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Day–night variation of intertidal flat sediment properties in relation to sediment stability
Estuarine, Coastal and Shelf Science 58 (2003) 663–675
Day–night variation of intertidal flat sediment properties in relation to sediment stability P.L. Friend*, M.B. Collins, P.M. Holligan School of Ocean and Earth Science, University of Southampton, Southampton Oceanography Centre, European Way, Southampton SO14 3ZH, UK Received 31 March 2003; accepted 4 June 2003
Abstract The majority of investigations that have measured sediment properties related to intertidal sediment stability have been undertaken during daylight subaerial exposure periods. As a consequence, models based upon such data represent only partially the intertidal flat surface conditions within any 24 h period. In this contribution, a comparison is made between surface sediment properties related to sediment stability measured during six consecutive (day/night), semi-diurnal subaerial exposure periods, at three stations on an intertidal sand flat in late March 1999. The study site was selected on the basis of its suitability for sampling and data collection at night, with special regard to safety and logistics. Seawater temperatures ranged from 4.1 to 9.6 C, and salinities from 33.9 to 34.8. Eleven parameters related to intertidal flat sediment stability were measured, or derived. These variables included the critical erosion shear stress (sc), chlorophyll a, phaeopigment, and colloidal carbohydrate content, mean grain size and settling velocity of the surface (0–1 mm) sediment fraction. Bed elevation was described using an acretion/erosion parameter (AEP) (West and West, 1991), whilst additional physical terms included ambient seawater salinity and temperature, as well as tidal range and wind speed, during the preceding immersion periods. One-way ANOVA was used to detect significant differences between day- and night-time emersion periods; similarly, principal components analysis (PCA) was applied to detect continuous variation between properties. The results show a high degree of temporal and spatial variability between day- and night-time intertidal flat variables, the PCA differentiating clearly between day and night conditions. Surface sediments across the intertidal flat exhibited varying degrees of biostabilisation. The maximum biostabilisation coefficient (18) was recorded at night in high microalgal biomass areas; the minimum (5) occurred during both day and night, in areas with lower microalgal biomass. All surface sediment parameters varied rhythmically between day- and night-time. Significant differences were found between day- and night-time biostabilisation coefficients, however, differences between day- and night-time sc values were not detected. It is suggested that sediment stability at night is enhanced in high microalgal biomass areas as a result of degradation products from bound extra-cellular polysaccharides (EPS) not easily detected using standard extraction procedures. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: semi-diurnal; intertidal flat; sediment stability; day/night; biostabilisation; EPS; Pilsey Sands; Chichester Harbour
1. Introduction A great deal of scientific attention has been focussed recently upon the exchange of materials between the land and the sea, e.g. Land–Ocean Interaction Study
* Corresponding author. E-mail address: [email protected] (P.L. Friend). 0272-7714/03/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0272-7714(03)00178-1
(LOIS) (Leeks and Jarvie, 1998). Interest stems from: (1) the fact that cohesive and non-cohesive sediments can act as both sources and sinks for natural and anthropogenic materials that enter coastal waters via rivers and estuaries (Biggs and Howell, 1984); and (2) the need to address the potential impact of climate change on the stability and effectiveness of natural coastal defences (e.g. beaches, intertidal flats, salt marshes). Sediments stored in the intertidal area protect the coastline through frictional dissipation of wave and
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current energy. The stability and transport of intertidal sediments is dominated by a number of physical driving forces, especially the action of tidal, wave and winddriven currents, acting at different spatial and temporal scales. Biological processes have an important influence on sediment stability through the enhancement of either erosion or deposition (Widdows et al., 1998; Austen et al., 1999; Yallop et al., 2000; Andersen, 2001; de Deckere et al., 2001; Defew et al., 2002). The possibility that a difference exists between dayand night-time sediment stability was first suggested by Montague (1986) after: (1) cohesive sediment erodability was observed to increase with increasing pH of the eroding water; and (2) photosynthesis in algal mats was found to cause a rise in the ambient pH during the day, whilst respiration at night lowered the pH. These studies implied that intertidal sediments with a benthic microalgal population should be more stable at night. Results from the small number of in situ investigations that have compared day- and night-time sediment stability properties are extremely scarce; they are either unpublished or have been collected as ancillary data during part of a larger study. In contrast to the suggestions of Montague (1986), the results of a study in April 1996, in which an in situ flume was used on a cohesive intertidal flat in Southampton Water, UK, suggested that a significant reduction in sediment stability occurred at night (Herrington, pers. comm.). In another study in Venice Lagoon, day- and night-time critical erosion shear stress (sc) values, derived using an annular field flume, were found to be similar (Amos et al., in press). In the latter case, seawater temperature (27 C) and salinity (28) values during the day were the same as those recorded at night. Recent studies of the causes of microalgal biostabilisation have tended to concentrate upon the production of extra-cellular polysaccharides (EPS) by benthic diatoms, either during in situ (light/dark) experiments during daylight emersion, or during laboratory studies (e.g. Underwood and Smith, 1998; Staats et al., 2000). Results suggest that soluble EPS are produced during both light and dark conditions (Smith and Underwood, 2000), whilst bound (labile) EPS are produced only during the light period (de Brouwer et al., 2002). On intertidal mudflats, bound EPS have been observed to decrease during darkness and immersion periods (Staats et al., 2000; de Brouwer and Stal, 2002). It is suggested that the diatoms themselves degrade bound EPS into soluble EPS with a high uronic acid content; in turn, uronic acids are implicated in the binding of EPS to the substratum (Dade et al., 1990; de Brouwer and Stal, 2002). Significant correlations between EPS and sediment stability have been found in the laboratory (Holland et al., 1974; Tolhurst et al., 2002), and for specific seasons and habitats in the field (Friend et al., in press). The majority of in situ correlations, however, are
poor or non-existent (Paterson et al., 2000; Defew et al., 2002). It is unclear if this is due to sampling methods (Kelly et al., 2001; Perkins et al., 2003), extraction methods and organisation of the EPS by the organisms themselves (de Brouwer et al., 2002), or some other factor such as sediment armouring (Tolhurst et al., in press). The present paper aims to compare selected day- and night-time properties of surface sediments, related to sediment stability, for consecutive emersion periods on an intertidal sand flat. The study area was selected on the basis of the absence of features liable to be hazardous during night-time sampling (e.g. creeks, gullies). Relationships between physical and biological properties of the surface sediments are examined, and a new objective method is proposed for determining the biostabilisation index (Manzenrieder, 1983) on sandy intertidal sediments using the Cohesive Strength Meter (CSM) (Tolhurst et al., 1999).
2. Materials and methods 2.1. Study area Three stations, representative of lower, middle, and upper intertidal flat conditions, were investigated on Pilsey Sands, Chichester Harbour, UK (Fig. 1) over six consecutive exposure periods, between March 24 and 26, 1999, during the annual occurrence of relatively high microalgal biomass (in comparison with summer values) at this location. Pilsey Sands is a flood-dominated, accretionary sand flat of approximately 2 km2 in area, located within the mouth of the eastern-most estuary of the Solent estuarine system. The estuary is ebbdominated at the mouth, with mean spring and neap tidal ranges of 3.9 and 1.9 m, respectively (Hydrographic Office, 1998). Maximum spring tidal current velocities, measured at 0.15 m above the bed, were 0.4 m s1. The mean surface (0–1 mm) sediment grain size is 180 lm (fine sand) and the mean surface dry bulk density is 1300 kg m3. Throughout the year, surface chl a contents decrease, from a maximum of 28 lg gDW1 in December/January to a minimum of 0.2 lg gDW1 in June/July. On an annual time-scale, surface colloidal carbohydrate contents tend to vary inversely with chl a, with maximum values of 350 lg glucose-equivalent (GE) gDW1 in August and minimum values of <50 lg GE gDW1 in December. Over one to three month periods with relatively high diatom biomass, chl a and colloidal carbohydrate tend to covary. 2.2. Sampling Push cores (n ¼ 4) for pigment, colloidal carbohydrate and grain size analysis of the surface (0–1 mm) sediment fraction were collected at low water for each
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Fig. 1. Location map of Pilsey Sands intertidal flat, Chichester Harbour, UK.
