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Hydrodynamics and structure: Interactive effects on meiofauna dispersal
J. Exp. Mar. Biol. Ecol., 1986, Vol. 104, pp. 53-68 Elsevier
53
JEM 00790
Hydrodynamics
and structure: interactive effects on meiofauna dispersal* Margaret
A. Palmer
Department of Biology. Wabash College, Crawfordsville, IN 47933, U.S.A. (Received
29 May 1986; revision
received
11 August
1986; accepted
4 September
1986)
Abstract: Studies have shown that meiofauna occur in the water column due to both passive erosion processes and behavioral emergence. The relative importance of these two processes varies between habitats. I tested the hypothesis that two aspects of the physical environment, hydrodynamics and aboveground structure, interactively influence meiofauna dispersal. Experiments were performed in a recirculating laboratory flume using sediment boxcores from two sites - from within a vegetated (Spartina) region and from an adjacent unvegetated mudflat. A 2 x 3 factorial design was used with three flow levels and structure either present or absent. For each treatment combination the number of meiofauna drifting was determined. The number of meiofauna emerging was determined under no flow conditions with structure present and with structure absent. Flow and structure did influence drift from the mud boxcores with the greatest number of fauna drifting at high flows and when structure was present. For the Spartina boxcores, the number of meiofauna drifting increased as flow increased but was not influenced by the presence of structure. For both mud and Spartina boxcores, emergence of copepods was enhanced when structure was present. It is concluded that, for both sites, surface activity of copepods was enhanced in the presence of structure, yet the copepods were able to minimize downstream transport when Spartina culms were present. Nematode emergence was not influenced by structure. Key words: Meiofauna;
Meiofaunal
dispersal;
Hydrodynamics
and dispersal;
Structure
and dispersal
INTRODUCTION
Study of the interactions between soft-bottom marine macrobenthos and flowing water has received much recent attention by benthic ecologists. Hydrodynamics may determine feeding modes of benthos (Taghon et&, 1980; Carey, 1983) and may influence benthic recruitment (Eckman, 1983; Hannan, 1984) and subsequent survival (Grant, 198 1). Interest in the influence of flow on marine animals has not been limited to macrofauna. Field studies have shown that meiofauna are influenced directly by flow, with meiofauna occurring at the sediment/water interface and in the water column (Bell & Sherman, 1980; Hagerman & Rieger, 1981; Sibert, 198 1). While some studies have shown that the occurrence of meiofauna in the water column results primarily from the erosion of sediment-dwelling animals by tidal currents (Palmer & Gust, 1985) or wave action (Hagerman & Rieger, 1981), other studies have concluded that meiofauna
* Contribution
No. 640 from the Belle W. Baruch
0022-0981/86/$03.50
0
1986 Elsevier
Science
Institute
Publishers
for Marine
Biology and Coastal
B.V. (Biomedical
Division)
Research.
54
MARGARET
A. PALMER
migrate into the water column (Arndt et al., 1982; Walters & Bell, 1986). Such different results are likely due to both habitat differences and species-specific beha~or~ differences (Fleeger et al., 1984). Two aspects of the physical environment, hydrodynamics and aboveground structure, may interactively influence the abundance of meiofauna in the water. There is certainly ample evidence that flow influences meiofauna. Some meiofauna have been observed to burrow as flow increases (Rhoads et al., 1977; Palmer, 1984), to move into interstices and secrete mucous “dams” (Crenshaw, 1980), to vertically migrate in sand in response to vibrations simulating surf(Boaden, 1968) or migrate down in response to water flow (Palmer & Molly, 1986), and to be eroded by tidal currents (Palmer & Gust, 1985). Evidence that structure influences sediment-dwelling meiofauna mostly refers to belowground interactions, such as, meiofaunal abundance around plant or seagrass shoots (e.g., Bell et al., 1978; Osenga & Coull, 1983). Fleeger et al. (1984) provided evidence that meiofauna do enter the water actively and passively in vegetated areas. We do not know if meiofaunal entrance into the water is related to the vegetation. Indirect evidence that meiofaunal activity in the water is influenced by aboveground structure includes studies showing that sediment recolonization, some of which has been shown to be via the water, is influenced by the presence of artificial or natural structures (Bell & Coen, 1982; E&man, 1983; Kern, pers. comm.). Such structure may decrease the risk of predation (Woodin, 1978) or may enhance food availability to the benthos (Thistle et al., 1984). However, structure may also enhance sediment erosion (Eckman et al., 1981) and, thus, the effect of structure on meiofaunal dispersal may be complex. The role of flow and structure in meiofauna dispersal, both active emergence and passive transport, cannot be investigated in the field alone, since adequate control of flow throughout an experiment is difficult, if not impossible. Thus, laboratory flume experiments were designed to control flow and manipulate habitat structure. I tested the hypothesis that the occurrence of mud-dwelling meiobenthos in the water column is related to above~ound structure and Ilow in an interactive way. ~rou~out this paper, meiofauna refers specifically to typical sediment-dwellers, not epibenthic or phytal meiofauna well known to reside on seagrasses or other structures (e.g., Mukai, 1971; Hicks, 1977). For these fauna, I examined drift (passive erosion) and behavioral emergence. Since meiofauna may be eroded as passive particles and erosion of meiofauna &if. may be enhanced by protruding structures, meiofaunal drift was examined in the presence and absence of structure. My hypothesis was that drift will be greatest when critical friction velocities are exceeded and structure is present. EmergePrce. Since meiofa~a from this study site minimize activity on the sediment surface as flow is increased, emergence was examined when flow was absent. My hypothesis was that emergence will be enhanced when structure is present.
MEIOFAUNADISPERSAL
55
METHODS AND MATERIALS STUDYSITE AND FLUME The study site was in the North Inlet Estuary, near Georgetown, South Carolina (33”22’N: 79”lO’W). The high-salinity, well-mixed estuary (1.4 m tidal range) is characterized by wide expanses of Spartina altemSfloruLoisel marsh grass and meandering tidal creeks. Experimental cores were collected from the center of an intertidal vegetated mudflat and from an adjacent unvegetated region. Mean flow (U) over the mudflat reached a maximum of approximately 15 cm * s _ ’ and friction velocities (u * ) may exceed 1.00 cm +s- ’ (u * = (z/p)“‘, where z = shear stress and p = fluid density). , where U = mean stream flow, d = flow depth, and Y y = kinematic fluid viscosity) vary from 20000 to over 200000 depending on stage of the tide. The sediment surface is burrowed by the fiddler crab, Uca sp., with surface roughness most pronounced immediately adjacent to Spartina culms. The experiments were conducted in July 1984 in a recirculating laboratory flume that is 2.4 m long, 30 cm wide and 30 cm deep. The flume has a removable section 1.5 m downstream from the inflow where boxcores (20 x 20 cm cross section, 15 cm deep) can be inserted so that the sediment surface is flush with the bottom of the flume. Fresh sea water was supplied to the flume from a 200 gal. headtank and water flow was controlled by PVC ball valves. Water flow was measured using temperature-compensating hot bead probes (modified from LaBarbera & Vogel, 1976). Water depth was maintained at 6 cm and friction velocities were calculated using vertical velocity profiles taken just upstream of the experimental cores. Eight points were measured within the log layer and no flow measurements were made below z = 1 mm. Least squares regression was used to calculate slopes (~*/k) from plots of u vs. In(z), where u* = friction velocity, k = von Karman’s constant, and z = distance off the bottom. All r2 values from the regressions were greater than 0.90. As calculated by Schlichting’s (1979) equation for boundary layer thickness (Eqn. 21.8, p. 638), the boundary layer, at the point flow was measured, was still growing and the shear velocities I measured slightly exceeded those occurring further downstream. Thus, friction velocities were overestimated by ~7% (see Schlichting’s Eqn. 21.5, p. 637) and the error was consistent throughout the experiments. Flow Reynolds numbers in the flume varied from 40 000 at low flow to 91000 at the highest flow settings. Boxcores of stratigraphically undisturbed sediment were collected from the study sites and returned to the flume laboratory and placed in holding tanks. Experiments began after boxcores had equilibrated for 3 h. All boxcores were collected just after the mudflat was exposed at low tide so there was no water on the surface, yet a fairly high water content in the sediment. This minimized disturbance caused by surface cracks and runoff. Meiofauna field abundances were measured at the time boxcores were collected using small cores (1.9 cm diameter). The field abundances were not signifi-
Flow Reynolds numbers (Re = g
56
MARGARET
A. PALMER
cantly different from abundances measured in boxcore sediment after equilibration in the flume (t-test, P > 0.05). EXPERIMENTAL
DESIGN
The experiment was originally designed as a 4 x 3 factorial in which structure and flow were manipulated. Natural structure (S. alternzfloru) was absent, present, removed, or artificial structure was added. Flow was absent (no flow), below critical erosion velocity for the sediments (low), or above critical erosion velocity for the sediments (high). The 4 x 3 factorial design was meant to allow a comparison of the effects of natural and artificial structures on the same meiofauna community. Differences in meiofaunal abundances and composition were found between boxcores collected from the vegetated area and the unvegetated adjacent mudflat. Thus, for each area (hereafter, Spartina site and mud site), a 2 x 3 factorial design was used with structure present or absent and the three flow levels described above. Five replicate boxcores were used per treatment, thus a total of 30 boxcores per site were tested. For the Spartina site, structure-absent cores were prepared by clipping Spartina culms at the sediment surface. For the mud site, structure-present cores were prepared by implanting soda straws (artificial mimics) in otherwise featureless sediment boxcores in a tixed pattern. Since isolated Spartina culms and clusters of large and small culms are present in the field, the artificial mimics were arranged as isolated elements and in clusters. Spacing, height, and diameter of the structures were similar to natural Spartina stands (Table I). Structures were trimmed to a 4 cm height and water level was maintained at 6 cm. This scaling was selected somewhat arbitrarily since the culm height to water depth ratio changes constantly in the field depending on tidal height. In the field, there are periods when the Spartina is covered and periods when it protrudes through the water surface. After cores had equilibrated, they were exposed to their appropriate flow setting for 1 h and then faunal samples were collected. In the low flow treatments, midstream U was ‘Icm.s- ’ throughout the l-h period. This flow level was chosen because it was
