The relationship between concentrations of suspended particulate material and tidal processes in the Irish Sea

ContinentalShelfResearch,Vol. 13, No. 12, pp. 1325-1334, I993.

0278-4343/93 $6.00+ 0.00 ~) 1993PergamonPressLtd

Printed in Great Britain.

The relationship b e t w e e n c o n c e n t r a t i o n s o f s u s p e n d e d particulate material and tidal processes in the Irish Sea A. R.

WEEKS,*

J. H.

SIMPSONt

and D. BOWERSJ"

(Received 18 December 1991; in revisedform 20 August 1992; accepted 16 February 1993) Abstract The time-dependence of suspended particulate material (SPM) and its control by tidal currents was investigated in the Irish Sea from April to October in 1987. Recording transmissometers and current meters were deployed in mixed water (h = 80 m) in an area where strong reflectance in visible band imagery indicated high SPM concentrations. A seasonal signal was observed in the data with a reduction in beam attenuation (c) from late May to September. During spring and autumn a semi-diurnal variation in beam attenuation was interpreted as due to the advection of a west-east gradient in SPM. There was also evidence of local resuspension since c was positively correlated with current speed. Both processes contributed to generally higher values of c being attained during spring tides than at neap tides. There was no evidence of the influence of windstirring on SPM concentrations even when windspeeds exceeded 12.5 m s 1. A model to simulate the tidal control of SPM concentrations by resuspension and tidal excursion showed good agreement with the data during spring tides. It was less successful for the neap periods when the concentrations of SPM remained low.

INTRODUCTION

StJSPENOED particulate material (SPM) has an important role in the environment of the

shelf seas. Absorption and scattering by particulates limit the amount of radiation available to phytoplankton for photosynthesis. Chemical absorption and desorption of elements and compounds by particles modify the concentration of these substances in the surrounding sea water and in the particles. These processes are known to be crucial in the transport of some heavy metals (e.g. CARPENTERet al., 1981), while the surfaces of particles are recognized as important sites of microbial activity (HoPPE, 1984). The transport of fine particulates in the shelf seas, which may remain semi-permanently in suspension, is vital to understanding the constraints on phytoplankton growth and the fate of certain anthropogenic pollutants. There is considerable evidence indicating that tidal processes are important in initiating and maintaining the suspension of fine particulate material, although seasonality in the supply of particulates to the water column may also be important, especially during the summer. Concentrations of SPM were found to be positively correlated with current speed in

*Department of Oceanography, The University of Southampton, SO9 5NH, U.K. tSchool of Ocean Sciences, University College of North Wales, Menai Bridge, Anglesey, Gwynedd LL59 5EY, U,K. 1325

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A . R . WEEKS et al.

estuaries in studies by THORN(1975), OFFICER(1980), KIRBYand PARKER(1985) and PEJRUP (1986). JAGO (1981) showed that SPM varied with current speed in his study of the nearshore environment off the northeast coast of the UK. ROmNSONand SRISAENGTHONG (1981) found coherence between patterns of reflectance from remotely sensed images (Landsat) and semi-diurnal tidal cycles in their study of the Solent estuary. JOSEPH (1955) found measurements of beam attenuation varied with current velocities over a tidal cycle in water 29 m in depth at a site in the North Sea. Variations in SPM concentrations over the spring to neap tidal cycle in estuaries have been noted by ALLENet al. (1977), in straits by BUCHANet al. (1967) and in the sub-tidal environment by ZoaLISS (1977). Optical measurements of the diffuse attenuation coefficient of light by NEWTON (1986) showed higher values during springs than neaps in the Menai Straits. The seasonal cycle in SPM concentrations has been observed by others, notably BUCHAN et al. (1967) who found lower values during the summer months in the Menai Straits during 1961 and 1962. MITCHELSON (1984) noted a sharp reduction in SPM concentrations between April and June in 1981 and 1982, in mixed and frontal water in the northern Irish Sea. ROBINSON and SRISAENGTHONG(1981) noted a seasonal cycle in reflectance from Landsat visible band imagery, with high values in the winter in their study in the Solent estuary. In this paper we describe some recent results from a single point time-series of observations in a tidally energetic region of the northern Irish Sea and present a simple model that accounts for much of the observed variability.

