynthesis, also called blue-green algae. Macrophyte Aquatic plants that are large enough to b e Cyanobacteria Bacteria that obtain energy through lenses). by animals and plants. muddy sediments enclose isolated sandy layers (sand Bioturbation Mixing and reworking of sediment particles Lenticular beds are typical of tidal environments where surface. point and thins away in every direction, forming a lens. green algae) and their extracellular products bound to a Lenticular bed A sedimentary layer that is thickest at o ne Biofilm Microorganisms (diatoms, cyanobacteria, and deposits are larger than the mud deposits. ocean bottom. intermittent flows in tidal environments. In flasers, the sa nd Benthic macroalgae Macroscopic algae that cover the ripples partly infilled with mud, usually created by of organic material from roots and rhizomes. Flasers Sedimentary structures characterized by sand Belowground production The belowground production environment. Glossary helps to structure the sediment microbial References Acknowledgments 216 216 Tre r 2013 Elsevier Inc. All rights reserved. .13.1 Physiography, Sedimentology, and Stratigrap .13.1.1 Tidal Flats Deposits .13.2 Biofilms in Tidal Flat Sediments .13.2.1 What are Biofilms? .13.2.2 Diatom Biofilms .13.2.3 Cyanobacterial Biofilms .13.2.4 Green Algal Biofilms .13.2.5 Sediment Stabilization by Biofilms .13.2.6 Extracellular Polymeric Substances .13.2.7 Effects of Biofilms on Physical Properties an .13.2.8 Biotic Mediation of Bedforms .13.2.9 Destabilization – Buoyant Biofilms .13.2.10 Biofilms and Rainfall .13.2.11 Biofilms as Geomorphological Agents .13.2.12 Biofilms Biogeochemistry .13.3 Tidal Flats Vegetation and Sediment Transpo .13.3.1 Modification of Near-Bed Hydrodynamics atise on Geomorphology, Volume 12 http://dx.doi.org/10.1016/B978-0-12-374 of Tidal Flats 20 20 20 20 20 20 20 20 20 rocesses 21 21 21 21 21 21 nteractions 21 21 739-6.00403-6 201 s. Ti rans ics nd nde nam ntr oce d se in deltaic regions (i.e., Irrawaddy Delta, Myanmar and Niger Delta, ern An macro Michel Tidal fl abund accreti where have b sedime erned coast from onshore, longshore, and offshore sources, or fo ac (F be se in pr ge high terrigenous input, such as the Niger Delta and the mouth in the nergy com- vated, mud. ound, sized long; tidal flats are highly separated from one another (Paranagua, Parana, Brazil, 40-km long, Netto and Lana, 1997) (Figure 3). tidal flats gradually coarsens toward the mouth of the estuary, reflecting an increase in wave and tidal energy (Figure 4, Netto c s rt n e e s 202 Ecogeomorphology of Tidal Flats rmed in place biogenically or chemically. Sedimentation fills commodation space creating the broad tidal flat platform igures 1 and 2); flats also form as a thin veneer overlying the drock or other planar surfaces (Semeniuk, 1981). Although tidal flats exist in low wave-energy settings, dimentation processes are primarily a product of wave- duced erosion and subsequent transport by tidal currents. In otected environments, waves are small and locally wind nerated. Wave energy at tidal flats along open coasts is also and Lana, 1997). Likewise, sorting increases, whereas organi content decreases. Early studies of tidal flat sedimentation used the concept of settling and scouring lags to explain the landward transpo of particles on tidal flats (Postma, 1961, 1967; van Straate and Kuenen, 1957). Postma (1967) showed that th time–velocity asymmetry of the currents could also help in th landward transport of sediment. Since that time, numerou modation space and (2) volume of sediment delivered to the As seen in the Paranagua Estuary, the mean grain size of the nt (Ruz et al., 1998). The extent of tidal flats is gov- by: (1) antecedent topography, which dictates accom- Nigeria), landward of wide shallow shelves (i.e., west- dros Island, Bahamas), and along open coasts having tidal ranges (i.e., Bay of Fundy, Nova Scotia; Mont Saint , France; and German Bight, North Sea) (Figure 1). ats are primarily depositional environments where an ant supply of sediment produces vertical and lateral on (Reineck, 1972). An exception occurs in Hudson Bay the coast is undergoing isostatic rebound and tidal flats een created through the erosion of uplifted deepwater of the Amazon. Typically, tidal flats exhibit a zonation grain size that reflects a gradual decrease in wave e whereby the lower, more exposed parts of the flat are posed of sand and the increasingly landward, more ele and lower wave-energy portions of the flats transition to This trend is apparent in small bays (Manitounuk S Hudson Bay, 0.7-km wide; Ruz et al., 1998), moderately (18-km long; Lee et al., 2004; Jade, Germany, 15-km Reineck and Singh, 1980), and in deep estuaries where Rhizomes An underground stem, usually horizontal, from which roots and shoots can grow. Ridge and runnel Mudflat bedforms characterized by parallel ridges separated by shallow troughs (runnels). These bedforms are usually equispaced with a depth of tens of centimeters. Root scalping The mechanical removal of the canopy mat, including roots, from a surface (for example, from a salt marsh platform). Abstract Coastal landscapes are often dominated by extensive tidal flat are typically depositional environments that store sediments t support a diverse biota that modifies the erosive characterist Biofilms are a ubiquitous feature of intertidal mudflats a substances and formation of a tough layer protecting the u important role in regulating near-bed flow and particle dy bilization, and turbidity of the water column ultimately co chapter, an overview of the sedimentological and physical pr effects of biofilms and seagrasses on tidal flat substrates an 12.13.1 Physiography, Sedimentology, and Stratigraphy of Tidal Flats Tidal flats are low-relief sedimentary environments that are flooded and drained during the rise and fall of the tides. They are composed of a variety of sediment types and are found throughout the world in a wide range of coastal settings having low wave energy, such as behind barrier islands (i.e., Friesian Islands, North Sea; Outer Banks, North Car- olina), inside deep embayments (i.e., Jade, Germany; Skagit Bay, Washington; Shark Bay, Australia; and Missionary Bay, Hinchinbrook Island, Australia), at the mouth of estuaries (i.e., Chesapeake Bay, East Coast of USA and The Wash, UK), Seagrass meadows Marine flowering plants, with long and narrow leaves, that grow in large patches (meadows). Skimming flow The flow above a vegetation canopy. In a skimming flow the entire velocity profile, and related boundary layer, is displaced upward. Wavy bedding Sedimentary structures characterized by alternating rippled sand and mud layers. In wavy bedding, mud and sand deposits are equal. dal flats are characterized by near-horizontal topography and ported by rivers and nearshore currents. These environments of the substrate and mediates sediment transport processes. stabilize the bottom by secretion of extracellular polymeric rlying sediments. Seagrasses on subtidal flats also play an ics. A positive feedback between seagrasses, substrate sta- ols the morphological stability of these landforms. In this sses acting on tidal flat sediments have been presented. The diment transport processes are then introduced. low because waves are attenuated as they propagate across wide low-gradient inner shelves. High wave-energy events can occur due to the passage of infrequent, large magnitude storms, such as in Shark Bay when hurricanes rework tidal flat sediments to form chenier ridges (M. O’Leary, Personal Communication). Tidal flats are locally affected by small tidal creeks (Figure 2) or can be dominated by tidal processes where the tidal flats abut or are surrounded by major tidal channels, such as those that occur within macrotidal embay- ments (i.e., Bay of Fundy; Dalrymple and Zaitlin, 1989). In temperate zones