Hydrobiologia 497: 39–51, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.

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Role of hydrodynamic conditions on quantity and biochemical composition of sediment organic matter in sandy intertidal sediments (NW Atlantic coast, Iberian Peninsula) M. Incera, S.P. Cividanes, J. L´opez & R. Costas Departamento de Ecolox´ıa e Biolox´ıa Animal, Universidade de Vigo, 36200 Vigo, Spain Tel: +34 986 812588. Fax: +34 986 812556. E-mail: [email protected] Received 15 February 2002; in revised form 25 February 2003; accepted 8 March 2003

Key words: organic matter composition, intertidal sediment, biochemical composition

Abstract Ten beaches subjected to different wave action exposure were studied on the northwest coast of the Iberian Peninsula. According to descriptions of beach types (from very sheltered to very exposed) given by McLachlan (1980), five of the beaches studied were classified as sheltered and the other five as exposed beaches. The biochemical composition (proteins, lipids and carbohydrates) of sedimentary organic matter was analysed from the surface sediment down to a depth of 25 cm, between July and September 1997, at three tidal levels: high, medium, and low, at ebb tide. Biochemical compound concentrations were significantly higher in the sheltered than in the exposed beaches. Concentrations were, on average two, three and four times higher (for proteins, carbohydrates and lipids, respectively) in sheltered than in exposed sediments. The low hydrodynamic conditions of the sheltered beaches favoured the settlement of sedimentary organic matter. This is supported by the higher protein to carbohydrate ratio found in the exposed (12.3), in contrast with the sheltered localities (5.2). Sheltered but not exposed sediments were characterised by clear vertical profiles of protein, carbohydrate and lipid concentrations, with values on top approximately twice as high as in the deeper sediment layers. On the other hand, there were significant differences in the biochemical compound concentrations among tidal levels for both groups of intertidal localities, except for the protein concentrations in sheltered localities. Biochemical compound concentrations were higher at medium and low tidal levels for both sheltered and exposed beaches. In the three tidal levels, there was a significant negative relationship between the biopolymeric carbon and the intertidal slope. Thus, the biopolymeric carbon concentration decreased as the intertidal slope increased.

Introduction There is a large number of studies in intertidal sediments, mainly focusing on benthic organisms (i.e., meiofauna, macrofauna, bacteria) that have been studied in terms of biomass, diversity, richness and/or zonation. In recent years, some studies have focussed on the patchy spatial distribution of organisms at several spatial scales (Fleeger & Decho, 1987; Fleeger et al., 1990). It has been shown that food availability is a major structuring factor of marine benthos (Pearson & Rosenberg, 1978; Pearson & Rosenberg, 1987), and that species diversity in intertidal soft-

sediments is strongly correlated with food variability (Withlaton, 1981). Moreover, food resources are one of the most probable explanations for marine population patchiness (Decho & Fleeger, 1988) and for benthic community distribution, seasonal cycles and metabolism (Montagna et al., 1983; Rudnick et al. 1985). Food availability is linked to the biochemical composition of organic matter (Tenore & Hanson, 1980; Danovaro et al., 1993). Thus, determining organic matter composition is crucial in assessing food quality and quantity in benthic ecological studies. The biochemical composition of organic matter is the result of the dynamic equilibrium between external