station. Samples for pigment and colloidal carbohydrate analysis were freeze-dried (light excluded), and maintained at 80 C prior to analysis within one week of sample collection. Sediment chl a (microalgal biomass proxy) and phaeopigment (grazing/senescence indicator) contents were measured spectro-fluorometrically; they were quantified using the modified equations of Parsons et al. (1984). Colloidal carbohydrate (EPS proxy) was quantified using a double saline extraction procedure (Underwood et al., 1995), followed by a phenol– sulphuric acid assay (Dubois et al., 1956). 2.3. Physical measurements Sediment stability measurements (n ¼ 5) were made using the CSM (Paterson, 1989). The CSM is an in situ erosion device, which employs the stress induced by a perpendicular water jet to erode the surface of exposed intertidal sediments. An empirical calibration based on the equations of Bagnold (1966), modified by McCave (1971), allows the eroding pressure to be expressed in terms of an equivalent horizontal bed shear stress. For a full description of the CSM, see Tolhurst et al. (1999). The ambient temperature and salinity of the seawater used in the CSM were obtained using a standard, precalibrated field salinometer manufactured by the technical department of the School of Ocean and Earth Science, University of Southampton. Bed elevation was described using an accretion/erosion parameter (AEP) (West and West, 1991) obtained from two pairs of steel
poles placed 1 m apart at each station. Distances to the bed (n ¼ 8) were measured sufficiently far from the poles, to minimise any scour effects. Vertical accuracy was 2 mm. Grain size analysis was by a wet sieving method, which gave repeatable results using only a small sample size (<5 g), and which did not require any pretreatments (Wheatcroft and Butman, 1997). Meteorological data were obtained from a meteorological office weather station located adjacent to the intertidal flat. Tidal information was obtained from Admiralty tide tables (Hydrographic Office, 1998). During the study period, the weather remained overcast, with occasional sunny intervals: there was a light to moderate S to SW wind (mean speed of 3.9 m s1), and no rain fell during emersion. Maximum wind speed during preceding immersion periods was 5.4 m s1. Throughout the study period, the tidal range decreased from 4.4 to 3.9 m. 2.4. Statistical analyses Paired t-tests were used to examine significant differences (p < 0:05) between day- and night-time parameters. The AEP was converted to relative bed elevation for the paired t-tests, to avoid the use of negative values. Parameters were tested for normality of distribution and homogeneity of variance and transformed, where necessary, using standard transformations. Principal components analysis (PCA) was used to detect continuous variation in the distribution of
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variates between sampling times. Unlike most statistical analyses (which use a deductive approach), PCA uses an inductive approach to generate hypotheses. A series of eigenvalues and their corresponding eigenvectors (components) are produced from a matrix of similarities. By replacing the original variates of the raw data by the first few components with large eigenvalues, the data can be summarised and inherent patterns detected. Multiplication of each eigenvector with the original data matrix, produces a new data set of component scores. The components provide graphical co-ordinates (loadings) for the attributes of the data matrix, and the component scores for the individuals on the components. After suitable numerical scaling, the two sets of points are superimposed in a biplot, to assist in the interpretation. In the present study, the attributes were the variates of the data set, and the individuals were the consecutive sampling periods. Eleven PCs were computed using the MinitabÒ 12.2 computer program (Minitab Inc., PA, USA). The physical forcing terms, wind speed and tidal range during the preceding immersion, were included in the PCA. An adaptation of the MacArthur (1960) model, based upon species effectiveness, was used to determine the number of significant PCs. 2.5. Index of biostabilisation A coefficient of biological stabilisation (Yallop et al., 1994; Tolhurst et al., 1999) was calculated from the ratio of sc derived using the CSM (sc bio), to sc for the resuspension of abiotic sand (sc abio); this was sand with a diameter equal to the mean grain size of the 0–1 mm depth fraction, at each station during the CSM deployment. sc abio was calculated using the Shields criterion (Shields, 1936) and the Bagnold (1966) equation, as modified by McCave (1971), for the resuspension of sand. Settling velocities used in the modified Bagnold equation were calculated from the equations of Gibbs et al. (1971) for quartz spheres, as corrected by Baba and Komar (1981) for naturally worn quartz grains. The ambient temperature and salinity of the seawater used in the CSM were measured at the time of each CSM deployment, and were used in the density and settling velocity equations.
variable at the lower and upper stations, but increased generally at the middle station. During the medium-term study, no significant difference was detected in mean sc values between stations (one-way ANOVA), and sc was positively correlated with chl a at the lower station (r ¼ 0:704, p ¼ 0:011). Between March and April at the middle station, sc was correlated positively with chl a for six consecutive weeks (r ¼ 0:922, p ¼ 0:009). For the pooled data set, no significant correlation was found between sc and colloidal carbohydrate, or chl a. In the day-night study, considerable temporal and spatial variation of sediment properties was found at the lower, middle and upper intertidal stations (Fig. 3). A rhythmic variation between day- and night-time values of sc, chl a, and colloidal carbohydrate was apparent at the lower station; similarly, for the AEP at the middle station. At the lower station, sc increased at night and decreased during the day, varying inversely with both chl a and colloidal carbohydrate. Chl a, phaeopigments and colloidal carbohydrate covaried at the lower station, whilst other more subtle covariation between phaeopigments, AEP, and grain size was evident for the middle station. At the upper station, surface phaeopigment contents increased at night and decreased during the day. A list of parameters relative to sampling period and station is provided in Table 1. Ambient seawater salinity varied between 33.9 and 34.8. The minimum seawater temperature occurred at night (4.1 C), with the maximum during the day (9.4 C) (Table 2). The mean sc varied between 0.9 (0.2) N m2, at the middle station during the night, and 1.5 (0.1) N m2 at the lower station in the night (Fig. 4). The highest mean chl a content was 5.3 (0.3) lg gDW1 (lower station, during the day), whilst the lowest was 1.8 (0.3) lg gDW1 (middle station, at night). The mean colloidal carbohydrate content was consistently higher during the day, compared with the night, particularly at the lower and middle stations. The mean bed elevation increased during the night at the middle station. The highest mean phaeopigment contents occurred at the lower station during the day. Interestingly, phaeopigment contents at the middle and upper stations were greater during the night, than in the day. The mean grain size varied from 150 to 160 lm at the lower and upper stations; it was 180 lm at the middle station.
3. Results Data collection and sampling were nested within a medium-term, weekly sampling program of intertidal flat properties related to sediment stability (Fig. 2). The day-night study occurred during a general decrease in chl a measured, between March–May 1999, at the lower, middle and upper intertidal flat stations; similarly, during a decrease in colloidal carbohydrate at the lower and upper stations, but an increase at the middle station. Trends in sc before and after the day-night study were
3.1. Comparison between day- and night-time emersion periods Paired t-tests identified significant differences between day- and night-time colloidal carbohydrate (p ¼ 0:004) and chl a (p ¼ 0:011) content at the lower station, phaeopigment content (p ¼ 0:027) and bed elevation (p ¼ 0:048) at the middle station, and phaeopigment content (p ¼ 0:017) at the upper station. Significant
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Fig. 2. Results of the medium-term, weekly study (March–May 1999) of sc, chl a and colloidal carbohydrate at lower (A), middle (B), and upper (C) intertidal stations. The location of the day-night investigation reported in this paper is shown by the hatched area.
differences between day- and night-time sediment stability (sc) were not detected, for any of the stations. For the lower and middle stations, PCs 1 and 2 were significant; for the upper station, only PC 1 was significant (Table 3). PCs 1 and 2 accounted for 76% of the total variation at the lower station, and 73% of total variation at the middle station. PC 1 accounted for 48% of the upper station total variation. The lower station PC 1 (Fig. 5) discriminated clearly between dayand night-time n-scores. Emersion periods ÔDay 3Õ (D3) and ÔDay 5Õ (D5) showed a close correlation with phaeopigments and seawater temperature, and a weaker correlation with chl a, colloidal carbohydrate, grain size,
and settling velocity. D1 was affiliated closely with AEP and tidal range. Night n-scores (particularly N2 and N4) were affiliated closely with sediment stability (sc). For PC 2, tidal range and AEP were the most important attributes. PC loadings showed that chl a, colloidal carbohydrate, grain size, settling velocity and seawater temperature were correlated closely; phaeopigments were weakly positively correlated with this group. AEP and tidal range were correlated clearly. As with the lower station, the middle station PC 1 discriminated clearly between day- and night-time n-scores (Fig. 6). However, night n-scores were differentiated by phaeopigments and AEP, whilst D3 and D5
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Fig. 3. Temporal and spatial variation of surface sediment properties (SE mean) for consecutive day (D) and night (N) emersion periods: March 24–26, 1999.
n-scores were differentiated by seawater temperature, colloidal carbohydrate content, together with sc. D1 was affiliated closely with chl a, grain size and tidal range. For PC 2, tidal range was the most important attribute. PC loadings showed that temperature, colloidal carbohydrate content and (to a lesser extent) sediment stability were positively correlated; likewise, chl a, grain size and tidal range. Positive correlations were also apparent for phaeopigments and AEP. In contrast with both the lower and middle stations, the upper station PC 1 failed to discriminate clearly between day and night n-scores; instead, n-scores were discriminated by the (non-significant) PC 2 axis (Fig. 7). D1 was affiliated closely with chl a; similarly, N6 with phaeopigments. Mean wind speed, seawater salinity, grain size, tidal range, AEP and sediment stability were correlated closely, as were settling velocity and temperature.
3.2. Sediment stability Coefficients of biostabilisation for night-time emersion periods (average 16.7) at the lower station were significantly higher (paired t-test: p ¼ 0:011) than daytime coefficients (average 10.5) (Table 4). At the middle and upper stations biostabilisation indices were more variable, with no significant differences detected between day and night. The maximum biostabilisation coefficient (17.8) occurred at the lower station during emersion period N4; the minimum biostabilisation (4.6) occurred at the middle station, during D3 and N4.