and high latitudes, intertidal flats consist primarily of siliciclastics, contrasting to the tropics where they are dominated by carbonates, except in regions of Ecogeomorphology of Tidal Flats 203 Shark Bay studies have focused on the process–response of tidal flats and the resulting sedimentary structures and facies relationships, including work along the German coast (Reineck and Singh, 1980; Reineck and Wunderlich, 1968), at the Wash in England (Evans, 1965), and in the Bay of Fundy (Klein, 1967; Knight and Dalrymple, 1975). In addition, there have been several reference compilations dealing with tidal flats deposits (Reineck and Singh, 1980; Klein, 1977; Ginsburg, 1975). Niger Delta (a) (b) (c) Figure 1 Aerial photographs of tidal flats around the world: (a) Shark Bay Delta, Nigeria; (d) Coos Bay, OR, USA. 12.13.1.1 Tidal Flats Deposits The inundation and draining of tidal flats dictates that tidal currents in channels and creeks are constantly accelerating and decelerating and wave-induced shear stresses on flats are likewise increasing and decreasing as a function of water depth and wave energy. This cyclic regime in energy results in alternating transport conditions between the bedload and suspended load. Sand is deposited during the periods of Coos Bay (d) Pleasant Bay , western Australia; (b) Pleasant Bay, Cape Cod, MA, USA; (c) Niger (a) (b) (c) (d) (e) (f) Figure 2 Tidal flat ground photographs: (a) Brewster tidal flats; (b) Stewart Island flats New Zealand; (c) Rills on mud; (d) Mudflat; (e) Fiddler crabs on flat; (f) Ghost shrimp holes on tidal flat. 204 Ecogeomorphology of Tidal Flats 5 Ecogeomorphology of Tidal Flats 205 37°00′N N current or wave-induced bedload transport and mud is deposited during low energy and slack water conditions. Interbedded and interlaminated sand and mud are one of the diagnostic characteristics of tidal flat deposits. Reineck and Wunderlich (1968) have classified the primary sedi- mentary structures of mixed-sediment flats as having two end 36°55′N 36°50′N 126°25′E 0 1 2 3 Km 4 5 Sand Muddy sand Sandy mud 126°20′E Mud 5 8° 55°32′N Low tide zone: sand Mid tidal flat zone: silty sand Channel zone: sand Upper tidal flat zone: silt clay Manitounuk 55°33′N 77°16′W (a) (c) Figure 3 Sediment zonation in tidal flats: (a) Garolim Bay. Reproduced from macrotidal flats in Garolim Bay, west Korea: significance of wind waves and (b) Jade, North Sea. Reproduced from Reineck, H.E., Singh, I.B., 1980. Depo and Gadow, S., 1970. Sedimente und Chemismus. In: Reineck, H.-E. (Ed.), D pp. 23–35, with permission from Springer; (c) Manitounuk Bay. Reproduced Sedimentology and evolution of subarctic tidal flats along a rapidly emerging 1242–1254. 3°31′50″N N members: (1) high-energy ripple cross-bedded sand with in- frequent mud flasers and (2) low-energy muddy layers having occasional sandy lenticular beds. Transitioning between flasers and lenticular beds is wavy bedding in which sand and mud layers alternate to form continuous thin beds or laminae (Figure 5). 3°23′51″N 03′05″E 8°18′50″E 0 1 2 3 Km 4 Fine sand Muddy sand Mud N Kuugaapik River 77°17′W Meters 0 500 (b) Lee, H.J., Jo, H.J., Chu, Y.S., Bahk, K.S., 2004. Sediment transport on asymmetry of tidal currents. Continental Shelf Research 24, 821–832; sitional Sedimentary Environments. Springer-Verlag, Berlin, 439 pp, as Watt, Ablagerungs- und Lebenstraum. W. Kramer, Frankfurta. M., from Ruz, M., Allard, M., Michaud, Y., He´quette, A., 1998. coast, Eastern Hudson Bay, Canada. Journal of Coastal Research 14, 48° 40′W 25° 25′S 30′ 20′ Br az il 1 2 Antonina 3 4 5 6 8 9 10 11 13 Cotinga Island N Mel Island 12 15 14 16 17 18 20 19 Paranagua Bay ParanaguaPedras Island 5 Km 7 30′ 35′ 35 30 25 20Sa lin ity 15 10 1 5 10 15 20 30 1 5 10 15 20 25 20 15 O rg an ic co nt en t (% ) 10 5 0 2.5 2 1.5 1 0.5 So rti ng 0 1 5 10 15 20 6 5.5 5 4.5 4 3.5 G ra in s ize (φ ) 3 2.5 2 1 5 10 15 20 100 1 5 10 15 20 80 60 40 Sa nd (% ) 20 0 100 80 60 40Si lt (% ) 20 0 1 5 10 15 20 5 4 3 2 Cl ay (% ) 1 0 1 5 10 15 20 2 1 5 Inner Station number 10 15 20 Outer 1.6 1.2 0.8 Ca rb on at e (% ) 0.4 0 Figure 4 Sediment trends in the Paranagua Estuary in Parana, Brazil. Reproduced from Netto S.A, Lana, P.C., 1997. Influence of Spartina alterniflora on Superficial sediment characteristics of tidal flats in Paranagua Bay in Southeastern Brazil. Estuarine Coastal and Shelf Science 44, 641–648. 206 Ecogeomorphology of Tidal Flats Ecogeomorphology of Tidal Flats 207 In creeks and tidal channels within tidal flats, migrating bedforms produce cross-bedding indicative of the dominant current direction. Bedforms range in size from ripples (lo0.6 m) to megaripples (0.6olo6.0 m). In channels where ebb and flood current velocities are nearly equal and migrating, bedforms are partly preserved, herring-bone stratification is produced, and is a diagnostic characteristic of a tidal environment. Klein (1970) showed that in sandy tidal flat systems, bedforms migrate in the direction of the stronger current velocity, but when the current reverses, the bedform crest is partially eroded producing a trun- cated surface. This becomes a reactivation surface when the bedform resumes its dominant migration trend (Figure 6). (a) (c) Figure 5 Examples of primary sedimentary structures of tidal flats: (a) Fla bottom; (c) Wavy bedding; (d) Lenticular bedding. Reproduced with permis Environments. Springer-Verlag, Berlin, 439 pp. Tidal flats are reworked by the migration and meandering of small creeks (depth o2 m) as well as major tidal channels (depth¼4 to 410 m). Reineck (1972) reports that com- parison of historical maps behind Wangerooge Island along the German North Sea indicates that 58% of the tidal flats in this region were reworked during a 68-year period (1879–1947) to depths of 0.5 to 46 m. Commonly, the surface of tidal flats is also reworked by organisms, which can completely or partially destroy the primary sedimentary structures. Typically, bioturbation structures are most abun- dant in muddier sediments rather than the sandier portion of the flats due to the greater energy, which inhibits (b) (d) ser bedding; (b) Flasers at the surface and wavy bedding at the sion from Reineck, H.E., Singh, I.B., 1980. Depositional Sedimentary rre inan Ripple mi Dom 208 Ecogeomorphology of Tidal Flats (a) (b) Destructional event Subord Erosion Constructional event R Constructional event Dominant cu organisms; however, mixed sediments can be highly biotur- bated as well. The upper portion of tidal flats transitions to supratidal regions have a range of environments from salt marshes (temperate regions) and mangroves and sabkhas (tropics) to carbonate platforms and beaches and barriers. In a regressive system, tidal flats exhibit a fining upward sequence containing a basal shelly lag produced by reworking of tidal channel sediments. This layer is overlain by cross-bedded sand transi- tioning to a mixed sediment containing flasers, wavy bedding, and lenticular beds. Channel cut and fills can also be present. The sequence is capped by mud and finally supratidal sedi- ment. Carbonate settings produce tidal flat facies dominated by fine-grained sediment, shells, cemented layers, algal laminations, intraclasts, bioturbation structures, channel cut, and fills (Shinn et al., 1969). 