The Ecology of Seashores - Chapter 3 pps

85 3 Soft Shores CONTENTS 3.1 Soft Shores as a Habitat 87 3.1.1 Beach Formation 88 3.1.2 Sediment Characteristics 88 3.1.3 Currents, Wave Action, and Beach Formation 90 3.1.4 Exposure Rating 91 3.2 The Physicochemical Environment 92 3.2.1 Interstitial Pore Space and Water Content 92 3.2.2 Temperature 94 3.2.3 Salinity 94 3.2.4 Oxygen Content 94 3.2.5 Organic Content 95 3.2.6 Stratification of the Interstitial System 96 3.3 Soft Shore Types 97 3.4 Estuaries 98 3.4.1 What is an Estuary? 98 3.4.2 Special Features of Estuarine Ecosystems 101 3.4.3 Estuarine Geomorphology 104 3.4.3.1 Coastal Plain Estuaries (Drowned River Valleys) 104 3.4.3.2 Coastal Plain Salt Marsh Estuaries 105 3.4.3.3 Lagoon Type Bar-Built Estuaries 105 3.4.3.4 Lagoons 105 3.4.3.5 Fjords 105 3.4.3.6 Estuaries Produced by Tectonic Processes 105 3.4.4 Estuarine Circulation and Salinity Processes 105 3.4.4.1 Circulation Patterns 105 3.4.4.2 Classification of Circulation and Salinity Patterns 106 3.4.5 Estuarine Sediments 106 3.5 Soft Shore Primary Producers 107 3.5.1 The Microflora 107 3.5.1.1 Sand Beaches 107 3.5.1.2 Mudflats and Estuaries 107 3.5.1.3 Benthic Microalgal Biomass and Production 108 3.5.1.4 Factors Regulating Benthic Microalgal Distribution, Abundance, and Production 110 3.5.1.5 A Model of Estuarine Benthic Microalgal Production 111 3.5.1.6 Surf-Zone Phytoplankton 111 3.5.1.7 Epiphytic Microalgae 114 3.5.2 Estuarine Phytoplankton 115 3.5.2.1 Introduction 115 3.5.2.2 Composition of the Phytoplankton 116 3.5.2.3 Distribution and Seasonal Variation in Species Composition 116 3.5.2.4 Biomass and Production 117 3.5.2.5 Factors Regulating Estuarine Primary Production 119 3.5.2.6 A Model of Estuarine Phytoplankton Production 122 © 2001 by CRC Press LLC 86 The Ecology of Seashores 3.5.3 Estuarine Macroalgae 123 3.5.3.1 Composition and Distribution 123 3.5.3.2 Biomass and Production 124 3.5.3.3 Impact of Benthic Macroalgal Mats on the Benthic Fauna 125 3.5.4 Sea Grass Systems 126 3.5.4.1 Introduction 126 3.5.4.2 Distribution and Zonation 127 3.5.4.3 Biomass and Production 127 3.5.4.4 Factors Affecting Sea Grass Production 130 3.5.4.5 Fate of Sea Grass Primary Production 131 3.5.5 Salt Marshes 132 3.5.5.1 Introduction 132 3.5.5.2 Development, Distribution, and Zonation 132 3.5.5.3 Primary Production 133 3.5.5.4 Factors Affecting Production 136 3.5.5.5 Marsh Estuarine Carbon Fluxes 139 3.5.6 Mangrove Systems 143 3.5.6.1 Introduction 143 3.5.6.2 Distribution and Zonation 144 3.5.6.3 Environmental Factors 146 3.5.6.4 Adaptations 146 3.5.6.5 Biomass and Production 148 3.5.6.6 Litterfall 148 3.5.6.7 The Fate of Mangrove Leaf Litter 150 3.5.7 Relative Contribution of the Various Producers 153 3.6 Soft Shore Fauna 153 3.6.1 Estuarine Zooplankton 153 3.6.1.1 Introduction 153 3.6.1.2 Composition and Distribution 154 3.6.1.3 Temporal and Spatial Patterns 156 3.6.1.4 Biomass and Production 157 3.6.1.5 Factors Influencing Distribution and Production 158 3.6.2 Interstitial Fauna 159 3.6.2.1 The Interstitial Environment 159 3.6.2.2 The Interstitial Biota 159 3.6.2.3 The Meiofauna 160 3.6.2.4 Meiofaunal Recruitment and Colonization 162 3.6.2.5 Meiofaunal Population Density, Composition, and Distribution 162 3.6.2.6 Role of Meiofauna in Benthic Systems 164 3.6.2.7 Factors Involved in the Structuring of Meiofaunal Communities 166 3.6.3 Soft Shore Benthic Macrofauna 166 3.6.3.1 Introduction 166 3.6.3.2 Macrofaunal Zonation Patterns 167 3.6.3.3 Diversity and Abundance 173 3.6.3.4 Distribution Patterns of Estuarine Macrofauna 174 3.6.3.5 Epifauna 175 3.6.3.6 The Hyperbenthos 175 3.6.3.7 Soft Shore Macrofaunal Feeding Types 175 3.6.4 Estuarine Nekton 178 3.6.4.1 Introduction 178 3.6.4.2 Taxonomic Composition 178 3.6.4.3 Nektonic Food Webs 179 3.6.4.4 Patterns of Migration 180 3.6.4.5 The Estuary as a Nursery 181 © 2001 by CRC Press LLC Soft Shores 87 3.7 Biological Modification of the Sediment 181 3.7.1 Bioturbation and Biodeposition 181 3.7.2 Impact of Bioturbation on the Benthic Infauna 183 3.7.3 Influence of Macrofaunal Activity on the Chemistry of the Sediments 184 3.7.4 Influence of Macrofaunal Activity on Microbial Activities