4. Discussion The complex interplay between physical and biological properties related to sediment stability is
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Table 1 Mean parameter values at the lower (L), middle (M) and upper (U) stations for consecutive day (D) and night (N) sampling periods, numbered 1–6: March 24–26, 1999 (Coll carbo: colloidal carbohydrate; Phaeo: phaeopigments; Set vel: settling velocity) Sampling period
Station
sc (N m2)
Coll carbo (lg GE gDW1)
Chl a (lg gDW1)
Phaeo (lg gDW1)
AEP (cm)
Grain size (lm)
Set vel (cm s1)
D1 D1 D1 N2 N2 N2 D3 D3 D3 N4 N4 N4 D5 D5 D5 N6 N6 N6
L M U L M U L M U L M U L M U L M U
1.15 1.62 1.75 1.36 0.94 1.26 1.06 0.75 0.65 1.76 0.65 1.25 1.06 1.84 1.36 1.37 1.16 1.46
115.1 92.5 91.9 81.0 103.7 134.3 124.1 131.2 134.5 88.4 99.7 85.0 110.5 105.1 120.5 81.5 68.6 109.6
5.8 2.2 3.1 3.9 2.1 2.3 5.4 2.0 2.1 3.5 2.2 1.9 4.8 1.8 2.2 3.4 1.2 1.8
0.4 0.1 0.1 0.2 0.2 0.2 0.5 0.2 0.1 0.3 0.3 0.2 0.4 0.1 0.2 0.4 0.3 0.3
0.2 0.3 0.2 0.3 0.4 0.2 0.3 0.1 0.4 0.3 0.4 0.5 0.3 0.2 0.5 0.3 0.4 0.6
157 189 162 150 184 158 153 183 157 153 177 159 155 178 157 153 176 159
1.3 1.7 1.3 1.1 1.5 1.2 1.3 1.7 1.3 1.3 1.5 1.3 1.3 1.6 1.3 1.2 1.5 1.3
illustrated in the rhythmic temporal and spatial patterns across the intertidal flat (Fig. 3). During the course of the investigations, cyclical variations between day and night were observed for all the parameters at (at least) one of the stations. It is unlikely that the reasons for such variations can be attributed to a single causal mechanism. However, the main difference between day and night, the light/dark condition, is likely to be an important factor, particularly in areas of high microalgal biomass. The PCA differentiated clearly between day- and night-time conditions for the lower and middle intertidal flat (Figs. 5 and 6). However, the reasons for the similarity between the lower and middle stations, together with the dissimilarity between these two stations and the upper station (Fig. 7), are unclear; nonetheless, they may be related in some way to the proximity to the upper station of a ÔhardÕ shoreline sea defence. A similar difference between lower and middle station n-scores, and upper station n-scores was found in a PCA of 32 intertidal flat parameters, carried out as part of a long-term study into biological influences on sediment stability on Pilsey Sands. 4.1. Biostabilisation An index of biological stabilisation (Manzenrieder, 1983), based upon the ratio of the biotic to abiotic critical threshold of sediment motion, is a useful way of comparing biostabilisation between different sites and different erosion instruments. For cohesive sediments, a lack of understanding of the physics of particle erosion, together with selection of a suitable abiotic control site (Yallop et al., 1994), means that such indices
are relative; as such, they are dependent upon qualitative decisions made by the erosion device operator. For example, indices based upon ÔbiofilmÕ and Ônon-biofilmÕ areas (e.g. Tolhurst et al., 1999) are likely to underestimate the extent of biostabilisation. In the present study, the temperature and salinity of the ambient seawater used in the CSM at the time of deployment were measured, along with the mean grain size of sediment immediately adjacent to the test area. The theoretical abiotic suspension threshold could then be calculated, using the equations of Bagnold (1966) and McCave (1971), for the resuspension of sand. Hence, as the CSM was calibrated using the same resuspension equations (Tolhurst et al., 1999), an objective method of deriving the sc bio =sc abio ratio was achievable. Furthermore, the method is easily repeatable elsewhere, allowing direct comparison of biostabilisation coefficients between non-cohesive intertidal flats. On Pilsey Sands, all of the sediments showed some degree of biostabilisation (Table 4). The highest
Table 2 Ambient seawater salinity and temperature, together with mean wind speed and tidal range during preceding immersion for consecutive day (D) and night (N) sampling periods, numbered 1–6: March 24–26, 1999 Sampling period
Salinity
Seawater temperature ( C)
Wind speed (m s1)
Tidal range (m)
D1 N2 D3 N4 D5 N6
34.8 34.5 34.1 34.6 33.9 34.5
8.2 4.1 9.4 8.3 9.6 6.1
5.4 4.7 2.1 5.3 3.2 3.9
4.4 4.2 4.1 4.0 3.9 3.9
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Fig. 4. Mean parameter values (SE mean) for day- and night-time emersion periods at the lower, middle and upper stations.
biostabilisation coefficient was 17.8 at the lower station during emersion period N4; the lowest coefficient (4.6) occurred at the middle station during D3 and N4. A comparison of biostabilisation indices at Pilsey Sands with indices from other studies is somewhat restricted because the CSM has been little used on non-cohesive Table 3 PCA of day/night parameters for each station. Significant eigenvalues are highlighted in bold. Also illustrated are the proportion of variance explained by each eigenvector, and the minimum variance required for significance Station
PC
Eigenvalue
Variance (%)
Cumulative variance (%)
Minimum variance (%)
Lower
1 2 3 1 2 3 1 2 3
5.45 3.72 1.58 4.62 3.40 1.76 5.22 2.66 2.38
45.4 31.0 13.2 42.0 30.9 16.0 47.5 24.2 21.7
45.4 76.4 89.6 42.0 72.9 88.9 47.5 71.7 93.3
27.5 26.6 25.7 27.5 26.6 25.7 27.5 26.6 25.7
Middle
Upper
sediments. However, Ôbubble matÕ sediments with a mean particle size of 257 lm on the island of Texel, Netherlands, showed a biostabilisation coefficient (ratio of biotic sediment to sediment with organic matter removed), measured with the CSM, of 21.7 (Yallop et al., 1994). In the same study, a coefficient of 9.6 was reported for non-bubble mat sediments, of mean grain size 154 lm; this size, however, was below the CSM resolution for abiotic sediment (Tolhurst et al., 1999). The high biostabilisation coefficients found at Pilsey Sands agree with Amos et al. (1997), in which it was suggested that the effect of biostabilisation increased with grain size. It should also be noted that coefficients of biological stabilisation derived using the CSM (with the exception of measurements made on plane bed surfaces) are likely to be higher than coefficients derived using other erosion devices (e.g. Microcosm, SedErode). This is because the CSM integrates erosion over a smaller area, incorporating less spatial heterogeneity of biochemical parameters and bed roughness elements (Tolhurst et al., 2000). Furthermore, the method of calculating biostabilisation coefficients used in this study
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Fig. 5. PCs 1 and 2 and scaled component scores for the lower station. Emersion periods are labelled consecutively (1–6), according to day (D) or night (N) occurrence.
is likely to produce higher values than those derived from in situ comparisons of ÔbioticÕ and ÔabioticÕ sediment. 4.2. Day- and night-time sediment stability Very few in situ observations of day- and night-time parameters related to sediment stability have been made, making comparisons with other studies difficult. In the Southampton Water investigation the significant reduction in sc observed for cohesive intertidal flat sediments at night was attributed to a lack of vertical migration by benthic diatoms during the dark; hence, a reduction in migration-related EPS production (Herrington, pers. comm.). However, EPS production for diatom migration is expected to be independent of light conditions (de Brouwer and Stal, 2002), occurring in response to both light and tidal rhythms (Seroˆdio et al., 1997; Smith and Underwood, 1998). Furthermore, the
Southampton Water study was restricted to four consecutive day/night emersion periods (with n ¼ 2 replicates) and, whilst higher chl a values are reported for day-time emersion, EPS was not measured. In the present study, biostabilisation of the relatively high microalgal biomass, lower intertidal flat sediments was greater at night than during the day, despite lower chl a and EPS contents at night (Table 4). Reasons for this are unclear, but may be related to: (1) sediment binding by uronic acids produced during degradation of bound EPS at night (de Brouwer et al., 2002); (2) poor detection of uronic acids using a double saline extraction procedure; and (3) the presence of acetyl or other group organic polymers which promote substrate adhesion, but are not detectable using the phenol–sulphuric acid assay (de Brouwer, pers. comm.). In addition, other influences not considered in this study, e.g. the production of EPS by bacteria and benthic fauna, as well as the role of animal feeding and water surface tension may be
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Fig. 6. PCs 1 and 2 and scaled component scores for the middle station.
important. Whilst a significant difference existed between day- and night-time EPS (Fig. 3), no significant difference between sc values was detected, possibly as a result of the resolution of the CSM Mk III used in this study. 4.3. Temperature and other effects The possibility exists that lower temperatures (of the ambient seawater) during the night were responsible for less EPS dissolution compared with day-time values, causing the apparent night-time increase in the stability of the lower intertidal sediments (Fig. 3). For example, an increase in the temperature of incubation water can increase the amount of extracted polysaccharides (Staats, pers. comm.). Other, more subtle temperature effects at night were expected to affect: (1) particle settling velocities, causing a relative increase in grain size (Kro¨gel and Flemming, 1998); and (2) bed elevation, causing a relative decrease in bed level. Note the close correlation between seawater temperature and: (1) settling velocity, grain size, colloidal carbohydrate and chl a at the lower station (Fig. 5); (2) sc, colloidal carbohydrate and (to a lesser extent) settling velocity at the middle station (Fig. 6); (3) settling velocity at the upper station (Fig. 7). No such temperature effects, however, were detected; in fact, at the middle station during night emersion, bed elevation increased whilst grain size decreased. Phaeopigments at the lower station increased during the day and decreased at night, covarying with chl a (Figs.