12.13.2 Biofilms in Tidal Flat Sediments Estuaries and other low-energy coastal environments are often characterized by intertidal mudflats that were once considered wastelands (Stal and De Brouwer, 2003). It is now known that these are dynamic, highly productive environments, capable of supporting a diverse biota, and provide numerous ecosystem (c) R R Figure 6 Formation of reactivation surface by bedform migration, erosion, a 1970. Depositional and dispersal dynamics of intertidal sand bars. Journal of t current direction gration inant current direction R nt direction services. Each day, tidal flat sediments experience strong variations in light intensity as well as alternating periods of tidal flooding and ebbing, and aerial exposure. How these factors interact determines the geomorphology, ecology, and biogeochemistry of tidal flats. Biofilms are a ubiquitous feature of intertidal sand and mudflats, which are particularly im- portant because of their role as primary producers and in me- diating numerous processes. The microorganisms in biofilms mediate both small scale (i.e., chemical zonations) and large scale (i.e., sediment stability) properties and processes of tidal flats. Although the presence of biofilms on tidal flats is com- mon, still much is left to learn about how these organisms interact with each other and the surrounding environment. For example, how biofilms and tidal flats will respond to anthro- pogenic impacts such as nutrient loading and climate change (i.e., increased water temperature, rising sea level, etc.) is cur- rently of particular relevance. This section will primarily con- sider the effects of photosynthetic biofilms on intertidal flats. 12.13.2.1 What are Biofilms? Biofilms are a prominent (although not always visible to the naked eye) feature of tidal flats. They are composed of both autotrophic and heterotrophic microorganisms. The photo- synthetic eukaryotic and cyanobacteria group of organisms in Reactivation surface nd subsequent bedform migration. Reproduced from Klein, G. de V., Sedimentary Petrology 40, 1095–1127, with permission from Springer. A major component of biofilms is water, anywhere up to 99%. On intertidal flats, biofilms are a generally thin (mm to mm across the sediment–water/atmosphere interface making them izing effect often results in diatom biofilms being raised up tough layer or ‘skin’ protecting underlying sediments (see et al., 1993; Tolhurst et al., 2002; Friend et al., 2003; de Ecogeomorphology of Tidal Flats 209 compared with surrounding areas without a visible biofilm (Figure 7(a)). Biofilm-forming diatoms tend to be found in muddier sediments, but can occur in sandy sediments. 12.13.2.3 Cyanobacterial Biofilms Cyanobacteria are bacteria that can photosynthesize, getting their name from the bluish pigment phycocyanin (Figure 7(c)). Colonial species form filaments that ramify through the sedi- ment surface forming thick leathery biofilms (Figure 7(d)). The filaments are able to move, migrating toward light (Stal, 2010). Cyanobacteria also secrete EPS and are particularly effective at stabilizing sediment. Intertidal cyanobacteria tend to be found in sandier sediments, but can occur in muddy sediments. 12.13.2.4 Green Algal Biofilms Although arguably not biofilms in the strictest sense, fila- mentous green algae can form mats at the surface, which in vital components in the structure, functioning, and dynamics of intertidal flats. There are a variety of different types of biofilm organisms, some of which are described below. 12.13.2.2 Diatom Biofilms Diatoms are unicellular algae with a silica cell wall, which form extensive, generally brown or golden brown coloured, biofilms (Figures 7(a) and 7(b)). They are motile and can move from the sediment surface deeper into the sediment. They often migrate below the sediment surface just before the tide comes in to avoid being eroded and washed away, or to obtain essential nutrients (Tolhurst et al., 2003). When ex- posed to the air during low tide, they can migrate into the sediment to avoid high light intensities, which could damage the cell (Mouget et al., 2008). Although the exact mechanism remains poorly understood, their movement seems to rely on the secretion of extracellular polymeric substances (EPSs) (Edgar and Pickett-Heaps, 1984). They also secrete EPS as a part of unbalanced growth and as a food source for later use (de Brouwer and Stal, 2002a). These EPS secretions play a vital role in stabilizing sediments (see later sections). This stabil- thick) surface layer, consisting of the microbes, their exudates, and sediment particles. Photosynthetic intertidal biofilms must be exposed to sufficient sunlight for the cells to be able to photosynthesize, so they form at or near the sediment surface. Here they are alternately exposed to overlying water and air over a tidal cycle (although some organisms are motile and can descend below the sediment surface if required). Biofilms on intertidal sediments act as a barrier between the sediment and the overlying atmosphere/water and are often described as a ‘skin.’ They mediate biological, physical, and chemical processes biofilms are known as benthic algae or microphytobenthos (MPB). There are a variety of definitions of the term ‘biofilm’ (or microbial mat), but they are basically microorganisms and their extracellular products bound to a surface (Neu, 1994). Brouwer et al., 2002b, 2005). The EPS forms a cohesive matrix that helps to structure the sediment microbial environment, which controls the physical properties of the biofilm that can range from a loose slime to a compact, cohesive gel (Decho, 2000). EPS acts to both enhance the physicochemical cohesive forces between cohesive sediments and, at a larger scale, coats sediment particles, binding them together like a glue. This has two main effects (1) it acts to encourage flocculation of fine sediment, enhancing its deposition and (2) it increases the strength of the sediment bed (Decho, 1990; Sutherland et al., 1998a, b; Underwood and Paterson, 1993). The effects of EPS on sediment strength are obvious when natural sediments are cleaned to remove the EPS. Natural muddy sediments are viscoplastic, but when cleaned and Paterson, 1997 and Black et al., 2002 for more details). These often occur in concert making the relative contri- bution of each difficult to elucidate. The erodability and sta- bility of sediments also depends on the sedimentological properties of the substrate, including particle size, water con- tent, bulk density, organic content, depositional history, and air exposure (Willows et al., 1998; Tolhurst et al., 2006a). 12.13.2.6 Extracellular Polymeric Substances Biofilm organisms, which include heterotrophic bacteria, have been shown to increase sediment stability (Grant and Gust, 1987; Neumeier and Amos, 2006; Lanuru et al., 2007; Lundk- vist et al., 2007; Friend et al., 2003). Many of these organisms secrete a mucilaginous extracellular carbohydrate matrix col- lectively called EPS (Underwood et al., 1995). Many studies have shown a positive correlation between sediment stability and extracellular carbohydrates (Dade et al., 1992; Madsen many ways resemble true biofilms (Figures 7(e) and 7(f)). Filamentous green algae are not motile, but grow across the surface of the sediment. They cannot, therefore, migrate back to the sediment surface if they are buried by deposited sedi- ment. Filaments ramify below the sediment surface, pre- sumably representing older cells that have been gradually buried by deposition of sediment as the algae grow. These algae do not secrete copious amounts of EPS, so they do not stabilize the substrate as effectively as diatoms or cyano- bacteria, but their filaments are still able to trap and stabilize sediment. This results in raised areas of sediment very similar to those formed by diatoms or cyanobacteria (Figure 7(e)). Intertidal green algal biofilms are found in both muddy and sandy sediments. 