in Intertidal Sediments 186 3.8 Microbial Ecology and Organic Detritus 188 3.8.1 Introduction 188 3.8.2 Organic Matter 189 3.8.2.1 Sources of Organic Matter 189 3.8.2.2 Quantities of Particulate Organic Matter (POM) 192 3.8.2.3 Quantities of Dissolved Organic Matter (DOM) 193 3.8.3 River Input of Organic Carbon 193 3.8.4 Microbial Processes in Coastal Waters 195 3.8.4.1 Microbial Standing Stocks 195 3.8.4.2 Role of Microorganisms in Coastal Food Webs 195 3.8.5 Aerobic Detrital Decomposition 198 3.8.6 Microbial Processes in the Sediments 199 3.8.6.1 Sediment Stratification and Microbial Processes 199 3.8.6.2 Sediment Microbial Standing Stocks and Activity 200 3.8.6.3 Anaerobic Processes in the Sediments 203 3.8.7 Microorganisms and Detritus as a Food Resource 206 3.9 Nutrient Cycling 209 3.9.1 Introduction 209 3.9.2 The Nitrogen Cycle 209 3.9.2.1 Transformations of Nitrogen 209 3.9.2.2 The Coastal Nitrogen Cycle 210 3.9.2.3 Nitrogen Fixation, Nitrification, and Denitrification 211 3.9.3 Phosphorus Cycle 212 3.9.4 Sediment-Water Interactions in Nutrient Dynamics 214 3.9.4.1 Nutrient Fluxes across the Sediment-Water Interface 214 3.9.4.2 Causes and Mechanisms of the Migration of Nutrients at the Sediment-Water Interface 217 3.9.5 Nutrient Cycling in a High-Energy Surf-Zone Beach 218 3.9.6 Nutrient Cycling in Estuaries 219 3.9.7 Nutrient Cycling in Salt Marsh Ecosystems 219 3.9.8 Nutrient Cycling in Sea Grass Ecosystems 223 3.9.9 Nutrient Cycling in Mangrove Ecosystems 225 3.9.10 Models of Mangrove-Nutrient Interactions 227 3.10 Estuarine Shelf Interactions 228 3.10.1 Introduction 228 3.10.2 Some Case History Investigations 230 3.10.2.1 North Inlet, South Carolina 230 3.10.2.2 Beaufort, North Carolina 231 3.10.2.3 Mangrove Forest Systems 232 3.10.3 Conclusions 233 3.1 SOFT SHORES AS A HABITAT As mentioned previously, there is a gradation in shore type from rock through pebble and sand to mud, although mixed shores of sand or mud with rocky outcrops are common. The characteristics of the flora and fauna of hard (rocky) shores has already been discussed in detail. We now turn our attention to the other shore types, which are characterized by their relative instability. They are com- posed of particles of various sizes ranging from pebbles through coarse sands, fine sands to muds (silt and clay). In this chapter we shall consider the ways in which soft © 2001 by CRC Press LLC 88 The Ecology of Seashores shores are formed, their physical and chemical character- istics, the nature of the communities found in the various shore types, and their dynamic functioning. 3.1.1 BEACH FORMATION Hard shores are erosion shores cut by wave action. Soft shores, on the other hand, are depositing shores formed from particles that have been carried by water currents from other areas. The material that forms these depos- iting shores is in part derived from the erosion shores, but the bulk of the material, especially the silts and clays, is derived from the land and transported down the rivers to the sea. Beaches generally consist of a veneer of beach mate- rial covering a beach platform of underlying rock formed by wave erosion. On sand beaches the two main types of beach material are quartz (or silica) sands of terrestrial origin and carbonate sands of marine origin (particles weathered from mollusc shells and the skele- tons of other animals). Other materials that may con- tribute to beach sands include heavy minerals, basalt (of volcanic origin), and feldspar. On sand-mud and mud- flats, silts and clays of terrestrial origin and organic material derived from river input and from the remains of dead animals and plants contribute to the composition of the sediment. 