Interactions between hydrodynamics, benthos and sedimentation in a tide-dominated coastal lagoon

Marine Geology, 82 (1988) 61 81 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

61

INTERACTIONS BETWEEN HYDRODYNAMICS, BENTHOS AND SEDIMENTATION IN A TIDE-DOMINATED COASTAL LAGOON G A I L M. A S H L E Y 1 a n d R A Y M O N D

E. G R I Z Z L E 2.

1Department of Geological Sciences, Rutgers - - The State University of New Jersey, New Brunswick, N J 08903 (U.S.A.) 2Center for Coastal and Environmental Studies, Rutgers - - The State University of New Jersey, New Brunswick, N J 08903 (U.S.A.) (Received June 1, 1987; revised and accepted January 10, 1988)

Abstract Ashley, G.M. and Grizzle, R.E., 1988. Interactions between hydrodynamics, benthos and sedimentation in a tidedominated coastal lagoon. In: G.M. Ashley (Editor), The Hydrodynamics and Sedimentation of a Back.Barrier Lagoon-Salt Marsh System, Great Sound, New Jersey. Mar. Geol., 82: 61-81. Great Sound is a 6 km 2 shallow (average depth = 0.6 m) lagoon fringed by salt marsh and connected directly to the ocean by two large (5 and 10 km long) flood-dominated tidal channels which enter at opposite ends. The lagoon can be divided into three subenvironments: flood tidal delta and channel, transition area, and basin. Tidal deltas containing several distributary channels are deposited from expanding jet flow as tidal currents enter the lagoon. A physical process study (velocity profiles, salinity, temperature, suspended load and box cores) indicated that sedimentation patterns are determined by: (1) proximity to the point sources of sediment, the tidal channels, (2) spatial changes in tidal current strength and abundance of benthic organisms (primarily sand-trapping macroalgae), (3) dispersion of particulates by tidal and wave-generated currents, and (4) postdepositional processes including bioturbation and resuspension by waves. Tidal currents ranged from Umax> 40 cm/s (U0ma,= 4.0 cm/s) in the tidal delta channels, to Urea,< 20 cm/s (U.m,, = 2.6 cm/s) in transition areas to Ureax< 10 cm/s (U.oa, < 1.0 cm/s) in the basins. Umax,U,ma, = maximum current velocity and maximum shear velocity respectively. Typical suspended sediment loads were 10-50 mg/1 near the bottom; however, sediment transport was enhanced by wind-generated waves (up to 0.5 m wave height) which eroded the bottom creating concentration spikes of greater than 300 mg/1. Bottom sediments ranged from well-sorted fine sand in most delta and channel areas to basins composed of 90% silt and clay. Factors affecting sediment supply, transportation, deposition and resuspension interact in complex ways so that wide variability in the textural characteristics of deposited sediment occurs and no single factor (e.g., velocity) is well correlated with sediment characteristics. A conceptual model illustrating these spatial relations between hydrodynamics, benthos and sedimentation in the Great Sound system, was developed. The model should be useful in other tide-dominated coastal lagoons as a guide for design of benthic and sedimentary studies.

Introduction

latter

Estuaries are coastal water bodies which v a r y in g e o m o r p h o l o g y from deep t e c t o n i c and glaciated basins to relatively shallow drowned river mouths and back-barrier lagoons. The *Present address: Jackson Estuarine Laboratory, University of New Hampshire, Durham, NH 03824 (U.S.A.) 0025-3227/88/$03.50

type classified by P r i t c h a r d

(1967) as

"a bar-built estuary" covers environments f o u n d o n b o t h m i c r o t i d a l ( t i d a l r a n g e = 0 - 1 m) a n d m e s o t i d a l ( t i d a l r a n g e = 1 - 4 m) c o a s t l i n e s ( H a y e s , 1979). A s t i d a l r a n g e is a d o m i n a n t factor affecting physical processes on passive m a r g i n c o a s t l i n e s , o n e s h o u l d e x p e c t significant differences between back-barrier lagoons on the high and low ends of the tidal range

© 1988 Elsevier Science Publishers B.V.

62 spectrum. For example, microtidal coasts have long barriers with low elevations and widely spaced inlets. Tidal flow is weak and sediment transport is dominated by waves and the occasional storm which overtops the barrier forming washover fans (Kraft, 1971; Leatherman, 1975; Hayes, 1979) and storm-surge platforms (Boothroyd, 1986). Mesotidal barriers which have greater elevations, are shorter and have more closely spaced tidal inlets, allowing a greater volume of flow (tidal prism) under both fair-weather and storm conditions. Larger tidal ranges increase the flow strength causing many geomorphic and sedimentological processes to be dominated by tidal effects. Tidal currents carry organic particulates and dissolved nutrients, as well as terrigenous sediment (mineral and rock fragments) into the back-barrier environments where bidirectional flow aids lagoonal circulation. Biological productivity is typically high in coastal lagoons, producing an organic component to the accumulating sediment (Barnes, 1980). Benthic macrophytes are often abundant in the shallow areas where adequate light reaches the bottom and strong tidal currents do not inhibit their attachment and growth. These benthic macrophytes can substantially affect current flows and sedimentation patterns (Ginsburg and Lowenstam, 1958; Scoffin, 1970; Ward et al., 1984). Suspension feeders dominate the faunal benthos in some areas, and they can greatly accelerate sedimentation rates via biodeposition (Haven and Morales-Alamo, 1966). Most infaunal species affect bottom sediments by bioturbating activities which change sediment characteristics such as erodibility and compaction (Rhoads, 1974; Rhoads and Boyer, 1982). In the past, these lagoons were probably sinks for landward-migrating fine-grained sediments during the Late Pleistocene sea-level rise (Meade, 1969; Boothroyd et al., 1985), and thus, their recognition provides important information on position of paleo-sea levels. However, identification of lagoon deposits in the sediment record is not always straightforward (Frey and Howard, 1986).