40 inputs, autochthonous production and heterotrophic utilisation (Fabiano & Danovaro, 1994). Organic matter in marine sediments is composed of labile and refractory compounds (Fabiano & Danovaro, 1994). Simple sugars, fatty acids and proteins that are rapidly mineralised have been used to assess the labile portion of organic matter (Daumas et al., 1983; Fichez, 1991; Danovaro et al., 1993). Conversely, refractory organic matter, comprising humic and fulvic acids and complex carbohydrates, is characterised by lower degradation rates (Handa et al., 1972; Robinson et al., 1982; Sargent et al., 1983; Wilson et al., 1986; Buscail et al., 1990; Biddanda & Riemann, 1991). However, only labile compounds have been used to estimate the nutritional value of the sediment (Buchanan & Longbottom, 1970). The biochemical composition of sedimentary organic matter has been widely researched in many marine ecosystems, such as deep sea (Danovaro et al., 1993, 1995a,b), semi-enclosed marine systems (Pusccedu et al., 1999), subtidal sandy sediments (Fabiano et al., 1995), seagrass bed (Danovaro et al., 1994) or estuarine environments (Fabiano & Danovaro, 1994). However, in spite of the importance of the biochemical composition of the sedimentary organic matter (proteins, lipids and carbohydrates), as mentioned above, there is a conspicuous lack of information about concentrations and variability of these compounds in intertidal sediments. Due to their dynamic ecotonal location, these environments display strong spatial and temporal variability of major physicochemical characteristics, thus offering permanently disturbed conditions to inhabiting and/or visiting organisms. One of the main factors determining the character of a beach is the different degree of wave action (Brown & McLachlan, 1990). By way of an example, filtration of sea water through sand (wich concentrates organic material in the sediment), depends upon wave action (exposure gradient) (Brown & McLachlan, 1990). Therefore, concentrations of organic matter in beach sediments would be expected to differ between sheltered and exposed intertidal sediments. Exposed and sheltered intertidal sediments also differ in terms of grain size, shape and sorting, which are important factors in fixing sedimentary porosity and permeability which, in turn, influence drainage (McLachlan, 1983). Drainage, which generally increases with increasing exposure (McLachlan, 1983), is critical in determining moisture content, oxygen, organic input and depth of reduced layers. Under high-energy condi-

tions, the oxygen input and the high drainage maintain the sediments fully oxygenated, and no steep vertical gradients in oxygen and oxidation rates are evident. Under sheltered conditions, i.e., where the wave action decreased, drainage is so slow that the sand is constantly saturated. In both sheltered and exposed sediments, conditions change with sediment depth, but changes are slight in the coarser sand whereas they are dramatic in the finer sand. Consequently, sediment depth related patterns in sedimentary organic matter quantity and composition would be sharper in sheltered than in exposed intertidal sediments. Sheltered and exposed intertidal localities also differ in their beach profiles. Exposed beaches, where wave action is strong, usually have steep slopes; whereas sheltered beaches, where wave action is relatively mild, have gentler slopes. Thus, it may be predicted that organic matter concentration at different tidal levels would exhibit higher spatial variability in exposed than in sheltered localities. In this study, we analysed the general pattern of spatial variability in the sedimentary organic matter composition in intertidal sediments. Our main aims were to: (1) compare the variability in sediment organic matter composition between localities subjected to a different wave action exposure (exposed vs. sheltered) (2) assess the influence of sediment depth and tidal level on the biochemical compounds concentrations within beaches, and (3) study the influence of hydrodynamic conditions on the biopolymeric carbon concentrations.

Materials and methods Study area and sampling Sediment sampling was carried out at ten localities on the Northwest coast of the Iberian Peninsula (Fig. 1). These are influenced by a macrotidal regime with a medium tidal range of three meters. According to their geographical position in the ria system, the localities were subjected to a different degree of wave exposure. Five of these, namely Cesantes, Lourido, Bamio, Barraña and Testal, located in the inner part of the rias, were characterised by low wave exposure, shallow reduced sediment layers and the presence of macrofaunal burrows. The others, namely América, Espiñeirido, Carnota, Llas and Peñarronda, located

41

Figure 1. Study area and localities. Sheltered sites are within grey rectangles and exposed in white rectangles.

in the outer part of the rias, were characterised by moderate to strong wave action, deeper reduced layers and the absence of macrofaunal burrows. According to McLachlan (1980), the first five sites were classified as ‘sheltered’ whereas the other five sites were classified as ‘exposed’. Sampling was carried out once per site, between July and September 1997, during spring tide (i.e., tidal range >3 m). Sediment samples were collected by hand coring (167.7 cm2 surface area) at three tidal levels (high, medium and low) after ebbing. Immediately after recovery, samples were vertically sliced into five layers (0–5, 5–10, 10–15, 15–20, 20–25 cm depth). Each layer was homogenized by hand mixing and subsamples were taken for the analysis of lipids, proteins, carbohydrates, and water content. All subsamples were deep-frozen at −30 ◦ C until analysis, except those for water content. Sediment samples were collected in triplicate, at each tidal level and depth.