12.13.2.5 Sediment Stabilization by Biofilms Various processes and properties of intertidal flats are mediated by biofilms, but probably one of the most important influences of biofilms on the geomorphology of intertidal flats is to sta- bilize intertidal sediments, making them harder to erode (Paterson, 1989; Black et al., 2002; Larson et al., 2009). This is achieved by a number of mechanisms and processes, among them: (1) secretion of EPS, (2) smoothing of the sediment surface roughness, (3) network effects, and (4) formation of a 210 Ecogeomorphology of Tidal Flats reconstituted to the same water content they become fluid, with a consistency similar to that of milk or thin cream. EPS appears to be vital in maintaining the viscoplastic structure of natural cohesive sediments, imparting plasticity to the (a) (b) (c) (d) (e) Figure 7 (a) Diatom biofilm from Biezelingsche-Ham mudflat, the Netherlan surrounding areas without a visible biofilm. Scale bar is 4 cm. Image courtes of the surface of a diatom biofilm from the Biezelingsche-Ham mudflat, the N strands of EPS and a few small flocs of sediment (middle right hand side). S Note the blue–green colour and oxygen bubbles forming on the surface of th microscope image of Lyngbya spp. cyanobacteria filaments. Scale bar 40 mm colour and how the mat is raised up compared with areas without a mat, wh green algal filaments covering the sediment surface. Scale bar is 5 cm. sediment, even at large water contents. This is probably due, in part, to the action of these molecules in stabilizing water (the pore water) as well as their ability to increase the physico- chemical cohesion between sediment particles. (f) ds. Note the brown colour and how the biofilm is raised up above the y of Stal, L. (b) Low-temperature scanning electron microscope image etherlands. It consists of a number of different diatom species with cale bar¼10 mm. (c) Cyanobacterial biofilm from Cairns, Australia. e biofilm under a thin film of water on the far left hand side. (d) Light . (e) Green algal mat from Brays Bay, Australia. Note the bright green ere water pools. Small holes are crab burrows. (f) Close up of the Diatom secretion of EPS is strongly correlated to the motility of the diatom (Underwood and Smith, 1998). EPS secretion rates vary with species, growth phase, and physiological state (Decho, 1990; Smith and Underwood, 1998). In addition, the chemical composition of the secreted exopolymer can be dif- ferent between different species or change during growth and this has been connected to changes in sediment stability (Decho, 1990; de Brouwer et al., 2005). Finally, EPS production is intimately linked to the vertical migration patterns of epipelic diatoms (Smith and Underwood, 1998), which has been linked to temporal changes in sediment stability over minutes and hours, as the diatoms migrate to the surface during the day, or stay below the sediment surface at night (Tolhurst et al., 2003, 2006a; Friend et al., 2005). The production and extrusion of exopolymers is metabolically costly to organisms but it is also important in protecting the organisms against a variety of stresses (i.e. desiccation, Daniel et al., 1987; Savage and Fletcher, 1985 and toxin resistance, Decho, 1990). EPS pro- duction rates have been found to increase under environmental stresses (Yallop et al., 2000). For example, as nutrients become limiting in a mature biofilm, exopolymer production rates in- crease (Decho, 1990; Mylkestad, 1974). 12.13.2.7 Effects of Biofilms on Physical Properties and Processes One of the most notable effects of biofilms is the way they alter physical properties and processes in sediments. In abiotic muddy sediments, as water content decreases the strength (or stability) of the sediment increases. This is because as water is removed from the sediment, the fine-grained clay particles are brought closer together, increasing the strength of the inter- particle forces and leading to an increase in cohesion. Thus, as a muddy sediment becomes drier, it becomes stronger and more resistant to erosion. The presence of a biofilm can reverse this relationship. As a biofilm grows and becomes thicker, it in- creases the water content of the surface sediment (Tolhurst et al., 2008). It can do this in three different ways; first, the water contained within the cells themselves will increase the water content of any sample of sediment that includes the biofilm; second, by enhancing the deposition of loose fine- grained sediment flocs that have a large amount of pore space filled with water; and finally by ‘stabilizing’ water within the biofilm with EPS. Thus, biofilms tend to have large water contents. However, because this water is ‘contained’ (either rai a d sion Ecogeomorphology of Tidal Flats 211 EPS can also bind and therefore concentrate a variety of metal ions, including Cd2þ , Cu2þ , Cr3þ , Pb2þ (Decho, 2000). The binding affinity strength varies and depends on the composition of the EPS, the pH, and salinity. In estuarine environments, pH varies greatly because of freshwater inputs as well as photosynthesis and respiration (Decho, 2000). Typically, in acidic conditions (low pH) ions are released and conversely during basic conditions (high pH) ions are bound or chelation is favored. In the surface of intertidal sediments, chelation processes have distinct diel patterns that match the cycle of photosynthesis and respiration. Metal binding effi- ciency has also been related to the physical characteristics of the secretion. EPS in a gel state have been observed to more strongly bind metals than a looser, slimy EPS state. Thus, biofilms are vital in mediating heavy metal dynamics in intertidal flats. 0 200 400 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 G Er os io n th re sh ol d (N m − 2 ) Figure 8 Erosion thresholds for natural beach sands with and without stabilize the sediment decreases. The empirical curve is the Shields ero within the cells or stabilized by EPS), the sediment remains plastic at much larger water contents than it would if there was no biofilm. This results in natural sediments often having large water contents and being very strong (Tolhurst et al., 2008), the opposite of what would be expected in abiotic sediments. Diatoms tend to be found in muddy sediments, although there is something of a chicken and the egg situation; are diatoms preferentially living in muddy sediments, or are they merely enhancing the deposition and retention of fine grains? Experiments have shown that diatoms are good at stabilizing fine-grained sediments, but less effective in stabilizing coarse- grained sediments (Figure 8). It is currently unclear why this is, but it has probably something to do with the surface area to volume (and hence mass) relationship; as particles become larger, the effect of the EPS binding becomes proportionally less. Measurements in the Ems Dollard show that patches of 600 800 1000 Empirical curve Diatom biofilm No biofilm n size (μm) iatom biofilm. As grain size increases, the ability of the biofilm to threshold for sand. biofilms (Kornman and de Deckere, 1998). This is possible because the diatoms are capable of migrating up through processes acting to maintain the ridge–runnel feature. In sandier sediments, biofilms can act to prevent the 12.13.2.10 Biofilms and Rainfall sent), it would seem that more finer grained sediment is de- 12.13.2.12 Biofilms Biogeochemistry 212 Ecogeomorphology of Tidal Flats sediment bed from responding as expected to changes in hydrodynamic conditions (Grant et al., 1986; Friend et al., 2008). Ripple marks can become colonized by diatoms, which stabilize the bedform so that it no longer moves under hydrodynamic forcing; the sediment bedform is no longer determined by the existing hydrodynamics, but is a relic of previous conditions. Alternatively, biofilms can stabilize flat sediments; changes in hydrodynamic conditions result in ripples forming in adjacent areas, which then migrate over the top of the stabilized biofilm sediments. 