In modern lagoons sedimentation incorporates sediment-borne and dissolved pollutants moving in the surface water (Kelley, 1975; Nadeau and Hall, this issue). Thus, an understanding of the complex interactions that occur in them between physical and biological processes is important. The present study is based on field research in subtidal areas of Great Sound, New Jersey (Fig.l). The paper focuses on the interactions between tide- and wave-generated currents and benthos on sediment dispersal and accumulation in a tide-dominated lagoon. Specific objectives include: (1) characterizing tidal current velocities and sedimentary processes which result from tidal flow entering and exiting via large ocean-to-lagoon channels, and (2) assessing the nature of interactions between tide- and wave-generated currents, benthic organisms and sedimentation.

Geomorphic setting Great Sound is 6 km 2 in area and roughly circular in plan (Fig.l). The lagoon is shallow with an average depth of 0.6 m at mean low water. Intertidal to subtidal sand flats (flood tidal deltas) incised by small distributary channels occur at the three points where tidal flow enters the open water body; Ingram Thorofare in the northeast and Cresse Thorofare and Gull Island Thorofare, both in the south. These appear to be areas of rapid sedimentation formed under expanding jet flow as flood currents enter the lagoon (Fig.2). An elongate E - W oriented deep area originally separated the two tidal delta complexes (Ashley, this issue). This deep area appears to have been due to a lower sedimentation rate (compared to the deltas) because of its distal location from the tidal channels, the point sources of sediment. Dredging of the Intracoastal Waterway in 1913 and subsequent maintenance dredging has created drastic changes in bathymetry. A straight, 2 - 3 m deep channel now cuts across from north to south dividing the deep central area into two

63

Fig.1. Map of Great Sound showing location of data collecting sites and boundaries of subenvironments.

smaller basins. Side-casted dredge m a t e r i a l s line the I n t r a c o a s t a l W a t e r w a y with an artificial levee. T h e l a g o o n can be divided into t h r e e sube n v i r o n m e n t s based on g e o m o r p h o l o g y and w a t e r depth: (1) t h r o u g h - f l o w i n g delta distribut a r y c h a n n e l s r a n g i n g from > 5 m deep to i n t e r t i d a l and a s s o c i a t e d tidal delta shoals r a n g i n g from <0.5 m at m e a n low w a t e r to intertidal; (2) t r a n s i t i o n a l areas, 0.5-1 m deep; and (3) basins, d e e p e r t h a n 1 m (Fig.l).

Methods Field methods C u r r e n t velocities were m e a s u r e d d i r e c t l y with a M a r s h - M c B i r n e y 201M E M c u r r e n t m e t e r at the twelve study sites (Fig.l). Measurem e n t s were made with a probe a t t a c h e d to a fixed and sliding rod a p p a r a t u s w h i c h allows a c c u r a t e ( + 1 cm) e s t i m a t i o n of probe h e i g h t above the bed. Most v e l o c i t y m e a s u r e m e n t s

64

Fig.2. The tidal delta located in the northeast part of Great Sound at low tide (August 6, 1986). Through-flowingIngram Thorofare leads to Townsends Inlet in the distance.

were made at five-eight heights, from 2 cm above the bottom to within 5-10 cm of the water surface. The meter has a 20 s time constant and thus displays the velocity averaged over a 20 s period. At each height, six instantaneous readings were taken at 10 s intervals and averaged. Each site was visited on three-five different days, and tidal ranges encountered varied from neap (1.0 m) to spring tide (1.5 m) conditions. Most measurements were made on relatively calm days so that the wave component of the measured velocities was minimal. Bottom sediments were collected from each of the twelve study sites with a 5 cm diameter PVC piston corer. Only the upper 2-3 cm of sediment was used to determine sediment t e x t u ral characteristics. Most sites were also sampled with a clear acrylic box corer (internal dimensions 4 x 13 x 35 cm) in order to X-ray sediments for sedimentary structures, and for sediment profile imaging (Rhoads and Germano, 1982). Salinity and t e m p e r a t u r e were measured in situ at eight sites (Fig.l, Table 1) using a Yellow Springs I n s t r u m e n t Company meter

with the probe placed 5 - 1 0 c m above the bottom. Water samples from the eight sites (Fig.l) were collected with a horizontal Alpha bottle 2-15 cm from the bed. Wat er was filtered t h r o u g h 200 pm mesh plastic cloth to remove large zooplankton and detritus and kept cold until analyzed.

Laboratory methods All water samples were filtered within 1 day after collection. Triplicate aliquots (100500 ml) for each analysis were washed under vacuum t h r o u g h W h a t m a n GF/C glass fiber filters (pore size 1.2 pm) which had been previously ashed for 3 h at 475°C and weighed. Dry weight of total suspended particulate m a t t e r (SPM) was determined by drying the preweighed filter with SPM at 100°C for 3 h. P a r t i c u l a t e inorganic m a t t e r (PIM) and particulate organic m a t t e r (POM) fractions were determined after ashing the dried filter with SPM at 475°C, also for 3 h. Grain-size analysis of bottom sediments was carried out on an Elzone Analyzer (par-

65 TABLE 1 W a t e r q u a l i t y d a t a from n e a r - b o t t o m s a m p l e s t a k e n b e t w e e n A u g u s t 14 a n d O c t o b e r 4 1986 a n d b e t w e e n M a y 26 a n d S e p t e m b e r , 1986 Site

S P M (mg/1) Min.

Max.

P I M (mg/1)

P O M (mg/1)

Salinity (%o)

T e m p e r a t u r e (°C)

,~

Min.

