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Sediment-level fluctuation in a mussel bed on a ‘protected’ sand-gravel beach
Estuarine,
Coastal
and Shelf
Sediment-level on a ‘ Protected
Science (1988) 26,255-267
Fluctuation ’ Sand-Gravel
in a Mussel Beach
Bed
John Landahl” Departmerzt
of Zoology,
Received
University
16 Jurze 1986 and in revised
Keywords:
of Washingtorz, form
16 Augztst
sediment movement; mussels; Mytilzts;
Seattle,
WA
98195,
U.S.A.
1987
storms
Studies of a dense population of the blue mussel (Mytilus edulis) between an abrupt lower boundary at + 0.6 m above MLLW, and an indistinct upper limit at + 2.3 m above MLLW on a sand-gravel beach at Quartermaster Harbor, Vashon Island, WA, evaluated the effects of physical and biological factors on patterns of abundance and distribution. Winter storms caused little sediment movement high on the shore either inside or outside patches of mussels, but large fluctuations ( > 3 cm) at mid- and low-shore levels sometimes caused burial and mortality. Even high mussel biomass (25 kgmm2) did not prevent large-scale sediment level change. Introduction The annual cycle of sediment movement on exposed temperate sand beaches, in which the level of the beach face falls in winter and rises in summer, is well known (Bascom, 1980), and its role in the ecology of sand-beach communities has been investigated with respect to the adaptations of shore plants (Markham &Newroth, 197 1). Seasonal changes in sediment elevation on intertidal flats in protected inshore waters have also been documented (e.g. Frostick & McCave, 1979; Pickrill, 1979; Anderson et al., 1981). However, as the annual range of sediment fluctuation reported in these habitats is typically 5 cm or less, it is tempting to assume that sediment movement is not a problem for epifauna, flora, or infauna. It is commonly held that beds of animals or plants exert a stabilizing effect on sediment even on unprotected bottoms subject to strong wave action or water currents by slowing water movement and preventing scouring (e.g. Fager, 1964; Young & Rhoads, 1971; Bailey-Brock, 1979), but this has been shown to depend on the density of the bed (Eckman et al., 1981; Eckman, 1982). Protruding ‘roughness elements ’ such as worm tubes, seagrass blades, or rocks, can either stabilize or destabilize sediment in turbulent flows for reasons discussed in detail by Nowell and Church (1979); Eckman et al. (1981); and Eckman (1982). These investigators concluded that sparse or isolated ‘roughness elements ’ actually increase scouring of sediment in their immediate vicinity, although at high densities (i.e. at least one-twelfth of the bed area occupied by ‘ roughness elements ‘), a ‘ skimming ’ flow which does not erode the bed can indeed form. Eckman et al. (1981 j “Present Service,
address: Environmental Conservation Division, National 2725 Montlake Blvd E., Seattle, WA 98112, U.S.A.
Marine
Fisheries
255 0272-7714,‘88/030255+
13 $03.0010
@ 1988 Academic
Press Limited
256
J. Landahl
and Eckman (1982) argue that natural benthic plant and animal densities are usually too low to produce hydrodynamic sediment stabilization, and suggest that mucus binding of sediment may produce the stabilization of sediment reported by various investigators in association with beds of tubes or seagrasses. Among the benthic invertebrate species forming beds in intertidal and shallow subtidal soft-sediment habitats is Mytilus edulis, the blue, bay, or edible mussel (e.g. Field, 1921; Verwey, 1952). Beginning in 1977, I conducted a series of studies of the ecology of an intertidal mussel bed on a sand-gravel beach at Quartermaster Harbor, Vashon Island, Washington. These studies concerned distribution patterns and interactions of mussels, their associated epifauna, and co-occurring infauna (see Landahl, 1985). Initial observations suggested that the level of the beach face might fluctuate in a seasonal manner similar to that reported for protected intertidal mudflats (e.g. Frostick & McCave, 1979; Anderson et al., 1981). As such seasonal beach-level fluctuations could have had important implications for mussel persistence and distribution, sediment level was monitored within and outside patches of mussels at several shore levels for periods of up to 20 months during 1978 and 1979. Sediment accumulation among mussels due to increased siltation resulting from slowing of water movement could also have had effects on nearby infauna. Mortality of mussels resulting from burial beneath sediment had been observed at this locality in November 1977 (Landahl, 1985). Storms are known to have major impacts on soft-sediment benthic invertebrate assemblages (McCall, 1978; Yeo & Risk, 1979; Thistle, 1981; Orensanz & Gallucci, 1982), are reported to destroy mature mussel beds in Europe (Newell, 1970,1979), and are known to result in passive transport of at least one sand-beach bivalve (Mya arenaria; Matthiessen, 1960). However, few studies have attempted to assess the effects of storms on soft-sediment mussel beds in North America. Monitoring of sediment level provided information on storm impacts on M. edulis as well as on siltation within mussel beds.
