The character and motion of suspended particulate matter over the shelf edge and upper slope off Cape Cod

Continental Shelf Research, Vol. 8, Nos. 5-7. pp. 789--809.1988.

0278-4343/88$3.00 + 0.00 ~) 1988PergamonPress plc.

Printed m Great Britain.

The character and motion of suspended particulate matter over the shelf edge and upper slope off Cape Cod JAMES H . C H U R C H I L L , * PIERRE E . B I S C A Y E t a n d FRANK A I K M A N

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(Received 7 July 1986; in revisedform 12 February 1987; accepted 12 August 1987) AbstractwTime series of suspended particulate matter (SPM) concentration made by transmissometers deployed on the shelf edge and upper slope off New England are examined together with data from companion sensors. At the shelf-edge instrument cluster (on the 125-m isobath) the highest SPM concentrations were apparently due to sediment resuspended further onshore and advected to the transmissometer by offshore motion of the shelf-edge density front. This was due to a tendency for more frequent episodes of high bottom stress, and thus more sediment resnspention, progressing onshore from the outer shelf. The decrease of surface wave current amplitude with depth was primarily responsible for this cross-shelf gradient in bottom stress. On one occasion very turbid water was observed at the shelf edge when the bottom stress was low and the frontal motion was small. Data obtained from the U.S. National Marine Fisheries Service showed unusually intensive bottom trawling activity in the vicinity of the transmissometer during this event. This data further indicates that resnspension by trawlers may be an important source of suspended material on the shelf edge. The record of the transmissometer deployed over the upper slope indicates that transport of particulate material in suspension from the near bottom on the shelf into the slope region occurred primarily along density surfaces, and was not continuous but intermittent.

1. I N T R O D U C T I O N

THE S E E P (Shelf Edge Exchange Professes) experiment involved d e p l o y m e n t of a m o o r e d instrument array along 70°55'W in waters of the continental shelf and slope. D a t a were collected from this array, the configuration of which is shown in Fig. 1, from S e p t e m b e r 1983 to O c t o b e r 1984, with a hiatus of about 3 weeks during April 1984 for instrument recovery and redeployment. A primary objective of this experiment was to test the hypothesis that the continental slope of the Mid-Atlantic Bight is a m a j o r sink for fine-grained particles entering or produced within water of the continental shelf. The fate of these particles is of practical importance because they carry a large portion of the contaminants discharged into coastal and shelf waters. CSA~ADY et al. (1988) found that high speed near-bottom currents occur much less frequently on the slope than over the adjacent continental shelf and rise, suggesting that the slope environment is most favorable for sediment deposition. If the slope is a region of net sediment accumulation, then it is of interest to determine the pathways and estimate by what rate material moves from the shelf onto the slope. BISCAYE et al. (1988) have presented analysis of data from sediment traps on the S E E P array which provides insight as to the flux of material into and through the slope domain. This p a p e r is devoted to estimates of suspended * Woods Hole Oceanographic Institution, Woods Hole, MA 02543, U.S.A. t Lamont-Doherty Geological Observatory of Columbia University, Palisades, NY 10964, U.S.A. 789

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particulate concentration made by two transmissometers deployed on the SEEP array, one at 10 m above the bottom on the shelf edge and the other near the middle of the water column on the upper slope (see Fig. 1 for exact locations). Here we shall use the transmissometer data together with measurements from nearby temperature, salinity and velocity sensors to examine the spatial and temporal distribution of suspended particulate matter (SPM) over the shelf edge and upper slope. We shall also investigate causes of sediment resuspension in this region and what role the shelf-edge density front plays with regard to the seaward transport of material. Presentation of this analysis will be preceeded by a description of the oceanographic and sedimentological setting in the vicinity of the SEEP array, and a discussion of the transmissometer's operation and calibration. 2. O C E A N O G R A P H I C SETTING

2.1. Hydrography The hydrography over the shelf and slope of the Mid-Atlantic Bight exhibits a seasonal character which is well documented (e.g. BIGELOW, 1933; KETCHUMand CORWLX. 1964; CRESSWELL, 1967; LWE and CSANADY, 1984; BEARDSLEYet al., 1985). During the winter and late autumn, intense storms and convective mixing through surface cooling vertically mix shelf water to the bottom and slope water to a depth of about 150 m. This creates a sharp surface to bottom front at the shelf edge separating shelf water from warmer, more saline and denser slope water. From late spring to early autumn, weather systems are generally less intense than during winter and solar radiation provides buoyancy to nearsurface waters. As a result a pycnocline develops below a surface mixed layer. A typical summertime hydrographic transect, taken along the SEEP mooring line, is shown in Fig. 2. Note that the pycnocline of this section is nearly horizontal across the shelf and slope. There is, however, a near-bottom density front which intersects the bottom at the shelf edge, in the vicinity of the transmissometer on SEEP mooring 2. From examining hydrographic transects of the Mid-Atlantic Bight compiled by LYNEand CSANADY(1984), we find that the near-bottom density front persists throughout the year, and that its center typically intersects the bottom at about the 100-m isobath (somewhat shoreward of the location of the shelf-edge transmissometer). This front always coincides with a nearbottom salinity front, and in most sections also with a front in temperature. 2.2. Dynamics AIKMAN el al. (1988) and HOUGHTONet al. (1988) have examined velocities and frontal displacements at the shelf edge using data from the SEEP moorings. Their findings which are pertinent to the ensuing analysis of the transmissometer measurements are briefly reviewed below. Near-bottom displacements of the shelf-edge front were of order 20 kin, much larger than had been anticipated. The salinity measured by near-bottom sensors at the shelf edge (on moorings 1 and 2) was highly correlated and close to 90* out of phase with the north, or cross-isobath, velocity component. The above result implies that much of the salinity variance at these sensors was due to cross-isobath excursions of the nearbottom salinity/density front. At subtidal frequencies, cross-isobath velocities measured at the shelf edge were highly correlated with the along-isobath component of wind stress. Phases between these velocities and the wind stress are indicative of a classical