THE OBSERVATIONS Figure l(a) shows the site where the continuously recording instruments were deployed from April to October 1987 in water of depth 80 m. A transmissometer and current meter, at 20 m below the sea-surface, were positioned 0.5 m apart in order to make simultaneous measurements of beam transmittance and current speed and direction. Water samples were taken alongside the mooring at intervals to allow the transmissometer data to be interpreted in terms of SPM concentrations. The instruments were recovered and redeployed at monthly intervals. In order to define the physical structure of the water column and the spatial variation in SPM in the region, vertical profiles of conductivity, temperature, depth and beam transmittance were taken at a grid of stations [Fig. l(b)]. A full account of the survey results is given in WEEKSand SIMPSON(1991) and WEEKS(1990). To measure the SPM concentration we used beam transmissometers which recorded the data onto Aanderaa thermistor loggers (TR-1) at 30 min intervals. The beam transmissometer, designed and manufactured at University College of North Wales, had a path length of 0.25 m and a red LED (light emitting diode, 660 nm, + 11 nm) was employed as a light source in order to minimise the effect of light absorbance by gelbstoff (JERLOV, 1976). A full description of the design of the transmissometer is given by WEEKS(1990). The data were used to determine the beam attenuation coefficient, c (m-l), according to c = -In r

where I is the measured radiant flux, Io is the radiant flux from the source and r is the

1327

Concentrations of SPM and tidal processes in the Irish Sea

(a)

54 ~

( 53' 6"

5 °

3"

4"

Ib)

5~o

•

o

•

53'6.

5°

71]

~o

3°

Fig. 1. (a) Position of the mooring from 6 April 1987 to 12 May 1987 and bathymetry of the region. (b) The area of the grid survey, Station positions = O; mooring position = ®; surface values of suspended particulate material (g m -3) between 30 April and 4 May 1987.

1328

A.R. WEEKSetal.

pathlength of the light beam (m). The recording current meters, of the rotor type, were made by Aanderaa (model 4) with sensors to measure speed, direction and depth. The CTD and transmittance surveys were carried out at monthly intervals, when the mooring was serviced. Profile measurements of beam attenuation were made with a transmissometer of 0.3 m pathlength (WEEKS, 1990) which was interfaced to a Plessey 9400 conductivity, temperature and depth probe. The concentration of the SPM was measured by gravimetric analysis using GF/C Whatman glass fibre filter papers (nominal retention 1.2 ~tm) by the method of STRICKLANDand PARSONS (1972). Algorithms were developed to convert the beam transmittance measurements to SPM (Total SPM = 3.94 + 3.0. c, r = 0.66) (WEEKS,1990). The seabed in the region of the mooring is predominantly gravel and rock, but pockets of muds and silts are present in sheltered areas. A study of the particle size distribution at a variety of mixed and stratified stations showed that in mixed regions there was no significant difference between surface and near bottom waters (WEEKS, 1990). The temperature and salinity data in our study showed that at the site of the mooring the waters remained mixed throughout a tidal period. We found that there was no change in particle size distribution or particle type throughout the water column by our analysis of scanning electron microscope photographs of the SPM taken from different depths at a similarly well-mixed station, east of the mooring. The particles were a mixture of clay and organic material aggregates, fragments of phytoplankton skeletal material, and single clay particles. Both the size distribution (Coulter Counter) and SEM studies showed that the particle size range was typically between 1 and 20 ~tm. It is possible that we have underestimated the upper size range, as the Coulter Counter breaks up aggregates bigger than 5-10% of the aperture size. Furthermore, not only are the SEM samples from a very small volume but the filtering process may also break up aggregates. RESULTS There was a strong seasonal signal in the time series of beam attenuation at 20 m below the surface from April to October. Beam attenuation was highest during April and decreased during the summer months. Values of c increased again in September and October. The records from 6 April to 12 May and from 12 September to 20 October showed a strong spring to neap tidal periodicity, reaching higher values of c at springs than at neaps. This can be seen in the record from 6 April to 12 May (shown in Fig. 2). Comparison with the wind speed measurements from R A F Valley in Anglesey [Fig. 2(d)] did not indicate any significant correlation of beam attenuation with windstress. This suggests neither windstress patterns nor locally generated waves are a primary control of sediment concentration at this location. By contrast, the influence of the tide can be clearly seen in the record with a greater range in values of c at springs than at neaps [Fig. 2(a)-(c)]. In order to examine the shortterm variability more closely a section of the record was expanded [Fig. 3(a)]. The 4-day section of the data taken during a time of spring tides from 26 April to 30 April showed that the beam attenuation record was continuously dominated by tidal forcing, with peaks twice daily. Superimposed on this is a pattern of 4-daily peaks in c. This tidal variation can be understood in terms of SPM variations due to the erosion of sediment from the seabed by tidal currents, visualised as 4-daily peaks [e.g. I and II in Fig.