Environmental data Sediment redox potential (Eh) was measured in situ at 5 cm intervals down to 25 cm sediment depth, using an Eh-electrode (Metrohm 740) connected to an mV meter. Sediment shear strength was measured by means of a shear vane meter, Pilcon EDECD, at 5 cm intervals down to 20 cm depth. Beach profile was measured at low tidal level according to Emery’s method (1961) with a LEICA NA820 theodolite, from the dry zone to the lower limit of the saturation zone according to Salvat (1964). The beach face slope was measured by the inverse of the ratio between the intertidal width and the height of the high tidal level. (Slope = 1/R; R= intertidal width height of the high intertidal level). Water content in the sediments was estimated as the difference between wet and dry weight (60 ◦ C, 72 h) and was expressed as a percentage.

42 Sedimentary parameters Biochemical analysis Macrofaunal organisms (i.e., more than 2 mm) were removed from the sediment with a pair of tweezers under binocular before analysis. All biochemical analyses were conducted on sediment samples previously oven dried at 60 ◦ C until constant weight and finely powdered with a pestle (Pulverisette 2, FRITSCH). Depending on the organic content or nature of sediments, 0.5 – 2.5 grams of sediment were used for each biochemical analysis. Total lipids were extracted from dried sediment samples, by direct elution, with a chloroform-methanol solution (2:1, v/v) and analysed according to Zöllner & Kirsch (1962). After a previous extraction with 5% trichloroacetic acid (TCA), total carbohydrates were analysed according to Dubois et al. (1956) and expressed as glucose equivalents. Total protein analyses were carried out following an extraction with NaOH and were determined according to Lowry & Rosebrough (1951), as modified by Markwell et al. (1978). Protein concentrations are given as bovine serum albumin (BSA) equivalents. Data were normalised to sediment dry weight. Sediment samples, combusted at 500 ◦ C for 6 hours and processed as describe above, were used as blanks for all biochemical analysis. Quality of the organic matter The nutritional value of the sediment was evaluated by combining the three main biochemical classes of sedimentary organic matter. Carbohydrate, protein and lipid concentrations were converted to carbon equivalents assuming a conversion factor of 0.45, 0.50, and 0.70, respectively (Fabiano et al., 1995). The sum of lipid, protein and carbohydrate carbon was reported as the biopolymeric carbon (BPC sensu Mayer, 1989; Fabiano & Danovaro, 1994; Fabiano et al., 1995). The BPC was assumed as an estimate of the labile fraction of total sediment organic matter, i.e., the fraction that was potentially available to deposit feeders. The protein to carbohydrates ratio was also calculated to assess the age of the organic material (Cauwet, 1978; Fabiano et al., 1997). Data analysis We used a three-way ANOVA to identify the influence of tidal level, sediment depth and wave action exposure on physicochemical characteristics of the study

sites. The variances of organic matter composition were not the same for exposed and sheltered localities (Levene’s test p < 0.001), so that two independent tests were conducted. We used mixed-model ANOVA, allowing us to incorporate independent variables as random effects in the models. Thus, since the same locality was monitored across tidal and depth levels, we fitted locality as random term and tidal level and sediment depth as fixed terms in mixed-model ANOVAs. The relationship between biopolymeric carbon and exposure degree across tidal level was analysed by ANCOVA. All statistical analyses were performed using SPSS version 8.0 (SPSS, 1997).