TABLE I
Number of structures, structure diameter, and per cent boxcore surface area occupied by structure in the Spartina treatments and in the artificial mimic treatments: values are means (n = 15 boxcores) and standard errors are in parentheses; for Spartina, diameter was measured at culm base. Structure Spartina Number/boxcore Cluster diameter (cm) Per cent area occupied
by structure
11.7 (0.82) 0.87 (0.22) 1.9 (0.3)
Artificial
mimics
13.0 (0) 0.96 (0.12) 2.8 (0)
MEIOFAUNA
DISPERSAL
57
well below critical erosion velocity for the sediments and represents a flow level typical of early ebb or late flood tide in the field, In the high flow ~eatments, flow was set just above critical erosion threshold for the sediments (hereafter referred to as critical friction velocity, u*~,,~~). Since u*_,,~~varied between boxcores, it was determined separately for each boxcore used in the high flow treatments. Critical friction velocity was determined by slowly increasing flow while an observer watched sediment particles through a stereomicroscope until 10 or more particles were observed rolling in contact with the bed (after Rhoads et al., 1978). The transported particles consisted primarily of loose clay aggregates. Because flow measurements were made just upstream of the test section (structures preclude measurement within the boxcore), u *_critvalues do not represent actual critical friction velocities for the sediments. As in Eckman (1983) and Luckenbach (1986), the erosional thresholds measured in this manner provide relative measures of u * _f.it useful in comp~ng treatments. SAMPLING
AND ANAI.YSES
At the time boxcores were collected, surficial sediment samples were collected in the field for g-ranulometry measurements. Gram sizes were measured by wet-sieving the sediment and an aliquot of the silt-clay fraction was removed for pipette analysis (Buchanan & Kain, 1971). Median grain size and sorting coefftcients were calculated following Inman (1952). Meiofauna that were transported downstream in the flume (drift fauna) were collected from a 50 pm drift net that had filtered ali outgoing flume water during the 1 h experiments period. For the no flow treatments, animals that were in the water above the boxcore (emergent fauna) were sampled at the end of the l-h period and after removing the drift net. The emergent fauna were sampled using a 10 cm diameter funnel attached to a small diaphragm pump (intake velocity = 4 cm * s- ‘). Positioned 1.5 cm from the sediment surface, the intake was passed over the boxcorc once using a sweeping motion. Pump intakes sampled the areas around the culms and were not placed over culms. The technique depends on quick timing and agility; however, the manipulation is not difficult since the apparatus is fully visible through the side of the flume. Preliminary experiments with fluorescein-sea water and dyed sediments showed that the pump collected all suspended material and loose sediment aggregates on the surface. Meiofauna active at the sediment-water interface were also collected. The maxinlum swimming speed for meiofauna is approximately 0.2 cm . s - ’ (P~mer, 1984) and, thus, they could not evade the intake, All fauna1 samples were preserved in Rose Bengal-formalin for later enumeration. Parametric statistical tests were used after the data were examined to determine that appropriate assumptions were valid. Analysis of variance (ANOVA) was utilized to dete~ine if the abundance of meiofauna drifting was si~~c~~y affected by flow and structure. ANOVA was performed using the General Linear Model techniques (Helwig & Council, 1979) and multiple comparisons were performed when significant main
MARGARET A. PALMER
58
effects were present. t-Tests were performed to determine if the abundance of meiofauna emerging was significantly greater when structure was present or absent.
RESULTS STUDY
SITE COMPARISONS
Median grain size among the Spartina c&m was 38 pm and at the mud site was 45 pm. For the low flow treatments mean friction veiocity (u * ) was 0.25 cm - s - I. Mean critical friction velocity (u *_crit) of the sediments in the mud boxcores was 1.02 cm . s _ ’ and was significantly greater than a mean u *_ctit of 0.44 cm * s- ’ for the Spartina boxcores (Table II). When artificial structure was added to the mud boxcores, u*_,,~~ decreased and when structures were clipped in the Spartina boxcores, u *_critincreased significantly (ANOVA, significant structure effect, P -C0.05; Scheffe’s test, experiment wise error rate = 0.05). The abundance of meiofauna (Table III) and the copepod species composition (Table IV) differed between samples collected at the mud site and those collected among
TABLE II Critical friction velocities (u *_& of sediments for each of the four structure treatments: values are means based on five boxcores per treatment; standard errors are shown in parentheses. U*_ctit(cm.s-‘)
Treatment
1.02 0.71 0.61 0.44
Mud Mud with artificial structure Spartina removed Spartina
(0.05) (0.09) (0.04) (0.02)
TABLE III Composition of sediment meiofauna among Spartina culms and in areas free of culms (mud): values are mean number per 10 cm2 and are based on cores (1.9 cm diameter) collected at the times of boxcoring; n = 45 cores per site; standard errors in parentheses. Site _.__ Spartina
Nematoda Copepoda Nauplii Foraminifera Turbeliaria Others*
3540.6 540.1 296.5 310.6 24.7 260.5
(246.8) (65.0) (43.6) (43.6) (3.6) (25.0)
* Includes ostracods, mites, kinorhynchs, and juvenile annelids.
Mud 1680.3 190.6 218.9 133.8 23.3 86.5
(100.2) (19.8) (16.5) (13.9) (4.9) (10.1)
MEIOFAUNA
DISPERSAL
59
TABLE IV Species composition of sediment copepods among ,Spartina culms and in areas free of cuims (mud): values are mean number per 10 cm’ and are based on cores (1.9 cm di~eter) collected at the times of boxcoring; abundances are only given for those species that occurred in densities > 5 per 10 cm2; *, indicates rarely occurred; -, absent; standard error in parentheses. Site Taxon
Spartina
Mud
Ameriidae Nitocra lacustris (Schmankevitsch) Nitocra typica (Boeck)
10.6 (2.7) *
*
10.6 (1.9)
*
105.9*(19.1) 67.1 (13.6)
24.7 (6.4) 17.7 (2.7)
Canthocamptidae Mesochra pygmaea (Claus)
Cletodidae C~eiocamp~ushelabi~ Fleeger Enhydrosoma propinquum (Brady) Nannopus palustti (Sars)
Cyclopidae Halicyclops coulli Herbst
56.5 (11.8)
Cylindropsyllidae Paraleptastacus sp.
*
Diosaccidae Pseudostenhelia wellsi (Co1111& Fleeger) Roberisonia propinqua (T. Scott) Schizopera knabeni Lang Stenhelia (D.) bifdia (Coull)
*
* 17.6* (4.5) 222.4 (26.4)
116.5*(10.9)
14.1* (3.6)
6.1* (2.2)
Ectinosomatidae Ectinosoma sp. Halect~osoma ~~nonae Coull
Laophontidae Esoia sp. Normanella sp. Onychocamptus mohammed (Blanchard & Richard) Paronychocamptus wilsoni Cot111 Paralaophonte sp. Quinquelaophonte cap~l~ata{Wilson)
* *
* * * *
*
Taehididae Microarthridion linoraie (Poppe)
*
5.2 (1.4)
Thalestridae Diarthrodes aegideus (Brady)
24.7 (5.5)
Tisbidae Tisbe holothuriae (Humes)
*
the Spartina culms. Copepod species diversity was greater and meiofaunal abundances among the Spartina culms were twice those in the mud area. DRIFT
To test the hypothesis that the presence of meiofauna in the water column is a function of flow and structure, ANOVAs were performed for copepods, nematodes, and total
MARGARET A. PALMER
60
meiofauna. The results for nematodes and total meiofauna were the same; thus results for total meiofauna will not be reported. At the Sprtina site, the number drifting increased with increasing flow (Fig. 1) and there was no statistically significant structure effect (Table V). At the mud site, structure did influence drift, but the effect depended on flow (Table V). Here, the number drifting at a given flow level (Fig. 2) was greater when structure was present (Scheffe’s test, experimentwise error rate = 0.05). Copepods were the dominant taxon in the drift samples. In the Spartina boxcores, regardless of the structure treatment, 0.2 to 0.7% of the s~iment copepods drifted in low and high flow respectively. At the mud site, 0.2 to 0.4% drifted from no structure
Soartina
180
COPEPODS
site
180
/
NEMATODES /
0 z:
140
t: E p
100
6 9
60
60
i= 20 NF
LOW
NF
HIGH
LOW
HIGH
FLOW
Fig. I. Abundance of copepods and nematodes drifting from .Spartina boxcores (20 x 20 cm): dashed lines (NS) are for cores without structure and solid lines (S) are for cores with structure (Sparrina); values are mean number drifting from a boxcore; n = 5 boxcores per data point; standard error bars are shown.
TABLE V Analysis ofvariance results for the effect of flow and structure on the abundance of copepods and nematodes drifting: F values are given for each ANOVA; *, indicates significance at P = 0.05; **,significance at
P = 0.01.
Flow
Structure
Interaction
Spartina site
Copepods Nematodes
47.68* 16.44**
Mud site Copepods Nematodes
17.26** 18.05**
0.01 1.28
15.2X** 19.17**
3.88* 2.14
7.02** 5.57*
MEIOFAUNA
61
DISPERSAL
Mud site NEMATODES
COPEPODS 10
4
S
60
20 I NF
LOW
HIGH
NF
LOW
HIGH
FLOW of copepods and nematodes drifting from mud boxcores (20 x 20 cm): dashed lines (NS) are for cores without structure and solid lines(S) are for cores with structure (artificial mimics in mud cores); values are mean number drifting from a boxcore; n = 5 boxcores per data point; standard error bars are shown. Fig. 2. Abundance
boxcores in low and high flow respectively, while 0.7 to 1.1% drifted in boxcores with artificial structure. At the species level, all copepods present in the sediment were found in drift samples (Table VI). For both sites, Nitocra lucustri~ (Schmankevitsch), Schizopera knabeni Lang, and Microarthridion littorale (Poppe) were the most abundant copepods drifting relative to their sediment abundances; however, none of the sediment species were disproportionately represented in the drift to a significant extent. An examination of copepod species composition in structure vs. no structure treatments revealed that: (1) no one species was responsible for the higher number of copepods drifting from mud boxcores when structure was present, and (2) the species which are common to both sites did not drift in higher numbers from Spartina boxcores when structure was present. EMERGENCE
The number of copepods collected by pumping the water above the boxcores at the end of each no flow flume run was significantly greater when structure was present (Table VII). For nematodes, the number collected above structure cores was not statistically different from numbers above no structure cores. At the mud site, 0.2 to 0.4% of the sediment copepods were present in the water above no structure and structure cores, respectively. At the Spartina site, 0.6 to 1.2% of the sediment copepods were present in the water above no structure and structure cores, respectively. At the species level, Nitocra lacustris, Mesochra pygmaea (Claus), Hallicyclops coulli Herbst, Schizopera knabeni, and Microarthridion littorale emerged in greatest numbers relative to the sediment abundances (Table VIII).