Max.

,~

Min.

Max.

.~

Min.

Max.

X

Min.

Max.

Channel and delta A 49.0 180.0 B 47.9 80.5 C 48.2 187.6 1A 23.2 319.3 22 19.6 102.0

69.8 63.0 77.8 64.2 48.7

38.3 39.7 39.2 19.7 17.1

147.5 68.6 152.0 271.3 83.6

56.8 51.6 63.2 53.6 40.4

7.7 7.1 9.0 3.5 2.5

32.5 16.9 35.6 48.0 18.4

12.9 11.4 14.5 10.5 8.3

26.5 26.0 27.0 24.5 26.5

33.5 33.0 32.5 32.0 32.0

30.4 30.2 30.2 29.7 30.0

19.0 19.5 18.5 19.0 18.0

28.5 28.5 28.0 27.0 26.5

24.7 24.4 24.2 24.0 22.4

Transition 1B 22.6 13 25.3

307.3 414.4

67.9 99.1

19.2 21.7

255.7 353.7

57.3 84.1

3.4 3.3

51.6 60.7

10.5 14.9

23.5 25.0

32.0 32.5

29.6 29.8

19.0 19.5

27.0 27.0

23.9 23.9

Basins 3 24.6

411.7

79.7

21.3

346.0

67.0

3.3

65.7

12.6

24.0

32.5

29.8

19.5

27.0

24.1

SPM -- total suspended particulate matter. PIM -- particulate inorganic matter. POM -- particulate organic matter.

ticle counter). Each sample was treated with concentrated hydrogen peroxide to remove organics, and four replicates were run for each sample. Total organic matter (TOM) was determined by drying triplicate aliquots of each sample for 12 h at 100°C, and then ashing at 475°C for 4 h (Byers et al., 1978). Analytical methods

Mean velocities taken on the several sampling days (at each site) were plotted relative to time within the 12 h 25 min tidal cycle. Thus, a '~composite" tidal cycle was obtained which was used to characterize variations in shape of the tidal curve by subenvironment. The "vertical profile" method was used to calculate shear velocity (U.) and roughness height (Zo) using the logarithmic profile equation (Sternberg, 1972; Gross and Nowell, 1983; Middleton and Southard, 1984): v.

the bottom and z o = h e i g h t at which current velocity theoretically becomes zero. Confidence intervals (CI) were calculated for each U. from eqn.(1) using methods in Gross and Nowell (1983) and Grant et al. (1984). All current velocity data were plotted as vertical profiles (Uz against z), and semi-logarithmically (Uz against In z). Regression analysis was performed on each semi-log plot. Each semilog plot and its correlation coefficient were inspected to determine how well the profile conformed to a logarithmic profile. It was also determined if elimination of data point(s) in the upper part of the profile would cause a better log fit. This trial and error inspection, elimination of data point(s) and new regression analysis was done on all vertical profiles. Those not allowing confidence bands to be within plus or minus 50% of the estimated U. from eqn.(1) were omitted. Results and discussion

z

Uz = -~- In Zo

(1)

where U z = current velocity at height z, K = Von Karman's c o n s t a n t = 0.4, z = height above

Wind

Long-term (1923-1952) data from the National Weather Service station at Atlantic City

66

(45 km north of Great Sound) show t h a t the strongest (> 30-35 km/h) winds come from the northeast and moderate winds (20-30km/h) come from the south and west-northwest. The land elevation surrounding Great Sound does not exceed many meters above sea level, thus, most areas in the exposed lagoon are potentially affected by wind. Waves (0.3-0.5 m high) were observed to resuspend bottom sediments and result in very turbid waters (Fig.3), especially if they occurred near low tide when water depths are < 1 m in most areas. The shallow areas of the lagoon are probably affected frequently during the year by winds producing wind-generated waves that resuspend bottom sediments, as observed in other localities

,+°1

(Moore and Slinn, 1984; Shideler, 1984; Ward et al., 1984; Ward, 1985).

Water quality Salinity in Great Sound approximates coastal oceanic concentrations, and there is little spatial variation. Nineteen measurements taken at each of eight sites (1A, 1B, 3, 13, A, B, C and 22; Fig.l, Table 1) from May 26 to September 1, 1986 ranged from 26 to 33 ppt. On a given day, salinities at the eight sites never varied by more than 1-2 ppt. Minimum salinities of 23-24 ppt were recorded on 3-4 October 1985 just after passage of a hurricane which resulted in substantial rainfall over the

180moiL ~ (NE.34)

A

175mg/L~o (SSE.24) ,/

C

(SSW.40) 57

3 lgmglL ~

1A

(ssw.,o)

J'L,

1201

lOOJ E

8o.

~

8o.

1

4020-

1986

1986 M

l

J

'

J

i

A

S

M

i

i

J

J

'

A

i

S

M'

307mglL~

120"

1985

J

I I~'S'OI M' J

'

J

' A 'S (NE,34) ($SW.40) 227mo/L__~414 ~ 251

13 (ssw,s,):~

cssw,,o) ,~

140- 1B

1986

~,

1986

'J'

A 'S

22

ll

100"

I

"-.

.

•

it

!

I

;+o-. I\ 0..

I

A

/?,,/ ~ I l l , J =

20

I I I

I

A

1985 I

985 I

1986

1985

I

r

I

i

AS'OI M I

J

i

J' A

S

S'O:M' I

I : L

1986 J

'J'

A+S

I~S'O

1986

1I M'

J

985 A

S

A'S'O~IM ' J I

1986 'J'

A 'S

Fig.3. Suspended particulate matter (SPM, mg/1) against time t a k e n during all stages of the tidal cycle at eight of the sampling sites (Fig.l). Letters on the x-axis represent months. Each point represents one sample. Unusually high concentrations and the associated maximum wind conditions measured at Atlantic City, New Jersey are labeled showing direction and velocity (km/h).