12.13.2.9 Destabilization – Buoyant Biofilms Diatom and cyanobacterial biofilms often have a line of weak- ness below the main biofilm along which erosion can occur. This may represent the old sediment bed surface. In some cases, this line of weakness is formed (or enhanced) by the formation and trapping of oxygen bubbles within the biofilm. Underwater, these become buoyant and can lift away whole chunks of sediment deposited on top of them. Scaled up to a whole estuary, this represents tons of sediment that would otherwise be in the water column. All this would suggest that diatoms are acting to deposit, stabilizes, and retain fine sediments, with processes occurring at a scale of centimeters on the tidal flat being able to affect SSC estuary wide. It seems likely, therefore, that although diatoms may recruit to finer grained sediments, the development of a biofilm also acts to enhance fine-grained deposition and retention, reducing the grain size of the sedi- ments where diatoms grow. 12.13.2.8 Biotic Mediation of Bedforms One thing that should always be borne in mind when in- vestigating intertidal flats is that the various biological, phys- ical, and chemical processes are not acting in isolation, they may be synergistic or antagonistic. An example of this seems to be the formation of ridge–runnel systems, shore normal channels, and humps found on some intertidal mudflats. Diatoms occur in greater amounts on the ridges, which drain quickly and are therefore drier than the runnels, which have less diatoms and are wetter (often containing a surface film of water). It has been shown that sediment submerged under- water is less stable than the same sediment exposed to air and able to drain (Tolhurst et al., 2006b). Thus, the physical and biological features of ridges are acting in concert to increase their stability, whereas in runnels, they are acting to decrease stability; there is a synergy between the physical and biological biofilm contain considerably more fine-grained sediment than patches a few centimeters away with no visible biofilm. The biofilm areas are also raised a few centimeters above the ad- jacent sediments. Measurements of suspended sediment con- centrations (SSCs) in the estuarine water column have shown that SSC can decrease as a diatom biofilm forms, suggesting that the diatom biofilms are acting to strip sediment from the water column and retain it on the bed, creating the raised The importance of MPB in mediating properties and processes on intertidal flats makes understanding how and when they grow of vital importance. MPB growth on tidal flats can be responsible for more than 50% of total estuarine primary production (Underwood and Kromkamp, 1999). MPB primary posited on intertidal flats than would be the case if the biofilms were not present. The implication is that intertidal flats would be less extensive and sandier were not for biofilms. A study in Sydney, Australia, where algal biofilms were re- moved with an algicide, showed that erosion of fine-grained sediment was enhanced, leaving bare sandier sediment (un- published data). This is how intertidal sediments may have been before life and biofilms evolved. The effect of biofilms on intertidal sediments has con- siderable implications for the interpretation of sedimentary features in the geological record. Ripples may have been re- tained under hydrodynamic conditions very different to those they were formed in. Extensive muddy sediments may have been deposited under much more energetic conditions than those expected if purely physical processes were in action. Thus, biofilms are an integral component of the complex intertidal flat system, mediating numerous properties and processes (Murphy and Tolhurst, 2009). The interaction of biofilms with physical processes can be complex. Rain falling on intertidal flats erodes and mobilizes sediments, particularly fine-grained sediment (Mwamba and Torres, 2002; Pilditch et al., 2008; Tolhurst et al., 2006c, 2008). It might be expected that biofilms would act to prevent this, stabilizing the sediment against rainfall in a similar way to that by which they stabilize sediment against tidal flows. However, measurements show that rainfall is very effective at negating the stabilizing effect of diatoms (Paterson, 1997; Tolhurst et al., 2003). It is unclear exactly why this is, but it seems to have something to do with the combined effects of the physical impact of the rain drops and the introduction of freshwater reducing physicochemical bonding between particles, which is particularly dispersive to intertidal muddy sediments. Reduction in biofilm biomass and increase in rainfall in winter have been proposed as significant contributors to seasonal changes in sediment budgets on tidal flats (Pilditch et al., 2008). 12.13.2.11 Biofilms as Geomorphological Agents Given the various ways in which biofilms act to stabilize intertidal sediments, and given that all natural sediments contain these organisms (even if visible biofilms are not pre- biofilm with attached sediments under flow conditions much lower than that would otherwise be expected (Sutherland et al., 1998a; Tolhurst et al., 2008). We have collected chunks of diatom biofilm floating downstream after being disturbed by walking over the substrate, it is easy to see a similar process occurring due to disturbance from fish and/or wading birds. �2 �1 observed variability in MPB. Typically, muddy sediments are celerates, is a declining function of vegetation density, and this influences the variation in mean current flow between seagrass Ecogeomorphology of Tidal Flats 213 dominated by diatoms, whereas sandy sediments have more diversity that includes cyanobacteria and euglenids (Jesus et al., 2009). The depth distribution of benthic algae is also determined, in part, by the sediment type. Generally, in the top 2 mm of muddy sediment, a strong exponential decay of chlorophyll (a proxy for algal biomass) is seen, but in sandy sediments the chlorophyll is more evenly distributed and generally down to a few centimeters (Jesus et al., 2006). These differences are attributed to light attenuation patterns, as photosynthetically active radiation (PAR) is restricted to the top 2 mm (or less) in muddy sediments and several centi- meters deep in sandy sediments (Jesus et al., 2009). It has also been commonly observed that many MPB ex- hibit vertical migration often associated with water level and/ or light levels (Consalvey et al., 2004; Kromkamp et al., 1998). In response to these daily migrations and to the subsequent diurnal cycle of production and respiration, the entire sedi- ment microbial community and chemical zonations also show strong diurnal vertical migrations (Fenchel, 1969). In fact, vertical sediment profiles of pH, oxygen, hydrogen sul- fide, and nutrients exhibit sharp contrasts between illuminated and aerobic surface sediments to dark anaerobic subsurface sediments (Macintyre et al., 1996 and references therein). MPB communities have been described as a ‘filter’ of nu- trients at the sediment–water interface (Henriksen et al., 1980; Sundback and Miles, 2000; Sundback et al., 2000). They have been shown to alter nutrient fluxes by oxygenating surface sediments through photosynthesis and nutrient assimilation (An and Joye, 2001; Eyre and Fergusan, 2002; Sundback and Miles, 2000; Sundback et al., 2000; Tobias et al., 2003). MPB are dependent on water column nutrients and light and are thus sensitive to the daily changes in surface irradiance and tidal height. However, in contrast to other estuarine primary pro- ducers, they can remain active throughout the year (Kristensen, 1993). On an annual scale, MPB incorporate large amounts of nitrogen (N) and thus provide an important nitrogen retention mechanism in systems. In some cases, the photosynthetic pro- duction of oxygen by MPB promotes coupled nitrification– denitrification making the sediment a nitrogen sink. In this scenario, ammonium is oxidized to nitrate, while nitrate is re- duced to dinitrogen (N2) gas and diffuses out of the system (Sundback et al., 2004; Sundback and Miles, 2000). However, in systems with low nitrogen availability, MPB can effectively outcompete denitrifying bacteria for nitrate and in doing so decrease denitrification (Risgaard-Petersen, 2003; Sundback et al., 2004; Sundback and Miles, 2000). 