62
MARGARET A. PALMER TABLE VI
Species composition ofcopepods drifting at high flow: values are mean number drifting from unmanipulated boxcores (20 x 20 cm); n = 5 boxcores per mean; *, indicates rarely occurred; -, absent; standard error in parentheses. Site Spartina
Mud
Taxon” Ameriidae (0.9)
2.5 (0.9)
1.5 (1.4)
1.8 (1.2)
16.8 * (5.6) 14.7 (7.4)
7.2 (7.6) 2.4 (1.6)
3.2 (1.4)
4.0 (3.2)
Niiocra lacustrtk Nitocra typica
5.2 *
Canthocamptidae Mesochra pygmaea
Cletodidae Cletocamptus helobius Enhydrosoma propinquum Nannopus palustris
Cyclopidae Halicyclops coulli
Cylindropsyllidae
*
Paraleptastacus sp.
Diosaccidae Pseudostenhelia wellsi Robertsonia propinqua Schizopera hnabeni Stenhelia (D.) bifidia
* 7.6 * (2.8) 36.6 (13.4)
1.0 ;0.5) 8.8 (3.9)
5.2 *(0.3)
* *
Ectinosomatidae Ectinosoma sp. Halectinosoma winonae
Laophontidae Esola sp. Normanella sp. Onychocamptus mohammed Paronychocamptus wilsoni Paralaophonte sp. Quinquelaophonte capillata
2.2 * (1.8) 1.8 (0.4) *
1.0 * (0.8)
Tachididae Microarthridion littorale
3.5
(1.6)
5.0
(2.0)
4.0 (3.2)
Thalestridae Diarthrodes aegideus
Tisbidae
*
Tisbe holothuriae
a See Table IV for taxonomic authorities. DISCUSSION
Significant habitat differences between the two areas was indicated by the greater meiofaunal abundances and diversity at the Spartina site vs. the mud site (Tables III and IV). These faunal differences may be part of the zonation patterns described in Coull et al. (1979); however, my two sites were at the same tidal height and had similar
MEIOFAUNA
63
DISPERSAL
TABLE VII Abundance of emergent copepods and nematodes for Spartina and mud boxcores (20 x 20 cm) when no flow was present: values are mean number collected per boxcore; n = 5 boxcores per mean; standard error in parentheses; *, indicates structure means are significantly greater than no structure means at P = 0.05 (see text for details). Treatment Structure
No structure
Copepods Nematodes
258.4 (14.1)* 69.2 (14.2)
121.6 (22.2) 37.6 (12.0)
Mud site Copepods Nematodes
26.9 (3.8)* 17.8 (10.1)
16.0 (1.5) 10.6 (2.2)
Spartina site
grain sizes. Microtopographic relief was somewhat greater at the Spartina site. The faunal differences may be related to the presence of the Spartina culms. Similar increased abundances around biogenic structures have been observed for macrofauna (Woodin, 1981) and meiofauna (Bell, 1983; Eckman, 1983). Enhanced faunal abundances may be related to the role of structure in providing refugia from predators (Woodin, 1978) to flow-induced effects which result in enhanced food availability around structures (Thistle et al., 1984; Eckman, 1985) or to enhanced supply rates of organisms (Eckman, 1979,1983). Some studies have not shown enhanced abundances near structures (e.g., Decho et al., 1985) and the results of Bell & Woodin (1984) suggest that a structure-refuge effect is uncertain in some habitats. Critical friction velocities are of interest in this study because, despite behavioral and morphological factors that influence meiofaunal dispersal, water column abundances are related to friction velocities (Palmer & Gust, 1985). In the present study, sediment grain sizes were not significantly different, yet the mud site had a significantly higher critical friction velocity than the Spurtinu site (Table II). For both the mud and Spurtina boxcores, the presence of structure resulted in a lowering of the critical friction velocity for the sediments by z 30%. Structures protruding from a sediment bed may cause sediment destabilization due to the entrainment of higher momentum fluid downward near the structure (Eckman et al., 1981). Destabilization due to the physical presence of protruding structures is not necessarily the rule (Luckenbach, 1986). Structure diameter, structure height/water depth, the degree of microbial or mucous binding of sediment particles, and the boundary shear stress imposed by the external flow, all determine the precise effects structure will have on sediment transport (Eckman & Nowell, 1984). In the present study, the 0.87 cm diameter of the structure clusters and the structure height (4 cm), which approached the water depth (6 cm) in the flume, resulted in local scour. This scour may be further enhanced in the field since culm height
MARGARET A. PALMER
64
TABLE VIII
Species composition of copepods emerging at no flow: values are mean number emerging from a boxcore; n = 5 boxcores per mean; *, indicates rarely occurred; -, absent; standard error in parentheses. Site Spartina
Taxon”
No structure
Mud Structure
No structure
Structure
Ameriidae Nitocra lacustris Nitocra typica
4.2
(1.0)
10.2 (2.3) *
(1.3)
8.9 (1.7)
*
*
12.8 * (1.3) 1.2 (0.8)
19.2 * (4.3) 2.2 (0.4)
2.0 (0.5) 0.8 (0.5)
2.8 (0.5) 0.8 (0.4)
26.4 (11.8)
62.2 (17.0)
2.2 (0.8)
3.3 (2.1)
*
*
*
*
9.0 * (3.8) 35.6 (16.1)
29.8 * (9.7) 75.5 (11.4)
1.2 iO.4) 4.5 (0.9)
2.5 *(0.8) 13.6 (3.4)
* *
3.2 * (1.6)
* *
* *
1.2 *(0.9) 1.3 (0.5) *
2.4 * (1.6) 2.0 (0.2) *
1.2 * (0.3)
0.8 * (0.6)
4.3
(1.8)
*
Canthocamptidae Mesochra pygmaea
4.2
Cletodidae Cletocamptus helobius Enhydrosoma propinquum Nannopus palustris
Cyclopidae Halicyclops coulli
Cylindropsyllidae Paraleptastacus sp.
Diosaccidae Pseudostenhelia wellsi Robertsonia propinqua Schizopera knabeni Stenhelia (D.) bifdia
Ectinosomatidae Ectinosoma sp. Halectinosoma winonae
Laophontidae Esola sp. Normanella sp. Onychocamptus mohammed Paronychocamptus wilsoni Paralaophonte sp. Quinquelaophonte capillata
2.2 ;0.7)
1.2*(0.4)
8.9 (2.1)
1.8 (0.9)
3.0 (0.7)
9.2 (6.2)
19.2 (11.8)
*
*
*
*
Tachididae Microarthridion littorale
Thalestridae Diarthrodes aegideus
Tisbidae Tisbe holothuriae
a See Table IV for taxonomic authorities.