67 sound. Royer (1980), during a 2 year study showed salinities for Great Sound typically ranging from 25 to 32 ppt. However, one set of measurements taken after heavy rainfall ranged from 20.0 to 21.5 ppt. Such salinities reflect the minimum freshwater input to the system. Temperatures recorded in the present study (late spring-early fall) ranged from 18.0 ° to 28.5°C for the eight study sites (Table 1). Allen et al. (1978) show seasonal fluctuations typical of temperate coastal waters (for a site near our site 22) with readings ranging from 2°-4°C in J a n u a r y to 25°-27°C in J u l y - A u g u s t , from 1973 to 1977. Royer (1980) reports a range of 0°-30°C from quarterly measurements at four sites along the western side of Great Sound. Tidal currents

Figures 4-7 show mean (0.4 depth measured from bottom) velocities, (each curve is compiled from several days of data), and typical vertical profiles from four of the twelve study sites. Site 6 (water depth < 1 m at mean low water) and site 2 (water depth < 0.5 m at mean low water) represent conditions on the tidal channel margin and on a tidal delta, respectively. Site 19 is typical of the transition subenvironment and site 3 represents the basins (Fig.l). The shape of the mean velocity curves varied relative to location in the lagoon. Velocity curves from delta and channel areas ranged from nearly symmetrical with respect to time (site 6, Fig.4a, and site C) to asymmetrical (site 2, Fig.5a, and sites A, B and 1A), having velocity peaks early in the flood and late in the ebb. All sites in the transition (site 19, Fig.6a, and site 1B) and in the basin (site 3, Fig.7a, and site 9) areas had a similar time asymmetry with peaks early in the flood and late in the ebb. Maximum mean velocities by site ranged from < 10 cm/s at basin sites to > 40 cm/s at delta and channel sites (Table 2). It is clear from the velocity data that Great Sound is dominated by flood-oriented currents. In nearly all sites peak flood velocities are greater than peak ebb velocities. Flood domi-

nance was noted in both ocean-to-lagoon channels entering Great Sound, Great Channel and Ingram Thorofare (Ashley and Zeff, this issue) as has been noted in other mesotidal environments (Groen, 1967; Postma, 1967; Ward, 1978, 1981; Boothroyd et al., 1985). Vertical profiles varied from site to site because of differences in depth, velocity magnitude and bottom roughness (Figs.8 and 9). Site 2 (Figs.5b and c) is illustrative of the range of profile shapes observed. At low velocities (e.g., profiles 123, 127 and 133) a "kinked" profile was observed with nearly uniform velocities < 5 cm/s below the 10 cm height, and rapidly increasing velocities at heights > 10 cm. Ten centimeters corresponded to the approximate height of the algal canopy (see section on Benthos). Profile 139 was apparently measured at a spot locally bare of algae. As velocities increased the canopy was apparently pushed downward, so that at maximum velocities (U0.4d=15-20 cm/s) the entire profile was approximately logarithmic (e.g., profiles 138 and 135). Fonseca et al. (1982) found similar responses of sea grasses to increasing velocity. At sites 6 and 22, profiles were usually parabolic or logarithmic (Fig,4c); these sites represent delta and channel areas with biogenically "smooth" beds where velocities were swift and the sandy sediments appeared to be regularly transported as bedload. Profiles from sites in the transition and basin areas were variable and often approximated the kinked shape at low velocities (Figs.6c and 7c). Macroalgae and worm tubes represent large roughness elements (Figs.8a and 9), and probably were largely responsible for the kinked velocity profiles, especially under low-velocity conditions. Such segmented profiles are expected when large roughness elements such as bedforms (e.g., Dyer, 1970; Ashley, 1978) or macrophytes (e.g., Fonseca et al., 1982; Hiscock, 1983) occur and they result in stacked logarithmic layers, each with a different shear force (Nowell and Church, 1979; Chriss and Caldwell, 1982). Thus, calculation of U. (and Zo) most relevant to deposited sediments must include all near-bottom measurements. For our

68 40-

161?

30-

// 2

204

t

a.

-

U 0.4d Uo.4dmex

\

~

b.

SITE 6 =19.0

=42.2 =4.00

U'max

167

100-

162

A

E

/

158/61

0 v

N

10-

1020I

162 158

12011010090B070~60-

Uo.4d =

11.7

,67

C.

,,o]

20cm/s

,6, I

U0.4d= 4 2 . 2

162 U0,4d

_

u..~21.1

q

=2B.B r

D

I

N 50

--

40-

10

J 5 10

" 1'0 2tO 3'0 4'0

1~0 2'0

10 20 30

U (cm/s)

Fig.4. Velocity characteristics of the proximal tidal delta channel subenvironment with a biogenically "smooth" bed (site 6). a. U0.4 d against time. b. Semi-log depth profiles keyed to corresponding U0.4 d measurement in Fig.4a by a number at the top of the profile, c. Arithmetic depth profiles corresponding to profiles in Fig.4b. calculations all of the lowermost (within 20 cm of the bed) velocities were used. Even with a large (95% CI within plus or minus 50% of the estimated value) acceptable error range, only 58% (118 of 203) of the vertical profiles could be used for U, and Zo calculations. Such variability could be high, but few papers give error estimates. Grant et al. (1984) note that only 30% of their profiles had 95% CI's within 25% of the estimated /_7,, but over two-thirds were within 50%. It seems

likely that in the present study the major problem centered around the use of a single probe, as opposed to a fixed array where more optimum time averaging could have been done (Nowell, 1983). However, our estimates of U, are similar to published values for similar current conditions (e.g., Nowell et al., 1981; Fonseca et al., 1983; Grant et al., 1984). Roughness heights (Zo) were widely variable at all sites (Table 2), and no clear trends among subenvironments were evident. Roughness

69

SITE 2

a. flood 20-

,,~ 7.5

Uo.4 d

138 •

b.