3 and 4). Grazing by herbivores can limit the standing stock of benthic microalgae, leading to a decrease in sediment stability (e.g. Gerdol and Hughes, 1994). The surface phaeopigment content suggests that herbivoral grazing was an important factor in the lower intertidal area; it may explain the observed decrease in sediment stability, during day-time emersion periods. Higher phaeopigment contents at the middle and upper stations at night, compared with day-time values, probably indicated increased grazing activity at these stations at night.
5. Conclusions The in situ investigation of intertidal flat sediment properties, as related to sediment stability, remains central to many of the recent and ongoing multidisciplinary intertidal flat projects, e.g. LISP-UK (Black and Paterson, 1998); INTRMUD (EU MAS3-CT950022); F-ECTS (EU MAS3-CT97-0145); BIOPTIS (EU MAS3-CT97-0158). In such studies, data and sample collection is undertaken normally during daylight emersion periods (e.g. Amos et al., 1988; Paterson et al., 2000; Herman et al., 2001). However, very few studies have examined intertidal flat sediment stability at night (and in different seasons, for that matter); this is probably as a result of the safety and logistical considerations required for data and sample collection during the hours of darkness.
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Fig. 7. PCs 1 and 2 and scaled component scores for the upper station.
In this study, surface sediment stability (sc) was investigated for six consecutive day- and night-time emersion periods on a sandy intertidal flat in spring. Rhythmic variations in day- and night-time sediment stability and related parameters (chl a and EPS) occurred in the higher microalgal biomass areas. Here, the highest biostabilisation coefficients of 15–18 were recorded at night, compared with maximum day-time coefficients of 11. Significant differences in day- and night-time biostabilisation coefficients existed in high microalgal biomass areas, however, differences between day- and night-time sc were not detected. Enhancements in sediment stability at night in high biomass areas may have been due to the presence of uronic acids in soluble EPS, not easily detected with the extraction procedure used in the present study. It is recommended that future day/night EPS studies use Table 4 Coefficients of biostabilisation for consecutive day (D) and night (N) sampling periods: March 24–26, 1999 Sampling period
Lower station
Middle station
Upper station
D1 N2 D3 N4 D5 N6
10.8 17.4 10.8 17.8 10.0 14.9
9.4 6.9 4.6 4.6 11.9 8.8
16.3 13.4 5.9 11.5 12.3 14.1
extraction and analysis procedures more likely to detect uronic acids (see de Brouwer and Stal, 2002). Rhythmic variations also occurred between day- and night-time values for other physical and biological properties in areas with lower microalgal biomass. These variations were difficult to explain, illustrating the complexity of the temporal and spatial variability of intertidal flat parameters. The results show that significant differences between day- and night-time sediment stability may exist, but that improvements in the CSM resolution as well as a more extensive sampling program are required for their in situ detection. Note that the new version of the CSM (Mk IV) offers enhanced resolution with smaller incremental pressure increases (Black, pers. comm.). Recent studies have found a poor correlation between carbohydrate and sediment stability (e.g. Paterson et al., 2000; de Brouwer et al., 2002; Defew et al., 2002). Others indicate that differential production and degradation rates exist for bound and soluble EPS fractions during exponential and stationary diatom growth phases (Smith and Underwood, 2000; de Brouwer and Stal, 2002). It is therefore recommended that sediment stability during different growth phases and under varying EPS solubility is investigated using laboratory microcosm flumes. It is further recommended that future in situ investigations examine high microalgal biomass areas, not only on sandy intertidal flats, but also on muddy intertidal flats where day/night differences may be more easily
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detectable, and may be especially significant where there are migratory biofilms (Tolhurst et al., in press). In addition, comparative studies at the same sites are required in different seasons in order to elucidate the effects of temperature changes and associated variations in EPS production. Furthermore, it is hoped that the development of new in situ instrumentation will permit the investigation of parameters during submersion, as well as emersion periods. Results from this study have implications for the accuracy of time-dependent estuarine modelling studies, which are calibrated using sc values obtained during day-time emersion periods only.
Acknowledgements This study was funded by the UK Natural Environment Research Council and ABPmer Ltd. under award no. GT04/97/269/MAS. The authors wish to thank: the Ministry of Defence, Thorney Island, for providing unlimited access to Pilsey Sands; the UK Meteorological Office for the provision of meteorological data; and K. Davis for drawing the location map. J. de Brouwer is thanked for his helpful advice throughout the preparation of this paper, and B.W. Flemming, T.J. Tolhurst and one anonymous reviewer are thanked for their comments and suggestions in the review stage. This paper is one of a series of publications produced for the Estuary Processes Research Project (EstProc) (DEFRA/ Environment Agency project no. FD1905).
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Kennedy, V.S. (Ed.), The Estuary as a Filter. Academic Press, Orlando, pp. 107–129. Black, K.S., Paterson, D.M., 1998. LISP-UK littoral investigation of sediment properties: an introduction. In: Black, K.S., Paterson, D.M., Cramp, A. (Eds.), Sedimentary Processes in the Intertidal Zone, Special Publications 139. The Geological Society, London, pp. 1–10. de Brouwer, J.F.C., Stal, L.J., 2002. Daily fluctuations of exopolymers in cultures of the benthic diatoms Cylindrotheca closterium and Nitzschia sp. (Bacillariophyceae). Journal of Phycology 38, 464– 472. de Brouwer, J.F.C., Wolfstein, K., Stal, L.J., 2002. Physical characterisation and diel dynamics of different fractions of extracellular polysaccharides in an axenic culture of a benthic diatom. European Journal of Phycology 37, 37–44. Dade, W.B., Davis, J.D., Nichols, P.D., Nowell, A.R.M., Thistle, D., Trexler, M.B., White, D.C., 1990. Effects of bacterial exopolymer adhesion on the entrapment of sand. Geomicrobiological Journal 8, 1–16. de Deckere, E.M.G.T., Tolhurst, T.J., de Brouwer, J.F.C., 2001. Destabilisation of cohesive intertidal sediments by infauna. Estuarine, Coastal and Shelf Science 53, 665–669. Defew, E.C., Tolhurst, T.J., Paterson, D.M., 2002. Site-specific features influence sediment stability of intertidal flats. Hydrology and Earth System Sciences 6, 971–981. Dubois, M., Gilles, K.A., Hamilton, J.K., Rebers, P.A., Smith, F., 1956. Colorimetric method for determination of sugars and related substances. Analytical Chemistry 28, 350–356. Friend, P.L., Ciavola, P., Cappucci, S., Santos, R. Bio-dependent bed parameters as a proxy tool for sediment stability in intertidal areas. Continental Shelf Research (Nearshore and Coastal Oceanography) (special issue), in press. Gerdol, V., Hughes, R.G., 1994. Effect of Corophium volutator on the abundance of benthic diatoms, bacteria and sediment stability in two estuaries in south-eastern England. Marine Ecology Progress Series 114, 109–115. Gibbs, R.J., Matthews, M.D., Link, D.A., 1971. The relationship between sphere size and settling velocity. Journal of Sedimentary Petrology 41, 7–18. Herman, P.M.J., Middelburg, J.J., Heip, C.H.R., 2001. Benthic community structure and sediment processes on an intertidal flat: results from the ECOFLAT project. Continental Shelf Research 21, 2055–2071. Holland, A.F., Zingmark, R.G., Dean, J.M., 1974. Quantitative evidence concerning the stabilisation of sediments by marine benthic diatoms. Marine Biology 27, 191–196. Hydrographic Office, 1998. Admiralty Tide Tables, United Kingdom and Ireland including European Channel Ports, vol. 1. Hydrographic Office, Taunton, UK, 434 pp. Kelly, J.A., Honeywill, C., Paterson, D.M., 2001. Microscale analysis of chlorophyll a in cohesive intertidal sediments: the implication of microphytobenthic distribution. Journal of the Marine Biological Association of the United Kingdom 81, 151–162. Kro¨gel, F., Flemming, B.W., 1998. Evidence for temperature-adjusted sediment distributions in the back-barrier tidal flats of the East Frisian Wadden Sea (southern North Sea). In: Alexander, C.R., Davis, R.A., Henry, V.J. (Eds.), Tidalites: Processes and Products, SEPM Special Publication 61. SEPM, Tulsa, OK, pp. 31–41. Leeks, G.J.L., Jarvie, H.P., 1998. Introduction to the land–ocean interaction study (LOIS): rationale and international context. The Science of the Total Environment 210, 5–20. MacArthur, R.H., 1960. On the relative abundance of species. American Nature 94, 25–36. Manzenrieder, H., 1983. Die biologische Verfestigung von Wattfla¨chen aus der Sicht des Ingenieurs. Miteilungen des Leichtweiss-Instituts fu¨r Wasserbau, Technische Universita¨t Braunschweig 79, pp. 135– 193.