exceeds water depth for most of the tidal cycle. Even though structure treatments had lower u *_crit values, adding artificial structure to the mud boxcores did not reduce u *_crit to that determined for the Spartina boxcores (0.44 cm. s- ‘). This suggests that there are differences in erodibility of sediments between the mud and Spartina site that can
MEIOFAUNA
DISPERSAL
65
not be explained by structure alone. The lower u *_critvalues for the Spartina cores may be related to the increased abundance of infauna in these cores. Faunal activities are known to alter sediments in a number of ways which influence the erodibility of sediments. Biogenic reworking is known to lower critical erosion velocities (Grant et al., 1982), as are increases in surface roughness (noted qualitatively in Spartina cores in this study) and decreases in consolidation in sediments (Nowell et al., 1981). Luckenbach (1986) found no direct hydrodynamic effect of structures on sediment destabilization and emphasized that structures (Diopatra tubes in his study) may have purely biologically-mediated effects on sediment destabilization. My results may be due to both hydrodynamic structure effects and biologically-mediated sediment effects. The number of meiofauna drifting from mud and Spartina boxcores was influenced by flow, with higher numbers drifting as critical friction velocities were exceeded (Table V, Figs. 1 and 2). Thus meiofauna are acting like passive particles and are subject to erosion by tidal currents. This is not suprising since other studies have suggested that meiofauna may be in the water due to tidal suspension (Bell & Sherman, 1980; Palmer & Brandt, 1981) and the number of mud meiofauna in the water column has been shown to increase as friction velocity increases (Palmer & Gust, 1985). For the mud boxcores, the number of meiofauna drifting was also influenced by structure. In these cores, more animals drifted when structure was present. This was despite the fact that flow settings for structure vs. no structure boxcores were comparable. The treatments consisted of no flow, flow below u.+,,~~ (low), and flow above u*_,,~~(high), with the last flow set independently for each boxcore. This design ensured that each boxcore was subjected to the same potential for erosion. Thus increased drift in the presence of structure, with the susceptibility to erosion comparable to no structure boxcores, suggests that the meiofauna were not acting like purely passive particles. Somehow the animals increased their likelihood of downstream transport when structure was present. Either they were emerging into the water and were transported, or they exhibited more surface activity which resulted in a u *_crit for the meiofauna that was below critical friction velocities for the sediments. Both interpretations require behavioral events. For the Spartina boxcores, structure had no significant effect on numbers drifting. The lack of a significant structure effect here compared to the mud boxcores may be due, in part, to the very high variability in numbers drifting from the Spartina boxcores vs. the mud boxcores (note relative magnitudes of error bars in Fig. 1 and 2). In addition, fauna may not exhibit higher drift rates in the presence of Spartina if the culms represent a refuge from flow. The meiofauna may emerge here but may not be transported. Animals may retreat into spaces between blades or within the boundary layer of the structures where shear stresses are reduced (Statzner, 1981; Silvester & Sleigh, 1985). The artificial structures may not have been a suitable refuge from flow since, unlike the Spartina culms which are textured and with grass blades, the soda straws are essentially smooth cylinders. Further data are required to investigate the potential role of Spartina culms as flow refugia. Different results for the mud area vs. the Spartina area may also be related to the fact
66
MARGARETA.PALMER
that the two areas do differ with respect to community composition (Table IV) and meiofaunal abundances (Tabie III). An ex~ination of species patterns for the drift data (Table VI) does not clarify the picture. ~it~cru Iacust~, Schizopera k~a~~i, and Microarthridion lirrorale are the most common drift fauna at both sites. The structure effect at the mud site was basically due to all species; however, the species occurring at both the mud and Spar&a sites did not exhibit a structure response in Spartina boxcores. Thus, apparently, the same species are doing different things in different habitats! The number of copepods emerging (determined from pump samples) was a function of structure at both sites (Tables VII and VIII), with a significantly higher number of copepods in the water or on the sediment surface when structure was present. This result supports my previous interpretation of the drill data. For both sites, copepods emerge when st~cture is present and yet suffer higher risks of transport only at the mud site. With the addition of ~allicyciops cot& at both sites and Mesochra py~ueu at the Spa&a site, the dominant emergent species are the same as those most common in the drift (Nitocra lacustris, Schizopera knabeni, Microarthridion littorale). The fact that copepods emerged in greater numbers than nematodes and responded to structure reflects the copepods’ agility in the water and on the sediment surface. Nemat~es are known to avoid the sediment surface perhaps because they are poor swimmers and may be subjected to even higher risks of transport (Palmer, 1984). This study was designed to test the hypothesis that the occurrence of mud-dwelling meiobenthos in the water column is related to aboveground structure and flow in an interactive way. There was a significant structure x flow interaction for meiofauna d~fting from mud boxcores. Flow atTected the number of mud meiofauna drifting; however, this effect depended on whether or not structure was present. The number of fauna drifting from mud boxcores was greatest when both flow and structure were present. Since structure and no structure cores were subjected to comparable flows, mud meiofauna are not acting as purely passive particles but are increasing their risk of transport when structure is present. The emergence data support my su~estion that, for both sites, surface activity of copepods is enhanced in the presence of structure, yet the copepods are able to minimize downstream transport only at the Spartina site. It is hypothesized that natural structure (Spartim) may serve as refugia from flow for the meiofauna. Meiofauna dispersal is clearly a complex process. Structure may influence meiofauna dispersal but its effect varies depending on flow, the nature of the structure, and the species ~om~sition of the meiofauna community.
ACKNOWLEDGEMENTS
This work was superb by the Byron K. Trippet fund of Wabash College and by a grant from the National Science Foundation’s Biological Oceanography Program (OCE-8509904). I thank the Belle W. Baruch Institute for the use of laboratory facilities
MEIOFAUNA
DISPERSAL
61
and Mike Molloy for field and laboratory assistance. Special thanks go to Drs. Susan Bell, Betsy Brown, Cheryl Ann Butman, Jim Beckman and Mark Luckenbach for critical comments which significantly improved the manuscript.
REFERENCES ARNDT, II., K. J~ACHIM, A. MICHALK,F. WRONNA & R. HERKLOB,1982. Untersuchungen zur Vertikalwanderung epibenthischer Copepoden in einem Rachen Kustengewasser mit Hilfe seiner Planktonreuse ~o~r~~se~cha~~i~e Reihe, Vol. 6, pp. 57-60. BELL,S. S., 1983.An experimental study ofthe relationship between below-ground structure and meiofaunal taxa. Mar. Biol., Vol. 76, pp. 33-39. BELL,S.S.& L.D.COEN,1982. Investigations on epibenthic meiofauna. II. Influence of microhabitat and macroalgae on abidance of small invertebrates on ~iopaf~a cupreu (Polychaeta : Onuphidae) tube caps in Virginia. $. Exp. Mar. B&Z. Eeoi., Vol. 61, pp. 175-188. BELL, S. S. & K. S. SHERMAN,1980. Tidal resuspension as a mechanism for meiofauna dispersal. Mar. Ecol. Prog. Ser., Vol. 3, pp. 245-249. BELL,S. S. & S. A. WOODIN, 1984. Community unity: experimental evidence for meiofauna and macrofauna. J. Mar. Res., Vol. 42, pp. 60.5-632. BELL,S. S., M. C. WATZIN& B.C. COULL,1978. Biogenic structure and its effect on the spatial heterogeneity of meiofauna in a salt marsh. 3. Exp. Mar. Biol. Ecol., Vol. 35, pp. 99-107. BOADEN, P.J.S., 196X. Water movement - a dominant factor in interstitial ecology. Sartiu, Vol. 34, pp. 125-136. BUCHANAN,J.B & J.M. KAIN, 1971. Measurement of the physical and chemical environment. In, IBP Handbook No. 16: Methods forthe study of marine benthos, edited by N. A. Holme & A. D. McIntyre, Blackwell, Oxford. pp. 30-58. CAREY,D.A., 1983. Particle resuspension in the benthic boundary layer induced by flow around polychaete tubes. Can. J. Fish. Aquat. Sci., Suppl. 1, pp. 301-308. COULL, B.C., S. S. BELL, A.M. SAVORY& B. W. DUDLEY, 1979. Zonation of meiobenthic copepods in a southeastern United States salt marsh. Esruarine Coastal Mar. Sci., Vol. 9, pp. 181-188. CRENSHAW,D.G., 1980. How interstitial animals respond to viscous flows. Ph.D. dissertation, Duke University, Durham, North Carolina, 105 pp. DECHO, A. W., W. D. HUMMON & J. W. FLEEGER, 1985. Meiofauna-sediment interactions around subtropical seagrass sediments using factor analysis. J. Mar. Res., Vol. 43, pp. 237-255. ECKMAN,J. E., 1979. Small-scale patterns and processes in a soft-substratum, intertidal community. f. Mar. Res., Vol. 37, pp. 437-457. ECKMAN, J.E., 1983. Hydrodynamic processes affecting benthic recruitment. Limnol. Oceanogr., Vol. 28, pp. 241-257. ECKMAN,J.E., 1985. Flow disruption by an animal-tube mimic affects sediment bacterial colonization. J. Mar. Res., Vol. 43, pp. 419-435. ECKMAN, J.E. & A.R.M. NOWELL, 1984. Boundary skin friction and sediment transport about an animal-tube mimic. Sedimento~o~, Vol. 3 1, pp. 85 l-862. ECKMAN,J. E., A. R. M. NOWELL& P. A. JUMARS, 1981. Sediment destabilization by animal tubes. J. Mar. Rex, Vol. 39, pp. 361-374. FLEEGER,J. W., G.T. CHANDLER,G.R. FITZHUGEI& F.E. PHILLIPS, 1984. Effects of tidal currents on meiofauna densities in vegetated salt marsh sediments. Mar. Ecol. hog. Ser., Vol. 19, pp. 49-53. GRANT, J., 1981. Sediment transport and disturbance on an intertidal sandflat: infaunal distribution and recolonization, Mar. Ecol. Prog. Ser., Vol. 6, pp. 249-255. GRANT,W. D., L. F. BOYER& L. P. SANFORD,1982. The effects of bioturbation on the initiation of motion of intertidal sands. .r, Mar. Res., Vol. 40, pp. 659-677. HAGERMAN, GM. & R. M. RIEGER, 1981. Dispersal of benthic meiofauna by wave and current action in Bogue Sound, NC., U.S.A. P.S.Z.N.I. Mar. Ecoi., Vol. 2, pp. 245-270. HANNAN, C. A., 1984. Planktonic larvae act like passive particles in turbuIent near-bottom flows.Limnol. Oceanogu., Vol. 29, pp. 1108-l 115.