Materials
and Methods
Study area Quartermaster Harbor is, as the name suggests, a protected embayment, It is formed by Maury Island and the south end of abutting Vashon Island (latitude 47”23’N, longitude 122’29’W). These islands lie in central Puget Sound, a fjord connected with the Pacific Ocean by the Strait of Juan de Fuca (Figure 1). The study was conducted on an alluvial fan at the mouth of a small creek midway between the mouth and the back of the harbor. This fan was well protected except from south winds. Studies were conducted on a wide beach on the northern side of a sand-gravel point (area approximately 1 km2) which was entirely submerged at high tide. Winds at Seattle-Tacoma International Airport (about 15 km to the north-east of the study area) were from the south and south-west about 35”,, of the time between 1960 and 1969, but reached speeds in excess of 30 kmph (16 kt) from these directions only about 2’,, of the time (Turnbeaugh, 1975). This site is most exposed to a wind from the south, because its longest fetch (10 km) is in that direction (Turnbeaugh, 1975). Turnbeaugh (1975) calculated that a 65 km h-r (35 kt) south wind of >3 h duration would produce waves 1.8-m high with a period of 5 s at this site. However, his study suggests that considerably smaller waves (0.7 m) more frequently occurred during storms in the period 1960-69, based on the wind observations at the airport.
257
Sediment-level fluctuation
122O40'
122'30'
122"20'
122°10'
48"OO'
47030'
47'20'
47"20'
122'40'
122"30'
122"20'
122'10'
Figure 1. Map of Puget Sound, Washington, showing lcjca tion of the study site at Quartermaster Harbor (QH), Vashon Island. Latitudes are N; longin Ides are W
Although apparently well protected on the basisof its geography, the presenceof up to 50”,, gravel in the substrate (Landahl, 1985) suggestedthat the study areawas exposed to more wave and current action than the innermost region of the harbor, where the shores were considerably muddier. The beach surface wasirregular and characterized by rolling topography consisting of ridges and swaleswith a few shallow ponds.
To study possible seasonalbeach-level fluctuations, gaugesconsisting of square cedar stakes 1.9 cm on a side and 0.6-m long to which plastic centimetre scaleshad been glued (Plate 1) were used to monitor sediment level. Gauges of this nature are not ideal in that they constitute isolated, protruding ‘ roughness elements ’ and hence can be expected to increase sediment scour in their immediate vicinity while submerged if current velocity is sufficiently high (Eckman et al., 1981). However, no scour pits were noted at the bases of
the gauges,indicating that current velocities were usually low at the shore levels at which they were placed, or that scour pits were filled in by wave action during the advance and retreat of the tide. Gauges were set in place in groups of three in areasof high mussel density (60-loo”,, mussel cover; estimated ‘field weight ’ biomass 12-25 kg rn-‘, seebelow) at three shore levels ( + 0.5, + 1.4 and + 1.9 m) to permit assessmentof the effect of musselson sediment movement. One group was set in place in December 1977, one in January 1978, and one in July 1978(Table 1). In July and August 1978, four more groups of gaugeswere set in place
258
.7. Landahl
-
--
Plate 1. Sediment-level gauge at Quartermaster encrusted mussels and shell debris.
TABLE
groups groups
Harbor
surrounded
by barnacle-
1. Dates of establishment and numbers of observations for sediment-level gauge at Quartermaster Harbor, Washington; each group consisted of three gauges; all were in slope microhabitats except group 6, which was in a swale
Group
Shore level (m)
Mussel cover (“0)
1 2 3 4 5 6 7
+2,4 +1.5 +0,6 +0.3 + 1.9 + 1.4 f0.5
0 0 0 0 60 95 100
Estimated biomass (km m- ‘) 0 0 0 0 12 23 25
Distance between gauges (ml 10 1 2 10 2 2 1
Starting date 14 Jan. 29 Aug. 29 Aug. 29 Aug. 19 Dec. 15 Jan. 23 Jul.
1978 1978 1978 1978 1977 1978 1978
Ending date 11 Aug. 29 June 29 June 11 Aug. 10May 11 Aug. 11 Aug.