Concentrations of SPM and tidal processesin the Irish Sea (a )

1329

8

E; x 5 (b )

',C)

~5

~

20

25

30

35

Bays

O 2

o

-r

(d)

IO

, . .

vvvvvvvvvvvvvvVVVVVVVVVVVVVVVVvVVVVVVVVI VVVVVVVVwvv v

,

20

: 5

IO

15

20

25

30

35

Days

Fig. 2. Timeseries from 6 April to 12 May 1987of (a) beam attenuation, c (m-1); (b) modulusof velocity, lul (m s-l); (c) eastward velocity, u E (m s-l); (d) wind speed from Valley, Anglesey, W (knots). Time marks are at midday on the day indicated.

3(a)] and the displacement of a horizontal gradient of SPM concentrations, visualised as 2-daily peaks [e.g. III and V in Fig. 3(a)]. The tidal excursion which did not exceed 10 km, was almost rectilinear in the region of the mooring. The parallel measurements of current speed and of eastward velocity, interpreted as eastward displacement, are shown in Fig. 3(b) and (c).

A CONCEPTUAL MODEL The combination of displacement and resuspension processes is represented in the schematic of Fig. 4. The values ofc at a point vary as a horizontal gradient in SPM oscillates in a rectilinear flow in the region of the mooring. With higher concentrations to the east, and the direction of the tide from east to west, maximum values of c occur when the tidal displacement is furthest to the west, conversely, minimum values of c are at the time of the greatest easterly displacement of water past the mooring. Values of c also increase with current speed [Fig. 4(b)], and reach maximum values every 6.2 h, in the tidal flow. When the two graphs are combined [Fig. 4(c)], the form of the curve shows a qualitative agreement with the observed signal (Fig. 3). A horizontal gradient in c was observed in the region with higher values to the northeast, during the CTD surveys at the beginning and the end of the deployment (WEEKS and SIMPSON, 1991). On the basis of this model, the variation of c at a fixed point in a tidal flow can be represented as

1330

A . R . WEEKSet al.

where d is the predicted beam attenuation coefficient, x is the tidal displacement of an east-west gradient in c, lul is the modulus of velocity and a, fl, ~, and n are constants to be determined, a is the mean gradient of c; fl is a measure of the efficiency of resuspension and ), is a constant representing the low frequency background signal.

(a)

7 6 A T E co

5 4 5 2 I I

0

(b)

7 6 5

E

4 5 2 I 0

H

(c)

-?t 'J (d) 6

vE

4 2

O

(e)

6

'E

v

~a

-2

L-

April.

1987

Fig. 3. Expanded time series from 26 April to 30 April 1987 at 20 m below the sea-surface; (a) beam attenuation (m-l); (b) current speed (m s-l); (c) eastward tidal displacement (km); (d) predicted beam attenuation (m-l); (e) the difference between measured and predicted beam attenuation (m-l). I and II are times of maximum current speed, lul (m s-l); III, IV and V are times of minimum current speed. IV is at the maximum displacement to the west, whereas III and V are at times of maximum displacement to the east. The lines I - V have been drawn solely to emphasize the similarity between the beam attenuation record and current speed and tidal excursion.

1331

Concentrations of SPM and tidal processes in the Irish Sea

Application of the model to the data Values of lul were taken from the measurement of current speed, x was calculated from velocity m e a s u r e m e n t s as x=

.dt.

(a)

w

I

{j .

.

.

.

.

E

b)

.--.

'E L~

W

(c)

I

E (a

LW l

HW i

Max.

Max

ebb

fl.ood

,

,

Fig. 4. The conceptual model: (a) the effect of the oscillation of a horizontal gradient in beam attenuation, increasing to the east, in a rectilinear tidal flow, on values of c; (b) the effect of tidal resuspension of sediment on values of c; and (c) the combined effect of (a) and (b) on beam attenuation.

1332

A . R . WEEKS et al.

Table 1.

Values o f a and fl when n = I f r o m the period 6 April 1987 to 12 May 1987

Days after start

y (m -1)

a (m -1 k m -1)

0-36 0-4 4-8 8--12 12-16 16-20 20-24 24-28 28-32 32-36

0.800 1.66 2.87 4.00 1.58 1.33 1.94 0.842 0.402 0.0537

-0.0857 -0.0127 -0.149 -0.214 -0.051 -0.020 -0.089 -0.013 -0.0235 -0.002

fl (m -2 s)

r2 (%)

Days(~) after spring tide

2.05 0.569 1.13 0.403 1.15 0.949 0.926 0.997 -0.001 0.417

33.2 30.0 53.2 71.8 21.3 33.3 73.8 29.3 16.7 66.3

N/A +6 .... -5 .... -1-.. +3... +8 .... - 3 . -+ 1•-+5 .... -6-..