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HELWIG,J.T. & K.A. COUNCIL, 1979. SAS u.rer’.rguide. SAS Institute Inc., Raleigh, N.C., 494 pp. HICKS, G. R. F., 1977. Species comparisons and zoogeography of marine phytal harpacticoid copepods from Cook Strait and their contribution to total phytal meiofauna. N.Z. J. Mar. Freshwater Res., Vol. 11, pp. 441-469. INMAN,D. L., 1952. Measures for describing the size distribution of sediments. J. Sediment. Petrol., Vol. 22, pp. 122-145. LABARBERA,M. & S. VOGEL, 1976. An inexpensive thermistor flowmeter for aquatic biology. Limnol. Oceanogr., Vol. 21, pp. 750-756. LUCKENBACH,M., 1986. Sediment stability around animal tubes: the roles of hydrodynamic processes and biotic activity. Limnol. Oceanogr., Vol. 31, pp. 778-787. MUKAI, H., 1971. The phytal animals on the thalli of Sargassum serrufijdium in the Sargassum region, with reference to their seasonal fluctuations. Mar. Biol., Vol. 8, pp. 170-182. NOWELL,A. R. M., P. A. JUMARS& J. E. ECKMAN, 1981. Effects of biological activity on the entrainment of marine sediments. Mar. Geol., Vol. 42, pp. 133-153. OSENGA, G. A. & B.C. COULL, 1983. Spartina alternifloru Loisel root structure and meiofauna abundance. J. Exp. Mar. Biol. Ecol., Vol. 67, pp. 221-225. PALMER,M.A., 1984. Invertebrate drift: behavioral experiments with intertidal meiobenthos. Mar. Behuv. Physiol., Vol. 10, pp. 235-253. PALMER,M.A. & R.R. BRANDT,1981. Tidal variation in sediment densities of marine benthic copepods. Mar. Ecol. Prog. Ser., Vol. 4, pp. 207-212. PALMER,M.A. & G. GUST, 1985. Dispersal of meiofauna in a turbulent tidal creek. J. Mar. Res., Vol. 43, pp. 179-210. PALMER,M. A. & R. M. MOLLOY,1986. Flow and the vertical distribution ofmeiofauna: a flume experiment. Estuaries, Vol. 9, pp. 225-228. RHOADS,D. C., R. C. ALLER & M. B. GOLDHABER,1977. The influence of colonizing benthos on physical properties and chemical diagenesis of the estuarine seafloor. In, Ecology of marine benthos, edited by B.C. Coull, University of South Carolina, Columbia, pp. 113-138. RHOADS,D. C., J. Y. YINGST& W. J. ULLMAN,1978. Seafloor stability in central Long Island Sound. Part I. Temporal changes in erodibility of tine-grained sediment. In, Estuarine interactions, edited by M. L. Wiley, Academic Press, New York, pp. 221-242. SCHLICHTING,H., 1979. Boundary layer theory. McGraw-Hill, New York, 7th edition, 817 pp. SIBERT, J.R., 1981. Intertidal hyperbenthic populations in the Nanaimo Estuary. Mar. Biol., Vol. 64, pp. 259-265. SILVESTER,N.R. & M.A. SLEIGH, 1985. The forces on microorganisms at surfaces in flowing water. Freshwater Biol., Vol. 15, pp. 433-449. STATZNER,B., 1981. The relation between “hydraulic stress” and microdistribution of benthic invertebrates in a lowland running water system, the Schierenseebrooks (North Germany). Arch. Hydrobiol., Vol. 91, pp. 192-218. TAGHON, G.L., A.R.M. NOWELL & P.A. JUMARS, 1980. Induction of suspension feeding in spionid polychaetes by high particulate fluxes. Science, Vol. 210, pp. 562-564. THISTLE, D., J.A. REIDENAUER,R.H. FINDLAY& R. WALDO, 1984. An experimental investigation of enhanced harpacticoid (Copepoda) abundances around isolated seagrass shoots. Oecologia (Berlin), Vol. 63, pp. 295-299. WALTERS,K. & S. S. BELL, 1986. Die1 patterns of active vertical migration in seagrass meiofauna. Mar. Ecol. Prog. Ser., in press. WOODIN, S.A., 1978. Refuges, disturbance, and community structure: a marine soft-bottom example. Ecology, Vol. 59, pp. 274-284. WOODIN, S.A., 1981. Disturbance and community structure in a shallow water sandflat. Ecology, Vol. 62, pp. 1052-1066.
53
JEM 00790
Hydrodynamics
and structure: interactive effects on meiofauna dispersal* Margaret
A. Palmer
Department of Biology. Wabash College, Crawfordsville, IN 47933, U.S.A. (Received
29 May 1986; revision
received
11 August
1986; accepted
4 September
1986)
Abstract: Studies have shown that meiofauna occur in the water column due to both passive erosion processes and behavioral emergence. The relative importance of these two processes varies between habitats. I tested the hypothesis that two aspects of the physical environment, hydrodynamics and aboveground structure, interactively influence meiofauna dispersal. Experiments were performed in a recirculating laboratory flume using sediment boxcores from two sites - from within a vegetated (Spartina) region and from an adjacent unvegetated mudflat. A 2 x 3 factorial design was used with three flow levels and structure either present or absent. For each treatment combination the number of meiofauna drifting was determined. The number of meiofauna emerging was determined under no flow conditions with structure present and with structure absent. Flow and structure did influence drift from the mud boxcores with the greatest number of fauna drifting at high flows and when structure was present. For the Spartina boxcores, the number of meiofauna drifting increased as flow increased but was not influenced by the presence of structure. For both mud and Spartina boxcores, emergence of copepods was enhanced when structure was present. It is concluded that, for both sites, surface activity of copepods was enhanced in the presence of structure, yet the copepods were able to minimize downstream transport when Spartina culms were present. Nematode emergence was not influenced by structure. Key words: Meiofauna;
Meiofaunal
dispersal;
Hydrodynamics
and dispersal;
Structure
and dispersal
INTRODUCTION
Study of the interactions between soft-bottom marine macrobenthos and flowing water has received much recent attention by benthic ecologists. Hydrodynamics may determine feeding modes of benthos (Taghon et&, 1980; Carey, 1983) and may influence benthic recruitment (Eckman, 1983; Hannan, 1984) and subsequent survival (Grant, 198 1). Interest in the influence of flow on marine animals has not been limited to macrofauna. Field studies have shown that meiofauna are influenced directly by flow, with meiofauna occurring at the sediment/water interface and in the water column (Bell & Sherman, 1980; Hagerman & Rieger, 1981; Sibert, 198 1). While some studies have shown that the occurrence of meiofauna in the water column results primarily from the erosion of sediment-dwelling animals by tidal currents (Palmer & Gust, 1985) or wave action (Hagerman & Rieger, 1981), other studies have concluded that meiofauna
* Contribution
No. 640 from the Belle W. Baruch
0022-0981/86/$03.50
0
1986 Elsevier
Science
Institute
Publishers
for Marine
Biology and Coastal
B.V. (Biomedical
Division)
Research.
54
MARGARET
A. PALMER
migrate into the water column (Arndt et al., 1982; Walters & Bell, 1986). Such different results are likely due to both habitat differences and species-specific beha~or~ differences (Fleeger et al., 1984). Two aspects of the physical environment, hydrodynamics and aboveground structure, may interactively influence the abundance of meiofauna in the water. There is certainly ample evidence that flow influences meiofauna. Some meiofauna have been observed to burrow as flow increases (Rhoads et al., 1977; Palmer, 1984), to move into interstices and secrete mucous “dams” (Crenshaw, 1980), to vertically migrate in sand in response to vibrations simulating surf(Boaden, 1968) or migrate down in response to water flow (Palmer & Molly, 1986), and to be eroded by tidal currents (Palmer & Gust, 1985). Evidence that structure influences sediment-dwelling meiofauna mostly refers to belowground interactions, such as, meiofaunal abundance around plant or seagrass shoots (e.g., Bell et al., 1978; Osenga & Coull, 1983). Fleeger et al. (1984) provided evidence that meiofauna do enter the water actively and passively in vegetated areas. We do not know if meiofaunal entrance into the water is related to the vegetation. Indirect evidence that meiofaunal activity in the water is influenced by aboveground structure includes studies showing that sediment recolonization, some of which has been shown to be via the water, is influenced by the presence of artificial or natural structures (Bell & Coen, 1982; E&man, 1983; Kern, pers. comm.). Such structure may decrease the risk of predation (Woodin, 1978) or may enhance food availability to the benthos (Thistle et al., 1984). However, structure may also enhance sediment erosion (Eckman et al., 1981) and, thus, the effect of structure on meiofaunal dispersal may be complex. The role of flow and structure in meiofauna dispersal, both active emergence and passive transport, cannot be investigated in the field alone, since adequate control of flow throughout an experiment is difficult, if not impossible. Thus, laboratory flume experiments were designed to control flow and manipulate habitat structure. I tested the hypothesis that the occurrence of mud-dwelling meiobenthos in the water column is related to above~ound structure and Ilow in an interactive way. ~rou~out this paper, meiofauna refers specifically to typical sediment-dwellers, not epibenthic or phytal meiofauna well known to reside on seagrasses or other structures (e.g., Mukai, 1971; Hicks, 1977). For these fauna, I examined drift (passive erosion) and behavioral emergence. Since meiofauna may be eroded as passive particles and erosion of meiofauna &if. may be enhanced by protruding structures, meiofaunal drift was examined in the presence and absence of structure. My hypothesis was that drift will be greatest when critical friction velocities are exceeded and structure is present. EmergePrce. Since meiofa~a from this study site minimize activity on the sediment surface as flow is increased, emergence was examined when flow was absent. My hypothesis was that emergence will be enhanced when structure is present.