P.L. Friend et al. / Estuarine, Coastal and Shelf Science 58 (2003) 663–675 McCave, I.N., 1971. Sand waves in the North Sea off the coast of Holland. Marine Geology 10, 199–225. Montague, C.L., 1986. Influence of biota on erodability of sediments. In: Mehta, A.J. (Ed.), Estuarine Cohesive Sediment Dynamics: Lecture Notes on Coastal and Estuarine Studies 14, pp. 251–269. Parsons, T.R., Maita, Y., Lalli, C.M., 1984. Determination of Chlorophylls and Total Carotenoids, A Manual of Chemical and Biological Methods for Seawater Analysis. Pergamon Press, Oxford, pp. 101–112. Paterson, D.M., 1989. Short-term changes in the erodability of intertidal cohesive sediments related to the migratory behaviour of epipelic diatoms. Limnology and Oceanography 34, 223–234. Paterson, D.M., Tolhurst, T.J., Kelly, J.A., Honeywill, C., de Deckere, E.M.G.T., Huet, V., Shayler, S.A., Black, K.S., de Brouwer, J., Davidson, I., 2000. Variations in sediment stability and sediment properties across the Skeffling mudflat, Humber estuary, UK. Continental Shelf Research 20, 1373–1396. Perkins, R.G., Honeywill, C., Consalvey, M., Austin, H., Tolhurst, T.J., Paterson, D.M., 2003. Changes in microphytobenthic chlorophyll a resulting from sediment compaction due to dewatering: opposing patterns in concentration and content. Continental Shelf Research 23, 575–586. Seroˆdio, J., da Silva, J.M., Catarino, F., 1997. Non-destructive tracing of migratory rhythms of intertidal benthic microalgae using in vivo chlorophyll a fluorescence. Journal of Phycology 33, 542–553. Shields, A., 1936. Anwendung der A¨hnlichkeits-Mechanik und der Turbulenzforschung, auf die Geschiebe-Bewegung, Preuss. Versuchanstalt fu¨r Wasserbau und Schiffbau, Berlin, 20 pp. Smith, D.J., Underwood, G.J.C., 1998. Exopolymer production by intertidal epipelic diatoms. Limnology and Oceanography 43, 1578–1591. Smith, D.J., Underwood, G.J.C., 2000. The production of extracellular carbohydrates by estuarine benthic diatoms; the effects of growth phase and light and dark treatment. Journal of Phycology 36, 321– 333. Staats, N., Stal, L.J., De Winder, B., Mur, L.R., 2000. Oxygenic photosynthesis as driving process in exopolysaccharide production of benthic diatoms. Marine Ecology Progress Series 193, 261–269. Tolhurst, T.J., Jesus, B., Brotas, V., Paterson, D.M. Diatom migration and sediment armouring—an example from the Tagus Estuary, Portugal. Hydrobiologia, in press. Tolhurst, T.J., Black, K.S., Shayler, S.A., Mather, S., Black, I., Baker, K., Paterson, D.M., 1999. Measuring the in situ erosion shear stress
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of intertidal sediments with the Cohesive Strength Meter (CSM). Estuarine, Shelf and Coastal Science 49, 281–294. Tolhurst, T.J., Black, K.S., Paterson, D.M., Mitchener, H., Termaat, R., Shayler, S.A., Gust, G., 2000. A comparison and measurement standardisation of four in situ devices for determining the erosion shear stress of intertidal sediments. Continental Shelf Research 20, 1397–1418. 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, Proceedings in Marine Science 5, 409–425. Underwood, G.J.C., Smith, D.J., 1998. In situ measurements of exopolymer production by intertidal epipelic diatom-dominated biofilms in the Humber estuary. In: Black, K.S., Paterson, D.M., Cramp, A. (Eds.), Sedimentary Processes in the Intertidal Zone, Special Publications 139. The Geological Society, London, pp. 125–134. Underwood, G.J.C., Paterson, D.M., Parkes, R.J., 1995. The measurement of microbial carbohydrate exopolymers from intertidal sediments. Limnology and Oceanography 40, 1243–1253. West, M.S., West, J.R., 1991. Spatial and temporal variations in intertidal zone sediment properties in the Severn estuary, UK. In: Elliott, M., Ducrotoy, J.P. (Eds.), Estuaries and Coasts. Olsen and Olsen, Fredensborg, pp. 25–30. Wheatcroft, R.A., Butman, C.-A., 1997. Spatial and temporal variability in aggregated grain size distributions, with implications for sediment dynamics. Continental Shelf Research 17, 367–390. Widdows, J., Brinsley, M.D., Elliott, M., 1998. Use of in situ flume to quantify particle flux (biodeposition rates and sediment erosion) for an intertidal mudflat in relation to changes in current velocity and benthic macrofauna. In: Black, K.S., Paterson, D.M., Cramp, A. (Eds.), Sedimentary Processes in the Intertidal Zone, Special Publications 139. The Geological Society, London, pp. 85–97. Yallop, M.L., De Winder, B., Paterson, D.M., Stal, L.J., 1994. Comparative structure, primary production and biogenic stabilisation of cohesive and non-cohesive marine sediments inhabited by microphytobenthos. Estuarine, Coastal and Shelf Science 39, 565–582. Yallop, M.L., Paterson, D.M., Wellsbury, P., 2000. Inter-relationships between rates of microbial production, exopolymer production, microbial biomass and sediment stability in biofilms of intertidal sediments. Microbial Ecology 39, 116–127.
Day–night variation of intertidal flat sediment properties in relation to sediment stability P.L. Friend*, M.B. Collins, P.M. Holligan School of Ocean and Earth Science, University of Southampton, Southampton Oceanography Centre, European Way, Southampton SO14 3ZH, UK Received 31 March 2003; accepted 4 June 2003
Abstract The majority of investigations that have measured sediment properties related to intertidal sediment stability have been undertaken during daylight subaerial exposure periods. As a consequence, models based upon such data represent only partially the intertidal flat surface conditions within any 24 h period. In this contribution, a comparison is made between surface sediment properties related to sediment stability measured during six consecutive (day/night), semi-diurnal subaerial exposure periods, at three stations on an intertidal sand flat in late March 1999. The study site was selected on the basis of its suitability for sampling and data collection at night, with special regard to safety and logistics. Seawater temperatures ranged from 4.1 to 9.6 C, and salinities from 33.9 to 34.8. Eleven parameters related to intertidal flat sediment stability were measured, or derived. These variables included the critical erosion shear stress (sc), chlorophyll a, phaeopigment, and colloidal carbohydrate content, mean grain size and settling velocity of the surface (0–1 mm) sediment fraction. Bed elevation was described using an acretion/erosion parameter (AEP) (West and West, 1991), whilst additional physical terms included ambient seawater salinity and temperature, as well as tidal range and wind speed, during the preceding immersion periods. One-way ANOVA was used to detect significant differences between day- and night-time emersion periods; similarly, principal components analysis (PCA) was applied to detect continuous variation between properties. The results show a high degree of temporal and spatial variability between day- and night-time intertidal flat variables, the PCA differentiating clearly between day and night conditions. Surface sediments across the intertidal flat exhibited varying degrees of biostabilisation. The maximum biostabilisation coefficient (18) was recorded at night in high microalgal biomass areas; the minimum (5) occurred during both day and night, in areas with lower microalgal biomass. All surface sediment parameters varied rhythmically between day- and night-time. Significant differences were found between day- and night-time biostabilisation coefficients, however, differences between day- and night-time sc values were not detected. It is suggested that sediment stability at night is enhanced in high microalgal biomass areas as a result of degradation products from bound extra-cellular polysaccharides (EPS) not easily detected using standard extraction procedures. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: semi-diurnal; intertidal flat; sediment stability; day/night; biostabilisation; EPS; Pilsey Sands; Chichester Harbour
1. Introduction A great deal of scientific attention has been focussed recently upon the exchange of materials between the land and the sea, e.g. Land–Ocean Interaction Study
* Corresponding author. E-mail address: [email protected] (P.L. Friend). 0272-7714/03/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0272-7714(03)00178-1
(LOIS) (Leeks and Jarvie, 1998). Interest stems from: (1) the fact that cohesive and non-cohesive sediments can act as both sources and sinks for natural and anthropogenic materials that enter coastal waters via rivers and estuaries (Biggs and Howell, 1984); and (2) the need to address the potential impact of climate change on the stability and effectiveness of natural coastal defences (e.g. beaches, intertidal flats, salt marshes). Sediments stored in the intertidal area protect the coastline through frictional dissipation of wave and
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current energy. The stability and transport of intertidal sediments is dominated by a number of physical driving forces, especially the action of tidal, wave and winddriven currents, acting at different spatial and temporal scales. Biological processes have an important influence on sediment stability through the enhancement of either erosion or deposition (Widdows et al., 1998; Austen et al., 1999; Yallop et al., 2000; Andersen, 2001; de Deckere et al., 2001; Defew et al., 2002). The possibility that a difference exists between dayand night-time sediment stability was first suggested by Montague (1986) after: (1) cohesive sediment erodability was observed to increase with increasing pH of the eroding water; and (2) photosynthesis in algal mats was found to cause a rise in the ambient pH during the day, whilst respiration at night lowered the pH. These studies implied that intertidal sediments with a benthic microalgal population should be more stable at night. Results from the small number of in situ investigations that have compared day- and night-time sediment stability properties are extremely scarce; they are either unpublished or have been collected as ancillary data during part of a larger study. In contrast to the suggestions of Montague (1986), the results of a study in April 1996, in which an in situ flume was used on a cohesive intertidal flat in Southampton Water, UK, suggested that a significant reduction in sediment stability occurred at night (Herrington, pers. comm.). In another study in Venice Lagoon, day- and night-time critical erosion shear stress (sc) values, derived using an annular field flume, were found to be similar (Amos et al., in press). In the latter case, seawater temperature (27 C) and salinity (28) values during the day were the same as those recorded at night. Recent studies of the causes of microalgal biostabilisation have tended to concentrate upon the production of extra-cellular polysaccharides (EPS) by benthic diatoms, either during in situ (light/dark) experiments during daylight emersion, or during laboratory studies (e.g. Underwood and Smith, 1998; Staats et al., 2000). Results suggest that soluble EPS are produced during both light and dark conditions (Smith and Underwood, 2000), whilst bound (labile) EPS are produced only during the light period (de Brouwer et al., 2002). On intertidal mudflats, bound EPS have been observed to decrease during darkness and immersion periods (Staats et al., 2000; de Brouwer and Stal, 2002). It is suggested that the diatoms themselves degrade bound EPS into soluble EPS with a high uronic acid content; in turn, uronic acids are implicated in the binding of EPS to the substratum (Dade et al., 1990; de Brouwer and Stal, 2002). Significant correlations between EPS and sediment stability have been found in the laboratory (Holland et al., 1974; Tolhurst et al., 2002), and for specific seasons and habitats in the field (Friend et al., in press). The majority of in situ correlations, however, are
poor or non-existent (Paterson et al., 2000; Defew et al., 2002). It is unclear if this is due to sampling methods (Kelly et al., 2001; Perkins et al., 2003), extraction methods and organisation of the EPS by the organisms themselves (de Brouwer et al., 2002), or some other factor such as sediment armouring (Tolhurst et al., in press). The present paper aims to compare selected day- and night-time properties of surface sediments, related to sediment stability, for consecutive emersion periods on an intertidal sand flat. The study area was selected on the basis of the absence of features liable to be hazardous during night-time sampling (e.g. creeks, gullies). Relationships between physical and biological properties of the surface sediments are examined, and a new objective method is proposed for determining the biostabilisation index (Manzenrieder, 1983) on sandy intertidal sediments using the Cohesive Strength Meter (CSM) (Tolhurst et al., 1999).