3.1.2 SEDIMENT CHARACTERISTICS The most important feature of beach material particles is their size. Particle size is generally classified according to the Wentworth scale in phi units, where q = –log 2 diameter (mm). The Wenthworth classification is summarized in Table 3.1. A classification scheme (Figure 3.1) is generally used to describe differences in sediment texture by refer- ence to the proportion of sand, silt, and clay. Such classi- fications are essentially arbitrary and many such gradings can be found in the engineering and geological literature. TABLE 3.1 Wentworth Scale for Sediments Generic Name Wentworth Scale Size Range Particle Diameter (mm) Gravel Boulder <–8 > 256 Cobble –6 to –8 64 to 256 Pebble –2 to –6 4 to 64 Granule –1 to –2 2 to 4 Sand Very coarse 0 to –1 1.0 to 2.0 Coarse 1 to 0 0.50 to 2.0 Medium 2 to 1 0.25 to 0.50 Fine 3 to 2 0.125 to 0.50 Very fine 4 to 3 0.0625 to 0.125 Mud Silt 8 to 4 0.0039 to 0.0625 Clay > 8 < 0.0039 FIGURE 3.1 Classification scheme for sediment texture according to percentage composition of silt, clay, and sand. (Redrawn from Parsons , T.R., Takashi, M., and Hargrave, B.T., Biological Oceanographic Processes, 2nd. ed., Pergamon Press, Oxford, 1977, 193. With permission.) © 2001 by CRC Press LLC Soft Shores 89 Analysis of sediment size fractions is generally car- ried out by passing the dried sediment through a set of sieves of varying sizes and weighing the fractions retained on the sieves. Following this, further graphical analysis is generally carried out by plotting cumulative curves on probability paper and calculating the parameters listed in Table 3.2 (Folk, 1966). An important property is the degree to which the sediments are sorted, i.e., of uniform particle size or varying mixtures of different sized parti- cles. Unless the sands are badly skewed, median and mean particle diameters are very similar, and for most ocean beaches are in the range of fine to coarse sand. The inclu- sive graphic standard deviation is the best index of the sorting of the sediments. Values below 0.5 indicate good sorting, values between 0.5 and 1.0 moderate sorting, and values above 1.0 poor sorting, with a wide range of par- ticle sizes present. Skewness measures the asymmetry of the cumulative curve, and plus or minus values indicate excess amounts of fine or coarse material, respectively. The inclusive graphic skewness is the best measure of this. Values between –0.1 and +0.1 indicate near symme- try, values above +0.1 indicate fine skewed sediments, while values below –0.1 indicate coarse skewed sedi- ments. For normal curves, K G (Kurtosis) is 1.0, while leptokurtic curves with a wide spread have values over 1.0, and platykurtic curves, with little spread and much peakedness, have values below 1.0. The type of beach developed in any particular locality is dependent on the velocity of the water currents and the particle sizes of the available sediments. This is due to the fact that particles carried in suspension fall out when the current velocity falls below a certain level. This relation- ship is shown in Table 3.3. From this table it can be seen that sands and coarse material settle rapidly, and any sed- iment coarser than 15 µm will settle within a tidal cycle. For finer particles, the settling velocities are much lower. Consequently, the waters in estuaries and enclosed inlets tend to be turbid as silt and clay are carried in suspension until they settle on mudflats, as they will not be deposited unless the water is very still. Thus, pebble beaches are formed only in areas of strong wave action, with sand beaches where wave action is moderate, and muddy shores are characteristic of quiet waters of semienclosed bays, deep inlets, and estuaries. The relationship between current speed and the erosion, transportation, and deposition of sediments is shown in Figure 3.2. From this figure it can be seen that for pebbles 10 4 µm (1 cm) in diameter, erosion of the sediments will take place at current speeds over 150 cm sec –1 . At current speeds between 150 and 90 cm sec –1 , the pebbles will be transported by the current, while at speeds of less than 90 cm sec –1 they will be deposited. Similarly, for a fine sand of 10 2 µm (0.1 mm) diameter, erosion will occur at speeds greater than 30 cm sec –1 and deposition will occur at speeds less than 15 cm sec –1 . For silts and clays, a similar relationship exists. However, erosion velocities are affected by the degree of consolidation of the sediment, which is a function of its water content. Throughout the intertidal area of a beach there is a gradient in substratum texture of finer particles at low tidal TABLE 3.2 Measures of Sediment Parameters 1. Measures of average size (a) Median particle diameter (Mdq) is the diameter corresponding to the 50% mark on the cumulative curve (q50). (b) Graphic mean particle curve (q50). M Z = (q16 + q50 + q84)/3. 