MEIOFAUNADISPERSAL
55
METHODS AND MATERIALS STUDYSITE AND FLUME The study site was in the North Inlet Estuary, near Georgetown, South Carolina (33”22’N: 79”lO’W). The high-salinity, well-mixed estuary (1.4 m tidal range) is characterized by wide expanses of Spartina altemSfloruLoisel marsh grass and meandering tidal creeks. Experimental cores were collected from the center of an intertidal vegetated mudflat and from an adjacent unvegetated region. Mean flow (U) over the mudflat reached a maximum of approximately 15 cm * s _ ’ and friction velocities (u * ) may exceed 1.00 cm +s- ’ (u * = (z/p)“‘, where z = shear stress and p = fluid density). , where U = mean stream flow, d = flow depth, and Y y = kinematic fluid viscosity) vary from 20000 to over 200000 depending on stage of the tide. The sediment surface is burrowed by the fiddler crab, Uca sp., with surface roughness most pronounced immediately adjacent to Spartina culms. The experiments were conducted in July 1984 in a recirculating laboratory flume that is 2.4 m long, 30 cm wide and 30 cm deep. The flume has a removable section 1.5 m downstream from the inflow where boxcores (20 x 20 cm cross section, 15 cm deep) can be inserted so that the sediment surface is flush with the bottom of the flume. Fresh sea water was supplied to the flume from a 200 gal. headtank and water flow was controlled by PVC ball valves. Water flow was measured using temperature-compensating hot bead probes (modified from LaBarbera & Vogel, 1976). Water depth was maintained at 6 cm and friction velocities were calculated using vertical velocity profiles taken just upstream of the experimental cores. Eight points were measured within the log layer and no flow measurements were made below z = 1 mm. Least squares regression was used to calculate slopes (~*/k) from plots of u vs. In(z), where u* = friction velocity, k = von Karman’s constant, and z = distance off the bottom. All r2 values from the regressions were greater than 0.90. As calculated by Schlichting’s (1979) equation for boundary layer thickness (Eqn. 21.8, p. 638), the boundary layer, at the point flow was measured, was still growing and the shear velocities I measured slightly exceeded those occurring further downstream. Thus, friction velocities were overestimated by ~7% (see Schlichting’s Eqn. 21.5, p. 637) and the error was consistent throughout the experiments. Flow Reynolds numbers in the flume varied from 40 000 at low flow to 91000 at the highest flow settings. Boxcores of stratigraphically undisturbed sediment were collected from the study sites and returned to the flume laboratory and placed in holding tanks. Experiments began after boxcores had equilibrated for 3 h. All boxcores were collected just after the mudflat was exposed at low tide so there was no water on the surface, yet a fairly high water content in the sediment. This minimized disturbance caused by surface cracks and runoff. Meiofauna field abundances were measured at the time boxcores were collected using small cores (1.9 cm diameter). The field abundances were not signifi-
Flow Reynolds numbers (Re = g
56
MARGARET
A. PALMER
cantly different from abundances measured in boxcore sediment after equilibration in the flume (t-test, P > 0.05). EXPERIMENTAL
DESIGN
The experiment was originally designed as a 4 x 3 factorial in which structure and flow were manipulated. Natural structure (S. alternzfloru) was absent, present, removed, or artificial structure was added. Flow was absent (no flow), below critical erosion velocity for the sediments (low), or above critical erosion velocity for the sediments (high). The 4 x 3 factorial design was meant to allow a comparison of the effects of natural and artificial structures on the same meiofauna community. Differences in meiofaunal abundances and composition were found between boxcores collected from the vegetated area and the unvegetated adjacent mudflat. Thus, for each area (hereafter, Spartina site and mud site), a 2 x 3 factorial design was used with structure present or absent and the three flow levels described above. Five replicate boxcores were used per treatment, thus a total of 30 boxcores per site were tested. For the Spartina site, structure-absent cores were prepared by clipping Spartina culms at the sediment surface. For the mud site, structure-present cores were prepared by implanting soda straws (artificial mimics) in otherwise featureless sediment boxcores in a tixed pattern. Since isolated Spartina culms and clusters of large and small culms are present in the field, the artificial mimics were arranged as isolated elements and in clusters. Spacing, height, and diameter of the structures were similar to natural Spartina stands (Table I). Structures were trimmed to a 4 cm height and water level was maintained at 6 cm. This scaling was selected somewhat arbitrarily since the culm height to water depth ratio changes constantly in the field depending on tidal height. In the field, there are periods when the Spartina is covered and periods when it protrudes through the water surface. After cores had equilibrated, they were exposed to their appropriate flow setting for 1 h and then faunal samples were collected. In the low flow treatments, midstream U was ‘Icm.s- ’ throughout the l-h period. This flow level was chosen because it was
TABLE I
Number of structures, structure diameter, and per cent boxcore surface area occupied by structure in the Spartina treatments and in the artificial mimic treatments: values are means (n = 15 boxcores) and standard errors are in parentheses; for Spartina, diameter was measured at culm base. Structure Spartina Number/boxcore Cluster diameter (cm) Per cent area occupied
by structure
11.7 (0.82) 0.87 (0.22) 1.9 (0.3)
Artificial
mimics
13.0 (0) 0.96 (0.12) 2.8 (0)
MEIOFAUNA
DISPERSAL
57
well below critical erosion velocity for the sediments and represents a flow level typical of early ebb or late flood tide in the field, In the high flow ~eatments, flow was set just above critical erosion threshold for the sediments (hereafter referred to as critical friction velocity, u*~,,~~). Since u*_,,~~varied between boxcores, it was determined separately for each boxcore used in the high flow treatments. Critical friction velocity was determined by slowly increasing flow while an observer watched sediment particles through a stereomicroscope until 10 or more particles were observed rolling in contact with the bed (after Rhoads et al., 1978). The transported particles consisted primarily of loose clay aggregates. Because flow measurements were made just upstream of the test section (structures preclude measurement within the boxcore), u *_critvalues do not represent actual critical friction velocities for the sediments. As in Eckman (1983) and Luckenbach (1986), the erosional thresholds measured in this manner provide relative measures of u * _f.it useful in comp~ng treatments. SAMPLING
AND ANAI.YSES
At the time boxcores were collected, surficial sediment samples were collected in the field for g-ranulometry measurements. Gram sizes were measured by wet-sieving the sediment and an aliquot of the silt-clay fraction was removed for pipette analysis (Buchanan & Kain, 1971). Median grain size and sorting coefftcients were calculated following Inman (1952). Meiofauna that were transported downstream in the flume (drift fauna) were collected from a 50 pm drift net that had filtered ali outgoing flume water during the 1 h experiments period. For the no flow treatments, animals that were in the water above the boxcore (emergent fauna) were sampled at the end of the l-h period and after removing the drift net. The emergent fauna were sampled using a 10 cm diameter funnel attached to a small diaphragm pump (intake velocity = 4 cm * s- ‘). Positioned 1.5 cm from the sediment surface, the intake was passed over the boxcorc once using a sweeping motion. Pump intakes sampled the areas around the culms and were not placed over culms. The technique depends on quick timing and agility; however, the manipulation is not difficult since the apparatus is fully visible through the side of the flume. Preliminary experiments with fluorescein-sea water and dyed sediments showed that the pump collected all suspended material and loose sediment aggregates on the surface. Meiofauna active at the sediment-water interface were also collected. The maxinlum swimming speed for meiofauna is approximately 0.2 cm . s - ’ (P~mer, 1984) and, thus, they could not evade the intake, All fauna1 samples were preserved in Rose Bengal-formalin for later enumeration. Parametric statistical tests were used after the data were examined to determine that appropriate assumptions were valid. Analysis of variance (ANOVA) was utilized to dete~ine if the abundance of meiofauna drifting was si~~c~~y affected by flow and structure. ANOVA was performed using the General Linear Model techniques (Helwig & Council, 1979) and multiple comparisons were performed when significant main
MARGARET A. PALMER
58
effects were present. t-Tests were performed to determine if the abundance of meiofauna emerging was significantly greater when structure was present or absent.
RESULTS STUDY
SITE COMPARISONS
Median grain size among the Spartina c&m was 38 pm and at the mud site was 45 pm. For the low flow treatments mean friction veiocity (u * ) was 0.25 cm - s - I. Mean critical friction velocity (u *_crit) of the sediments in the mud boxcores was 1.02 cm . s _ ’ and was significantly greater than a mean u *_ctit of 0.44 cm * s- ’ for the Spartina boxcores (Table II). When artificial structure was added to the mud boxcores, u*_,,~~ decreased and when structures were clipped in the Spartina boxcores, u *_critincreased significantly (ANOVA, significant structure effect, P -C0.05; Scheffe’s test, experiment wise error rate = 0.05). The abundance of meiofauna (Table III) and the copepod species composition (Table IV) differed between samples collected at the mud site and those collected among
TABLE II Critical friction velocities (u *_& of sediments for each of the four structure treatments: values are means based on five boxcores per treatment; standard errors are shown in parentheses. U*_ctit(cm.s-‘)
Treatment
1.02 0.71 0.61 0.44
Mud Mud with artificial structure Spartina removed Spartina
(0.05) (0.09) (0.04) (0.02)
TABLE III Composition of sediment meiofauna among Spartina culms and in areas free of culms (mud): values are mean number per 10 cm2 and are based on cores (1.9 cm diameter) collected at the times of boxcoring; n = 45 cores per site; standard errors in parentheses. Site _.__ Spartina
Nematoda Copepoda Nauplii Foraminifera Turbeliaria Others*
3540.6 540.1 296.5 310.6 24.7 260.5
(246.8) (65.0) (43.6) (43.6) (3.6) (25.0)
* Includes ostracods, mites, kinorhynchs, and juvenile annelids.
Mud 1680.3 190.6 218.9 133.8 23.3 86.5
(100.2) (19.8) (16.5) (13.9) (4.9) (10.1)
MEIOFAUNA
DISPERSAL
59
TABLE IV Species composition of sediment copepods among ,Spartina culms and in areas free of cuims (mud): values are mean number per 10 cm’ and are based on cores (1.9 cm di~eter) collected at the times of boxcoring; abundances are only given for those species that occurred in densities > 5 per 10 cm2; *, indicates rarely occurred; -, absent; standard error in parentheses. Site Taxon
Spartina
Mud
Ameriidae Nitocra lacustris (Schmankevitsch) Nitocra typica (Boeck)
10.6 (2.7) *
*
10.6 (1.9)
*
105.9*(19.1) 67.1 (13.6)
24.7 (6.4) 17.7 (2.7)
Canthocamptidae Mesochra pygmaea (Claus)
Cletodidae C~eiocamp~ushelabi~ Fleeger Enhydrosoma propinquum (Brady) Nannopus palustti (Sars)
Cyclopidae Halicyclops coulli Herbst
56.5 (11.8)
Cylindropsyllidae Paraleptastacus sp.
*
Diosaccidae Pseudostenhelia wellsi (Co1111& Fleeger) Roberisonia propinqua (T. Scott) Schizopera knabeni Lang Stenhelia (D.) bifdia (Coull)
*
* 17.6* (4.5) 222.4 (26.4)
116.5*(10.9)
14.1* (3.6)
6.1* (2.2)
Ectinosomatidae Ectinosoma sp. Halect~osoma ~~nonae Coull
Laophontidae Esoia sp. Normanella sp. Onychocamptus mohammed (Blanchard & Richard) Paronychocamptus wilsoni Cot111 Paralaophonte sp. Quinquelaophonte cap~l~ata{Wilson)
* *
* * * *
*
Taehididae Microarthridion linoraie (Poppe)
*
5.2 (1.4)
Thalestridae Diarthrodes aegideus (Brady)
24.7 (5.5)
Tisbidae Tisbe holothuriae (Humes)
*
the Spartina culms. Copepod species diversity was greater and meiofaunal abundances among the Spartina culms were twice those in the mud area. DRIFT
To test the hypothesis that the presence of meiofauna in the water column is a function of flow and structure, ANOVAs were performed for copepods, nematodes, and total
MARGARET A. PALMER
60
meiofauna. The results for nematodes and total meiofauna were the same; thus results for total meiofauna will not be reported. At the Sprtina site, the number drifting increased with increasing flow (Fig. 1) and there was no statistically significant structure effect (Table V). At the mud site, structure did influence drift, but the effect depended on flow (Table V). Here, the number drifting at a given flow level (Fig. 2) was greater when structure was present (Scheffe’s test, experimentwise error rate = 0.05). Copepods were the dominant taxon in the drift samples. In the Spartina boxcores, regardless of the structure treatment, 0.2 to 0.7% of the s~iment copepods drifted in low and high flow respectively. At the mud site, 0.2 to 0.4% drifted from no structure
Soartina
180
COPEPODS
site
180
/
NEMATODES /
0 z:
140
t: E p
100
6 9
60
60
i= 20 NF
LOW
NF
HIGH
LOW
HIGH
FLOW
Fig. I. Abundance of copepods and nematodes drifting from .Spartina boxcores (20 x 20 cm): dashed lines (NS) are for cores without structure and solid lines (S) are for cores with structure (Sparrina); values are mean number drifting from a boxcore; n = 5 boxcores per data point; standard error bars are shown.
