The role of current velocity in structuring eelgrass (Zostera marina L.) meadows

Estuarine, Coastal and Shelf Science (1983) 17, 367-380

The Role of Current Velocity in Structuring Eelgrass (Zostera marina L.) Meadowsa

Mark S. Fonsecabs, and John S. FisheP

Joseph C. Ziemanb,

Gordon

W. Thayer:

bDepartment of Environmental Sciences, Uni*~ersity of Virginia, Charlottesville, VA 22903, CNational Marine Fisheries Service, Southeast Fisheries Center, NOAA, Beaufort, NC 28516 and dDepartment of Civil Engineering, North Carolina State University, Raleigh, North Carolina, 27607, U.S.A. Received 11 May 1982 and in revised form 8 November 1982

Keywords:

currents; shear velocity; sediment distribution;

equilibrium

Measurements of velocity profiles, bathymetry, and surface sediment characteristics across eelgrass (Zostera marina L.) meadows yielded information on community development processes and functional attributes of this ecosystem. Height/length ratios of the meadows were positively correlated with tidal current velocity. Low, medium, and high current regimes were separated by surface current velocities of approximately 50 and 90cms-1. Z. marina can tolerate approximately 120-150cm/sec current velocitiesin the areasstudied.Per cent

silt-clay and organic matter content of the surface sedimentsare negatively associated with shearvelocity, suggestingthat meadows in highcurrent areasare sourceswhile meadowsin low current areasaresinksof autochthonousdetritus. Current velocity maintains seagrass meadows at different equilibrium levels (relative climaxes). We theorize these different equilibrium levels provide unequal habitat utilization potentials for the associated fauna1 community. Introduction

Eelgrass (Zostera marina), usually occurs as a monoculture, although it is occasionally mixed with shoalgrass(Halo&& wrightii), in the Beaufort, NC, area and widgeon grass (Ruppiu maritima), in portions of its distribution prone to low salinity. As a result, species successionis not as important in eelgrass development as it may be in tropical seagrasssystems. As described by McRoy and Lloyd (1981), ‘Zosteru is considered a coloniser aswell as a climax species-that is, the single speciesrepresents the entire range of system development (den Hartog, 1971). Since there are not the usual visual characters of succession(specieschange), other indicators must be sought. . .‘. A dominant indicator of marine ecosystem development is related to the degree of fluid motion. The processes of seagrassecosystem development appear to be driven to a point where an equilibrium is reached between the structure of the meadow and localized current flow dynamics. Petersen (1918) was the first to develop an interest in the role that seagrasseshave in stabilization of subtidal and intertidal habitats and Wilson (1949) described the influence ‘Contribution 82-03B of the National Marine Tresent address: National Marine Fisheries Beaufort, NC 28516, U.S.A.

Fisheries Service, Southeast Fisheries Service, Southeast Fisheries Center,

Center. NOAA,

367 0272-:714/83/100367

+ 14 $03.00/O

0 1983 Academic

Press Inc. (London)