2. Measures of uniformity of sorting (a) Phi quartile deviation (QDq), where QDq = (q75 – q25)/2 (b) Inclusive graphic standard deviation (qI), where 3. Measures of skewness (Skqq), where (a) Phi quartile skewness (Skqq), where Skq q = (q25 + q75 – 2 q 50)/2 (b) Inclusive graphic skewness (Sk 1 ), where 4. Measures of kurtosis or peakedness: Graphic kurtosis (K i ), where I q84 q16–() 4 q95 q5–() 6.6 += Sk 1 q16 q84 q50–+() 2 q84 q16–()() q5 q95 q50–+() 2 q95 q5–()() += K G q95 q5– 2.44 q75 q25–() = TABLE 3.3 Settling Velocities of Sediments Material Median Diameter (µm) Settling Velocity (m day –1 ) Fine sand 125–250 1040 Very fine sand 62–125 301 Silt 31.2 75.2 15.6 18.8 7.8 4.7 3.9 1.2 Clay 1.95 0.3 0.98 0.074 0.49 0.018 0.25 0.004 0.12 0.001 Source: After King, C.M., Introduction to Marine Geology and Geomorphology, Edward Arnold, London, 1975, 196. With permission. © 2001 by CRC Press LLC 90 The Ecology of Seashores levels to coarse particles higher up the shore. Figure 3.3 illustrates these processes on a sand beach at Howick in the Upper Waitemata Harbour, New Zealand. It is inter- esting to note that on the Zostera (eelgrass) flat there are finer deposits due to the reduction of water velocity by the leaves of the plants. 3.1.3 CURRENTS, WAVE ACTION, AND BEACH FORMATION ment on a temperate shore. Beyond the highest point reached by waves on spring tides is the “Dune Zone.” The “Beach Zone” extends from the upper limit of the drift line to the extreme low water level. It is subdivided into the “Bachshore Zone” above high water, which is covered only on exceptional tides, and the “Foreshore Zone” extending from low water up to the limit of high water wave swash. The “Nearshore Zone” extends from low water to the deepest limit of wave erosion. It is subdivided into an “Inner Turbulent Zone” covering the region of breaking waves and an “Outer Turbulent Zone.” The pro- file of a sand beach may exhibit structures such as “berms” and “ridges.” A berm is a flat-topped terrace on the back- shore, while a ridge is a bar running along the beach near low water. Below low water the corresponding features are a “bar” and a “trough.” The physical feature of beaches of importance to ecol- ogists can be found in King (1975), and McLachlan and Erasmus (1983). Water movement results in shear stress on the sea bed. This may move sediment off the bed, whereupon it may be transported by currents and waves. Cyclic water movement leads to the formation of ripples on the sand. Sand can be transported in two modes — as bed load and as suspended load. Bed load is defined as that part of the total volume of transported material mov- ing close to the bed, and not much above the ripple height. Suspended load is that part transported above the bed. Movement of material up and down beaches varies with the nature of the waves and shore level. As waves approach the beach and as the water becomes shallower and the breakpoint approached, more and more sand is caught up and transported. Inside the breakpoint the direc- tion of transport depends chiefly on the slope of the waves. Steep waves are destructive, tending to move material seaward, while flatter waves are constructive, tending to move particles up the beach. The slope of the beach face depends on the interaction of the swash/backwash pro- cesses planing it. Swash running up a beach carries sand with it and therefore tends to cause accretion and a steep beach face. Backwash has the opposite effect. If a beach consists of very coarse material such as pebbles, the uprunning swash tends to drain into the beach face, thus eliminating the backwash. The sediments are thus carried up the beach but not back again, resulting in a steep beach face. Fine sand and sand-mud beaches, on the other hand, stay waterlogged because of their low permeability, so that each swash is flattened by a full backwash, which flattens the beach by removing sand. Thus the beach