U o . 4 d m a x = 1 7.8

U,max

=2.22

100

138 E N10-

2 10

o ~......4 •

133 20-

135

ebb

;

127 133

il 4

//

135

1_ 20cm/s

123 Uo.4d= 4.8

Co 139 127 Uo.4d=9.4 Uo.4d=7.0

138 Uo.4d= 17.8

133 135 Uo.4d=12.0 Uo.4d:| 1.6

100 9080.

I

70"~60 ,~O 50-

N

-

30~ i

4.

i -~-

2

-j

I

,

i

105 ' 1'5

5

15

g ' 1'5

g ' 1'5

5

15

5

15

U (cm/s)

Fig.5. Velocity characteristics of the distal tidal delta with a biogenically "rough" bed (site 2). See Fig.4 for further explanation. heights measured in the field are often much greater than grain size of the bed because of factors such as biogenic roughness and bedload transport (Smith and McLean, 1977). (See Nowell (1983) and Jumars and Nowell (1984) for discussion of z 0 measurements from marine studies.) In the present study roughness elements of biogenic origin (macroalgae and worm tubes) were probably the major reason,

in conjunction with errors associated with the logarithmic profile method. However, current ripples are common on the delta (e.g., sites 6 and 22) during periods of high current velocities. Bottom sediment

The mean grain size of bottom sediments generally decreased progressively from the

70

SITE 19

a Uo.4d

b.

= 5.0

Uo.4dmax = 10.2 U,

flood

max

= 1.82

236

100-

239

I

S °- ~8-2'~9 230

246

/

A

E O

N10. 10-

246 ebb

!

I

20cm/s C°

238 239 230 Uo.4d= 8.6 Uo.4d= 7.8 Uo.4d--5.0

244 Uo.4d=7.4

236 Uo.4d=5.2

246 Uo.4d=lO.2

100-908070-

E6O~"50-N 4030-/, 2010-

/7 -, ij 5

15

I

,t

i5

5

5 ' 1'5

U (cm/s) Fig.6. Velocity characteristics of the transition subenvironment (site 19). See Fig.4 for further explanation.

tidal deltas (> 40 llm) through the transition environments (range of 31-3 pro) to the basin (< 10 ~n) (Fig.10). Similarly, the proportion of sand generally decreased ( > 80-10%) from the tidal delta to the basins whereas the TOM increases (1-2% to 8%). However, mud may settle locally within the channel and delta subenvironment (e.g., site A, Fig.10). Sites located at a distance from the thalweg or on the lee side of shoals where wave and tidal

energy is decreased accumulate fine muddy sand (Kelly, 1975; Zeff, this issue). There was also a trend in decreasing mean grain size with increasing distance laterally from the Intracoastal Waterway channel due to the dispersion of sandy dredge material side-casted along channel margins (Fass and Carson, this issue; Young et al., this issue). More spatially comprehensive data on bottom sediments are found in Fass and Carson (this issue).

71

b.

a.

SITE 3 U0.4d

=4.4

142

100

U 0.4dmax = 7.5 U.max =0.87

152

151

f

flood 10-

153 •

~lS

E

O10-

142

~'~

156

10-

- ~.~_...i 150 • 151 ebb

,,

1 '

152 Uo.td=4.5 1 4 07

153 Uo.4d=7.5

142 Uo.4d=5.9

150

C.

20cm/s ~

156

Uo.4d=4.0

150 Uo.4d=6.4

151 Uo.4d=7.2

I

130J

!1

1201101009080O~ 70N 60" 50-

q

40" 30" 20" 10"

q

F

i

i

t j / @

@

i

5

U (cm/s) Fig.7. V e l o c i t y c h a r a c t e r i s t i c s of t h e b a s i n s u b e n v i r o n m e n t (site 3). See Fig.4 for f u r t h e r e x p l a n a t i o n .

Suspended sediment Because current velocities were usually not measured when samples for water quality analyses were collected, it is not possible to directly compare these data. However, graphs of SPM in Fig.3 show similar concentrations, except on windy days when spikes occur. These data suggest that wind-generated waves are extremely important in resuspending sedi-

ments. Relatively low background SPM levels are typical and are produced by tidal currents alone during fair weather. It follows, then, that maximum transport within Great Sound would take place under combined flow (tides and waves) during storm events (Ashley and Zeff, this issue; Young et al., this issue). PIM:SPM ratios on means are all > 0.80. For individual samples the inorganic fraction (PIM) was rarely <70% of the SPM.

72 TABLE 2 Summary of tidal velocity data Site

Uo., d max

U,(cm/s)

95% CI/percent U,

Zo(Cm)

(cm/s) Channel and delta 1A 15.0 2 6

17.8 42.2

A

11.2

B C 22

19.7 14.3 40.0

Transition 1B 16.0 13 9.8 19 10.2 Basin 3

7.5 6.2

1.48 4.00 2.22 2.22 4.00 1.00 1.67 2.22 1.67 1.54 1.82

± 0.33/22.5% ± 2.85/71.3~/o ± 0.30/13.7% ± 0.65/29.5% _ 1.62/40.4% ±0.14/13.6% ± 1.10/66.5% ±0.40/18.0% ± 0.28/16.8% ± 0.26/16.8% ±0.69/37.8%

0.56 0.93 1.82 0.02 3.32 0.42 1.15 2.05 1.12 0.07 0.51

2.67 0.93 1.48 1.82

± 0.58/21.7% ± 0.16/16.8% ± 0.21/13.9% ± 0.59/32.2~/o

1.12 1.42 1.03 2.29

0.75 0.87 0.59

+ 0.16/21.8% ± 0.30/34.4% ± 0.25/42.0%

0.53 1.02 0.94

Fig.8. Tidal delta and channel subenvironment, a. Surface at extreme spring low tide showing abundant macrophytes (site C). b. X-ray from site 2 showing w o r m tubes and burrows. Scale bar = 1 cm.