Lmxred

368

M. S. Fonseca et al.

that loss of seagrasses, through the ‘Wasting disease’, had on shoreline composition. Ginsberg & Lowenstam (1958) incorporated seagrasses into the concept of biotic modilkation of sedimentary processes while other investigators (Molinier & Picard, 1952; Zieman, 1972; Schubel; 1973; Orth, 1977; Christiansen et al., 1981; and Fonseca et al., 1982) described how reduction in current flow affects sediment modification and growth pattern of seagrass meadows. Patriquin (1975) described the dynamics of erosion in seagrass meadow from wind-driven circulation, whereas Wood et al. (1969) conceptually integrated the functional role of current velocity in biological, physical, and chemical processes. Scoffin (1970) provided the best description of flow behavior in a seagrass meadow, but his description of current velocity is incomplete. First, he neglected to locate velocity measures by height above the bottom or distance into the meadow. Consideration of velocity profiles and reduction of flow by distance into the meadow are necessary to determine current velocity modifications by the meadow (Fonseca et al., 1982). Second, Scofti described the velocity as reaching zero at the sediment-water interface. Since velocity must go to zero at some point near that interface, measurements of this height above the bottom at which zero velocity is attained would provide a measure of sediment stability at a given site. Aside from the above publications, the effects of sediment modification and current reduction on seagrass meadow structure are unknown. Currents have different velocity ranges and characterize very different habitats. We hypothesize that as current increases in velocity, its influence on the structure of the biological community is greater. Structure refers here not only to the three-dimensional composition and configuration of the meadow, but also to the composition of meadowassociated fauna. When current flows over a seagrass meadow, the increased drag on the fluid motion by the seagrass leaves and shoots is a result of the seagrass creating a hydraulically rougher bottom. The rate of change of velocity with depth increases as the lower portion of the velocity profile is slowed by the seagrass shoots, and velocity over the seagrass canopy is increased as the canopy bends (Fonseca et al., 1982). Measures of shear velocity (U*) and roughness height (6 ‘) are derived from the velocity profile. Since shear velocity describes the rate of change of fluid velocity with depth, it therefore describes the shear force within the profile (Bagnold, 1941). The intercept of the logarithmic velocity profile with the depth axis (ordinate) is the height above the bottom (roughness height, b ‘) where the fluid velocity should reach zero. This is the ‘semi-motionless’ layer of water maintained by bottom friction described by Ginsberg i? Lowenstam (1958) in relation to seagrasses. Hence the velocity profile as characterized by U* and b ’ effectively describes the balance between the force from current flow and resistance afforded by seagrasses. By utilizing velocity profile measurements, we have been able to characterize flow dynamics along transects through meadows under a wide range of current regimes. Our objectives were to (1) defme current regime, (2) investigate modification of velocity proties across meadows, (3) compare predicted and observed velocities (accounting for bathymetry) across meadows, (4) survey sediment composition, and (5) predict canopy orientation. Although not presented here, a knowledge of U*, mean sediment particle size (mm), and water salinity and temperature provides the necessary information to generate the Shields entrainment function (Vanoni, 1964). Information from our studies can be used to determine stress limits of meadows and movement of detritus (Fisher et al., 1979) to characterize microhabitat conditions. Such information would he helpful in describing fauna1 assemblages, trophic interactions, and developmental processes of seagrass meadows.

Role of current velocity

Methods Current flow patterns as measuredby the velocity profile are different over a flat meadow than over a mounded one. As a bedform develops, current velocity against its leading edge increasesaswater depth decreases.Current velocity must decreaseon the downstreamedge near the bottom as flow separatesfrom the bedform. To account for the influence of raised bedforms across velocity profiles, the transects chosen in this study were arranged for analysis by grouping them in height/length (h/L) classes.A height/length classification of bedform is determined by measuringthe rise (or fall) of a meadowfrom where seagrass cover begins to where it ends along the major axis of current flow and dividing by the length of that vegetated area. We developed this technique to provide a relative classification schemethat reduces error of current flow description between habitats due to bedform variation. Transects were selected across seagrassmeadows with different maximum current velocities. Three transects were in Charlestown Pond breachway, Charlestown, RI, where maximum surface currents > 100cm s-1 are developed. Two transects were measuredin the Cape Lookout area of North Carolina where velocities up to 80 cm s-i were recorded. Four transects were located at Middle Marsh in Back Sound, near Beaufort, NC, where the average maximum velocity was 45 cm s-1. The transects were establishedrunning from unvegetated areasinto the seagrassmeadow. A topographic profile, from which h/L ratios were calculated, was determined for each transect by using a line transit. At fixed stations on each transect, plant density and biomass,sediment characteristics, and velocity profiles were measured. Stations were spacedso as to best characterize the flow situation in each bedform type. Velocity profiles were recorded several times at each station when current was flowing from bare into vegetated areas. Velocity measurements were made on the ebb or flood tide, depending on whichever gave the consistently highest velocities. Shoots from a 20 x 20cm quadrat at each station were separated into foliar and subsurface portions and dried at 100 “C for 24-36 h. A linear regression equation was used to convert dry weight to leaf area index [I,,, seagrasssurface area (m2) per m2 of bottom covered]. A plant surface area per volume of water column factor was used for correlation with flow dynamics: Z,,/m = Z,, (m’)/[(m’ bottom covered) x D] where D is the total water depth (m). Current velocity profiles were measured using a bi-directional, electromagnetic Marsh-McBirney water current meter (Model 511).= Measurements were made at 2 cm above the bottom, at the apparent plant canopy-water column interface, and at the free surface. Heights (2) above the bottom were recorded in each case. After having measuredcurrent velocity profiles along each transect, we calculated what the velocity should have been at each station within the vegetated area, incorporating only bathymetric changesand using a conservation of massequation: ub,l