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September 1983. upwelling-downwelling flow structure, with near-surface flow to the right of the alongisobath wind and opposing flow near the bottom. 3. S E D I M E N T O L O G I C A L

SETTING

Figure 1 displays the percent of silt plus clay (particle diameter <0.0625 ram) in surficial sediment over the Mid-Atlantic Bight, contoured from the data of I-L~THAWAY

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(1971). SEEP moorings on the shelf (moorings 1 and 2) were located in an area of fine surficial sediment (>30% silt plus clay) commonly referred to as the "Mud Patch". From acoustic and coring surveys, TWlCHELLet al. (1981) report that this feature is actually a 213 m thick lens of fine-grained material which overlies coarser sediment. Profiles of SPM over the Mud Patch have been measured by BISCAYEand OLSEN (1976), BOTHNERet al. (1982), B ~ et al. (1985) and as part of the SEEP experiment. A typical cross-section of transmissometer beam attenuation, which is roughly proportional to the concentration of suspended particles, is shown in Fig. 2. In general, near-bottom water over the Mud Patch has a relatively high concentration of SPM, most of which is non-combustible, inorganic material. For example, BOTHNERet al. (1982) found SPM in water samples taken during fairly calm atmospheric conditions at 1 m above the bottom from the Mud Patch to be more than 75% non-combustible and of concentration ranging between 1000 and 2000 I~g 1-1, roughly twice the typical concentration measured in near-bottom water samples from the shelf region outside the Mud Patch. Near-bottom slope water directly offshore of the Mud Patch has been found to be much less turbid than its counterpart over the shelf (cf. Fig. 2). The station density of surveys which have been carded out in this region is too sparse, however, to determine if a turbidity front actually coincides with the near-bottom density front at the shelf edge. An important concern regarding the signal from the transmissometer at mooring 2, which was 10 m above the bottom (mab), is: Would locally resuspended sediment extend in measurable concentration to this transmissometer? To address this question we refer to data presented by B ~ (1987) from a transmissometer deployed on the mid-shelf (67-m isobath) in a region of the Mud Patch with surficial sediment of size distribution very similar to that near mooring 2 (based on the data of HATHAWAY,1971). In Fig. 13.10C of Butman's report, the signal from the transmissometer, which was 15 mab, shows significant increases in suspended particulate concentration coincident with storminduced increases in bottom stress, indicating that locally resuspended sediment was measurable to more than 15 mab. 4. INSTRUMENTATION

Transmissometers deployed on the SEEP moorings (described in detail by BARTZ et al., 1978) measure the percent of light transmitted through a 25 cm long collimated beam. These instruments are relatively insensitive to temperature (0.1% error over a temperature range of 0-25°C), and use nearly monochromatic red light (660 nm wavelength at maximum power) which is not appreciably attenuated by dissolved organic matter (JERLOV, 1976). The light transmission, T, measured by the transmissometer is related to a beam attenuation coefficient, c, through T(r) = e

where r is the beam pathlength. A number of laboratory studies have shown that for particles with a small grain size range, transmissometer beam attenuation is linearly related to suspended mass concentration ( B ~ and MooDY, 1983; BAKER and LAWU.E, 1984). The proportionality constant, however, strongly depends upon the dominant grain size, smaller particles being more efficient light absorbers per unit mass than larger ones. Nevertheless, field calibrations carried out seaward of the energetic

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nearshore environment (at water depth >30 m) have found definite linear relationships between beam attenuation and suspended mass concentration, often over a large area and range of water depth (BAKERand LAVELLE,1984; GARDNERet al., 1985). Here we consider only the beam attenuation signal, but note that this should be roughly proportional to, or at least monotonically change with, the mass concentration of suspended particles. For those interested, the attenuations reported hear are approximately related to mass concentration, 7, through the calibration formula determined by GARDNER et al. (1985): y = 1208 c - 496, where c and 7 have units of m-z and pg l-z respectively. 5. N E A R - B O T T O M

BEAM ATTENUATION

SIGNAL AT THE SHELF EDGE

5.1. Event analysis The shelf-edge transmissometer was located on mooring 2, 10 mab at the 125-m isobath (Fig. 1). For this instrument, data analysis is restricted to measurements from the period 11 September to 15 December 1983. At other times either the transmissometer signal was seriously degraded due to bio-fouling of the light sensor, or the companion cluster of temperature, conductivity and velocity senors did not function. The beam attenuation time series of this period, shown in Figs 3b and 4c, is characterized by

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"events" of relatively high attenuation (or equivalently high SPM concentration) which last for a few days and are separated by periods of low attenuation. In this section we shall relate these events to measurements of other properties with the aim of determining by what mechanism and to what extent sediment in this region of the shelf edge is locally resuspended. In Figs 3 and 4, ten events of high beam attenuation are identified.Also shown in these figures are time series of hourly averaged speed and salinity measured by sensors 5 m below the transmissometcr, the root mean square (r.m.s.) of surface wave-induced horizontal velocity at 5 m below the transmissometer (computed using surface wave height spectra and bottom pressure data as described in Section 5.1.4), and the surface wind stress measured by N O A A buoy EB04 (located at 38°18'N, 71°42'W). H o w the attenuation events relate to these measurements is summarized by Table I and discussed in detail below. 5.1.1. Relationship with near-bottom speed. Of the 10 events considered, seven occurred during a period when the near-bottom speed exceeded 20 o n s'x for some