73

G

Fig.9. Acrylic box cores, a. Transition subenvironment (site 13). b. Basin subenvironment. Note several worm tubes protruding above the bed in the background (site 9). Scale bars = 1 cm.

Benthos Figures 8 and 9 show the sediment-water interface typical of sites sampled in the three subenvironments. Figure 8a shows a dense cover of macroalgae which occurred in a d e l t a - c h a n n e l area with a biogenically rough bed (site C) during spring and early summer 1986 (Grizzle, 1988). Sites 6 and 22 represent shallow-water areas in the delta and channel subenvironment where high current velocities (>40 cm/s) occurred (Fig.4) and the bottom was relatively smooth (biogenically). Only small scattered patches of macroalgae, and patches of ampeliscid amphipod tubes were observed. Current ripples produced by waveand tide-generated currents were commonly visible in shallow water at low tide and across the adjacent intertidal flats in these areas. At times, current ripples covered large areas of the delta. Frequently, the ripples were interrupted by patches of ampeliscid tubes which appeared to inhibit bedload movement. Other areas in the delta and channel subenvironment with lower current velocities (Umax=20 cm/s; e.g., site 2, Fig.5) possessed biogenically rough beds. The biogenic roughness was largely a result of abundant macroalgae, particularly

the green alga Ulva lactuca, which occurred in small patches t h a t covered 25-50% of the bed. The estimate of areal coverage was based on observations at site 2 from June 1985 to October 1986 (Fig.l). Large infaunal polychaetes were also abundant at site 2 (Fig.8b), but their tubes did not protrude above the bed (in contrast to tubes in transition and basin areas; Fig.9). Sites on the margins of flood tidal deltas are in areas of potentially high sand deposition rates. Sand moving as bedload and intermittent suspended load ceases to move when velocity decreases. This occurs when the crosssectional area increases upon entering the lagoon, and the flow encounters increased bottom roughness caused by dense macroalgae. At low tide the blades of algae in these areas (e.g., sites 1A and 2) looked as if they had been lightly "dusted" with snow because of the deposited fine sand. Such macrophytehydrodynamics interactions resulting in increased sedimentation rates is probably a common occurrence in coastal lagoons (Ginsburg and Lowenstam, 1958; Scoffin, 1970; Harlin et al., 1982; Ward et al., 1984). In most transition and basin areas the bottom was biogenically rough due to abundant macroalgae (and sponges at times) and

74 E 50,

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i n f a u n a l p o l y c h a e t e (maldanids and onuphids) w o r m tubes w h i c h p r o t r u d e several centim e t e r s into the w a t e r c o l u m n (e.g., site 13, Fig.9a). R h o d o p h y t e s were the most a b u n d a n t algae in t h e s e areas; the c h l o r o p h y t e Ulva was u s u a l l y m u c h less a b u n d a n t t h a n on the deltas. In the basin, the i n d i v i d u a l algal p a t c h e s were m u c h l a r g e r t h a n in t r a n s i t i o n areas, sometimes c o m p l e t e l y c o v e r i n g several s q u a r e m e t e r s of b o t t o m and s e p a r a t e d by b a r e areas. These algal p a t c h e s were observed at sites 3 and 9 at low tide, and were e s t i m a t e d to

p r o t r u d e 1 0 - 2 0 c m into the w a t e r column. M a c r o a l g a l p a t c h e s are a c o m m o n f e a t u r e of lagoons along the A t l a n t i c m a r g i n ( H a r l i n et al., 1982; N o r t o n and M a t h i e s o n , 1983; Boothr o y d et al., 1985). W o r m tubes of 1 cm diameter were observed p r o t r u d i n g in some cases several c e n t i m e t e r s above the b o t t o m (Fig.9b). An a v e r a g e areal d e n s i t y of these tubes was not determined, but at least two large (20 cm long) a d u l t worms were found in some box cores and as m a n y as f i v e - t e n large w o r m tubes o c c u r r e d in a single 10 cm wide core sample.

75

Faas and Carson (this issue) and Thorbjarnarson et al. (1985), show dense worm tubes and/or burrows from cores taken in the basin and transition areas. Thus, the most biogenically rough beds, consisting predominantly of worm tubes and algal patches, occurred in the deeper transition and basin subenvironments. Sedimentation rates Net sediment accumulation in Great Sound has been calculated by utilizing total Pb (Kelley, 1980) and Zl°pb and 137Cs (Thorbjarnarson et al., 1985) (Fig.l, Table 3). Values range from 0.05 cm/yr (site K15) to 1.0 cm/yr (sites K1 and K2) (Kelley, 1980) in the transition subenvironment to 0.18 cm/yr (site T14) in the basin, and to 0.54 cm/yr (site T l l ) in the tidal delta (Thorbjarnarson et al., 1985). The total Pb technique does not take bioturbation into consideration, thus estimates are probably high. The utilization of both 21°pb and 137Cs acts as a check; however, Thorbjarnarson et al. (1985) concluded that deep biological mixing affects 2lOpb ' so their calculated accumulation rates represent maximum estimates. Their sample sites were in a basin and a nearly abandoned tidal channel and are probably low for the lagoon as a whole. Accumulation rates should be higher in the tidal delta area. Sedimentation rates for other back-barrier lagoons along the Atlantic seaboard are generally low: Rusnak (1967), at 0.2-0.4 cm/yr and

Bartberger (1976), at 0.15 cm/yr in Chincoteague Bay.