12.13.3 Tidal Flats Vegetation and Sediment Transport Interactions Benthic vegetation on subtidal flats plays an important role in regulating near-bed flow and particle dynamics. Seagrasses, production rates range from 30 to 230 g C m yr (Krom- kamp and Forester, 2006). Such high productivity helps sustain a robust and diverse food web that provides critical food for migrating shorebirds and fish. MPB inhabit the uppermost surface of sediments at the sediment–water interface. Sediment type explains much of the patches of different sizes. Flow speeds within seagrass canopies typically are 2–10 times slower than outside the meadow (Gambi et al., 1990; Gacia et al., 1999), and also less variable (Heiss et al., 2000). Specific water velocities are often o10 cm s�1, but can be as high as 100 cm s�1 (Koch, 2001). Although there is general agreement that the slowing of current velocities enhances particle deposition within seagrass meadows, there are few direct measurements of sediment de- position and retention. Gacia et al. (1999) used sediment traps in Mediterranean P. oceanica meadows and showed that seagrass canopies slowed current velocities with intensities proportional to the canopy height, and that particle retention was up to 15 times higher than unvegetated sediment. They found that the particle trapping capacity of the meadow was related to the surface area of the leaves, leaf bending, and particle attachment to the leaf surface. attached or free-floating benthic macroalgae, and benthic microalgae (‘MPBs’) all influence sediment suspension and transport. Seagrasses are often referred to as ‘ecosystem engin- eers’ (Jones et al., 1994) because they can alter their physical environment by affecting currents, waves, and turbulence. The presence of dense vegetation in the benthic boundary layer alters the roughness of the bottom and affects the vertical vel- ocity profiles, reducing flows within the canopy as currents are deflected over the canopy. Gambi et al. (1990) distinguished between two hydrodynamic environments related to seagrass canopies, a canopy–water interface region characterized by high shear stress and high turbulence intensity, and a below- canopy region characterized by low shear stress and reduction of turbulence. The slower current velocities within the canopy result in decreased sediment suspension and increased particle deposition (e.g., Heiss et al., 2000; Peterson et al., 2004). 12.13.3.1 Modification of Near-Bed Hydrodynamics Seagrass shoot density and architecture influences how the aboveground vegetation slows flow velocities, traps sediment, and attenuates waves and turbulence (Fonseca and Fisher, 1986; Gambi et al., 1990; Gacia et al., 1999); and the extensive belowground roots and rhizomes help to stabilize the sedi- ment and increase resistance to storm and wave disturbance. Most studies on the effect of seagrasses on hydrodynamics and sediment transport have focused on larger subtidal species, such as Zostera marina, Thalassia testudinum, Syringodium fili- forme, and Posidonia oceanica (Fonseca and Fisher, 1986; Gambi et al., 1990; Gacia et al., 1999). However, smaller intertidal species (e.g., Zostera novazelandica) also can have a significant effect on sediment suspension (Heiss et al., 2000). Seagrass densities vary from patchy or low-density meadows (o100 shoots per square meter) in areas that are physically disturbed, have low water quality, or have been recently re- stored, to very dense meadows (41000 shoots per square meter). In flume studies, Peterson et al. (2004) showed that there were greater flow reductions inside the canopy with in- creasing vegetation density, and that there were significant differences in flow on the edges of meadows. Their model showed that the ‘edge effect,’ or the zone in which flow de- increased. This sediment stabilization is a seasonal phenom- enon at least in temperate systems, and the deposited material suspension (Friedrichs et al., 2000). For an individual seagrass shoot, vertical differences in velocities between the canopy and sediment surface, and horizontal differences in upstream and downstream velocities result in a pressure gradient downstream of the seagrass shoot. High pressure near the sediment surface and low pressure near the top of canopy where the currents are strongest leads to ascending flows on the downstream side of the shoot (Figure 9; Koch et al., 2006). This can result in bed scouring and enhanced sediment suspension, as well as pore water upwelling in permeable sediments (Nepf and Koch, 1999; Koch and Huettel, 2000). On tidal flats where benthic macroalgae are dominant, flow causes the thalli to move and scour the sediment when densities are low, increasing sediment suspension relative to bare sediments. Macroalgal transport as bedload has been documented in a number of systems, including the Lagoon of Venice, where Flindt et al. (1997) showed that measured transport rates of Ulva reached 300 gDWm�2 h�1 during peak tidal flows. This mechanism for sediment destabilization is akin to the well-documented phenomenon of saltating or abrading particles increasing erosion in cohesive sediments (e.g., Houser and Nickling, 2001; Thompson and Amos, 2002, 2004). Macroalgae that scrape the bed while moving across it can dislodge particles and increase sediment suspension and erosion. In cohesive beds, the critical stress required to initiate erosion is often greater than the stress required to maintain the sediment in suspension. Z U 214 Ecogeomorphology of Tidal Flats may be resuspended at times of the year when the microalgae are less productive (Widdows et al., 2002). 12.13.3.2 Vegetation Density Effects Most previous work on vegetation effects on sediment stabil- ization has been done on high-density populations, and less is known about the effects of lower densities on sediment fluxes, although it is common for seagrass and macroalgae to occur in low densities or to be patchy in distribution. Lawson (2008) found that low densities of both seagrass and macroalgae in- creased sediment suspension by as much as 97% compared with bare sediments, and that the threshold between destabil- izing and stabilizing effects of these benthic communities is density dependent and likely requires the generation of skim- ming flow. Dense seagrass meadows typically displace velocity vertically creating a protected area next to the sediment surface (Gambi et al., 1990). At low densities, flow is deflected hori- zontally around individual shoots and localized areas of high shear stress can develop around the shoots. The threshold between destabilizing and stabilizing effects on sediments in seagrass-vegetated tidal flats is likely dependent on the ratio of the distance between individual shoots and their height, as suggested by Vogel (1994) for emergent features. When the distance-to-height ratio is rela- tively large, the localized flow that develops around each feature is independent, whereas when the ratio is 1 or smaller, a vertically displaced skimming flow develops. This mech- anism has been used to explain the effects of other emergent features, such as polycheate worm tubes, on sediment The effect of the seagrass canopy on wave attenuation is still not well understood (Koch et al., 2006). Wave attenuation is likely highest when the canopy occupies a large proportion of the water column (Ward et al., 1984; Fonseca and Cahalan, 1992), but reduction in wave energy also has been docu- mented in deep meadows (Verduin and Backhaus, 2000; Granata et al., 2001). In wave-swept environments, seagrasses are exposed to more complex flows than when unidirectional flows are dominant. A long flexible shape is advantageous, as the leaves can sway back and forth with the water motion and thus minimize forces on