slope is a function of the relationships between particle size and wave action (Figure 3.5). Each grade of beach material has a characteristic angle of slope; a gravel or shingle beach has a depositional slope of about 12°, and a rubble beach may have a slope of about 20°. Fine sand and mud beaches may have a slope of under 2°. Material removed from a beach is carried out to sea in the undertow. Waves that break obliquely on the shore carry material up the shore at an angle, while the back- wash, with its contained sediment, runs directly down the beach. This means that with each breaking wave, some FIGURE 3.2 Erosion, transportation, and depositional velocities for different particle sizes. Also illustrated is the effect of the water content of the sediment on the degree of consolidation, which in turn modifies the erosion velocities. (Redrawn from Postma, H., in Estuaries, Lauff, G., Ed., Publication No. 83, American Association for the Advancement of Science, Washington, D.C., 1967, 158. With permission.) © 2001 by CRC Press LLC Figure 3.4 is a profile of a typical sandy beach environ- Soft Shores 91 material is carried a short distance along the beach by a process known as beach drifting. Beach material can thus be transported considerable distances along the shore. The longshore transport of sediments is assisted by longshore currents, which run parallel to the shore (Figure 3.6). Such currents are found when more water is brought ashore than can escape in the undertow; this leads to a piling up of water that escapes by running parallel to the beach. The interaction of surface gravity waves moving toward the beach, and edge waves moving along shore, produces alternating zones of high and low waves, which determines the position of rip currents. The classical pat- tern that results from this is the horizontal eddy or cell known as the nearshore circulation pool. Water filtration by the sediments: Large volumes of seawater are filtered by the intertidal and subtidal sedi- ments. In the intertidal this occurs by the swash flushing unsaturated sediments and in the subtidal by wave pump- ing, that is, by pressure changes associated with wave crests and troughs (Riedl, 1971; Riedl et al., 1972). Most filtration occurs on the upper beach around high tide. Water seeps out of the beach slowly by gravity drainage, mostly below the mean tide level. The volume of water filtered increases with coarser sands and steeper beaches (McLachlan, 1982). Tidal range also has an influence, with maximum filtration volumes associated with small to moderate tidal ranges. 3.1.4 E XPOSURE R ATING In the literature on beach ecology, the terms “exposed” and “sheltered,” or “high” and “low,” are used in a very FIGURE 3.3 Distribution of sediment particles analyzed according to the Wentworth scale for three types of beach (A, upper beach; B, lower beach; C, Zostera flat at Howick, Upper Waitemata Harbour, North Island, New Zealand), and a tidal mudflat in Lyttleton Harbour, South Island, New Zealand (D). In E the same information is presented as cumulative curves. (A to D redrawn from Morton, J. and Miller, M., The New Zealand Seashore, Collins, London, 1968, 441. With permission.) © 2001 by CRC Press LLC 92 The Ecology of Seashores subjective way. To one worker, a beach may be “exposed,” while to another it may be “moderately sheltered.” McLachlan (1980a) developed a more objective exposure rating for beaches on a 20 point scale. The parameters considered included wave action, surf zone width, percent of very fine sand, median particle diameter, depth of the reduced layer, and the presence or absence of animals with stable burrows (Table 3.4). On the basis of the total score, beaches were rated as shown in Table 3.5. 3.2 THE PHYSICOCHEMICAL ENVIRONMENT Differences in particle size and the degree of sorting result in important changes in the physicochemical properties of the sediments, which are reflected in the density and kinds of plants and animals that characterize the deposits. Among the most important of these are the interstitial pore space, water content, mobility, and depth to which the deposits are disturbed by wave action, the salinity and temperature of the interstitial water, the oxygen content, the organic content, and the depth of the reducing layer. 