2. Materials and methods 2.1. Study area Three stations, representative of lower, middle, and upper intertidal flat conditions, were investigated on Pilsey Sands, Chichester Harbour, UK (Fig. 1) over six consecutive exposure periods, between March 24 and 26, 1999, during the annual occurrence of relatively high microalgal biomass (in comparison with summer values) at this location. Pilsey Sands is a flood-dominated, accretionary sand flat of approximately 2 km2 in area, located within the mouth of the eastern-most estuary of the Solent estuarine system. The estuary is ebbdominated at the mouth, with mean spring and neap tidal ranges of 3.9 and 1.9 m, respectively (Hydrographic Office, 1998). Maximum spring tidal current velocities, measured at 0.15 m above the bed, were 0.4 m s1. The mean surface (0–1 mm) sediment grain size is 180 lm (fine sand) and the mean surface dry bulk density is 1300 kg m3. Throughout the year, surface chl a contents decrease, from a maximum of 28 lg gDW1 in December/January to a minimum of 0.2 lg gDW1 in June/July. On an annual time-scale, surface colloidal carbohydrate contents tend to vary inversely with chl a, with maximum values of 350 lg glucose-equivalent (GE) gDW1 in August and minimum values of <50 lg GE gDW1 in December. Over one to three month periods with relatively high diatom biomass, chl a and colloidal carbohydrate tend to covary. 2.2. Sampling Push cores (n ¼ 4) for pigment, colloidal carbohydrate and grain size analysis of the surface (0–1 mm) sediment fraction were collected at low water for each
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Fig. 1. Location map of Pilsey Sands intertidal flat, Chichester Harbour, UK.
station. Samples for pigment and colloidal carbohydrate analysis were freeze-dried (light excluded), and maintained at 80 C prior to analysis within one week of sample collection. Sediment chl a (microalgal biomass proxy) and phaeopigment (grazing/senescence indicator) contents were measured spectro-fluorometrically; they were quantified using the modified equations of Parsons et al. (1984). Colloidal carbohydrate (EPS proxy) was quantified using a double saline extraction procedure (Underwood et al., 1995), followed by a phenol– sulphuric acid assay (Dubois et al., 1956). 2.3. Physical measurements Sediment stability measurements (n ¼ 5) were made using the CSM (Paterson, 1989). The CSM is an in situ erosion device, which employs the stress induced by a perpendicular water jet to erode the surface of exposed intertidal sediments. An empirical calibration based on the equations of Bagnold (1966), modified by McCave (1971), allows the eroding pressure to be expressed in terms of an equivalent horizontal bed shear stress. For a full description of the CSM, see Tolhurst et al. (1999). The ambient temperature and salinity of the seawater used in the CSM were obtained using a standard, precalibrated field salinometer manufactured by the technical department of the School of Ocean and Earth Science, University of Southampton. Bed elevation was described using an accretion/erosion parameter (AEP) (West and West, 1991) obtained from two pairs of steel
poles placed 1 m apart at each station. Distances to the bed (n ¼ 8) were measured sufficiently far from the poles, to minimise any scour effects. Vertical accuracy was 2 mm. Grain size analysis was by a wet sieving method, which gave repeatable results using only a small sample size (<5 g), and which did not require any pretreatments (Wheatcroft and Butman, 1997). Meteorological data were obtained from a meteorological office weather station located adjacent to the intertidal flat. Tidal information was obtained from Admiralty tide tables (Hydrographic Office, 1998). During the study period, the weather remained overcast, with occasional sunny intervals: there was a light to moderate S to SW wind (mean speed of 3.9 m s1), and no rain fell during emersion. Maximum wind speed during preceding immersion periods was 5.4 m s1. Throughout the study period, the tidal range decreased from 4.4 to 3.9 m. 2.4. Statistical analyses Paired t-tests were used to examine significant differences (p < 0:05) between day- and night-time parameters. The AEP was converted to relative bed elevation for the paired t-tests, to avoid the use of negative values. Parameters were tested for normality of distribution and homogeneity of variance and transformed, where necessary, using standard transformations. Principal components analysis (PCA) was used to detect continuous variation in the distribution of
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variates between sampling times. Unlike most statistical analyses (which use a deductive approach), PCA uses an inductive approach to generate hypotheses. A series of eigenvalues and their corresponding eigenvectors (components) are produced from a matrix of similarities. By replacing the original variates of the raw data by the first few components with large eigenvalues, the data can be summarised and inherent patterns detected. Multiplication of each eigenvector with the original data matrix, produces a new data set of component scores. The components provide graphical co-ordinates (loadings) for the attributes of the data matrix, and the component scores for the individuals on the components. After suitable numerical scaling, the two sets of points are superimposed in a biplot, to assist in the interpretation. In the present study, the attributes were the variates of the data set, and the individuals were the consecutive sampling periods. Eleven PCs were computed using the MinitabÒ 12.2 computer program (Minitab Inc., PA, USA). The physical forcing terms, wind speed and tidal range during the preceding immersion, were included in the PCA. An adaptation of the MacArthur (1960) model, based upon species effectiveness, was used to determine the number of significant PCs. 2.5. Index of biostabilisation A coefficient of biological stabilisation (Yallop et al., 1994; Tolhurst et al., 1999) was calculated from the ratio of sc derived using the CSM (sc bio), to sc for the resuspension of abiotic sand (sc abio); this was sand with a diameter equal to the mean grain size of the 0–1 mm depth fraction, at each station during the CSM deployment. sc abio was calculated using the Shields criterion (Shields, 1936) and the Bagnold (1966) equation, as modified by McCave (1971), for the resuspension of sand. Settling velocities used in the modified Bagnold equation were calculated from the equations of Gibbs et al. (1971) for quartz spheres, as corrected by Baba and Komar (1981) for naturally worn quartz grains. The ambient temperature and salinity of the seawater used in the CSM were measured at the time of each CSM deployment, and were used in the density and settling velocity equations.
variable at the lower and upper stations, but increased generally at the middle station. During the medium-term study, no significant difference was detected in mean sc values between stations (one-way ANOVA), and sc was positively correlated with chl a at the lower station (r ¼ 0:704, p ¼ 0:011). Between March and April at the middle station, sc was correlated positively with chl a for six consecutive weeks (r ¼ 0:922, p ¼ 0:009). For the pooled data set, no significant correlation was found between sc and colloidal carbohydrate, or chl a. In the day-night study, considerable temporal and spatial variation of sediment properties was found at the lower, middle and upper intertidal stations (Fig. 3). A rhythmic variation between day- and night-time values of sc, chl a, and colloidal carbohydrate was apparent at the lower station; similarly, for the AEP at the middle station. At the lower station, sc increased at night and decreased during the day, varying inversely with both chl a and colloidal carbohydrate. Chl a, phaeopigments and colloidal carbohydrate covaried at the lower station, whilst other more subtle covariation between phaeopigments, AEP, and grain size was evident for the middle station. At the upper station, surface phaeopigment contents increased at night and decreased during the day. A list of parameters relative to sampling period and station is provided in Table 1. Ambient seawater salinity varied between 33.9 and 34.8. The minimum seawater temperature occurred at night (4.1 C), with the maximum during the day (9.4 C) (Table 2). The mean sc varied between 0.9 (0.2) N m2, at the middle station during the night, and 1.5 (0.1) N m2 at the lower station in the night (Fig. 4). The highest mean chl a content was 5.3 (0.3) lg gDW1 (lower station, during the day), whilst the lowest was 1.8 (0.3) lg gDW1 (middle station, at night). The mean colloidal carbohydrate content was consistently higher during the day, compared with the night, particularly at the lower and middle stations. The mean bed elevation increased during the night at the middle station. The highest mean phaeopigment contents occurred at the lower station during the day. Interestingly, phaeopigment contents at the middle and upper stations were greater during the night, than in the day. The mean grain size varied from 150 to 160 lm at the lower and upper stations; it was 180 lm at the middle station.