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Table 1. Summary of events observed by the mooring 2 transmissometer

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High beam attenuation (BA) is well related to high near-bottom speed (NBSP), and also corresponds to a drop in salinity. Occurs during a lull in the wind stress. NBSP exceeds 20 cm s-t only for a few hours at the outset of this event. Occurs during a period of high NBSP, yet spikes in BA and NBSP do not coincide. Unusually low wind stress and sluggish near-bottom currents during this event. Consists of two BA spikes: the first coinciding with high NBSP and NBWC, the second with low NBSP and NBWC. Occurs during low NBSP but shortly after a local maximum in NBWC. High BA of this event confined to a definite range of salinity, suggestive of a turbid cloud trapped within isopycnals of the front. Well correlated with change in salinity. Peak wind stress, NBSP, and NBWC occur about 12 h after the peak in BA, and 2.5 days after the event's initiation. A pulse of high NBSP appears to initiate this event. Coincides with a slight drop in salinity. The one event clearly related to windinduced NBSP and NBWC. Peak BA, however, occurs during a lull in the wind, NBSP and NBWC, but during a salinity minimum.

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t Wind stress. ~t Near-bottom surface wave currents. § The relationship is salinity increasing with decreasing beam attenuation, and vice versa.

portion of the event. With the exception of event 10, the high values of near-bottom speed were primarily due to motion at the semidiurnal tidal frequency. From this observation, one may suppose that the threshold for sediment resuspension in this region is near 20 cm s-1. However, on three occasions near-bottom speed exceeded 20 cm s-1 but the beam attenuation was low (these were only short duration pulses of speed, though). Also, the high-frequency spikes in beam attenuation seldom exactly coincided with spikes in near-bottom speed. 5.1.2. Relationship with salinity. Note in Fig. 3 that with the exception of event 4, which is considered later, elevation in beam attenuation coincided with a decrease in salinity. This trend was particularly dramatic from mid-October onward. Especially noteworthy are events 6 and 8, which coincided with a large reduction in salinity yet began during a time of low near-bottom speed. Recall from Section 2 that salinity measured at the shelf edge is a fairly good indicator of frontal position, and note that the salinity axis of Fig. 3 nearly spans the typical range of salinity which brackets the shelf-

Suspended particulate matter off Cape Cod

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edge front (Fig. 2). Beam attenuation at mooring 2 thus apparently increased during seaward excursions of the shelf-edge front, then shifted to low values as the front retreated towards shore. 5.1.3. Relationship with wind stress. Only one event, 10, may be attributed to strong near-bottom currents generated by the surface wind stress. However, even during this event the relationship between wind stress and beam attenuation is not clear-cut. The beam attenuation was actually greatest during a 1 1/2 day lull in the wind which began on about 12:00 of 5 December. 5.1.4. Relationship with surface waves. It is well established that oscillating currents due to surface waves significantly contribute to bottom stress and are a major factor in the resuspension of sediment over the inner and middle shelf of the Mid-Atlantic Bight, i.e. at the bottom depths <70 m (LAVELLEet al., 1978; BUTMANet al., 1979; LESHTet al., 1980; TWlCHELL et al., 1981; BUTMArq and MooDy, 1983; BUTMAN, 1987). To our knowledge there has been no reported experimental study regarding the effect of surface wave currents on the bottom environment over the outer shelf of the Mid-Atlantic Bight. The r.m.s, of surface wave velocity 5 mab at mooring 2 (125-m isobath) was estimated by applying linear wave theory to spectra of wave height from N O A A buoy EB08 (see Fig. 1 for location). Unfortunately, this buoy produced no wave height data from 20 November to 21 December 1983. Surface wave velocities for this period were determined by the following procedure using pressure measurements from U.S. Geological Survey (USGS) tripod T, which was deployed on the 100-m isobath, 60 km from EB08 and 80 km from mooring 2 (Fig. 1). The tripod's pressure sensor was located 1 mab and obtained a burst of 12 samples, spaced 4 s apart, every 7.5 min. Standard deviation of the burst pressure samples was highly correlated with the r.m.s, of near-bottom surface wave velocity computed from the wave spectra (corr. coef. = 0.87). Using the linear rclation between the two variables, determined by least-squares, we computed r.m.s, surface wave velocities from the burst pressure standard deviations over the time period when buoy data were missing. The composite time series of r.m.s, wave velocity is shown in Fig. 4a. Three events of high beam attenuation were initiated during a time of relatively strong near-bottom currents due to surface waves. However, there was no clear relation between beam attenuation and surface waves; attenuation was frequently low when nearbottom wave currents were strong, and vice versa. 5.1.5. Overview. Examination of the time series of Figs 3 and 4 has suggested that the highest beam attenuations recorded at mooring 2 were, in general, not due to locally resuspended sediment, but to particulate matter advected from further onshelf. A case in point is event 10. This event began on 4 December during a period of strong eastward wind stress which produced relatively vigorous flow and surface wave currents near the bottom. The largest attenuations of this event, however, occurred during a i 1/2 day lull in the wind stress, and when the near-bottom wave currents and hourly averaged speeds were particularly weak. The high attenuations during this lull also coincided with a local minimum in the salinity signal from mooring 2, which is indicative of maximum seaward excursion of the near-bottom shelf-edge front. Clearly, event 10 was the result of sediment resuspended by storm-induced bottom stress; but the most turbid water seen at the transmissometer during this event contained sediment resuspended further onshelf and advected to mooring 2 by offshore frontal motion.