Results Environmental characteristics The profile and beach face slopes for each locality are shown in Figure 2. Both profiles and slopes showed no considerable differences in sheltered or exposed localities. Between groups of beaches (i.e., sheltered vs. exposed) slopes were significantly different (t10 = 2.93, p = 0.019), being gentler in the sheltered than in the exposed localities. Among the sheltered group, the locality of Testal had a narrower and steeper profile than the other sheltered ones. However, Testal has been classified as a sheltered locality because it is located in the inner part of the ria and its characteristics coincide with those proposed by McLachlan (1980) for a sheltered beach (see ‘Study area’ in ‘Materials and methods’). There were significant differences in the sediment water content between the exposed and the sheltered localities (Table 1). Exposed localities had higher water content than sheltered ones. Sediment water content decreased significantly with the increase in tidal level (Table 1, Fig. 3). The interaction between tidal level and exposure type was not significant, thus among tidal levels there was a similar pattern of variability (Fig. 3). At high tidal levels, water content was low (from 5.3 to 22.5%); at medium tidal levels water content ranged from 16.6 to 35.3%, and at low tidal levels values ranged from 18.0 to 38.0%. No significant differences in sediment water content among sampling depths were observed (Table 1). Sediment redox potential (Eh) values displayed significant differences among tidal levels, sediment depths and exposure types (Table 1). In both the exposed and sheltered localities, Eh values decreased

43

Figure 2. Intertidal profiles of the sheltered (A) and the exposed (B) study localities. The 1/slope is shown in brackets (slope = 1/R; R = intertidal width height of the high intertidal level). Table 1. Three-way ANOVAs of the effect of the tidal level (i.e. low, medium, and high), the sediment depth (each 5 cm) and exposition (sheltered and exposed localities) on shear strength, water content and redox potential Shear strength Source of variation Tidal level Depth Exposition Tidal level × Depth Tidal level × Expos. Depth × Exposition Error

df 2 3 1 6 2 3 96

F 6.09 78.10 1.62 1.34 0.85 0.39

P 0.003 <0.001 0.206 0.245 0.430 0.760

from high to low tidal level. The highest values (> +150 mV) were measured at the high tidal level in all the localities. At this level, Eh values remained above

Water content df 2 4 1 8 2 4 120

F 77.82 0.10 23.35 0.57 2.84 0.07

Redox potential P <0.001 0.982 <0.001 1.000 0.062 0.991

df 2 5 1 10 2 5 125

F 87.04 9.02 39.86 3.25 14.90 0.51

P <0.001 <0.001 <0.001 0.001 <0.001 0.769

the oxic boundary (+100 mV, Hargrave et al., 1995) (Fig. 3). On the other hand, there was a significant interaction between tidal level and exposure type (Table

44

Figure 3. Vertical profiles of sediment water content (A), redox potential (B) and shear strength (C) at exposed and sheltered localities at the three tidal levels. Standard errors are indicated. () High tidal level, () medium tidal level and () low tidal level.