Hydrodynamics, benthos and sediments: A conceptual model The following discussion centers around Fig.11 (after Grizzle, 1988) which shows trends by subenvironment of the four major factors that interact to determine the characteristics of deposited sediments in most sedimentary systems (Davies and Gorsline, 1976). These factors include: (1) supply and size of sediment available, (2) maximum sediment size which can be transported, (3) sediment size which is deposited, and (4) postdepositional processes. This section will integrate information presented above into a conceptual model explaining variations in deposited sediments in Great Sound and the differences which occur between the subenvironments. Vertical tidal current profiles are drawn to scale relative to one another with the highest velocities (50 cm/s) in the delta and channel subenvironment (Fig.ll). Changes in the shape of current velocity profiles across the lagoon are determined by depth and continuity considerations (Young et al., this issue) in conjunction with biogenic roughness. The "deposition" description does not include biological processes and refers only to physical processes. Biodeposition by suspension-feeding hard clams (Mercenaria mercenaria) which are abundant in

TABLE 3 Sedimentation rates in Great Sound and neighboring areas Location

Subenvironment

Dating method

Sedimentation rate (cm/yr)

Reference

Great Sound Great Sound

Transition Basin Tidal delta Nondifferentiated lagoon Nondifferentiated lagoon

Total Pb : l ° P b and 13VCs 14C

0.05-1.0 0.18 0.54 0.2-0.4

Kelley (1980) T h o r b j a r n a r s o n et al. (1985) Rusnak (1967)

14C

0.15

Bartberger (1976)

East coast lagoons Chincoteague Bay

76

Transition

Channel/Delta

Basin

1. All grain sizes (near T h e l w e g )

1. All grain sizes

1. Silt and clay

2. All grain sizes

2. Silt and ¢lly

2, Mostly clay

3. Mostly sand 4.

Tidal and wave c u r r e n t s

3. Mostly sand and silt

3. All grain sizes

4. Waves regularly

4. Waves sDorsdicslly

I

Fig.11. Conceptualmodelof flow(velocityprofiles), benthos (algae and wormtubes), and sedimentation(bottomtypes). After Grizzle (1988). The four numbered items in each subenvironmentsummarizeconditions there for four factors determining deposit characteristics: (1) Supply and size of sediment, (2) transport, (3) sediment-sizedeposited and (4) postdepositional processes. most areas may be an additional important factor not considered here. Because tidal currents are of major importance in transporting and reworking sediments in Great Sound, it is reasonable to assume that if all four of the above factors were similar in all subenvironments there should be a good correlation between tidal current velocities and bottom sediment characteristics. However, if any of these factors are substantially different, there will be departures from any simple tidal current-grain-size relationship. There is a strong correlation between most velocity parameters and sediment grain-size characteristics (Figs.12a and b). However, a more complex relationship than a simple linear one is suggested by these graphs. Strong currents t h a t dominate sedimentary processes in the delta and channel subenvironment result in sandy deposits. In contrast, areas with low-velocity tidal regimes in the basins typically have muddy deposits because little sand is transported to these areas. Thus, at the extremes of the range of tidal velocities, currents are well correlated with sediments.

However, in areas with intermediate velocity regimes there is no correlation with currents and sediments (see sites with Uo.4 d maxof 10--30 cm/s and U.max of 1.5-3.0 cm/s in Fig.12). In these areas other factors are clearly of major importance. A few caveats are perhaps in order before discussing the model. All grain-size data represent disaggregated grains with organic matter removed. Thus, the size distribution of particles present in the bottom sediments where a large part of the deposit is likely to be aggregates in the form of fecal pellets (Rhoads, 1974; the present authors' observations on Great Sound samples) is not known. Further, much of the silt- and clay-sized particles transported in Great Channel (and thus potentially entering Great Sound proper) is transported as organic-mineral agglomerates or fecal pellets (Carney, 1982; Carson et al., this issue). Sand grains are transported mainly as individual grains, but compose a very small part (< 15%) of total load (Carson et al., this issue). Therefore, because agglomerates play a major role in sedimentary processes, work is

77

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Fig.12. M e a n g r a i n size (upper row) and p e r c e n t s a n d (lower row) a g a i n s t velocity, a. U, mm, (cm/s) w i t h 95% CI. b. Uo.4 d m,, (cm/s). Closed s q u a r e s - - b a s i n s u b e n v i r o n m e n t ; dots t r a n s i t i o n s u b e n v i r o n m e n t ; open s q u a r e s - - delta and c h a n n e l subenvironment.

needed on suspended and deposited sediments in their n a t u r a l forms to refine and quantify the model. Also, the temporally dynamic nature of coastal systems is well known, and must be considered in future studies. Temporal changes in both sediments and benthos are probably important. In its present form the model is meant to provide the conceptual basis for design of future studies related to sedimentary processes in Great Sound and other tidedominated coastal lagoons. The following is a discussion of the nature of the importance t h a t each of the four factors mentioned above has in each subenvironment in determining the characteristics of deposited sediments. Flood tidal deltas

The delta and channel subenvironment is the most heterogeneous of the three in that it includes deep narrow channels with swift tidal currents and deposits t h a t are rarely affected by wind-generated waves. It also contains

shallow flats which are periodically affected by waves. Because of such heterogeneity in geomorphological and hydrodynamic characteristics, factor 1, supply of sediment, also shows quite a range. Generally, however, all grain sizes up to fine sand (125 ~m) are potentially available but the availability of sand apparently decreases as distance from the thalweg increases (Kelley, 1975; Zeff, this issue). Factor 2, transport capabilities, is also variable. In the deep channels where current velocities are high all available grain sizes are easily transported in suspension or as bedload over much of the tidal cycle. However, along subtidal flats on the inside of bends in the channel, and in depressions (i.e., area with slightly lower elevations surrounded by sand bars; e.g., site A, Fig.l) where mud is apparently accumulating (Fig.10), tidal currents probably rarely resuspend sand or even silt. However, a positive feedback mechanism may be operating here in which macroalgal growth is enhanced in areas with moderate tidal current velocities.