No. of obs 1979 1979 1979 1979 1979 1979 1979 Total
26 16 14 15 24 29 19 143
at similar shore levels in areas lacking mussels (Ol’, cover, biomass of necessity 0 kg m-‘) (Table 1). In August 1978 the shore levels of 16 approximately equally-spaced permanent marker stakes along a transect perpendicular to the water line spanning an elevation range from + 0.3 m to + 4.2 m were determined with reference to MLLW using transit, stadia rod, and tape. Elevations of the sediment-level gauges were estimated with reference to
Sediment-levelfluctuation
259
thesemarkers. All groups were on ridge slopes(seeLandahl, 1985) except group 6, which was in a swale. Sediment-level gaugesat a given shore level were spacedequal distancesapart, but this spacing interval was not the sameat all shore levels (Table 1). Readings were recorded at intervals of 10-80 days (usually every month or every fortnight) until August 1979, when several gaugeswere destroyed by drift logs. Prior to this time a number of the wooden stakesdeteriorated or lost their scales,but it wasalways possibleto replace them with new gaugesset to the original depth in the samelocation. From the data on sediment level over time, the range of fluctuation for each group of gaugesover the entire study could be calculated. Mussel density
Aggregation of musselsinto clumps and the presence of individuals as small as a few millimetres in shell length within these clumps made non-destructive censusing of the mussel population impossible. In situations of this nature, Pielou (1974) has suggested measuring a correlate of population size rather than counting individuals. For this study, cover class(defined asthe proportion of the substrate in a quadrat occupied by a speciesof interest) was chosen as the primary index of musselpopulation density. Cover classis a measure commonly employed in plant ecology, but seemsappropriate for the study of populations of sessileorganisms in general. With regard to M. edulis, musselcover can be considered to vary with mussel population size and several other factors, including mussel-layer depth, mean body size, and degree of aggregation. As a first approximation, musselcover can be said to be roughly proportional to population size, but obviously this simplification will be most accurate when mussel-layer depth, meanbody size, and degree of aggregation are all constant. This will be true most often in comparing measurements collected at a single shore level in a single season.In the present study, measurements collected at different shore levels or in different seasonswill sometimesbe compared; in these casesthe approximation may be lessaccurate. To obtain an objective estimate of cover class,this parameter was measuredin 0.25m2 quadrats by placing a rectangular brassframe (dimensions approximately 0.4 m by 0.6 m) divided into 54 squares(‘ subquadrats ‘) each 7 cm on a sideon the substrate. Subquadrats were scored either as unoccupied or as occupied by one or more live mussels. This presence/absencetechnique was an efficient field procedure which overestimated mussel cover at low but non-zero densities, becausea single individual could be scored aspresent in several adjacent subquadrats. However, estimates were much more accurate either at high or at precisely zero musseldensities such asthose chosenin this study. Plate 2 shows an 0.25m2 area of beach estimated ashaving IOO”, musselcover by the presence/absence method. To determine the relationship between mussel-cover measurementsand mussel biomass, 19 quadrats between the + 1.2- and + 1.4-m shore levels were selected. In July 1978, musselcover in each quadrat wasestimated, after which all musselsin each quadrat, together with attached shell fragments and associateddebris, were collected and weighed. For convenience, this estimate of whole wet weight biomasswill be termed ‘ field weight ‘. Linear regression with transformed Y values (seeZar, 1984) was used to fit a curve to the measurements for mussel cover and biomass. As mussel cover is, of necessity, zero when biomassis zero, the regressionline wasforced through the origin (seeZar, 1984). As percentage cover (C) is a measureof area, which is proportional to length squared, while biomass( W) is a measureof volume, which is proportional to length cubed, the model
260
J. Landahl
Plate 2. 0,25-m2 absence method
area of beach estimated as having 100°, mussel using sampling frame divided into 54 subquadrats.
cover
by presence/
where b is the regressioncoefficient and 3/2 (which can alsobe expressedas 1.5) is the ratio between the length exponents 3 (for volume) and 2 (for area), was chosenasappropriate to fit the data. The appropriate Y transformation was therefore w’ = w?“i (2) After fitting to estimate the value of b, the slope of the regressionline, equation (1) was used to convert musselcover measurementsto estimatesof biomass. The Spearman rank correlation coefficient (see Zar, 1984) was used to assessrhe relationships between the range of sediment-level fluctuation and shore level, and between range of fluctuation and musselcover. Results Mussel dekty
The regression model W= bC”5 provided a good fit to the data for mussel cover as a function of biomass(r2 = 0.76; Figure 2). The estimate for b was 0.025 (upper and lower 95”” confidence limits 0.017 and 0.034, respectively). Although mussel cover did not exceed 50°, between the + 1.2- and + 1.4-m shore levels, it sometimesreached lOO”,, at the +0.6-m level, where maximum ‘ field weight ’ musselbiomassreached 40.3 kg m-’ in small areas(about 100 cm2; seeLandahl, 1985), indicating biomassestimates projected using this equation are realistic outside the range of the actual measurements.