45 1). Thus, the redox potential dropped sharply with decreasing tidal level in the sheltered localities, but only slightly decreased in the exposed ones (Fig. 3). Shear strength showed no significant differences between exposed and sheltered site types (Table 1), increasing significantly with increasing sediment depth and from low to high tidal level (Table 1, Fig. 3). Organic matter composition Carbohydrate, protein and lipid concentrations (Fig. 4) displayed significant differences between sheltered and exposed intertidal localities (carbohydrates: t10 = 4.93, p = 0.001; proteins: t10 = 3.36, p = 0.001 and lipids: t10 = 5.37, p = 0.001). In the sheltered sediments, protein, carbohydrate and lipid concentrations were on average two, three and four times higher respectively than in the exposed ones. However, in exposed and sheltered sites, proteins were the dominant compound (on average 84 and 72% for exposed and sheltered localities, respectively), followed by carbohydrates (12 and 23%) and lipids (4 and 6%). There were significant variations in the three biochemical compounds among localities, both in sheltered and exposed localities (Table 2). Protein concentrations ranged from 37.4 to 801 and from 37.6 to 4165 µg g−1 sed. d.w. in the exposed and sheltered sediments, respectively. Carbohydrates ranged from 5.7 to 266.5 and from 3.6 to 1783 µg g−1 sed. d.w. in the exposed and sheltered sediments respectively. Finally, lipid concentrations ranged from 0.6 to 56.3 and 3.8 to 189.9 µg g−1 sed. d.w., in the exposed and sheltered sediments respectively (Fig. 4). There was a significant interaction, in protein and carbohydrate concentrations, between beach and tidal level in both sheltered and exposed localities (Table 2), indicating that the distribution of the biochemical compounds was not homogeneous across-shore within beaches. Significant concentration differences of all the biochemical compounds among sediment depths in sheltered sediments were observed, (Table 2), with higher concentrations in the upper layers and lower in the deepest (Fig. 4). This difference was not observed in the exposed intertidal sediments. In sheltered localities, lipid and carbohydrate concentrations displayed significant differences among tidal levels, with higher values at medium and low tidal levels (Table 1, Fig. 4). On average, lipid concentrations were 5 times higher, carbohydrates 4 and 6 times higher at medium and low tidal level than at high tidal level. There was a significant interaction between

Figure 4. Overall mean carbohydrate, protein and lipid concentrations at different tidal levels and exposition (exposed: open bars, sheltered: closed bars). Standard errors are indicated.

the sediment depth and the tidal level for lipid concentrations. Lipid displayed higher variability with sediment depth at high rather than at low tidal levels (Fig.

46

Figure 5. Vertical distributions of the main biochemical classes of organic compounds, (carbohydrates, proteins and lipids), at exposed and sheltered localities in the three tidal levels. Standard errors are indicated. () High tidal level, () medium tidal level and () low tidal level.

47 Table 2. Mixed-model ANOVAs of the effect of the tidal level (i.e. low, medium, and high), the sediment depth (each 5 cm) and locality (random factor) on protein, lipid and carbohydrate concentrations. Two independent analyses were performed, one on sheltered localities and another on exposed localities. Degrees of freedom are shown for each factor and its associated error term. Error terms are calculated as a linear combination of variance components of the random effects ad quadratic terms of the fixed effects (SPSS, 1997)

Source of variation Sheltered localities Tidal level Depth Locality Tidal level × Depth Tidal level × Beach Depth × Beach Exposed localities Tidal level Depth Locality Tidal level × Depth Tidal level × Beach Depth × Beach