TABLE V Analysis ofvariance results for the effect of flow and structure on the abundance of copepods and nematodes drifting: F values are given for each ANOVA; *, indicates significance at P = 0.05; **,significance at
P = 0.01.
Flow
Structure
Interaction
Spartina site
Copepods Nematodes
47.68* 16.44**
Mud site Copepods Nematodes
17.26** 18.05**
0.01 1.28
15.2X** 19.17**
3.88* 2.14
7.02** 5.57*
MEIOFAUNA
61
DISPERSAL
Mud site NEMATODES
COPEPODS 10
4
S
60
20 I NF
LOW
HIGH
NF
LOW
HIGH
FLOW of copepods and nematodes drifting from mud boxcores (20 x 20 cm): dashed lines (NS) are for cores without structure and solid lines(S) are for cores with structure (artificial mimics in mud cores); values are mean number drifting from a boxcore; n = 5 boxcores per data point; standard error bars are shown. Fig. 2. Abundance
boxcores in low and high flow respectively, while 0.7 to 1.1% drifted in boxcores with artificial structure. At the species level, all copepods present in the sediment were found in drift samples (Table VI). For both sites, Nitocra lucustri~ (Schmankevitsch), Schizopera knabeni Lang, and Microarthridion littorale (Poppe) were the most abundant copepods drifting relative to their sediment abundances; however, none of the sediment species were disproportionately represented in the drift to a significant extent. An examination of copepod species composition in structure vs. no structure treatments revealed that: (1) no one species was responsible for the higher number of copepods drifting from mud boxcores when structure was present, and (2) the species which are common to both sites did not drift in higher numbers from Spartina boxcores when structure was present. EMERGENCE
The number of copepods collected by pumping the water above the boxcores at the end of each no flow flume run was significantly greater when structure was present (Table VII). For nematodes, the number collected above structure cores was not statistically different from numbers above no structure cores. At the mud site, 0.2 to 0.4% of the sediment copepods were present in the water above no structure and structure cores, respectively. At the Spartina site, 0.6 to 1.2% of the sediment copepods were present in the water above no structure and structure cores, respectively. At the species level, Nitocra lacustris, Mesochra pygmaea (Claus), Hallicyclops coulli Herbst, Schizopera knabeni, and Microarthridion littorale emerged in greatest numbers relative to the sediment abundances (Table VIII).
62
MARGARET A. PALMER TABLE VI
Species composition ofcopepods drifting at high flow: values are mean number drifting from unmanipulated boxcores (20 x 20 cm); n = 5 boxcores per mean; *, indicates rarely occurred; -, absent; standard error in parentheses. Site Spartina
Mud
Taxon” Ameriidae (0.9)
2.5 (0.9)
1.5 (1.4)
1.8 (1.2)
16.8 * (5.6) 14.7 (7.4)
7.2 (7.6) 2.4 (1.6)
3.2 (1.4)
4.0 (3.2)
Niiocra lacustrtk Nitocra typica
5.2 *
Canthocamptidae Mesochra pygmaea
Cletodidae Cletocamptus helobius Enhydrosoma propinquum Nannopus palustris
Cyclopidae Halicyclops coulli
Cylindropsyllidae
*
Paraleptastacus sp.
Diosaccidae Pseudostenhelia wellsi Robertsonia propinqua Schizopera hnabeni Stenhelia (D.) bifidia
* 7.6 * (2.8) 36.6 (13.4)
1.0 ;0.5) 8.8 (3.9)
5.2 *(0.3)
* *
Ectinosomatidae Ectinosoma sp. Halectinosoma winonae
Laophontidae Esola sp. Normanella sp. Onychocamptus mohammed Paronychocamptus wilsoni Paralaophonte sp. Quinquelaophonte capillata
2.2 * (1.8) 1.8 (0.4) *
1.0 * (0.8)
Tachididae Microarthridion littorale
3.5
(1.6)
5.0
(2.0)
4.0 (3.2)
Thalestridae Diarthrodes aegideus
Tisbidae
*
Tisbe holothuriae
a See Table IV for taxonomic authorities. DISCUSSION
Significant habitat differences between the two areas was indicated by the greater meiofaunal abundances and diversity at the Spartina site vs. the mud site (Tables III and IV). These faunal differences may be part of the zonation patterns described in Coull et al. (1979); however, my two sites were at the same tidal height and had similar
MEIOFAUNA
63
DISPERSAL
TABLE VII Abundance of emergent copepods and nematodes for Spartina and mud boxcores (20 x 20 cm) when no flow was present: values are mean number collected per boxcore; n = 5 boxcores per mean; standard error in parentheses; *, indicates structure means are significantly greater than no structure means at P = 0.05 (see text for details). Treatment Structure
No structure
Copepods Nematodes
258.4 (14.1)* 69.2 (14.2)
121.6 (22.2) 37.6 (12.0)
Mud site Copepods Nematodes
26.9 (3.8)* 17.8 (10.1)
16.0 (1.5) 10.6 (2.2)
Spartina site
grain sizes. Microtopographic relief was somewhat greater at the Spartina site. The faunal differences may be related to the presence of the Spartina culms. Similar increased abundances around biogenic structures have been observed for macrofauna (Woodin, 1981) and meiofauna (Bell, 1983; Eckman, 1983). Enhanced faunal abundances may be related to the role of structure in providing refugia from predators (Woodin, 1978) to flow-induced effects which result in enhanced food availability around structures (Thistle et al., 1984; Eckman, 1985) or to enhanced supply rates of organisms (Eckman, 1979,1983). Some studies have not shown enhanced abundances near structures (e.g., Decho et al., 1985) and the results of Bell & Woodin (1984) suggest that a structure-refuge effect is uncertain in some habitats. Critical friction velocities are of interest in this study because, despite behavioral and morphological factors that influence meiofaunal dispersal, water column abundances are related to friction velocities (Palmer & Gust, 1985). In the present study, sediment grain sizes were not significantly different, yet the mud site had a significantly higher critical friction velocity than the Spurtinu site (Table II). For both the mud and Spurtina boxcores, the presence of structure resulted in a lowering of the critical friction velocity for the sediments by z 30%. Structures protruding from a sediment bed may cause sediment destabilization due to the entrainment of higher momentum fluid downward near the structure (Eckman et al., 1981). Destabilization due to the physical presence of protruding structures is not necessarily the rule (Luckenbach, 1986). Structure diameter, structure height/water depth, the degree of microbial or mucous binding of sediment particles, and the boundary shear stress imposed by the external flow, all determine the precise effects structure will have on sediment transport (Eckman & Nowell, 1984). In the present study, the 0.87 cm diameter of the structure clusters and the structure height (4 cm), which approached the water depth (6 cm) in the flume, resulted in local scour. This scour may be further enhanced in the field since culm height
MARGARET A. PALMER
64
TABLE VIII
Species composition of copepods emerging at no flow: values are mean number emerging from a boxcore; n = 5 boxcores per mean; *, indicates rarely occurred; -, absent; standard error in parentheses. Site Spartina
Taxon”
No structure
Mud Structure
No structure
Structure
Ameriidae Nitocra lacustris Nitocra typica
4.2
(1.0)
10.2 (2.3) *
(1.3)
8.9 (1.7)
*
*
12.8 * (1.3) 1.2 (0.8)
19.2 * (4.3) 2.2 (0.4)
2.0 (0.5) 0.8 (0.5)
2.8 (0.5) 0.8 (0.4)
26.4 (11.8)
62.2 (17.0)
2.2 (0.8)
3.3 (2.1)
*
*
*
*
9.0 * (3.8) 35.6 (16.1)
29.8 * (9.7) 75.5 (11.4)
1.2 iO.4) 4.5 (0.9)
2.5 *(0.8) 13.6 (3.4)
* *
3.2 * (1.6)
* *
* *
1.2 *(0.9) 1.3 (0.5) *
2.4 * (1.6) 2.0 (0.2) *
1.2 * (0.3)
0.8 * (0.6)
4.3
(1.8)
*
Canthocamptidae Mesochra pygmaea
4.2
Cletodidae Cletocamptus helobius Enhydrosoma propinquum Nannopus palustris
Cyclopidae Halicyclops coulli
Cylindropsyllidae Paraleptastacus sp.