3.2.1 INTERSTITIAL PORE SPACE AND WATER C ONTENT The interstitial water of a beach is either retained in the interstices between the sand grains as the tide falls, or is replenished from below by capillary action. The quantity of water that is retained within the sediment is a function of the available pore space, which in turn is dependent on the degree of packing and the degree of sorting of the FIGURE 3.4 Profiles of a typical sandy beach environment, showing the areas referred to in the text. (After McLachlan, A., in Sandy Beaches as Ecosystems, McLachlan, A. and Erasmus, T., Eds., Dr. W. Junk Publishers, The Hague, 1983, 332. With permission.) FIGURE 3.5 The relationship between beach particle size, exposure to wave action, and beach face angle in the western U.S.A. (Redrawn from Brown, A.C. and McLachlan, A., Ecology of Sandy Shores, Elsevier, Amsterdam, 1990, 21. After Wiegel, 1964. With permission.) © 2001 by CRC Press LLC Soft Shores 93 FIGURE 3.6 Nearshore cell circulation consisting of feeder offshore currents, rip currents, and a slow mass transport returning water to the surf zone. (Redrawn from Brown, A.C. and McLachlan, A., Ecology of Sandy Shores, Elsevier, Amsterdam, 1990, 32. After Sheppard and Inman, 1950. With permission.) TABLE 3.4 Rating Scheme for Assessing the Degree of Exposure of Sandy Beaches Parameter Rating Score 1. Wave action Practically absent 0 Variable, slight to moderate, wave height seldom exceeds 1 m 1 Continuous, moderate, wave height seldom exceeds 1 m 2 Continuous, heavy, wave height mostly exceeds 1 m 3 Continuous, extreme, wave height less than 1.5 m 4 2. Surf zone width (if wave score exceeds 1) Very wide, waves first break on bars 0 Moderate, waves usually break 50 to 150 m from shore 1 Narrow, large waves break on the beach 2 3. % very fine sand (62 to 125 µm) 5% 0 1 to 5% 1 1% 2 4. Median particle diameter (µm) Intertidal slope: 1/10 1/10–1/15 1/15–1/25 1/25–1/50 1/50 >710 5 6 7 7 7 500–710 4 5 6 7 7 350–500 3 4 5 6 7 250–350 2 3 4 5 6 180–250 1 2 3 4 5 <180 0 0 1 2 3 5. Depth of reduced layers (cm) 0 to 10 0 10 to 25 1 25 to 50 2 50 to 80 3 80 or more 4 6. Animals with stable burrows Present 0 Absent 1 Maximum score 20 Minimum score 0 Source: From Brown A.C. and McLachlan, A., Ecology of Sandy Shores, Elsevier, Amsterdam, 1990, 38. With permission. © 2001 by CRC Press LLC 94 The Ecology of Seashores sediment. In poorly sorted sediments the smaller particles pack into the interstices between the larger particles and thus reduce the percentage pore space. Coarse, ill-sorted sandy beaches have a relatively low porosity (approxi- mately 20%), whereas in more sheltered localities where the deposits are well-sorted, the water retention may approach 45%. The rate of replacement of water lost by evaporation from the surface of the deposits is dependent upon the diameter of the channels between the sand grains. These channels decrease in size with a decrease in grain size so that capillary rise is greatest in fine deposits. Thus, on beaches with fine deposits where the slope of the shore is low and the water retention (porosity) is high, the sediment is permanently damp, whereas on coarse, ill-sorted beaches where the slope is steep and the water retention low, the sediment contains less water and dries out quickly. Related to the above characteristic of the sediments are the properties of “thixotrophy” and “dilatancy,” which affect the ease with which burrowing animals can pene- trate into the substratum (Chapman, 1949). Visitors to the seaside will have noticed the whitening of the sand that occurs underfoot. This is caused by water being driven from the interstices by the pressure applied until the sand becomes hard packed and dry. This property is called dilatancy, and such sands are called dilatant. These sands are difficult to penetrate because the application of pres- sure causes them to harden. Dilatant sands usually have a water content of less than 22% by weight. When the water content of the sand is greater than 25% the sands become thixotrophic, and consequently softer and easier to pene- trate. Thixotrophic sediments become less viscous upon agitation and show a reduction in resistance with increased rate of shear in contrast to dilitant sediments, which show an increase in resistance. The most notorious examples of thixotrophic sands are quicksands, which liquify when pressure is applied. In experiments with burrowing worms, e.g., Arenicola, it has been shown that the speed of bur- rowing is dependent on the water content of the sediments and their resistance to shear. 