3. Results Data collection and sampling were nested within a medium-term, weekly sampling program of intertidal flat properties related to sediment stability (Fig. 2). The day-night study occurred during a general decrease in chl a measured, between March–May 1999, at the lower, middle and upper intertidal flat stations; similarly, during a decrease in colloidal carbohydrate at the lower and upper stations, but an increase at the middle station. Trends in sc before and after the day-night study were
3.1. Comparison between day- and night-time emersion periods Paired t-tests identified significant differences between day- and night-time colloidal carbohydrate (p ¼ 0:004) and chl a (p ¼ 0:011) content at the lower station, phaeopigment content (p ¼ 0:027) and bed elevation (p ¼ 0:048) at the middle station, and phaeopigment content (p ¼ 0:017) at the upper station. Significant
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Fig. 2. Results of the medium-term, weekly study (March–May 1999) of sc, chl a and colloidal carbohydrate at lower (A), middle (B), and upper (C) intertidal stations. The location of the day-night investigation reported in this paper is shown by the hatched area.
differences between day- and night-time sediment stability (sc) were not detected, for any of the stations. For the lower and middle stations, PCs 1 and 2 were significant; for the upper station, only PC 1 was significant (Table 3). PCs 1 and 2 accounted for 76% of the total variation at the lower station, and 73% of total variation at the middle station. PC 1 accounted for 48% of the upper station total variation. The lower station PC 1 (Fig. 5) discriminated clearly between dayand night-time n-scores. Emersion periods ÔDay 3Õ (D3) and ÔDay 5Õ (D5) showed a close correlation with phaeopigments and seawater temperature, and a weaker correlation with chl a, colloidal carbohydrate, grain size,
and settling velocity. D1 was affiliated closely with AEP and tidal range. Night n-scores (particularly N2 and N4) were affiliated closely with sediment stability (sc). For PC 2, tidal range and AEP were the most important attributes. PC loadings showed that chl a, colloidal carbohydrate, grain size, settling velocity and seawater temperature were correlated closely; phaeopigments were weakly positively correlated with this group. AEP and tidal range were correlated clearly. As with the lower station, the middle station PC 1 discriminated clearly between day- and night-time n-scores (Fig. 6). However, night n-scores were differentiated by phaeopigments and AEP, whilst D3 and D5
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Fig. 3. Temporal and spatial variation of surface sediment properties (SE mean) for consecutive day (D) and night (N) emersion periods: March 24–26, 1999.
n-scores were differentiated by seawater temperature, colloidal carbohydrate content, together with sc. D1 was affiliated closely with chl a, grain size and tidal range. For PC 2, tidal range was the most important attribute. PC loadings showed that temperature, colloidal carbohydrate content and (to a lesser extent) sediment stability were positively correlated; likewise, chl a, grain size and tidal range. Positive correlations were also apparent for phaeopigments and AEP. In contrast with both the lower and middle stations, the upper station PC 1 failed to discriminate clearly between day and night n-scores; instead, n-scores were discriminated by the (non-significant) PC 2 axis (Fig. 7). D1 was affiliated closely with chl a; similarly, N6 with phaeopigments. Mean wind speed, seawater salinity, grain size, tidal range, AEP and sediment stability were correlated closely, as were settling velocity and temperature.
3.2. Sediment stability Coefficients of biostabilisation for night-time emersion periods (average 16.7) at the lower station were significantly higher (paired t-test: p ¼ 0:011) than daytime coefficients (average 10.5) (Table 4). At the middle and upper stations biostabilisation indices were more variable, with no significant differences detected between day and night. The maximum biostabilisation coefficient (17.8) occurred at the lower station during emersion period N4; the minimum biostabilisation (4.6) occurred at the middle station, during D3 and N4.
4. Discussion The complex interplay between physical and biological properties related to sediment stability is
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Table 1 Mean parameter values at the lower (L), middle (M) and upper (U) stations for consecutive day (D) and night (N) sampling periods, numbered 1–6: March 24–26, 1999 (Coll carbo: colloidal carbohydrate; Phaeo: phaeopigments; Set vel: settling velocity) Sampling period
Station
sc (N m2)
Coll carbo (lg GE gDW1)
Chl a (lg gDW1)
Phaeo (lg gDW1)
AEP (cm)
Grain size (lm)
Set vel (cm s1)
D1 D1 D1 N2 N2 N2 D3 D3 D3 N4 N4 N4 D5 D5 D5 N6 N6 N6
L M U L M U L M U L M U L M U L M U
1.15 1.62 1.75 1.36 0.94 1.26 1.06 0.75 0.65 1.76 0.65 1.25 1.06 1.84 1.36 1.37 1.16 1.46
115.1 92.5 91.9 81.0 103.7 134.3 124.1 131.2 134.5 88.4 99.7 85.0 110.5 105.1 120.5 81.5 68.6 109.6
5.8 2.2 3.1 3.9 2.1 2.3 5.4 2.0 2.1 3.5 2.2 1.9 4.8 1.8 2.2 3.4 1.2 1.8
0.4 0.1 0.1 0.2 0.2 0.2 0.5 0.2 0.1 0.3 0.3 0.2 0.4 0.1 0.2 0.4 0.3 0.3
0.2 0.3 0.2 0.3 0.4 0.2 0.3 0.1 0.4 0.3 0.4 0.5 0.3 0.2 0.5 0.3 0.4 0.6
157 189 162 150 184 158 153 183 157 153 177 159 155 178 157 153 176 159
1.3 1.7 1.3 1.1 1.5 1.2 1.3 1.7 1.3 1.3 1.5 1.3 1.3 1.6 1.3 1.2 1.5 1.3
illustrated in the rhythmic temporal and spatial patterns across the intertidal flat (Fig. 3). During the course of the investigations, cyclical variations between day and night were observed for all the parameters at (at least) one of the stations. It is unlikely that the reasons for such variations can be attributed to a single causal mechanism. However, the main difference between day and night, the light/dark condition, is likely to be an important factor, particularly in areas of high microalgal biomass. The PCA differentiated clearly between day- and night-time conditions for the lower and middle intertidal flat (Figs. 5 and 6). However, the reasons for the similarity between the lower and middle stations, together with the dissimilarity between these two stations and the upper station (Fig. 7), are unclear; nonetheless, they may be related in some way to the proximity to the upper station of a ÔhardÕ shoreline sea defence. A similar difference between lower and middle station n-scores, and upper station n-scores was found in a PCA of 32 intertidal flat parameters, carried out as part of a long-term study into biological influences on sediment stability on Pilsey Sands. 4.1. Biostabilisation An index of biological stabilisation (Manzenrieder, 1983), based upon the ratio of the biotic to abiotic critical threshold of sediment motion, is a useful way of comparing biostabilisation between different sites and different erosion instruments. For cohesive sediments, a lack of understanding of the physics of particle erosion, together with selection of a suitable abiotic control site (Yallop et al., 1994), means that such indices
are relative; as such, they are dependent upon qualitative decisions made by the erosion device operator. For example, indices based upon ÔbiofilmÕ and Ônon-biofilmÕ areas (e.g. Tolhurst et al., 1999) are likely to underestimate the extent of biostabilisation. In the present study, the temperature and salinity of the ambient seawater used in the CSM at the time of deployment were measured, along with the mean grain size of sediment immediately adjacent to the test area. The theoretical abiotic suspension threshold could then be calculated, using the equations of Bagnold (1966) and McCave (1971), for the resuspension of sand. Hence, as the CSM was calibrated using the same resuspension equations (Tolhurst et al., 1999), an objective method of deriving the sc bio =sc abio ratio was achievable. Furthermore, the method is easily repeatable elsewhere, allowing direct comparison of biostabilisation coefficients between non-cohesive intertidal flats. On Pilsey Sands, all of the sediments showed some degree of biostabilisation (Table 4). The highest
Table 2 Ambient seawater salinity and temperature, together with mean wind speed and tidal range during preceding immersion for consecutive day (D) and night (N) sampling periods, numbered 1–6: March 24–26, 1999 Sampling period
Salinity
Seawater temperature ( C)
Wind speed (m s1)
Tidal range (m)
D1 N2 D3 N4 D5 N6
34.8 34.5 34.1 34.6 33.9 34.5
8.2 4.1 9.4 8.3 9.6 6.1
5.4 4.7 2.1 5.3 3.2 3.9
4.4 4.2 4.1 4.0 3.9 3.9
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Fig. 4. Mean parameter values (SE mean) for day- and night-time emersion periods at the lower, middle and upper stations.