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An interesting trend seen in Figs 3 and 4 is that wind stress; beam attenuation, hourly averaged near-bottom speed, r.m.s, surface wave current, and changes in near-bottom salinity all tended to attain greater values during the latter portion of the records (excepting event 4 with regard to the beam attenuation record; this anomalous event is discussed in Section 5.2). A plausible explanation for this, which is supported by the forthcoming analysis, is as follows. The generally more intense wind stress episodes later in the record produced greater bottom stresses and thus more sediment resuspension over the shelf. Recalling the discussion of Section 2.2, these events of large wind stress also resulted in lengthy frontal excursions, thereby bringing to the transmissometer more turbid and less saline water from further onshelf. Several events of high beam attenuation in Fig. 3 actually consist of a number of abrupt spikes in the attenuation record. For example, both events 5 and 7 are made up of two beam attenuation spikes, one initiated during a time of strong near-bottom currents and the other occurring during a period of relatively tranquil flow near the bottom. Also, during the latter portion of event 6 a succession of beam attenuation spikes are seen. That these spikes often coincided with weak near-bottom currents indicates that they were not, in general, due to intermittent local resuspension of sediment. Rather they are evidence of distinct and relatively small turbid clouds. Some indication as to the size of these clouds may be gained from the displacement of water past the transmissometer coincident with the beam attenuation spikes. From the velocity record taken 5 m below the transmissometer, we found that most spikes of beam attenuation, more than 0.5 m-1 above the background signal, had coincident Eulerian water displacement in the range of a few hundred meters to 5 km, with the alongshelf component of displacement generally 2-3 times larger than the cross-shelf component. A notable exception to this was event 10, which was the only event clearly initiated during a storm. This event coincided with an alongshelf water displacement of more than 33 km. At present we can only speculate as to the cause of these clouds. Some may have been produced by spatially limited sediment resuspension. As will be demonstrated in the next section, this was likely the case for event 4 which appears to have been produced by bottom fishing activity. Another process which may produce turbid clouds of limited extent, and has been shown to induce sediment resuspension on the outer shelf, is shoaling of internal waves ( B ~ s , 1987). It is also possible that some clouds may have initially been part of a much larger region of SPM, as would be expected from storminduced sediment resuspension over the mid-shelf, which was broken up through horizontal velocity divergence and associated vertical motion. 5.2. Event 4--Can it be blamed on trawlers? Upon first consideration event 4 is puzzling. It had by far the highest beam attenuation of the record; yet it occurred during a time of particularly low wind stress, weak nearbottom flow, and small frontal displacement (salinity change) near mooring 2 (Figs 3 and 4). Although it persisted for more than 5 days, this event was apparently due to a relatively small and patchy cloud of SPM. The dimension of water which moved by the transmissometer during this event, estimated from the record of the near-bottom current meter at mooring 2, was approximately 3.5 km in both the alongshelf and cross-shelf directions. The spikes of beam attenuation which made up this event corresponded to water displacements of only a few hundred meters. Spikes of high beam attenuation coincident with sluggish near-bottom flow were also observed in transmissometer records

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from Long Island Sound by BOHLENand WII,n~ICK(1984), who found that they were due to sediment resuspended by nearby bottom fish trawls. In Corpus Christi Bay, SCrn~EL et al. (1978) found that relatively small shrimp trawls generate tremendous turbid plumes, attaining widths of over 100 m and maximum concentrations of up to 500 mg F t. To determine if event 4 may be due to sediment stirred up by trawling, we have obtained records of commercial fishing activity in the vicinity of mooring 2 from the U.S. National Marine Fisheries Service (NMFS) at Woods Hole, MA. These indicate that the region close to mooring 2 was intensively fished shortly before and during event 4. From 14 to 19 October 1983, 14 trawlers were operating in a 14 × 18.5 km area (10°lat × 10*long) upon which mooring 2 was nearly centered. During this time period these vessels trawled for a total of 207 h, which was 85% of the total trawling done in this 10' × 10' rectangle over the 11 September-15 December 1983 period of the transmissometer record. Multiplying the hours fished by the typical trawling speed of 5.5 km h-1 and by a typical trawl size of 40 m gives the total bottom area disturbed equal to 46 km 2, which is 18% of the 10' × 10' area. That such intense trawling occurred near mooring 2 during a time of particularly sluggish near-bottom currents (both steady and surface wave-induced) and small frontal motion was fortunate in that it has allowed us to conclude, to a good degree of confidence, that the beam attenuation spikes of event 4 resulted from sediment resuspension by trawling. 5.3. Spectral analysis The relationship of beam attenuation to other properties is quantified here using crossspectral analysis. Because event 4 accounts for much of the attenuation variance but is unrelated with any other measured property, it would unduly bias and has therefore been excluded from this analysis. This event fortuitously fell between two distinct phases of the period under consideration as was noted above; that is, generally greater wind stress, beam attenuation, near-bottom current and frontal motion after the event. Also, event 4 occurred less than 1 week prior to the so-called "fall overturn", i.e. the episodic homogenization of shelf water by convective mixing. Figure 5 displays coherence and phase spectra relating beam attenuation with nearbottom speed at mooring 2 for the period prior to event 4. Note that the coherence is generally much greater in the semidiurnal tidal band than at higher or lower frequencies. The phase difference between beam attenuation and speed is this band is dose to 90", which suggests that the high coherence in the band is not due to local sediment resuspension by tidal currents, as would be indicated by a near-0* phase difference. Coherence and phase relating beam attenuation at mooring 2 with water properties measured 5 m below the transmissometer are shown in Table 2. The estimates listed in this table are from calculations carried out over time series which bracket event 4, and have been summed into three frequency bands: subtidal (14 h - 10.7 day periods), semidiurnal tidal ( 1 1 . 4 - 1 4 h) and super-tidal (<11.4 h). (The diurnal tidal band contained very little velocity variance relative to the semidiurnal band and thus is not considered separately.) In the subtidal band, beam attenuation is much more highly correlated with salinity than with speed, indicating that low-frequency changes in suspended sediment concentration measured at mooring 2 were, in general, associated with movement of the shelf-edge front rather than with changes in near-bottom speed. This notion is supported by the high coherence and near-90* phase difference of attenuation with the north, or cross-shelf, velocity component in the subtidal band after

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Table 2.