3.2.2 TEMPERATURE The temperature within the sediments is determined by insolation, evaporation, wind, rain, tidal inundation, and the amount of pore water. In general there is a gradient across the intertidal zone with maximum and minimum values occurring at the high water mark and low water mark, respectively. Marked vertical temperature gradients can develop in the upper 10 cm of the sediments, below which the temperature is fairly uniform, approaching that of the overlying seawater. The vertical gradient is much steeper in the summer than in the winter in temperate regions. Thus, animals living in the sediment are buffered against the temperature extremes that can occur when the tide is out. 3.2.3 S ALINITY The salinity of the interstitial water of the sediments rep- resents an equilibrium between the overlying seawater and the fresh water seeping out from the land. In estuaries there is a horizontal salinity gradient from low water to high water. The nature of this gradient depends on the pattern of estuarine circulation and salinity stratification. There may be considerable differences between the inter- stitial salinities and those of the overlying water (see Fig- ure 3.7 for some data from the Avon-Heathcote Estuary, New Zealand). It can be seen that the interstitial salinity is considerably dampened when compared to that of the overlying water. Tube-building invertebrates that irrigate their burrows can play a significant role in maintaining the interstitial water salinity and other chemical properties so that it approximates that of the overlying water (Aller, 1980; Montague, 1982). Many such species cease irriga- tion when the salinity of the overlying water falls below a certain level. Interstitial salinity variations are greatest on intertidal flats. During exposure to air, the salinities of the surface sediment are subject to dilution by rain and concentration by evaporation. In a two-month study (September to Octo- ber) of salinity in a Salicornia- Spartina marsh at Mission Bay, San Diego, California, the water retained on the marsh had a higher salinity than that of the bay (ca. 34) for 75% of the time, exceeding 40 for 37% of the time, exceeding 45 for 10% of the time, and had a recorded maximum value of 50 (Bradshaw, 1968). 3.2.4 OXYGEN CONTENT The oxygen content depends to a large extent on the drain- age of water through the sediments. Porosity and drainage time increase sharply when there is 20% or more of fine TABLE 3.5 Beach Types and Descriptions (Scores as Awarded in Table 3.4) Score Beach Type Description 1 to 5 Very sheltered Virtually no wave action, shallow reduced layers, abundant macrofaunal burrows 6 to 10 Sheltered Little wave action, reduced layers present, usually some macrofaunal burrows 11 to 15 Exposed Moderate to heavy wave action, reduced layers deep, usually no macrofaunal burrows 16 to 20 Very exposed Heavy wave action, no reduced layers, macrofauna of highly mobile forms only Source: From Brown, A.C. and McLachlan, A., Ecology of Sandy Shores, Elsevier, Amsterdam, 1990, 31. With permission. © 2001 by CRC Press LLC [...]... friction with the bottom The salinity of the upper layer increases down-estuary while the salinity of the lower layer decreases up-estuary until the tip of the salt wedge is reached Exam- © 2001 by CRC Press LLC ples of this type of estuary are the James River in the U.S.A., the Mersey and the Thames in the U.K., and the Hawkesbury River in Australia c Vertically homogeneous estuaries: In these estuaries... for the oxidization of food causes anaerobic conditions; hence the steepness and depth of the RPD layer depend basically on the equilibrium ‘food: oxygen flow into the interstices’.” The depth of the RPD layer depends upon the organic content and grain size composition (mean size, sorting, % clay) of the sediment; an increase in the 96 The Ecology of Seashores FIGURE 3. 