Diosaccidae Pseudostenhelia wellsi Robertsonia propinqua Schizopera knabeni Stenhelia (D.) bifdia
Ectinosomatidae Ectinosoma sp. Halectinosoma winonae
Laophontidae Esola sp. Normanella sp. Onychocamptus mohammed Paronychocamptus wilsoni Paralaophonte sp. Quinquelaophonte capillata
2.2 ;0.7)
1.2*(0.4)
8.9 (2.1)
1.8 (0.9)
3.0 (0.7)
9.2 (6.2)
19.2 (11.8)
*
*
*
*
Tachididae Microarthridion littorale
Thalestridae Diarthrodes aegideus
Tisbidae Tisbe holothuriae
a See Table IV for taxonomic authorities.
exceeds water depth for most of the tidal cycle. Even though structure treatments had lower u *_crit values, adding artificial structure to the mud boxcores did not reduce u *_crit to that determined for the Spartina boxcores (0.44 cm. s- ‘). This suggests that there are differences in erodibility of sediments between the mud and Spartina site that can
MEIOFAUNA
DISPERSAL
65
not be explained by structure alone. The lower u *_critvalues for the Spartina cores may be related to the increased abundance of infauna in these cores. Faunal activities are known to alter sediments in a number of ways which influence the erodibility of sediments. Biogenic reworking is known to lower critical erosion velocities (Grant et al., 1982), as are increases in surface roughness (noted qualitatively in Spartina cores in this study) and decreases in consolidation in sediments (Nowell et al., 1981). Luckenbach (1986) found no direct hydrodynamic effect of structures on sediment destabilization and emphasized that structures (Diopatra tubes in his study) may have purely biologically-mediated effects on sediment destabilization. My results may be due to both hydrodynamic structure effects and biologically-mediated sediment effects. The number of meiofauna drifting from mud and Spartina boxcores was influenced by flow, with higher numbers drifting as critical friction velocities were exceeded (Table V, Figs. 1 and 2). Thus meiofauna are acting like passive particles and are subject to erosion by tidal currents. This is not suprising since other studies have suggested that meiofauna may be in the water due to tidal suspension (Bell & Sherman, 1980; Palmer & Brandt, 1981) and the number of mud meiofauna in the water column has been shown to increase as friction velocity increases (Palmer & Gust, 1985). For the mud boxcores, the number of meiofauna drifting was also influenced by structure. In these cores, more animals drifted when structure was present. This was despite the fact that flow settings for structure vs. no structure boxcores were comparable. The treatments consisted of no flow, flow below u.+,,~~ (low), and flow above u*_,,~~(high), with the last flow set independently for each boxcore. This design ensured that each boxcore was subjected to the same potential for erosion. Thus increased drift in the presence of structure, with the susceptibility to erosion comparable to no structure boxcores, suggests that the meiofauna were not acting like purely passive particles. Somehow the animals increased their likelihood of downstream transport when structure was present. Either they were emerging into the water and were transported, or they exhibited more surface activity which resulted in a u *_crit for the meiofauna that was below critical friction velocities for the sediments. Both interpretations require behavioral events. For the Spartina boxcores, structure had no significant effect on numbers drifting. The lack of a significant structure effect here compared to the mud boxcores may be due, in part, to the very high variability in numbers drifting from the Spartina boxcores vs. the mud boxcores (note relative magnitudes of error bars in Fig. 1 and 2). In addition, fauna may not exhibit higher drift rates in the presence of Spartina if the culms represent a refuge from flow. The meiofauna may emerge here but may not be transported. Animals may retreat into spaces between blades or within the boundary layer of the structures where shear stresses are reduced (Statzner, 1981; Silvester & Sleigh, 1985). The artificial structures may not have been a suitable refuge from flow since, unlike the Spartina culms which are textured and with grass blades, the soda straws are essentially smooth cylinders. Further data are required to investigate the potential role of Spartina culms as flow refugia. Different results for the mud area vs. the Spartina area may also be related to the fact
66
MARGARETA.PALMER
that the two areas do differ with respect to community composition (Table IV) and meiofaunal abundances (Tabie III). An ex~ination of species patterns for the drift data (Table VI) does not clarify the picture. ~it~cru Iacust~, Schizopera k~a~~i, and Microarthridion lirrorale are the most common drift fauna at both sites. The structure effect at the mud site was basically due to all species; however, the species occurring at both the mud and Spar&a sites did not exhibit a structure response in Spartina boxcores. Thus, apparently, the same species are doing different things in different habitats! The number of copepods emerging (determined from pump samples) was a function of structure at both sites (Tables VII and VIII), with a significantly higher number of copepods in the water or on the sediment surface when structure was present. This result supports my previous interpretation of the drill data. For both sites, copepods emerge when st~cture is present and yet suffer higher risks of transport only at the mud site. With the addition of ~allicyciops cot& at both sites and Mesochra py~ueu at the Spa&a site, the dominant emergent species are the same as those most common in the drift (Nitocra lacustris, Schizopera knabeni, Microarthridion littorale). The fact that copepods emerged in greater numbers than nematodes and responded to structure reflects the copepods’ agility in the water and on the sediment surface. Nemat~es are known to avoid the sediment surface perhaps because they are poor swimmers and may be subjected to even higher risks of transport (Palmer, 1984). This study was designed to test the hypothesis that the occurrence of mud-dwelling meiobenthos in the water column is related to aboveground structure and flow in an interactive way. There was a significant structure x flow interaction for meiofauna d~fting from mud boxcores. Flow atTected the number of mud meiofauna drifting; however, this effect depended on whether or not structure was present. The number of fauna drifting from mud boxcores was greatest when both flow and structure were present. Since structure and no structure cores were subjected to comparable flows, mud meiofauna are not acting as purely passive particles but are increasing their risk of transport when structure is present. The emergence data support my su~estion that, for both sites, surface activity of copepods is enhanced in the presence of structure, yet the copepods are able to minimize downstream transport only at the Spartina site. It is hypothesized that natural structure (Spartim) may serve as refugia from flow for the meiofauna. Meiofauna dispersal is clearly a complex process. Structure may influence meiofauna dispersal but its effect varies depending on flow, the nature of the structure, and the species ~om~sition of the meiofauna community.
ACKNOWLEDGEMENTS
This work was superb by the Byron K. Trippet fund of Wabash College and by a grant from the National Science Foundation’s Biological Oceanography Program (OCE-8509904). I thank the Belle W. Baruch Institute for the use of laboratory facilities
MEIOFAUNA
DISPERSAL
61
and Mike Molloy for field and laboratory assistance. Special thanks go to Drs. Susan Bell, Betsy Brown, Cheryl Ann Butman, Jim Beckman and Mark Luckenbach for critical comments which significantly improved the manuscript.
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HELWIG,J.T. & K.A. COUNCIL, 1979. SAS u.rer’.rguide. SAS Institute Inc., Raleigh, N.C., 494 pp. HICKS, G. R. F., 1977. Species comparisons and zoogeography of marine phytal harpacticoid copepods from Cook Strait and their contribution to total phytal meiofauna. N.Z. J. Mar. Freshwater Res., Vol. 11, pp. 441-469. INMAN,D. L., 1952. Measures for describing the size distribution of sediments. J. Sediment. Petrol., Vol. 22, pp. 122-145. LABARBERA,M. & S. VOGEL, 1976. An inexpensive thermistor flowmeter for aquatic biology. Limnol. Oceanogr., Vol. 21, pp. 750-756. LUCKENBACH,M., 1986. Sediment stability around animal tubes: the roles of hydrodynamic processes and biotic activity. Limnol. Oceanogr., Vol. 31, pp. 778-787. MUKAI, H., 1971. The phytal animals on the thalli of Sargassum serrufijdium in the Sargassum region, with reference to their seasonal fluctuations. Mar. Biol., Vol. 8, pp. 170-182. NOWELL,A. R. M., P. A. JUMARS& J. E. ECKMAN, 1981. Effects of biological activity on the entrainment of marine sediments. Mar. Geol., Vol. 42, pp. 133-153. OSENGA, G. A. & B.C. COULL, 1983. Spartina alternifloru Loisel root structure and meiofauna abundance. J. Exp. Mar. Biol. Ecol., Vol. 67, pp. 221-225. PALMER,M.A., 1984. Invertebrate drift: behavioral experiments with intertidal meiobenthos. Mar. Behuv. Physiol., Vol. 10, pp. 235-253. PALMER,M.A. & R.R. BRANDT,1981. Tidal variation in sediment densities of marine benthic copepods. Mar. Ecol. Prog. Ser., Vol. 4, pp. 207-212. PALMER,M.A. & G. GUST, 1985. Dispersal of meiofauna in a turbulent tidal creek. J. Mar. Res., Vol. 43, pp. 179-210. PALMER,M. A. & R. M. MOLLOY,1986. Flow and the vertical distribution ofmeiofauna: a flume experiment. Estuaries, Vol. 9, pp. 225-228. RHOADS,D. C., R. C. ALLER & M. B. GOLDHABER,1977. The influence of colonizing benthos on physical properties and chemical diagenesis of the estuarine seafloor. In, Ecology of marine benthos, edited by B.C. Coull, University of South Carolina, Columbia, pp. 113-138. RHOADS,D. C., J. Y. YINGST& W. J. ULLMAN,1978. Seafloor stability in central Long Island Sound. Part I. Temporal changes in erodibility of tine-grained sediment. In, Estuarine interactions, edited by M. L. Wiley, Academic Press, New York, pp. 221-242. SCHLICHTING,H., 1979. Boundary layer theory. McGraw-Hill, New York, 7th edition, 817 pp. SIBERT, J.R., 1981. Intertidal hyperbenthic populations in the Nanaimo Estuary. Mar. Biol., Vol. 64, pp. 259-265. SILVESTER,N.R. & M.A. SLEIGH, 1985. The forces on microorganisms at surfaces in flowing water. Freshwater Biol., Vol. 15, pp. 433-449. STATZNER,B., 1981. The relation between “hydraulic stress” and microdistribution of benthic invertebrates in a lowland running water system, the Schierenseebrooks (North Germany). Arch. Hydrobiol., Vol. 91, pp. 192-218. TAGHON, G.L., A.R.M. NOWELL & P.A. JUMARS, 1980. Induction of suspension feeding in spionid polychaetes by high particulate fluxes. Science, Vol. 210, pp. 562-564. THISTLE, D., J.A. REIDENAUER,R.H. FINDLAY& R. WALDO, 1984. An experimental investigation of enhanced harpacticoid (Copepoda) abundances around isolated seagrass shoots. Oecologia (Berlin), Vol. 63, pp. 295-299. WALTERS,K. & S. S. BELL, 1986. Die1 patterns of active vertical migration in seagrass meiofauna. Mar. Ecol. Prog. Ser., in press. WOODIN, S.A., 1978. Refuges, disturbance, and community structure: a marine soft-bottom example. Ecology, Vol. 59, pp. 274-284. WOODIN, S.A., 1981. Disturbance and community structure in a shallow water sandflat. Ecology, Vol. 62, pp. 1052-1066.