Coherence and phase relating beam attenuation with other properties measured by near-bottom sensors on mooring 2 S u b f i d a l band ( 1 4 h - 10.7 d a y s )

Per. 1 N Vel. E Vel. Temp.

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Per. 2 0.44 8 8 ° +_ 2 0 ° 0.56 129 ° + 14 ° 0.31 148 ° _+ 32 ° 0.51 1670 + 16 ° 0.26 16° +- 370

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( 1 1 . 4 - 14 h)

Per. 1 0.74 38 ° + 2 7 ° 0.55 100 ° + 4 8 ° 0.45*

_ 0.61 92 ° + 4 0 ° 0.56 - 9 6 ° _+ 470

Per. 2 0.71 2 0 ° + 22 ° 0.62 101 ° + 28 ° 0.50 93 ° _+ 4 0 ° 0.59 970 _+ 31 ° 0.51 - 1 2 9 ° + 39 °

( 1 1 . 4 h)

Per. 1 0.38 _ 6 ° _+ 12 ° 0.31 79 ° + 15 ° 0.10" _ 0.23 - 1 0 ° + 21 ° 0.11 8 0 ° _+ 5 0 °

Per. 2 0.33 - 2 6 ° _+ 11 ° 0.25 370 + 15 ° 0.19 61 ° + 20 ° 0.20 5 4 ° + 19 ° 0.08* -

* Not significantly different f r o m z e r o t o 9 5 % confidence. Positive phase indicates beam attenuation is leading. Confidence intervals of phase are at the 95% significance level. P e r i o d 1 is 9 S e p t e m b e r - 1 0 O c t o b e r 1983. P e r i o d 2 is 21 O c t o b e r - - 8 D e c e m b e r 1983.

event 4. (Note: the signal measured at a fixed point of a property with a constant gradient in a certain direction is 90 ° out of phase with the velocity component in the same direction.) Interestingly, the east velocity component is also highly correlated with beam attenuation after event 4, suggesting that the attenuation signal was also influenced by advection of suspended material in the alongshelf direction. In the semidiurnal tidal band the coherences of beam attenuation with speed are comparable to those with salinity; and, as pointed out above, phases between speed and attenuation are significantly different from zero, ruling out local resuspension as a major cause of beam attenuation variance in this band.

Suspended particulate matter off Cape Cod

801

5.4. Scatter plots

A straightforward means of examining the relationship between two variables at zero time lag is to simply graph one against the other in a so-called scatter plot. Figure 6 displays scatter plots of beam attenuation at mooring 2 against other variables. As in the cross-spectral calculations, values from event 4 have been excluded from these plots. In Figure 6a beam attenuation is plotted against near-bottom speed from mooring 2. This figure shows no discernible relationship between these two variables; i.e. elevated attenuation values occurred at both high and low speeds, as did low values of attenuation. Figure 6b shows a plot of beam attenuation against bottom stress at the 125-m isobath due to the combination of surface wave-induced currents and more slowly varying "steady" flow. The stress was determined by the method of L w ~ et al. (1988), which is based upon the theory derived by GRAm" and MA~S~.N (1979). This method computes stress using a measurement of "steady" near-bottom current together with the standard deviation of pressure due to surface waves. Hourly averaged speed from mooring 2 was used to approximate the steady current. The standard deviations of bottom pressure fluctuations were determined by applying linear wave theory to wave height spectra from buoy EB08, 2.5

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802

J . H . CHURCHILL a d .

with gaps in the buoy'S record filled using pressure data from USGS tripod T in the same manner as for the r.m.s, wave-velocity time series of Fig. 4a. The vertical roughness scale of the bottom, required by this method, was to a value appropriate for the Mud Patch of 0.5 cm (V. LvnE, personal communication). As with beam attenuation and near-bottom speed, no relationship between bottom stress and beam attenuation can be seen in Fig. 6b. Particularly noteworthy is that the highest beam attenuations often occurred when the bottom stress was near zero. Taken together Fig. 6a and b support the notion that many of the episodes of high SPM concentration seen at mooring 2 were not due to locally resuspended sediment. Figure 6c is a plot of beam attenuation against salinity measured 5 m below the mooring 2 transmissometer, which reveals that progressively higher beam attenuations were recorded in water of lower salinity. This is consistent with the idea that turbid water encountered by the transmissometer was predominantly advected from onshelf by frontal motion. In this diagram, however, low as well as high attenuations coincide with lower salinities, indicating that high turbidity in this region, although always associated with less saline water, occurred intermittently. The absence of high SPM concentration in water with salinity characteristic of slope water in Fig. 6c suggests that the region seaward of mooring 2 was relatively (but not completely) inactive with regard to sediment resuspension. It also implies that SPM was, to some extent, inhibited from crossing the near-bottom shelf-edge density front. Some idea as to the length of cross-shelf excursions experienced by turbid water observed at the mooring 2 transmissometer may be gained from Fig. 6d which shows beam attenuation plotted against the north component of water displacement at the transmissometer. The latter quantity is the cross-shelf position which a water parcel would attain if it moved with a velocity as measured by the transmissometer's companion current meter. For example, a cross-shelf velocity series, vi, measured at time interval At gives a displacement series, y,,, through: n