the root/rhizome systems that anchor the plants in the sediment (Koch et al., 2006). Benthic algal populations also can stabilize tidal flat sedi- ments at times of year when the algae are growing actively and populations are physically stable. Dense populations of mat- forming species such as Ulva rigida and Enteromorpha intesti- nalis stabilize sediments by decreasing shear flow at the sedi- ment surface (Escartin and Aubrey, 1995) and reducing sediment suspension (Sfriso and Marcomini, 1997; Romano et al., 2003). Thick mats (equivalent to 3.5–6.2 kg wet weight per square meter) displace velocities vertically and can deflect as much as 90% of the flow over the mat, with only 10% of the flow traveling through the mat (Escartin and Aubrey, 1995). Benthic microalgal mats also stabilize surface sediments by producing and extruding EPS (e.g., Paterson, 1989; de Brouwer et al., 2006). EPS bind sediment particles together so that the shear stress needed for erosion of the sediment is Figure 9 Vertical differences in velocities between the canopy and sediment surface and horizontal differences in upstream and downstream velocities result in a pressure gradient downstream of the seagrass shoot. High pressure near the sediment surface and low pressure near the top of canopy where the currents are strongest leads to ascending flows on the downstream side of the shoot. This can result in bed scouring and enhanced sediment suspension, as well as pore water upwelling in permeable sediments. Reproduced with permission from Koch E.W., Ackerman J., van Keulen M., Verduin, J., 2006. Fluid dynamics in seagrass ecology: from molecules to ecosystems. In: Larkum, A.W.D., Orth, R.J., Duarte, C.M. (Eds.), Seagrasses: Biology, Ecology and Conservation. Springer-Verlag, Dordrecht, The Netherlands, pp. 193–225. Overall, for both seagrass and macroalgae, sediment sus- pension is influenced largely by whether the flow interacts with the primary producers as one solid feature (i.e., the meadow or mat) or as isolated, individual structures (shoots or fronds), despite their great differences in morphology (Lawson, 2008). For seagrass, canopy closure over the sedi- ment as the leaves bend with the current depends on the morphology of the blades, shoot density, and flow rates, and this influences sediment suspension (Koch and Gust, 1999). Similarly, suppression of sediment suspension for sediments overlain by macroalgae occurs when flow is deflected around dense algal populations, whereas scouring of the sediment bed occurs when flow is through the macroalgae, causing move- ment of the algae (Lawson, 2008). 12.13.3.3 Feedbacks and Bistability On subtidal flats in temperate environments, the growth of seagrass is often limited by light availability, which is affected by turbidity from suspended sediments and phytoplankton concentrations, and by colored organic matter (Gallegos, 2001). In shallow systems, turbidity is influenced strongly by internal sediment resuspension caused by waves and tides, and this is the dominant controlling factor in lagoonal and coastal bays systems that lack riverine inputs (Lawson et al., 2007). Rooted vegetation exerts positive feedbacks on water clarity by reducing the susceptibility of sediments to resus- pension, enhancing deposition of fine sediment, and immo- phytoplankton growth (Folkard, 2005; McGlathery et al., 2007). The absence of seagrass would increase sediment resuspension and water column turbidity, thereby making conditions less hospitable for the seagrass growth. These feedbacks may lead to alternative states for the tidal flat eco- system – either seagrass meadows with high water column clarity and enough light to support seagrass growth, or bare sediment beds with high levels of suspended sediments in the water column and light conditions unsuitable for the seagrass growth (van der Heide et al., 2007; Carr et al., 2010). Carr et al. (2010) developed a one-dimensional hydro- dynamic model of vegetation–sediment–water flow inter- actions and used it to investigate the strengths of the positive feedbacks between seagrass cover, stabilization of bed sedi- ments, turbidity of the water column, and the existence of a favorable light environment for seagrasses. The model in- cludes attenuation of wave orbital velocities due to the sea- grass canopy, combined wave and current stresses, and an active-layer bed formulation. Modifications to the velocity profile due to drag of seagrass stems and stem deflection are accounted for in the model, and an empirical relationship defining the light attenuation coefficient from total suspended solids, chlorophyll a, and gilvin is used to calculate PAR at the canopy surface. The results indicated that under typical con- ditions in a temperate shallow coastal lagoon, seagrass was stable in water depths o2.2 m (51% of the bay bottom deep enough for seagrass growth) and bistable conditions existed for depths of 2.2–3.6 m (23% of bay), where the preferred state depended on initial seagrass cover. The remaining 26% 5 Dep pe cif ic −2 t de ate. pro l lag Ecogeomorphology of Tidal Flats 215 1 2 3 4 N et s Bi st ab le −3 −4 Figure 10 Specific growth rate as a function of depth for varying shoo growth were evaluated by looking at the sign of the net annual growth r (unfavorable) conditions for seagrass establishment and persistence. Re 2010. 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He received his Masters in Geological Oceanography at y at the University of South Carolina. He is a marine geologist who studies and coastal evolution of marshes, estuaries, river deltas, barrier islands, and hemes are presently focused on coastal response to accelerating sea-level rise, ouisiana coast, and climatic and oceanographic controls on strand plain n coastal geology is used both nationally and internationally. He is a Fellow and has received numerous teaching awards at Boston University. Sciences, Boston University. He Environmental, Maritime, Geotec sediment armouring - an example from the Tagus Estuary, Portugal. Hydrobiologia 503, 183–193. microbial production, exopolymer production, microbial biomass, and sediment stability in biofilms of intertidal sediments. Microbial Ecology 39, 116–127. Biographical Sketch Sergio Fagherazzi is an associate professor of surface processes and marine sciences at the Department of Earth obtained a Doctoral Degree in Hydrodynamics in 1999 at the Department of hnical, and Hydraulic Engineering, University of Padua, Italy. His primary study interest concerns the evolution of coastal environments. His research activities cover all aspects of coastal mor- phodynamics, including wetlands erosion, hurricanes impact on sandy beaches and mitigation, tsunami effects olut fluxes and preferential recycling of benthic microalgal nitrogen in a shallow macrotidal estuary. Marine Ecology-Progress Series 257, 25–36. Tolhurst, T.J., Defew, E.C., de Brouwer, J.F.C., Wolfstein, K., Stal, L.J., Paterson, D.M., 2006a. Small-scale temporal and spatial variability in the erosion threshold and properties of cohesive intertidal sediments. Continental Shelf Research 26, 351–362. Tolhurst, T.J., Defew, E.C., Perkins, R.G., Sharples, A., Paterson, D.M., 2006b. The effects of tidally driven temporal variation on measuring intertidal cohesive sediment erosion threshold. Aquatic Ecology 40, 521–531. Tolhurst, T.J., Friend, P.L., Watts, C., Wakefield, R., Black, K.S., Paterson, D.M., 2006c. The effects of rain on the erosion threshold of intertidal cohesive sediments. Aquatic Ecology 40, 533–541. Tolhurst, T.J., Gust, G., Paterson, D.M., 2002. The influence of an extracellular polymeric substance (EPS) on cohesive sediment stability. In: Winterwerp, J.C., Kranenburg, C. (Eds.), Fine Sediment Dynamics in the Marine Environment. Elsevier Science Bv, Amsterdam, pp. 409–425. Tolhurst, T.J., Jesus, B., Brotas, V., Paterson, D.M., 