biostabilisation coefficient was 17.8 at the lower station during emersion period N4; the lowest coefficient (4.6) occurred at the middle station during D3 and N4. A comparison of biostabilisation indices at Pilsey Sands with indices from other studies is somewhat restricted because the CSM has been little used on non-cohesive Table 3 PCA of day/night parameters for each station. Significant eigenvalues are highlighted in bold. Also illustrated are the proportion of variance explained by each eigenvector, and the minimum variance required for significance Station
PC
Eigenvalue
Variance (%)
Cumulative variance (%)
Minimum variance (%)
Lower
1 2 3 1 2 3 1 2 3
5.45 3.72 1.58 4.62 3.40 1.76 5.22 2.66 2.38
45.4 31.0 13.2 42.0 30.9 16.0 47.5 24.2 21.7
45.4 76.4 89.6 42.0 72.9 88.9 47.5 71.7 93.3
27.5 26.6 25.7 27.5 26.6 25.7 27.5 26.6 25.7
Middle
Upper
sediments. However, Ôbubble matÕ sediments with a mean particle size of 257 lm on the island of Texel, Netherlands, showed a biostabilisation coefficient (ratio of biotic sediment to sediment with organic matter removed), measured with the CSM, of 21.7 (Yallop et al., 1994). In the same study, a coefficient of 9.6 was reported for non-bubble mat sediments, of mean grain size 154 lm; this size, however, was below the CSM resolution for abiotic sediment (Tolhurst et al., 1999). The high biostabilisation coefficients found at Pilsey Sands agree with Amos et al. (1997), in which it was suggested that the effect of biostabilisation increased with grain size. It should also be noted that coefficients of biological stabilisation derived using the CSM (with the exception of measurements made on plane bed surfaces) are likely to be higher than coefficients derived using other erosion devices (e.g. Microcosm, SedErode). This is because the CSM integrates erosion over a smaller area, incorporating less spatial heterogeneity of biochemical parameters and bed roughness elements (Tolhurst et al., 2000). Furthermore, the method of calculating biostabilisation coefficients used in this study
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Fig. 5. PCs 1 and 2 and scaled component scores for the lower station. Emersion periods are labelled consecutively (1–6), according to day (D) or night (N) occurrence.
is likely to produce higher values than those derived from in situ comparisons of ÔbioticÕ and ÔabioticÕ sediment. 4.2. Day- and night-time sediment stability Very few in situ observations of day- and night-time parameters related to sediment stability have been made, making comparisons with other studies difficult. In the Southampton Water investigation the significant reduction in sc observed for cohesive intertidal flat sediments at night was attributed to a lack of vertical migration by benthic diatoms during the dark; hence, a reduction in migration-related EPS production (Herrington, pers. comm.). However, EPS production for diatom migration is expected to be independent of light conditions (de Brouwer and Stal, 2002), occurring in response to both light and tidal rhythms (Seroˆdio et al., 1997; Smith and Underwood, 1998). Furthermore, the
Southampton Water study was restricted to four consecutive day/night emersion periods (with n ¼ 2 replicates) and, whilst higher chl a values are reported for day-time emersion, EPS was not measured. In the present study, biostabilisation of the relatively high microalgal biomass, lower intertidal flat sediments was greater at night than during the day, despite lower chl a and EPS contents at night (Table 4). Reasons for this are unclear, but may be related to: (1) sediment binding by uronic acids produced during degradation of bound EPS at night (de Brouwer et al., 2002); (2) poor detection of uronic acids using a double saline extraction procedure; and (3) the presence of acetyl or other group organic polymers which promote substrate adhesion, but are not detectable using the phenol–sulphuric acid assay (de Brouwer, pers. comm.). In addition, other influences not considered in this study, e.g. the production of EPS by bacteria and benthic fauna, as well as the role of animal feeding and water surface tension may be
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Fig. 6. PCs 1 and 2 and scaled component scores for the middle station.
important. Whilst a significant difference existed between day- and night-time EPS (Fig. 3), no significant difference between sc values was detected, possibly as a result of the resolution of the CSM Mk III used in this study. 4.3. Temperature and other effects The possibility exists that lower temperatures (of the ambient seawater) during the night were responsible for less EPS dissolution compared with day-time values, causing the apparent night-time increase in the stability of the lower intertidal sediments (Fig. 3). For example, an increase in the temperature of incubation water can increase the amount of extracted polysaccharides (Staats, pers. comm.). Other, more subtle temperature effects at night were expected to affect: (1) particle settling velocities, causing a relative increase in grain size (Kro¨gel and Flemming, 1998); and (2) bed elevation, causing a relative decrease in bed level. Note the close correlation between seawater temperature and: (1) settling velocity, grain size, colloidal carbohydrate and chl a at the lower station (Fig. 5); (2) sc, colloidal carbohydrate and (to a lesser extent) settling velocity at the middle station (Fig. 6); (3) settling velocity at the upper station (Fig. 7). No such temperature effects, however, were detected; in fact, at the middle station during night emersion, bed elevation increased whilst grain size decreased. Phaeopigments at the lower station increased during the day and decreased at night, covarying with chl a (Figs.
3 and 4). Grazing by herbivores can limit the standing stock of benthic microalgae, leading to a decrease in sediment stability (e.g. Gerdol and Hughes, 1994). The surface phaeopigment content suggests that herbivoral grazing was an important factor in the lower intertidal area; it may explain the observed decrease in sediment stability, during day-time emersion periods. Higher phaeopigment contents at the middle and upper stations at night, compared with day-time values, probably indicated increased grazing activity at these stations at night.
5. Conclusions The in situ investigation of intertidal flat sediment properties, as related to sediment stability, remains central to many of the recent and ongoing multidisciplinary intertidal flat projects, e.g. LISP-UK (Black and Paterson, 1998); INTRMUD (EU MAS3-CT950022); F-ECTS (EU MAS3-CT97-0145); BIOPTIS (EU MAS3-CT97-0158). In such studies, data and sample collection is undertaken normally during daylight emersion periods (e.g. Amos et al., 1988; Paterson et al., 2000; Herman et al., 2001). However, very few studies have examined intertidal flat sediment stability at night (and in different seasons, for that matter); this is probably as a result of the safety and logistical considerations required for data and sample collection during the hours of darkness.
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673
Fig. 7. PCs 1 and 2 and scaled component scores for the upper station.
In this study, surface sediment stability (sc) was investigated for six consecutive day- and night-time emersion periods on a sandy intertidal flat in spring. Rhythmic variations in day- and night-time sediment stability and related parameters (chl a and EPS) occurred in the higher microalgal biomass areas. Here, the highest biostabilisation coefficients of 15–18 were recorded at night, compared with maximum day-time coefficients of 11. Significant differences in day- and night-time biostabilisation coefficients existed in high microalgal biomass areas, however, differences between day- and night-time sc were not detected. Enhancements in sediment stability at night in high biomass areas may have been due to the presence of uronic acids in soluble EPS, not easily detected with the extraction procedure used in the present study. It is recommended that future day/night EPS studies use Table 4 Coefficients of biostabilisation for consecutive day (D) and night (N) sampling periods: March 24–26, 1999 Sampling period
Lower station
Middle station
Upper station
D1 N2 D3 N4 D5 N6
10.8 17.4 10.8 17.8 10.0 14.9
9.4 6.9 4.6 4.6 11.9 8.8
16.3 13.4 5.9 11.5 12.3 14.1
extraction and analysis procedures more likely to detect uronic acids (see de Brouwer and Stal, 2002). Rhythmic variations also occurred between day- and night-time values for other physical and biological properties in areas with lower microalgal biomass. These variations were difficult to explain, illustrating the complexity of the temporal and spatial variability of intertidal flat parameters. The results show that significant differences between day- and night-time sediment stability may exist, but that improvements in the CSM resolution as well as a more extensive sampling program are required for their in situ detection. Note that the new version of the CSM (Mk IV) offers enhanced resolution with smaller incremental pressure increases (Black, pers. comm.). Recent studies have found a poor correlation between carbohydrate and sediment stability (e.g. Paterson et al., 2000; de Brouwer et al., 2002; Defew et al., 2002). Others indicate that differential production and degradation rates exist for bound and soluble EPS fractions during exponential and stationary diatom growth phases (Smith and Underwood, 2000; de Brouwer and Stal, 2002). It is therefore recommended that sediment stability during different growth phases and under varying EPS solubility is investigated using laboratory microcosm flumes. It is further recommended that future in situ investigations examine high microalgal biomass areas, not only on sandy intertidal flats, but also on muddy intertidal flats where day/night differences may be more easily
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detectable, and may be especially significant where there are migratory biofilms (Tolhurst et al., in press). In addition, comparative studies at the same sites are required in different seasons in order to elucidate the effects of temperature changes and associated variations in EPS production. Furthermore, it is hoped that the development of new in situ instrumentation will permit the investigation of parameters during submersion, as well as emersion periods. Results from this study have implications for the accuracy of time-dependent estuarine modelling studies, which are calibrated using sc values obtained during day-time emersion periods only.
Acknowledgements This study was funded by the UK Natural Environment Research Council and ABPmer Ltd. under award no. GT04/97/269/MAS. The authors wish to thank: the Ministry of Defence, Thorney Island, for providing unlimited access to Pilsey Sands; the UK Meteorological Office for the provision of meteorological data; and K. Davis for drawing the location map. J. de Brouwer is thanked for his helpful advice throughout the preparation of this paper, and B.W. Flemming, T.J. Tolhurst and one anonymous reviewer are thanked for their comments and suggestions in the review stage. This paper is one of a series of publications produced for the Estuary Processes Research Project (EstProc) (DEFRA/ Environment Agency project no. FD1905).
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