D,

=

ub,,

D,,

then ub,l -=

D, u b,2>

D2,

‘Mention NOAA,

of trade names does not imply endorsement or by the affiliations of the authors.

by National

Marine

Fisheries

Service,

370

M. S. Fonseca et al.

where Ub,l and Q,,* are the bottom velocities at 2 cm height at stations 1 (leading edge) and 2 (some station within the meadow), [respectively], and D, and D2 are the total water depths at stations 1 and 2, respectively. The predicted bottom velocities (2 cm height) were used for comparison with the observed data to demonstrate the reduction of velocity over different bedforms by the seagrass cover. Velocities used in the regression equations were based on maximums developed during a lunar cycle. All velocities, including those predicted from the above relation, were corrected, by graphically interpolating NOM tidal current tables (US National Ocean Survey, 1977) to what would be the approximate value achieved at a station during the maximum current velocity and tidal stage during a lunar cycle. These corrected numbers were used as input to multiple linear regression (MULREG) computer programs contained in an SPSS system. This correction was performed to add validity to correlations of sediment development with the seagrass meadow. We hypothesize that bedform and sediment distribution will develop to a point where it is in balance with the maximum velocity experienced during the lunar cycle. For all multiple linear regressions, the relative importance of the independent variables was inferred from the absolute values of the standardized regression coefficient (beta). This is acceptable, since the correlation matrix indicated no severe colinearity among the independent variables. Two sediment cores were taken at each station. Surface samples were removed from each core to a depth of 1 cm and frozen until analysed for per cent organic matter and silt-clay. Values from replicated samples were averaged. The relationship between these sediment parameters and distance into the meadow (X), leaf surface area (m2) per square meter of bottom (Z,J, and U*,, was examined by multiple linear regression. The bending angle of the seagrass shoots was determined from the equation: y=10.039

+ 6.307(log,,

x)

where x is the Froude number, Us- 1’2 Z-I, such that U is the velocity (cm s-l) at the apparent plant canopy-water column interface, g is acceleration due to gravity, and Z is the height above the bottom (cm) of the velocity measurement, while y is the bending angle (degrees) with 90” = normal to the bottom and 0” = prone or parallel to the bottom (Fonseca et al., 1982).

Results Current regime

The increase in current velocity and U* to adjust for the lunar maximum currents of these parameters was not constant for all transects. The correctioq for transects in the low current regime (h/L z 10-3) ranged from a 27% to an 81% increase (average increase 54%), while the medium and high current regimes required on average a 39% and a 5% increase, respectively. The relationship between meadow mounding (height/length ratio, h/L) and the lunar maximum current velocity (surface measure in cm s-1, Us, ,,) are shown for three characteristic meadows (Figure 1). The h/L ratio was arbitrarily divided into regimes where the ratio changed order of magnitude (Table 1). From the model (Figure l), the current velocities, Us,mx, separating these regimes are approximately 53 and 94 cm s-1 (Table 1).

Role of current

o’25-

2 \ 5 cl‘0L Jz T5 5 i i s? 2

0.20

-

o-15

-

Y A

=A exp =0~0004

B

=0.058

371

velocity

(BX)

Characterlsttc meadows under each current regime (current flows from left to right)

~2:O.f341

Vertkzal .vbew

Cross-sectlonal wew g?%y

p$J

Medbum

0.05

-InI

-

Low

60

40

0

Maximum

80

100

lunar cycle surface u s mox In cm s-’

120

velocity,

Figure 1. The h/L ratio of several Z. murinu meadows (dots) and one T. tesrudinum meadow (star, Scoffin 1970) are regressed on current velocity. Cross-sectional and vertical-view diagrams of the degree of mounding and coverage patterns respectively are on the right. Horizontal lines describe arbitrary numerical h/L limits for high, medium and low current regimes.