Yn = ~

vi At,

i=1

where the displacements are relative to an arbitrary origin. Figure 6d indicates that high beam attenuations are associated with southern water displacements, again implying that turbid water was brought to the transmissometer via offshore frontal motion. Possible causes for the patterns seen in Fig. 6c and d include: an increase in the proportion of fine grain, easily resuspended, particles in surficial sediment shoreward of mooring 2, and an increase in the frequency and intensity of high bottom stress episodes shoreward of this mooring. Based on the displacement vs attenuation plot of Fig. (xi, the most turbid water observed at mooring 2 was advected from 15 to 20 km onshore, or from the vicinity of the 90-m isobath. This is consistent with Fig. 6c: salinities associated with the minimum and maximum beam attenuations in this figure are separated by a cross-shore distance of ~20 km in typical hydrographic sections. The map of Fig. 1 shows that the proportion of silt plus clay in surficial sediment between mooring 2 and the 90-m isobath does increase in the shoreward direction. However, the overall variation in surficial sediment texture in this region is modest, and seems unfikely, by itself, to be responsible for significant spatial differences in the amount of sediment resuspension.

803

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In Fig. 7a estimated bottom stress at the 90-m isobath is plotted against bottom stress at mooring 2 (125-m isobath). The stresses were computed by the method of LWE e t al. (1988) using surface wave-induced bottom pressure fluctuations determined from wave buoy and USGS Tripod T data as discussed above. For the computation of stresses at 90 m, hourly averaged speed from the near-bottom sensor of mooring 1 (which was actually on the 82-m isobath) was used to represent the "steady" current. Values in this figure are limited to the time of 9 September-7 December during which the near-bottom current meters from moorings 1 and 2 both produced reliable data. This figure shows that bottom stress at 90 m was, in general, 3-4 times that at 125 m. Based on the extension of Shields diagram to small grain sizes by MASTZ (1977) the threshold for resuspension of 37 lam diameter particles, the median grain size of surface sediment near mooring 2 (HATHAWAY, 1971), is about 1 dyne cm -2. This is close to a sediment resuspension threshold of 1.2 dyne cm -2 determined from experiments in a region of silty bottom sediment (median grain size =30 I~m) off California by HICKEYe t al. (1986). Supposing a minimum bottom stress for sediment resuspension of 1.0 dyne cm -2, Fig. 7a indicates significantly more frequent episodes of sediment resuspension near the 90-m than near the 125-m isobath. We thus conclude that the tendency of higher SPM concentration at mooring 2 in water from further onshore, as seen in Fig. 6c and d, was primarily due to a general increase in bottom stress and sediment resuspension progressing onshore from the moo'ing. Figure 7b is a plot of hourly averaged near-bottom speed measured at mooting 2 against that from mooring 1, and shows little overall difference in the magnitude of nearbottom flow at these locations. Clearly, the reduction in estimated bottom stress at 125 m relative to that at 90 m wPs predominately due to the attenuation with depth of surface wave-induced currents. It should be noted that although the near-bottom environment at SEEP mooring 2 (125-m isobath) appears to be relatively tranquil, very energetic near-bottom flow has been measured elsewhere on the shelf edge. For example, CSASADVet a l . (1988) display a

804

J . H . CHURCHn.J. et al.

record from a near-bottom current meter located 80 km east of the SEEP line and on the 202-m isobath which has velocity magnitude frequently exceeding 30 cm s-~ and occasionally >50 cm s-t. Present data is inadequate, however, to determine if this disparity in the vigor of near-bottom flow is a regional difference or due to a cross-isobath trend. 6. E F F E C T OF S U R F A C E W A V E S AND BOTTOM T R A W L I N G

In the previous section it was demonstrated that currents due to surface waves were responsible for a dramatic seaward decrease in bottom stress between the 90- and 125-m isobaths. It is of interest to investigate how near-bottom, wave-induced currents vary over the continental shelf. To do this we computed r.m.s, velocity due to surface waves 1 mab at several water depths by applying linear wave theory to frequency spectra of wave height from N O A A buoy EB08. A total of 6980 spectra from the period 1 January31 November 1984 were used. The results are given in Fig. 8 which displays, as a function of bottom depth, the portion of time which the r.m.s, of the horizontal component of near-bottom wave-induced velocity exceeds specified levels. This figure may be put in the context of bottom stress using the theory of GRAr,rr and MADSEr~(1979). For example, for a bottom with a vertical roughness scale of 0.5 cm and with a steady current of magnitude 20 cm s-t at 5 mab, this theory predicts that a near-bottom wave current of 12 s period and amplitude of 0, 5, 10, 15 and 20 cm s-t will result in a bottom stress of 0.5, 1.5, 3.0, 4.8 and 7.0 dyne cm -2, respectively. In light of this example, Fig. 8 indicates that oscillating currents associated with surface waves should significantly contribute to bottom stress production, and hence sediment resuspension, over the inner shelf (bottom depths <60 m). By contrast, this figure shows that at bottom depths >120 m the nearbottom r.m.s, wave current very seldom exceeds 5 cm s-t, indicating that surface waves do not appreciably influence bottom stress over the outer shelf edge and upper slope. Given the infrequency of high bottom stress over the outer shelf, one may suppose that an activity such as trawling, which has been shown to generate large turbid plumes by SCHUBELet al. (1978) and here by event 4, may significantly contribute to the overall quantity of sediment resuspension. An indication of this may be inferred from side-scan sonographs of the bottom presented by TWlCHELLet al. (1981) (their Fig. 3). A sonograph from the Mud Patch at 100 m water depth shows a high density of trawl lines (presumably