df

Proteins F

p

df

Lipids F

p

df

Carbohydrates F

p

2/8 4/16 4/8.3 8/32 8/32 16/32

2.9 4.6 6.0 2.0 14.5 1.3

0.112 0.012 0.014 0.077 <0.001 0.236

2/8 4/16 4/8.4 8/32 8/32 16/32

16.7 19.1 11.5 3 7.4 1.2

0.001 <0.001 0.002 0.012 <0.001 0.286

2/8 4/16 4/8 8/32 8/32 16/32

15.4 9.3 17.8 1.7 13.2 0.9

0.002 <0.001 0.001 0.127 <0.001 0.532

2/8 4/16 4/8.1 8/31 8/31 16/32

8.5 0.07 5.4 1.9 52.1 1.4

0.011 0.992 0.021 0.101 <0.001 0.187

2/8 4/16 4/10.8 8/32 8/32 16/32

21.2 0.2 13.4 0.9 5.2 2.1

0.001 0.924 <0.001 0.500 <0.001 0.038

2/8 4/16 4/8.1 8/32 8/32 16/32

20.3 1.3 9.6 2.2 59 1.5

0.001 0.313 0.004 0.054 <0.001 0.141

5). The interaction between tidal level and locality was also significant in intertidal sheltered sediments, for all the biochemical compounds. In the exposed localities, significant differences were observed in the concentrations of proteins, lipids and carbohydrates among tidal levels (Table 2), with higher values at medium and low tidal levels. Moreover, the interaction between tidal level and locality was significant (Table 2). Biopolymeric carbon and hydrodynamic conditions The biopolymeric carbon was used as a summary of protein, lipid and carbohydrate concentrations. At the three tidal levels, there was a significant positive relationship between biopolymeric carbon concentrations and the inverse of the beach face slope (1/S) (Fig. 6), i.e., the biopolymeric carbon concentration decreased as the intertidal slope increased. Moreover, interaction between tidal level and intertidal slope was significant, thus the relationship was stronger at medium and low tidal levels than at high tidal level.

Discussion Sediment biochemical composition We found differences between sheltered and exposed

localities in biochemical composition variability. The mixed-model ANOVA allowed us to identify the relative contribution of sediment depth and tidal level on biochemical composition while controlling for the non-independence of samples from the same locality. Protein, lipid and carbohydrate concentrations showed significant differences between exposed and sheltered localities. The relative contribution of the biochemical compounds to the total organic pool was clearly dominated by proteins, followed by carbohydrates and lipids at both groups of intertidal sediments. However, the percentages were different between exposed and sheltered localities. Proteins represent 71% and 84%, carbohydrates 23 and 12% and lipids 6 and 4% of the total organic pool in sheltered and exposed localities respectively. These values are consistent with results from previous studies on subtidal sediments of coastal areas (Meyer-Reil, 1983; Sargent et al., 1983), but differ from deep-sea sediments, which are usually characterised by low protein and lipid concentrations and high carbohydrate concentrations (Danovaro et al., 1993), characteristic of highly oligotrophic or detritic (such as deep-sea) environments (Danovaro et al., 1993). On the other hand, the protein to carbohydrate ratio (PRT:CHO) has been used to assess the ‘age’ of sediment organic matter (Hobson, 1967; Cauwet, 1978). Since proteins are more readily used by bac-

48 teria than carbohydrates (Williams & Carlucci, 1976; Newell & Field, 1983), high PRT:CHO ratios indicate living organic matter or ‘newly-generated’ detritus (Danovaro et al., 1993). By contrast, low PRT:CHO ratios suggest the presence of aged organic matter (Danovaro et al., 1993) and the role of proteins as a potentially limiting factor for benthic consumers (Fabiano et al., 1995). This ratio ranges from lower than 0.1 in oligotrophic deep-sea sediments (500–2400 m depth in the eastern Mediterranean Sea, Danovaro et al., 1993) to higher than 10 in coastal Antarctic sediments (Pusccedu, 1997). In our study area, average PRT:CHO ratios of 12.27 in exposed and 5.24 in sheltered intertidal sediments ranked high when compared with ratios reported in the literature. This suggests that most of the sediment organic matter was recently produced and that protein is not a limiting element for consumer growth in the studied intertidal sediments. Moreover, the higher PRT:CHO ratios in exposed than in sheltered localities indicates that in the former there is little dead organic matter accumulation, probably due to the strong hydrodynamic condition of exposed beaches. The higher hydrodynamics permits the deposition of coarser sediments through which water runs easily, preventing the accumulation of organic matter. By contrast, the low hydrodynamics in sheltered beaches favours the accumulation of sedimentary organic matter because of the scarce renewal of interstitial water. Thus, with decreasing exposure, beaches tend to become more importers of organic matter (Little, 2000), with higher accumulations of organic matter. This conclusion must be applied with caution, because different methods for the calculation of protein and carbohydrate concentrations were used in the literature. Changes in organic matter composition with sediment depth and tidal level

Figure 6. Relationship between 1/(intertidal slope) and biopolymeric carbon concentrations (expressed in micrograms per gram [dry weight] of sediment) in the three tidal levels studied. Mean values for the 25 cm sediment depth are reported at each locality. () Sheltered localities and () exposed localities. ANCOVA; tidal level effect: F2,25 = 4.6, p = 0.012; intertidal slope [covariable]: F1,25 = 121.01, p < 0.001; tidal level ∗ intertidal slope: F2,25 = 25.59, p < 0.001).