TABLE current

1. Current velocity

h/L 0.1 0.1-0~09 0~001-0~009

regime

designations

in relation

Current (10-l) (10-Z) (10-S)

Visual observations that describe the continuity regimes are included in Figure 1.

regime

high medium low

to h/L

ratio,

maximum

Velocity

lunar

surface

(Us, max) ranges

94 cm s-l 53 cm s-l

of the meadow under the various current

Flow dynamics Shear velocity (U*) was predictable by multiple regressions (again, based on the correlation matrix which did not demonstrate any several colinearity) only when stratified by current regime (Table 2). Surface velocity (U,) displayed its requisite positive, significant correlation with shear velocity (U*). There was no predictable influence of seagrasses, expressed either as density (Q,) or leaf area (Z,,/m), on shear velocity under any current regime (Table 2). There was a trend for leaf (area) abundance to contribute more negatively (signs of B, Table 2) and to become more influential (beta values, Table 2) on the dependent variable, U* as current increased. Distance into the meadow under the low current regime had a significant and negative association with U*. There was no significant

M. S. Fonseca et al.

372

2. Multiple linear regressions for ent current regimes. u* = shear velocity, velocity at a given station in cm s-1, 4s area index (mz, two-dimensional surface sion coefficient for independent variables, dependent variables, F = F-statistic for different from 0 at the 0.01 level

TABLE

Dependent variable

Independent variable

Low current u*

High current u*

u*

Standard error of E

F

Significance

- 0.028 0.144 o.ooo - 3.319

- 0.320 0.833 0.026

0.009 0.016 0.000

8.850 76.458 0.056

X us Idm Constant

- 0.027 0.143 0.004 - 3.233

- 0.321 0.827 0.288

0.009 0.167 0.015

8.976 73.644 0.073

regime X us DSS Constant

0.035 0.112 0.000 - 3.796

0.131 0.914 0.312

0.259 0.114 0.000

1.833 96.958 0.107

X us Wm Constant

0.029 0.118 - 0.020 - 3.867

0.107 0.962 - 0.060

0.025 0.015 0.041

1.294 60.018 0.241

l **

4 4s

0.135 0.962 - 0.127

0.315 0.007 0.001

I.805 97.522 1.532

l **

Constant

0.423 0.072 - 0.001 - 1.438

X 4 Wm Constant

0.244 0.067 - 0.042 - 1.281

0.078 0.892 - 0.143

0.284 0.007 0.027

0.739 90.216 2.401

l **

current

u*

Value of beta

regime X us 48 Constant

u*

Medium u*

B

flow dynamics of 2. murina meadows under differX = distance into the meadow (m), Us = surface = density of seagrass shoots per ms, Ils/m = leaf area) per cubic meter of water column, B = regresbeta = standardized regression coefficient for inthat B, NS = not significant, l ** = significantly

regime X

***

R2

0.858

l ** NS

l **

t** NS

-

0.858

NS *** NS

0.904

NS

0.905

NS

NS

0.934

NS

NS

0.939

NS

associationof distance (X) with u* under medium and high current regimes (Table 2). An irregular positive relation between roughnessheight (b ‘) and u* is demonstrated in Figure 2. Figure 3 shows near-bottom velocities along transects selected from each of the three current regimes (2 cm height isotach) as predicted from bathymetrically-induced effects (predicted or P curve), and for observed velocities (observed or 0 curve, uncorrected for the lunar maxima). The most dramatic reduction in velocity, as shown by the departure of the P and 0 curves from each other, occurred immediately upon entering the meadow. This trend is consistent except in the highest current regime, where on the crest of the mound in thick seagrasscover the P curve risesdramatically due to the effect of the mound and the 0 curve nearly meets it, indicating the seagrasseffect on current reduction was almost completely exhausted.

373

Role of current velocity

W3

h/L

10-Z

High current regime

Medium current regime

Low current regfme

.

IO-’

. . . .

.

. I

.

I-O

3-o Shear

Figure 2. Roughness meadows increases.

Sediment

height

I

I

4-o

5.0

.

2-o velocity

becomes larger

(U*J

*

.

.

*,

6X

I” cm s-’

and more variable

as shear velocxty

in seagrass

dynamics

Sediment analysiswas performed on only high and low current regime meadowsbecause samplesof the medium current regime were lost after the field work was done. In the low current regime, the occurrence of organic matter in the surface 1 cm could be predicted by distance into the meadow (X) and plant standing stock as 1,, (Table 3). Organic matter distribution was not predictable in the high current regime. Per cent silt-clay had no predictable distribution in the low current regimes. In the high current regime, however, silt-clay distribution was predictable by an inverse relationship with lunar maximum shear velocity (U*,,,). Synoptic