2003. Diatom migration and Underwood, G.J.C., Smith, D.J., 1998. Predicting epipelic diatom exopolymer concentrations in intertidal sediments from sediment chlorophyll a. Microbial Ecology 35, 116–125. Verduin, J.J., Backhaus, J.O., 2000. Dynamics of plant-flow interactions for the seagrass Amphibolis antarctica: field observations and model simulations. Estuarine Coastal and Shelf Science 50, 185–204. Vogel, S., 1994. Life in Moving Fluids. Pergamon, New York. Ward, L.G., Kemp, W.M., Boynton, W.R., 1984. The influence of waves and seagrass communities on suspended particulates in an estuarine embayment. Marine Geology 59, 85–103. Widdows, J., Lucas, J.S., Brinsley, M.D., Salkeld, P.N., Staff, F.J., 2002. Investigation of the effects of current velocity on mussel feeding and mussel bed stability using an annular flume. Helgoland Marine Research 56, 3–12. Willows, R.I., Widdows, J., Wood, R.G., 1998. Influence of an infaunal bivalve on the erosion of an intertidal cohesive sediment: a flume and modeling study. Limnology and Oceanography 43, 1332–1343. Yallop, M.L., Paterson, D.M., Wellsbury, P., 2000. Interrelationships between rates of Robinson W Fulweiler is assistant professor of Earth Sciences at Boston University. She is a biogeochemist and coastal ecosystem ecologist. Her research included coastal watershed mass balances of major biogenic elements in New England (C, N, P, and Si), the biogeochemistry of nitrogen in coastal marine ecosystems, especially sedi- ments, and wetland ecology in coastal Louisiana. Her recent focus has been on how climate change may influence nitrogen fixation and denitrification in estuarine and shelf systems and anthropogenic impacts on the coastal silica cycle. erosion and accretion, sediment dynamics on tidal flats, effects of seagrass on turbidity in coastal lagoons, and the impacts of sea-level rise and climate change on the evolution of coastal landscapes. She currently serves as the chair of the Marine Working Group of the Community Surface Dynamics Modeling System. Ecogeomorphology of Tidal Flats 219 Karen J McGlathery is Professor of Environmental Sciences at the University of Virginia. Her research focuses on the effects of long-term change, including climate, sea-level rise, land-use and species invasions in coastal marine ecosystems. She received her BS from Connecticut College, her PhD from Cornell University, and she did her postdoctoral work at the University of Copenhagen and the National Environmental Research Institute in Den- mark. She is the Program Director of the Virginia Coast Reserve Long-Term Ecological Research site, which is one of 25 sites in the nation funded by the National Science Foundation to study long-term environmental changes in marine and terrestrial ecosystems. Karen serves on the editorial board of the journal Ecosystems. Patricia L Wiberg is Professor and Chair of Environmental Sciences at the University of Virginia, where she has taught since 1990. Wiberg received her PhD in oceanography from the University of Washington in 1987. Her research focuses on the mechanics of sediment erosion, transport, and deposition, as well as associated evolution of sediment bed properties and morphology. Her research topics include mechanisms and rates of saltmarsh Zoe Hughes is a senior postdoctoral research associate in the Department of Earth Sciences, Boston University. She received her PhD from the University of Southampton working at the National Oceanography Centre, South- ampton. Hughes uses a combination of modeling and field investigations to examine the impacts of waves, tidal currents and sea-level rise on coastal sediment transport, and geomorphology. Her recent work involves col- laborations with ecologists, undertaking interdisciplinary studies of feedbacks between hydrodynamics, sediment movement, and both flora and fauna. She is involved in a range of studies within coastal barrier and saltmarsh systems along the East and Gulf coasts of USA. James T Morris is the Director of the Belle Baruch Institute for Marine and Coastal Sciences, Professor of Biological Sciences, Distinguished Professor of Marine Studies at the University of South Carolina, and is an American Association for the Advancement of Science Fellow. He served as a Program Officer at the National Science Foundation in the Division of Environmental Biology from 2003 to 2005 and was a visiting professor at Aarhus University, Denmark in 1990. His academic background includes degrees in environmental sciences (BA, Uni- versity of Virginia), biology (MA, Yale), and forestry and environmental studies (PhD, Yale). He held a post- doctoral fellowship at the Marine Biological Laboratory, Woods Hole before taking a faculty position at the of St. Andrews, Scotland. His primary interests concern the sedimentology and ecology of intertidal sedimentary habitats. His research is multidisciplinary and field based, investigating the nature of direct and indirect inter- actions between sediments and biota, particularly how biota affects the erosion of sediments. He has helped develop various devices for measuring erosion, as well as remote sensing techniques for investigating microbial biofilms. advising nongovernmental organizations, especially Environmental Defense Fund, National Audubon Society, and National Wildlife Federation, on issues underlying the restoration of the Mississippi River delta in the Gulf of 220 Ecogeomorphology of Tidal Flats Mexico. David S Johnson is a research associate in the Ecosystems Center at the Marine Biological Laboratory in Woods Hole, MA, USA. He is a marine ecologist who is interested in species interactions and ecosystem processes. David is particularly fond of invertebrates. He received his PhD from Louisiana State University, Baton Rouge, LA, USA in 2008. Linda A Deegan is a Senior Scientist at The Ecosystems Center, Marine Biological Laboratory at Woods Hole. She obtained a doctorate in Marine Science in 1985 from Louisiana State University. Her primary interest is in how animals through their behavior and interactions shape ecosystem processes, including geomorphic dynamics and nutrient cycling. She works in many ecosystems, from temperate coastal wetlands to arctic and tropical streams. She has been a member of the editorial boards of Ecological Applications and Estuaries and Coasts. She currently serves on the Board of The Nature Conservancy and is a member of the Science and Engineering Special Team University of South Carolina in 1981. He currently serves on the Wetland Carbon Modeling working group at the National Center for Ecological Synthesis, the Special Science, Engineering, and Technology panel that is making recommendations on the restoration of the Mississippi River Delta, and at National Research Council Gulf Oil Spill committee. Trevor J Tolhurst is a lecturer in coastal processes at the School of Environmental Sciences, University of East Anglia, UK. He obtained a Doctoral Degree in Marine Biology in 2000 at the Gatty Marine Laboratory, University 12.13 Ecogeomorphology of Tidal Flats 12.13.1 Physiography, Sedimentology, and Stratigraphy of Tidal Flats 12.13.1.1 Tidal Flats Deposits 12.13.2 Biofilms in Tidal Flat Sediments 12.13.2.1 What are Biofilms? 12.13.2.2 Diatom Biofilms 12.13.2.3 Cyanobacterial Biofilms 12.13.2.4 Green Algal Biofilms 12.13.2.5 Sediment Stabilization by Biofilms 12.13.2.6 Extracellular Polymeric Substances 12.13.2.7 Effects of Biofilms on Physical Properties and Processes 12.13.2.8 Biotic Mediation of Bedforms 12.13.2.9 Destabilization - Buoyant Biofilms 12.13.2.10 Biofilms and Rainfall 12.13.2.11 Biofilms as Geomorphological Agents 12.13.2.12 Biofilms Biogeochemistry 12.13.3 Tidal Flats Vegetation and Sediment Transport Interactions 12.13.3.1 Modification of Near-Bed Hydrodynamics 12.13.3.2 Vegetation Density Effects 12.13.3.3 Feedbacks and Bistability Acknowledgments References