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Suspended particulate matter off Cape Cod

805

cut by the ~2 m wide trawl doors), we estimate as many as 20 lines over 100 m 2. This is evidence not only of intensive trawling, but also that the action of bottom currents in this region was relatively weak. The effect of commercial trawling on the sedimentary regime of the outer shelf may be gauged by the bottom area disturbed by trawl gear. From data compiled by the NMFS, we determined that during 1985 U.S. fishing vessels trawled a total of 452 vessel days in a region bounded by 70030'W and 71°30'W and by 40°N and 40°30'N, which extends roughly from the 70- to the 200-m isobath and includes much of the Mud Patch. More than 75% of this trawling occurred during January to April when fish stocks had migrated to the outer shelf from shallower waters. Multiplying the time of fishing by a typical trawl speed of 5.5 km h-l and trawl width of 40 m (as in Section 5.2) puts the total area trawled at 2390 km 2, roughly 50% of the region considered. Activity of foreign fishing vessels is catalogued into fishing windows by the NMFS. The window which overlaps the region dealt with above, window 4, is outlined in Fig. 1. Although fishing is permitted over the entire window, all trawling is done shoreward of the 160-m isobath. During 1985 foreign vessels trawled a total of 922 vessel days in this window. For the parameters listed above the corresponding bottom area disturbed is 4870 km 2, which is 80% of the region in this window subject to trawling (shallower than 160 m). Thus during 1985, which was not a year of unusually intensive fishing, U.S. and foreign fishing vessels operating over the shelf edge of the Mud Patch combined to trawl an area comparable to the region's overall size. It seems likely that trawling resuspends a large quantity of sediment in this area, and makes a significant contribution to the overall load of SPM at water depths where high bottom stress episodes are rare. The effect of trawling and other bottom fishing activity on sediment transport over the Mid-Atlantic Bight shelf is being investigated further and will be the topic of a later paper. 7. T R A N S M I S S O M E T E R SIGNAL FROM THE UPPER SLOPE

The transmissometer deployed on the upper slope mooring, mooring 4, was 115 m below the surface at the 500-m isobath (Fig. 1). The beam attenuation signal from this instrument (Fig. 9), although much smaller in magnitude, is similar to that from the shelf-edge transmissometer (Fig. 3) in that it consists mostly of abrupt spikes. These suggest that high turbidity in this region of the slope is of limited extent, either horizontally or vertically, or both. Statistically, and on an event-by-event basis, the record from this transmissometer is not significantly related to beam attenuation measured at mooring 2. This is of no surprise considering that the cross-shelf extent of turbid clouds seen at the shelf edge, as discussed in Section 5.1.5, was generally much smaller than the 30 km distance separating moorings 2 and 4. A number of hydrographic studies have demonstrated that during winter, following the formation of the surface to bottom shelf-edge front, waters of the outer shelf and upper slope exhibit a well-defined temperature and salinity relationship (BEARDSLEYand FLAGO, 1976; GOROONand AIgMA~, 1981; NIEDRAUER, 1981; LYNE and CSANADY, 1984). As a result temperature and density co-vary during this time period, and the form of the shelf-edge density field is nearly identical to that of temperature. From examining configurations of the wintertime shelf-edge thermal front, derived from SEEP mooring data, we found that the signal of the upper slope transmissometer was related to frontal orientation. This relationship is illustrated by Fig. 10, in which frontal formations

806

J. H. CHURCHILL et al.

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Fig. 10. Configurations of the thermal front at the shelf edge created using data from temperature sensors (dots) on SEEP moorings 1, 2, 4 and 6. Also shown are the maximum beam attenuations measured at the mooring 4 transmissometer (cross) on the given dates.

typically corresponding with high and low beam attenuations are shown. A temperature field which most often coincided with low beam attenuation, exemplified by Fig. 10a, has the front rising sharply seaward from the edge of the shelf, and has no isotherm joining the mooring 4 transmissometer with the shelf edge. A thermal configuration which frequently corresponded with high beam attenuation, displayed in Fig. 10b, has lower isotherms of the front extending nearly horizontally from the bottom at the shelf edge into the slope. Another frontal structure often associated with high beam attenuation at mooring 4 is shown in Fig. 10c. This has isotherms which vertically peak at the outer shelf and then descend seaward into the slope. These typifying examples indicate that beam attenuation at the mooring 4 transmissometer was generally high when an isotherm extended from the sensor to the near-bottom region of the shelf. Conversely, when there was no isotherm connecting the transmissometer with the shelf edge, beam attenuation was most often low. The large-scale slope of isotherms in the region of the mooring 4 transmissometer and the shelf edge should be roughly proportional to the difference between the temperature

807

Suspended particulate matter off Cape Cod

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measured by the 120 m deep sensors at moorings 2 and 4. For a time period which was after the formation of the surface to bottom shelf-edge front, beam attenuation at mooring 4 is plotted against this temperature difference in Fig. 11. This plot indicates that the highest beam attenuations occurred when the 120 m temperature at mooring 4 was nearly equal to, or slightly cooler than, the 120 m temperature at mooring 2, conditions corresponding to the thermal patterns of Fig. 10b and c. Only low beam attenuations were measured when the 120 m temperature was more than a degree warmer at mooring 4 than at mooring 2, which, referring to Fig. 10a, corresponds to a situation in which isotherms from the shelf edge passed far above the mooring 4 transmissometer. Noting again that temperature and density co-vary at the shelf edge during winter, the above implies that SPM observed at the mooring 4 transmissometer was primarily transported along isopycnal surfaces from the near-bottom environment of the shelf edge. From studies conducted in canyons off California, DRAKE and GORSLINE (1973) also found evidence of seaward sediment transport along isopycnals. The great scatter in beam attenuation at near-zero temperature difference in Fig. 11 suggests that the transport was intermittent. Furthermore, Figs 10 and 11 indicate there is relatively little cross-isopycnal transfer of sediment from near the bottom at the shelf edge into the slope region. This conclusion is supported by the plot of beam attenuation vs salinity data from the shelf edge (Fig. 6¢), as discussed in Section 5.4. 8.