current pow

diagrams

The seagrasscanopy in Figures 4(a) and (b) is drawn at the degree of bending predicted from Fonseca et al. (1982) in low and high current regimes, respectively. Canopy bending was visibly higher in the high current regime transect. The patterns of sediment distributions and velocity profiles distinctly separate the two transects. In the low current transect the height above the bottom of the maximum current velocity isotach (30 cm s 1: is stable across the meadow, rising only as the flow field enters the meadow. The flow field change effected at the meadow edge is demonstratedby the unusually shapedvelocity profile at this point. Silt-clay and organic matter content do not fluctuate substantially across the vegetated portion of the meadow [Figure 4(a)]. In contrast, the height above the bottom of the maximum current velocity isotach (80 cm s-i) varies inversely with the mound height and scours the meadowjust after the mound crest (as shown in the P versus 0 curve of Figure 3 (c)). The high current transect displays no change in silt-clay content in the sediment down its length, but demonstrates a pulse of organic matter content in the crest area of the mounded meadow just outside the area of maximum scour [Figure 4(b)], possibly representing the area of the meadow which provides consistent reduction in near-bottom velocities.

M. S. Fonseca et al.

374

( a)

Low currant

regime

50 .P

20 IO I 5

I 0

( b)

60

Medium

4 15

I IO

current

I 20

5

regime

t -

s -

7Ob 60-

N

I

i;

>I c

3

I

I

I

I

I

I

I

I

0

I

2

3

4

5

6

7

( c ) Hugh current

regime

90

70

P

60

30 2010 1 0

1 I

I

I

I

I

1

I

2

3

4

5

6

7

distance

(ml

Transect

Figure 3. Predicted versus observed current are shown here for three selected transects. regimes. Distance = 0 is the meadow edge.

velocities at the 2 cm height above the bottom Each transect is from one of the three current

Role of current

velocity

375

TABLE 3. Multiple linear regressions for sediment characteristics of Z. marina meadows under high and low current regimes. % OM = per cent organic matter in the surface 1 cm of sediment. % S-C = per cent silt-clay fraction in the surface 1 cm, U*,,, = maximum shear velocity on a lunar cycle, Z = distance into the meadow (m), 11, = leaf area index (mz, two-dimensional surface area) per square meter of meadow. B = regression coefficient for independent variables, beta = standardized regression coefficient for independent variables, F = F-statistic for that B, NS = not significant, l = significantly different from 0 at 0.10 level, l ** = significantly different from 0 at the 0.01 level

Dependent variable Lou> current %OM

96 s-c

High current %OM

% s-c

Independent variable

B

Value of beta

Standard error of B

F

Significance

R’

regime u* max X 4, Constant

O-005 0.050 0.395 I.024

0.005 0.545 0,440

0.193 0.022 0.185

0,001 4.982 4.550

NS * *

0 716

u* ma* X ha Constant

- 4.182 0.028 1.951 23.255

- 0,446 0,029 0,207

3.219 0.373 3.090

1.689 0,006 0.399

NS NS NS

0.284

regime rJ* max X 4, Constant

1,060 - 3,016 - 0.004 0,511

0,439 - 0.378 - 0,002

1.259 3,392 1.000

0,709 0.791 0.000

NS NS NS

0 217

cJ* ma* X 4, Constant

~ 3.182 - 1.822 - 0,047 21.559

- 0,908 - 0.157 ~ 0,016

0,529 1.426 0.421

t*t NS NS

0,934

36.166 1.632 0.012 1

Discussion Current and sediment dynamics of the seagrass meadow We have determined that the physical configuration of the seagrass meadow is correlated with tidal current flow in seagrass meadows without strong wave surge (Figure 1). Not only is there an increase in mounding with increased tidal current velocity, but the continuity of seagrass cover follows an inverse relation with current velocity. This mounding phenomenon, which appears to hold both in temperate and in tropical systems, has been described by Scoffm (1970) in relation to currents and Patriquin (1975) in relation to wave surge. We have observed much less precipitous mounding in high current areas of Puget Sound, Washington, where the substrate was composed of gravel and cobbles, as opposed to the medium-sized sand found in these study areas. Measuring leaf area (Z1,/m) and distance into the meadow (X) for prediction of U* are not useful. Only in the broad meadows of the low current regime was there an indication that U* was modified over distance (x) based on their relative beta values. In general, these data corroborate our earlier work (Fonseca et al., 1982) which demonstrated CT* changes associated with the leading edge of a meadow. In higher current regimes, distance (X) has no pronounced effect of shear velocity prediction because the flow field is rapidly re-formed after passing the meadow edge, and because the meadow is patchier and much narrower in the direction of current flow. This

376

M. S. Fonsaca et al.

100 cm s-’ Current

direction

Im

Current

directton

(b)

+

Figure 4(a). A scaled drawing of a low current regime Z. marina meadow. Velocity profiles at each sampling station are enlarged to the upper and left values of the scale. The velocity profiles and 30 cm s-1 isotach are &awn in an enlarged scale for better viewing. (b) As for 4(a), except that it is for a high current regime Z. murinu meadow with velocity profiles drawn to the scale of the drawing.