78 Increase in density of plant cover leads to increased sedimentation. Factor 3, deposition, is largely dependent on tidal current regime, but biodeposition by suspension-feeding benthos may be important. Sand is probably deposited in areas where currents are strong, with fine silts being deposited in areas with low tidal energies. However, the finer particles may be more readily deposited as aggregates (Carney, 1982; Carson et al., this issue). Biogenic roughness primarily caused by macroalgae can cause increased sedimentation in some areas by slowing current velocities in near-bed parts of the boundary layer (Ginsburg and Lowenstam, 1958; Scoffin, 1970; Harlin et al., 1982; Ward et al., 1984). Factor 4, postdepositional processes, includes bioturbation by infaunal benthos which affects erodibility (Rhoads, 1974; Rhoads and Boyer, 1982), foraging and feeding activities by benthic and epibenthic animals, and resuspension of deposits by wind waves and tidal currents. To briefly illustrate how variations in factors other than tidal currents can cause substantial differences in deposit characteristics, consider sites B and C (Fig.l). The mean and maximum U0.4d were 9.5 and 19.7 cm/s, respectively, at site B, as opposed to 8.7 and 14.3 cm/s at site C. U.max was 2.22 cm/s at site B compared to 1.67 cm/s at site C (Table 2). Clearly sediments at site B are potentially exposed to higher tidal current energies than those at site C. However, the sediment at site B contains only 21.0% sand compared to 70.5% at site C. The mean grain size at site B is 8 pm compared to 44 ~m at site C. Differences in sediment deposits at these sites are probably largely a result of differences in factor 1 (sediment supply) and factor 4 (postdepositional processes). Site C is on the margin of Cresse Thorofare which probably transports an abundant supply of sand. Site B is in the same channel but it is over 100 m from the thalweg, hence its supply of sand is probably less. Furthermore, no high SPM concentrations were recorded at site B, as opposed to several incidences of very high SPM values caused by high winds at site C (Fig.3). Thus,

resuspension of sediments and probable winnowing out of fine grain sizes (Shideler, 1984; Ward et al., 1984) occurs regularly at site C, but rarely at site B. Transition

Most areas in the transition subenvironment are similar with respect to the four factors, and deposits are of different combinations of sandy mud. Factor 1: All grain sizes are potentially available in most areas, but much of the sand has been deposited in the lower energy parts of the delta and channel subenvironment before reaching these areas (Young et al., this issue). Factor 2: Tidal currents are moderate (Fig.6), and most of the sand remaining in suspension is deposited. Thus, only silt and clay appear to be transported under non-storm conditions and they move as aggregates. Factor 3: Deposition of all particles occurs, and concerning factor 4, intense bioturbation (Fig.9) and resuspension of sediments by wind-generated waves (e.g., SPM data from sites 1B and 13 in Fig.3) are common. Basin

The basins are low-energy environments, and sediments are predominantly mud. Factor 1: Nearly all sand has been deposited so only silt and clay (probably as aggregates) remain in suspension as water moves into the basins (factor 2). Factor 3: All grain sizes in suspension can be deposited by physical processes or biodeposited by suspension-feeding benthos. Factor 4: Waves can sporadically resuspend sediments, but the abundant and dense patches of macroalgae may buffer the sediments from wave forces (Fig.ll). Conclusions

Both physical and biological processes typically affect sedimentary deposits resulting in complex distribution patterns in many coastal systems. We have shown that explanation of

79

variations in deposits in Great Sound requires consideration of at least four factors (sediment supply, transportation, deposition and postdepositional processes) that potentially interact to determine deposit characteristics. The assumption is sometimes made that a direct relationship exists between tidal currents and deposited sediments, but the present study has demonstrated that this is not the case in Great Sound. The presence of worm-tube complexes and macrophytes produces a rough bed affecting (reducing) near-bottom velocities, and causing increasing sedimentation rates. In areas of moderate velocity tidal current regimes (U0.4 d max= 10--30 cm/s) bottom sediment types range from mud to sand. In these areas factors other than tidal currents (sediment supply, bottom roughness and resuspension) were particularly important in determining bottom sediment characteristics. This finding has important implications for benthic research aimed at determining the relative importance of environmental factors in processes ranging from individual growth to community structure. Because a wide range of combinations of sediment characteristics and environmental conditions can occur it is important to understand how hydrodynamics and sediments are related to one another. Clearly it will be necessary in some cases to use an experimental approach in determining the relative importance of bottom sediment characteristics and tidal currents. We suggest that the conceptual framework explained herein can be used in tide-dominated coastal lagoons to characterize hydrodynamic-sedimentary relations and to help direct research concerned with these phenomena.

Acknowledgements This work is the result, in part, of research sponsored by the NOAA Office of Sea Grant, Department of Commerce, under Grant NA83AA-D-00034. Support was also received from the Department of Geological Sciences, the Center for Coastal and Environmental Studies, the Fisheries and Aquaculture Tech-

nology Extension Center, Rutgers University, as well as the Jackson Estuarine Laboratory (University of New Hampshire). Appreciation is extended to Marjorie Zeff for assistance with fieldwork, Andrew Meglis for grain-size data, and Cheryl Skuba and Carol Vadnos for the word processing.

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