6 1 +3 +7 3 +1 +5 6 +2

The residual current was filtered from the values of u so that only the tidal displacement was obtained. In order to determine a, fl and ~, a regression was calculated using the time series measurements of beam attenuation, current speed and direction from 4 April to 12 May, 1987. The tide was rectilinear in the region of the mooring and so the displacement was always in the same plane. The exponent n was varied between 0.25 and 2.0 but this did not significantly improve on the results with n = 1. The constants derived from the regression, with n = 1, were then used to calculate b, using the time-series of lu] and x. The results for the whole deployment show a coefficient of determination (r 2) of 33.2% for the fit of the model to the data (Table 1). The limited success of this fit is likely to be due in part to the variability of the horizontal gradient which is not allowed for in fitting the data with a single value of a. The model also will not account for the gradual seasonal decline in c, as it only simulates short term trends. To get around this problem, the model was applied to short (4-day) sections of the data and the procedure was carried out as before. When applied in this way the model explained - 7 0 % of the variance in the data at times of spring tides. At neaps when both displacement and resuspension were much reduced, it accounted for only 30% of the variance (Table 1). Predicted beam attenuation is compared with the measured signal in the plots of Fig. 3(a),(d) and (e). Values of the resuspension constant, fl, ranged from 0.4 to 1.15 m -2 s in all but one of the 4-day sequences of data, whereas values of the spatial attenuation gradient, a, ranged from - 0 . 0 0 2 to - 0 . 2 1 4 m -1 km -1. During spring tides the ratio of fl/a decreased, suggesting greater influence of tidal displacement due to an increase in horizontal gradients bringing clear or turbid water past the mooring; for example, resuspension elsewhere, yielding the gradient, increases in importance relative to resuspension at the measurement point. The gradual decline of the constant ~, over the period can be attributed to the seasonal fall-out of SPM observed in the time-series from April to September.

Concentrations of SPM and tidal processesin the Irish Sea

1333

DISCUSSION These first long-duration measurements of the suspended particulate concentration from moorings indicate the dominant control of SPM concentration by the strong tidal flow with little influence of windstress. The tidal control is exerted by the two different mechanisms illustrated schematically in Fig. 4. The displacement of horizontal gradients by tidal excursion results in semi-diurnal oscillations in concentration [Fig. 4(a)] while resuspension induced by ebb and flood flow produces a quarter-diurnal variation [Fig. 4(b)]. The combination of these two effects gives rise to the characteristic "twin peak" variation in the beam transmittance [Fig. 4(c)]. Regression analysis of 4-day sections of the record, shows that this conceptual model can explain over 70% of the section variance at the time of spring tides when tidal influence is greatest. With decreasing tidal range the percentage of variation explained by the regression diminishes to a minimum of 30% at neaps. The coefficient a, which is a measure of the horizontal gradient, exhibits considerable variability but is consistently negative corresponding to increasing concentrations in the north east direction. This is consistent with the sign of the gradients observed in the grid survey whose magnitude - 0 . 0 5 m -1 km -1 ( - 0 . 0 8 mg 1-1 km-1) is of the same order as the value of a deduced from curve fitting. The coefficient fl, which represents the efficiency of resuspension, is relatively consistent over the different data sets and is typically of order 1.0 m -e s. Surprisingly, there is no evidence of a phase lag between high values of beam attenuation and the current speed during deposition of SPM. A time-series of vertical profiles of beam attenuation and current speed taken during CTD casts over 13 h at this station and at a station 20 km to the east confirms that the peak in c coincided with the maximum current speed. Tidal resuspension and mixing may occur extremely rapidly over the whole water column. The mixing time for homogenous shear flow in 60 m depth is of the order of 20 min at maximum tidal currents. The water column was always well-mixed at the site of the mooring as shown by vertical profiles of c (WEEKS and SI~VSON, 1991). However, at other stations within the survey, where tidal currents were weaker, a vertical gradient in c was observed, with higher values deeper in the water column just as the flood tide was reaching its maximum speed (WEEKS, 1990). Although a 6-hourly signal in c is observed, it must be concluded that resuspension is either of material on the seabed or simply recruitment of SPM from deeper in the water column. As we have no information about values o f c very close to the seabed, it would be inappropriate to speculate further. An interesting observation is that values of c always reached the same low value ( - 2 m-1) at the point of furthest displacement of water to the east. This suggests the presence of a source of clear water which is not influenced by the magnitude of tidal stirring. If it is assumed that the greatest tidal excursion is 10 km, the source of the comparatively clear water is a deeper region to the southwest of the mooring. In contrast, the semi-diurnal peaks in c show a strong spring to neap periodicity. The inference from these observations is that waters to the east of the mooring in shallower waters are more subject to tidal resuspension of particulate material than the deeper waters to the west. A general decline in the value of 7 represents the seasonal decrease in the concentration of SPM that is frequently observed in the shelf seas at this season. The strong seasonal cycle of SPM in mixed water can be attributed to several processes. The reduction in SPM concentrations in mixed waters in the late spring and summer may be caused by a reduction