Role of current velocity ___-

377

eliminates any cumulative canopy effect and results in the velocity reduction properties of the canopy being regularly exceeded, as demonstrated by Fonsecaet al. (1982). Seagrassleaf area theoretically reduces shear on the bottom; we could not demonstrate this becausethe meadow aswe sampleit is a static portrait of the dynamic balanceof forces of flow and resistance. The canopy cannot develop in these high current areas to where it chokes out the flow because there is inevitably an overriding geomorphologic control accounting for the existence of high current in that area. If the grasseswere given to increase beyond their observed abundances, the over-meadow current velocity would necessarily be increased becausethe flow is usually constrained to passover the meadows (outside meadow)

(inside meadow)

where U, is the current velocity and D, is the total water depth of station n. Therefore, if D, is effectively decreased by increased grass abundance, U, must increase. As 0; increases, the canopy bends and compressesinto a dense mat. If current velocity were increased, shear forces being expended in bending the canopy would exceed the reduction properties of that canopy, shearwould be transferred to the sedimentsurface, and sediment erosion would eventually follow. In the low current areas, the characteristic broad and often shallow meadows slow currents and trap fine sediments. If the seagrassmeadow trapped sediment sufficiently, the depth would decrease to the upper limit of the seagrasses’tolerance, the seagrasseswould die back, and the sediment would be scoured away to accommodatethe discharge designated by the bathymetric configuration. In the absenceof rapid geomorphologic shifts, the seagrass-inducedcurrent reduction and scour exist in an equilibrium constrained by a larger bathymetric scale. It is for this reason of equilibrium conditions that we observe roughnessheight increasing, even though velocity is increasing (Figure 2). If those ambient velocities are exceeded, the effect of canopy on current reduction is exceeded (Fonsecaet al., 1982) and the roughness height must again be diminished. In the presence of these more erosive conditions, the meadow could be eroded. This process cannot be sampled under field conditions because no meadows exist when their critical erosion point is frequently reached. Also, these measurementsdo not account for the sedimentstability afforded by the root-rhizome system. In the range of seagrassstanding stocks we have observed, the maximum current velocity was 110 ems-1. Therefore, we propose that approximately 120-150crn~~~ 1s the maximum velocity that 2. marina can tolerate. This estimate agrees with estimates of critical erosion velocities by Scoffin (1970) for current-dominated tropical seagrass meadows (Thalassia and Sjringodium) and by Patriquin (1975) for wave-dominated tropical seagrassmeadows. Changesin shear and flow patterns in relation to the seagrasses also contribute largely to edaphic development. The distribution of organic material is predictable in low, rather than high current velocities, but the opposite relationship wasobserved for silt-clay. Much of the organic input to the surface sediment is derived from senescenceof the foliar portions of the seagrasses(Kenworthy, 1981; Kenworthy et al., 1982) and to trapping of suspendedallocthonous material by the seagrasscanopy (Thayer et al., 1975; Zieman, 1975). In high current areas leaves are washed away once they die, especially since the high current meadowoften experiences strong flushing from currents of opposite directions during tide changes. Flushing also diminishes any sediment organic build-up related to the presence of the seagrasscover. The importance of current reduction in retaining

378

M. S. Fonseca et al.

autochthonous organic material for the development of sediment nitrogen resources is discussed in Kenworthy et al. (1982). The silt-clay distribution in the sediment serves as an indicator of the depositional environment afforded by the seagrasses, since this particle size is generally considered as more susceptible to erosion than any other non-organic sediment fraction. The silt-clay fraction had no predictable pattern of distribution within the low current meadows (Table 3). With water depth increasing over the course of the tidal cycle, the grasses’ effect on sediment settling at the meadow edge is lessened, and silt-clay size particles probably settle more evenly over the whole meadow. In high current areas the silt-clay fraction is more closely correlated with the seagrass canopy, while inversely related to U*,,. This relation is well known through the entrainment function of Shields (Miller et ul., 1977). Therefore, we may consider the high current areas as sources and low current areas as sinks for seagrass-derived detrital material.