8 Schematic representation of Eh... (Figure 3. 14A) The volume of water between the high and low water levels is known as the tidal prism and as it increases in volume from neap to spring tides, so does the velocity of the tidal currents Generally there is some degree of eddy diffusion and turbulent mixing at the interface between the surface layer of freshwater and the seawater of the salt wedge Depending on the degree of mixing, the water... Florida, 1986a, 31 With permission.) major impact on the morphology of the present estuaries Much will depend on the future rates of sedimentation resulting in the infilling of the estuaries According to the scheme of Davies (19 73) there is a continuum of estuarine types At one end of the spectrum there are lagoons, which are produced by marine (wave action), while at the opposite end there are deltas,... Cassie and Cassie (1968) on the primary productivity of Chaetoceros armatum and Asterionella glacialis (= japonica) on the west coast of the North Island, New Zealand, little attention was paid to the 112 The Ecology of Seashores FIGURE 3. 16 An energy flow model of the interstitial system study of surf-zone diatoms until the late 1970s with the initiation of extensive studies along the Washington coast (Oregon,... (b) when there are irregularities in the channel bed, and (c) in shallow estuaries where the volume of the tidal prism is large compared with the total volume of the estuary basin The velocity of the upper, freshwater layer is greatest at the surface and decreases with depth until the interface with the bottom saltwater layer is reached when the velocity is zero In contrast, the velocity of the lower... action is slight, due to the low pitch of the beach and the reduced fetch and steepness of the waves The sediments have a large proportion of fine sand and very fine sand and often some of the silt/clay fraction 4 Protected mudflats: These occur at the upper ends of deep inlets and harbors or on the inner side of barrier islands Wave action is slight, enabling the deposition of fine sediments and organic... but the mixture of freshwater downward into the saltwater is minimal The degree of mixing is largely dependent on the on the volume of freshwater inflow A layer of mixed water of varying depth develops between the freshwater and the seawater with marked haloclines between them Such estuaries are often referred to a highly stratified estuaries The estuary of the Mississippi River and some fjords are of. .. starts at the head of the estuary and as the floccules grow they drift downstream with the water becoming increasingly turbid In many estuaries this produces a so-called “turbidity maximum.” The presence and magnitude of this turbidity maximum is controlled by a number of factors, including the amount of suspended material in the water, the estuarine circulation pattern, and the settling velocity of the available... relationships of the surface layer of the sediment Biomass — Joint (1978) investigated microalgal production on a mudflat in the River Lynher estuary, Cornwall, England The seasonal cycle of chlorophyll a content of the surface sediment is shown in Figure 3. 15 The increase in chlorophyll a in April coincided with an increased rate of photosynthesis The increase in the standing stock of the chlorophyll . 164 3. 6.2.7 Factors Involved in the Structuring of Meiofaunal Communities 166 3. 6 .3 Soft Shore Benthic Macrofauna 166 3. 6 .3. 1 Introduction 166 3. 6 .3. 2 Macrofaunal Zonation Patterns 167 3. 6 .3. 3. 1 73 3.6 .3. 4 Distribution Patterns of Estuarine Macrofauna 174 3. 6 .3. 5 Epifauna 175 3. 6 .3. 6 The Hyperbenthos 175 3. 6 .3. 7 Soft Shore Macrofaunal Feeding Types 175 3. 6.4 Estuarine Nekton 178 3. 6.4.1. 119 3. 5.2.6 A Model of Estuarine Phytoplankton Production 122 © 2001 by CRC Press LLC 86 The Ecology of Seashores 3. 5 .3 Estuarine Macroalgae 1 23 3.5 .3. 1 Composition and Distribution 1 23 3.5 .3. 2