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A . R . WEEKS et al.

in the supply of SPM to the water column from the seabed, due to less wind-stirring in shallow water, or by an increase in the organic component of the sediments, or by an increase in sequestration on tidal flats. Acknowledgements--This study was undertaken with the support of a NERC studentship award to A. R. Weeks, which is gratefully acknowledged. We thank S. Boudjelas for her work on the analysis of the SPM by scanning electron microscope. P. Dewes and K. Saull contributed to the artwork. We thank the Captain and crew of the Prince Madog.

REFERENCES ALLEN G. P., G. SAUZAY,P. CASTAINGand J. M. JOUANNEAU(1977) Transport and deposition of sediments in the Gironde Estuary. In: Estuarineprocesses, 2, M. WILEY, editor, Academic Press, New York, pp. 63-81. BUCHANS., G. D. FLOODGATEand D. J. CRISP(1967) Studies of the seasonal variations of suspended matter in the Menai Straits. Limnology and Oceanography, 12,419--431. CARPENTERR., J. T. BENNETr and M. L. PETERSON (1981) 21°pb activities and fluxes to sediments of the Washington continental shelf and slope. Geochimica et Cosochima Acta, 45(Suppl.), 119-224. HOPPE H. G. (1984) Attachment of bacteria: advantage or disadvantage for survival in the aquatic environment. In: Microbial adhesion and aggregation, K. C. MARSHALL, editor, Life Sciences Research report no. 131, Springer-Verlag, New York, pp. 283-303. JAGOC. F. (1981) Sediment response to waves and currents on the North Yorkshire shelf of the North Sea. Spec. Pubis int. Ass. Sediment, 5, 283-301. JERLOVN. G. (1976) Marine optics, Elsevier, Amsterdam. JOSEPH J. (1995) Extinction methods to indicate distribution and transport of watermasses. Proceedings of UNESCO Symposium of Physical Oceanography, Tokyo, pp. 59-75. KIRBY R. and W. R. PARKER(1985) Distribution and behaviour of fine sediment in the Severn Estuary and the inner Bristol Channel. Canadian Journal of Fisheries and Aquatic Sciences, 40, 81-95. MITCHELSON E. G. (1984) Phytoplankton and suspended sediment distribution in relation to physical structure and water-leaving colour signals. Ph.D. thesis, University of Wales. NEWTON A. (1986) Nitrogen utilisation and succession in phytoplankton. M.Sc. thesis, University of Wales. OFFICER C. B. (1980) Physical dynamics of estuarine suspended sediments. Marine Geology, 40, 1-14. PEJRUP M. (1986) Parameters affecting fine-grained suspended sediment in a micro-tidal estuary, Ho-Bugt, Denmark. Estuarine and Coastal Shelf Science, 22, 241-254. RORINSON I. S. and D. SRISAENGTrIONG(1981) The use of Landsat MSS to observed sediment distribution and movement in the Solent coastal area. Proceedings of EARSel-ESA Symposium, Voss, Norway, 19-20 May 1981. ESA-SP-167, 221-232. STRICKLANDJ. H. D. and T. R. PARSONS(1972) A practical handbook of sea water analysis. Bulletin of the Joint Fisheries Research Board of Canada, 167, 311 pp. THORN M. F. C. (1975) Monitoring silt movement in an estuary. International Association for Hydraulics Research, Proceedings of the 16th Congress, Sao Paulo, C71. TOPLISS B. J. (1977) A study of optical irradiance in coastal waters. Ph.D. thesis, University of Wales. WEEKS A. R. (1990) Seasonal and tidal cycles of suspended particulate material in the Irish Sea. Ph.D. thesis, University of Wales. WEEKS A. R. and J. H. SIMPSON (1991) The measurement of suspended particulate concentrations from remotely-sensed data. International Journal of Remote Sensing, 12,725-737.