DISCUSSION

The transmissometer record from mooring 2 has indicated that within the region of the shelf-edge front, near-bottom SPM concentration attains higher values in water of lower

808

J . H . CHURCHILLet al.

salinity. This appears to be due to a tendency of greater bottom stress, and thus more sediment resuspension, progressing onshore from the outer shelf. An upshot of the crossshelf gradient in bottom stress may be deduced by considering sediment resuspended at a particular isobath in the frontal zone, say at the 90-m isobath. This sediment will be subject to cross-shelf excursions, via frontal motion, of about 20 km. That which is advected to and settles out in deeper water is likely to be resuspended less frequently than sediment which is deposited in shallower water. The result should be a net seaward transport of sediment. The magnitude of this flux would depend upon the particle settling rate, and is likely to be greatest in regions of fine-grained sediment. On the outer shelf of the Mud Patch (water depths >110 m) where high bottom stress episodes are relatively rare, sediment resuspension by trawling should contribute significantly to this process. The transmissometer record from mooring 4 indicated that transport of particulate matter, in suspension, from the shelf edge to the mid-slope region occurs primarily along density surfaces. This transport was observed intermittently, which may have been due to randomness e~:her of the transport process or in resuspension at the shelf edge (the presumed source region). Acknowledgements--The series of frontal configurations referred to in Section 7 was provided by Dr Robert Houghton of L-DGO. Data of domestic and foreign fishing activity were supplied by Daryl Christeusen and Joan Palmer of the NMFS Woods Hole Office, and by Patricia Gerrier and John Ten'ill of the Goucester, MA Office. Dr Bradford Butman made USGS tripod data available and offered many suggestions during the course of this study. We are grateful to Dr Vincent Lyne for helpful advice and for supplying computer programs used to compute bottom stress. In addition, we have benefited from numerous discussions with Dr Gabriel Csanady. This research was funded by the Department of Energy through a contract entitled "Circulation and Exchange Processes over the Continental Shelf and Slope" with Contract No. DE-AC2-79EV10005 at WHOI and DEAC02-76EV 02185 GB at L-DGO. Woods Hole Oceanographic Institution Contribution No. 6276, and Lamont-Doherty Geological Observatory Contribution No. 4118. REFERENCES AIKMAI~ F. III, H. W. Ou and R. W. HOUGHTON (1988) Current variability across the New England continental shelf-break and slope. Continental Shelf Research, $, 625-651. BAKERE. T. and J. W. LAVELt~ (1984) The effect of particle size on the light attenuation coefficient of natural suspensions. Journal of Geophysical Research, 89, 8197-8203. BARTZ R., J. R. V. ZE~VEt.D and H. PAK (1978) A transmissometer for profiling and moored observations in water. Society of Photo Optical l~trumentation Engineering, 160, 102-108. BEARDSLEYR. C. and C. N. Ft.AGG (1976) The water structure, mean currents, and shelf-water/slope-water front on the New England continental shelf. Memoires Societe Royale des Sciences de Liege, 10, 209-225. BEARDSLEYR. C., D. C. CHAPMAn, K. H. BRINK, S. R. RAMP and R. SCHLITZ(1985) The Nantucket Shoals Flux Experiment (NSFE79). Part 1: A basic description of current and temperature variabilit2:'. Journal of Physical Oceanography, 15, 713-748. BIGELOWH. B. (1933) Studies of water on the continental shelf, Cape Cod to Chesapeake Bay. I. The cycle of temperature. Papers of Physical Oceanography and Meteorology, 2, 135 pp. BISCA~ P.E. and C. R. OtSEN (1976) Suspended particulate matter concentrations and compositions in the New York Bight. In: Middle Atlantic Continental Shelf and the New York Bight, M. G. GROSS, editor, ASLO Special Symposia No. 2, pp. 124-137. BISCAYE P. E., R. F. ANDERSONand B. L. DECK (1988) Fluxes of particles and constituents to the eastern United States continental slope and rise: SEEP--I. Continental Shelf Research, g, 855-904. BOHLE~ W. F. and K. B. Wn~IC~ (1984) Observations of near-bottom suspended matter concentration at the FVP site central Long Island Sound predisposal material area: April 18, 1983-June 29, 1983. Prepared for Science Applications Institute, New Port, R.I., 100 pp. B~R M. H., C. M. PARMENTER,R. R. REm~IGS, B. BUTMA~, L. J. POPPE and J. D. MH.LL'CAN(1982) Studies of suspended matter along the north and middle Atlantic outer continental shelf. U.S. Geological Survey Open F'de Report, 82-938, 41 pp. B~rMAN B. (1987) Physical processes causing surficial-sediment movement. In: Georges Bank, R. H. BACKUS and D. W. BOURNE,editors, MIT Press, Cambridge, MA, pp. 147-162.

Suspended particulate matter off Cape Cod

809

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