Seagrasshabitat developmentrelated to current frow dynamics

Although we present no data on fauna, these components are undoubtedly influenced by the flow dynamics in the seagrass meadow. In the low current environment, shoots tend to be more erect, providing a complex three-dimensional habitat that is utilized by epibenthic fauna and local fishes (Homxiak ef al., 1982). In high velocity areas, current flow intrudes deeper into the meadow, increasing scour. Fauna existing under those regimes need adaptations for clinging, being more massive (e.g. hermit crabs), or select a habitat in the sediment. These adaptations are readily observable. Increased bending and compression of the seagrass canopy by high current velocities must also direct a different feeding strategy by the local fish community, since as the canopy becomes denser and more impermeable, prey are probably less conspicuous than in low current areas. Energy expenditure for prey capture by fishes in these meadows could be considerably higher than in low current meadows. All seagrass meadows, therefore, probably do not provide equivalent habitat utilixation’potentials under different current regimes. The concept of h/L (Figure l), P and 0 curves, (Figure 4) and synoptic diagrams (Figure 4) depict the interaction of forces that create the different canopy configurations described above. Five major points are suggested by these data: (1) each meadow can be characterized by the h/L ratio and concomitant generalization of flow field reduction patterns and characteristic edaphic development (Kenworthy, 1981; Kenworthy ef al., 1982); (2) h/L ratios are useful in other seagrass studies-such as locating stock for transplanting (Fonseca ef al., 1979, Kenworthy et al., 1980); (3) seagrass meadows could be differentiated on the basis of h/L for faunal structure and detrital pathway characterization; (4) the seagrass-current flow generalizations seem to be accurate in tidal current-dominated areas over a wide geographic range; and (5) each h/L category denotes patterns of meadow development that may represent different equilibrium levels of maturity (relative climax). We have attempted to describe the current velocity in5uence on seagrass meadow development in Figure 5. As stated by Odum (1969: 251), succession (sensu, community development) ‘results from modification of the physical environment by the community, that is succession is community-controlled even though the physical environment determines the pattern, the role of change and often sets limits as to how far development can go’. This statement seems to describe appropriately the influence of current velocity as given here. The seagrasses develop to a point where they are in equilibrium with the reduction of current flow provided by their canopy, sediment stabilization afforded by their

379

Role of current velocity

Morphological response of plants to current, light, nutrient concentrohon

SCurrent t~;~entotron

P
/Shift finer Range levels

to a substrate

Cycle J

I of equilibrium (relative climax)

III

\

Local meadow development

of meadow

Seedlmgs \ establish a

germrnote

8

as

Oscrllotron of .cooulotion: . stabrlization by selectton for many resource drmensrons, eweciallv bolonce between light, current velocity and sediment stabilrty

Short Habitat

development

term population ex: Ice shear rapid land fill

reduction:

\Mature

---

raptd

a-.:_-

bothymetrlc \

In giw

shift \ ‘.\

Cllmotrc evolutlonory

and cycle?

\

:,\

IncreasIng magnitude of Influence

Figure 5. Theoretical influence Each circle designates different

of current velocity time scales.

in the development

of a seagrass meadow.

root-rhizome complex, and the increasein velocity causedby any mounding associatedwith their own presence. Acknowledgements We thank the University of Virginia and the National Marine Fisheries Service, Southeast Fisheries Center, Beaufort Laboratory, for providing facilities and logistic support. We alsothank Mr Alex Chester (NMFS) for consultation on statistical procedures, Dr Paulette Peck01 (Albion College), Dr Frederick Short (Harbor Branch Foundation), and Mr W. Judson Kenworthy (NMFS) for their helpful comments in review, Herbert Gordy for graphics, and Donna Hebert for typing. This work was supported in part by the National Science Foundation, International Decade of Ocean Exploration, SeagrassEcosystem Study Grant OCE77-27051, and in part through a cooperative agreement between the U.S. Department of Energy and NOAA, National Marine Fisheries Service, Southeast Fisheries Center, Beaufort Laboratory.

380

M.

S. Fonseca

et al.

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