Unlike exposed sites, sheltered localities showed differences in biochemical compounds (proteins, carbohydrates and lipids concentrations) distribution at depth (Table 2). These results could be explained by the different hydrodynamic conditions, i.e., the high wave action vs. the low wave action, ruling over the two different environments. Moreover, the depth variability in organic matter composition may depend on the pore water circulation through the sand. Filtered water increases with coarser sands and steeper beaches (McLachlan, 1982), i.e., from sheltered to exposed intertidal sediments. Thus, in sheltered localities, lower

49 water renewal rates and lower exchanges at the watersediment interface will favour the build up of concentration gradients. Conversely, in exposed localities the higher pore water circulation will limit the formation of sharp gradients in biochemical concentrations. This is well supported by the sediment redox potential data (Eh) (Fig. 3, B). Sheltered localities displayed a marked Eh decline with sediment depth, indicative of redox potential discontinuity layers (RPD) (Gray 1981), and exposed localities displayed no such trend. These patterns indicate that higher oxygenated conditions are present with increasing sediment depth at exposed than at sheltered environments, because of higher water filtration rates. On the other hand, except for protein concentrations in sheltered localities, there are differences in the biochemical compounds concentrations among tidal levels for both groups of intertidal localities (Table 1). Biochemical concentrations were higher at medium and low tidal levels in sheltered and exposed beaches (Fig. 4). This pattern observed among tidal levels coincides with the pattern observed on meiofauna abundance in Espiñeirido beach (Rodriguez, 1999) and on the macrofauna abundance in Carnota, América and Llas beaches (De la Huz, 1999). At these localities, meiofauna and macrofauna abundances were greater in low than in high tidal levels. This suggests that biochemical concentrations could be a proximal factor affecting fauna abundance. In fact, it has long been proposed that, in sediments, food quality plays an important role for bacteria (Deming & Yager, 1992), meiofauna (Danovaro et al., 1995b; Danovaro, 1996) and macrofauna (Rosenberg, 1995). However, experimental studies involving manipulation of sediment biochemical composition are required to test the cause-effect relationship between food (sedimentary organic matter) and fauna.

that the number of species, diversity (H ), density, and species richness increased with reduced exposure to wave action. It is generally admitted that sandy shores show an increase in species diversity (Little, 2000), abundances and biomass as exposure decreases (McLachlan, 1990). It is suggested that this increase in fauna in sheltered localities is caused by more favourable environmental conditions and sediment stability. From our study, we may conclude that biopolymeric carbon concentrations increased from beaches with steep intertidal slopes to beaches with flat slopes, i.e., a similar pattern that is shown by organisms. We suggest, therefore, that the control of beach fauna is complex and that the physical environment may also act indirectly by controlling inputs and storage of sediment biopolymeric carbon, thus influencing the available food resources to benthic fauna.

Hydrodynamic conditions and biopolymeric carbon

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Acknowledgements The authors would like to thank “Equipo de Bentos” from the Departamento de Ecoloxía e Bioloxía Animal of the Universidade de Vigo, for their invaluable assistance during sampling. The authors are grateful to Ian Emmett and Manuel Filgueira for their help with English. We are particularly grateful to Alberto Velando for priceless suggestions and constructive criticisms that greatly helped us to improve an earlier version of the manuscript. We are also endebted to two anonymous referees for their fundamental help in improving the manuscript. This research was supported by the Xunta de Galicia (XUGA 30105A98) and the Universidade de Vigo (64102C859).

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