261
Sediment-level$uctuation
A-
30
/
'E 252 2 P 0 3 0 z -c a, z 2-
Y= 0.025X R2=0.76
20-
/
/
‘I2
n=19
15IO5-
0
0
20 Area
40 covered
60 by mussels
60
100
P/.1
Figure 2. Relationship between estimated biomass (Y, ‘ field weight ‘-see text) and mussel cover (X). Line fitted by linear regression through the origin using transformed Y values; dashed lines indicate 95”,, confidence bands for the regression line. Cl, Observed values; 0, levels of mussel density investigated.
TABLE 2. Patterns of change observed Harbor, Washington, for the period Mussels Shore
level
+ 1.9-2.4
+ 14-1.5 +0.5-0.6 f0.3
SD, standard
deviation
absent
Mussels
Pattern
SD”
Constant Long-term decline Annual cycle Long-term increase
0.41 2.10
(in cm) for sediment
Patterns
for sediment-level gauge groups at Quartermaster 1 January 1978 to 1 October 1979
0.95 0.67
level fluctuation
present
Pattern Constant Long-term increase Long-term increase with ‘ spike ’ -
SD” 0.47 0.69 2.36 -
of each group
of sediment movement
Four temporal patterns of sediment movement were evident at individual locations at Quartermaster Harbor (Table 2). For easeof comparison, sediment-level records are presented beginning 1 January 1978. To reveal seasonalvariations, data are presented as the difference between the individual observations and the mean sediment level over the entire period of observation for each group of gauges.The region within 2 cm of the longterm mean for eachgroup of gaugesis indicated. The 2-cm region hasbeen chosenbecause Kuenen (1942) reported that burial by more than this amount of sediment is fatal to M. edulis.
The first pattern was constancy, in which mean sediment level varied little over the period of observation. Constancy was observed only at the highest shore levels, whether or not musselswere present [Figure 3(a,e)]. The second was an annual cycle of small amplitude (about 1 cm); this wasobserved only at the +0.6-m shore level in an area free of mussels[Figure 3(c)]. The third was a long-term decline (decreaseabout 5 cm); this was observed only at the + 1.5-m shore level in an area free of mussels[Figure 3(b)]. The fourth was a long-term increase (change about 3 cm); this was observed at the + 0.3- and
262
.7. Landahl
I5
(e)
(a)
IO -
............+.+.
...................... ...
$----J.’ .............
I
-10
I
I
I
I
15 -
I
I
t..
I
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.*.;;i;;;;‘;‘i”i’. .,
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l . +, ....... m
I
I
I
(b)
I (f)
0
I 3
0
3
6 Time
9 from
12 I January
1978
15 I6 (months)
I 6
I 9 Ttme from I January
I I I2 I5 1978bnonthsl
I I6
21
Figure 3. Relative fluctuation of readings of three gauges at Quartermaster Harbor, Washington, about their group mean during the period 1 January 1978 to 1 October 1979. (a) Constancy of sediment level at the f2.4m shore level (no mussel cover) (n = 78). (b) Long-term decline of about 5 cm in sediment level at the + 1.5-m shore level (no mussel cover) (n = 48). (c) Annual cycle of sediment level (amplitude about 1 cm) at the +0.6-m shore level (no mussel cover) (n = 42). (d) Long-term increase of about 2 cm in sediment level at the +0.3-m shore level (no mussel cover) (n = 45). (e) Constancy of sediment level at the + 1.9-m shore level (60”,, mussel cover; estimated biomass 12 kg mm ‘; fl= 72). (f) Long-term increase ofabout 3 cm in sediment level at the + 1.4-m shore level (95”. mussel cover; estimated biomass 23 kg mm’; n =87). (g) Long-term increase of about 3 cm in sediment level with ‘ spike ’ in December 1978 at the +0.5-m shore level (lOO”,, mussel cover; estimated biomass 25 kg m-a; n = 57); the ’ spike ’ reflects a sudden increase and subsequent sudden decrease in sediment level at two of the three gauges of the group, and was accompanied by the complete disappearance of mussels in the vicinity of the affected gauges. Symbols show relative fluctuation of readings of three gauges about their group mean over the period of observation, dashed line is relative fluctuation of three-gauge group average about long-term group mean, and dotted lines indicate region within 2 cm of long-term group mean.
263
Sediment-level fluctuation
Shore
level
cm 1
I
Figure mussel Harbor,
4. Range of sediment-level fluctuation as a function (a) of shore level and (b) of cover (range GlOO”,,; estimated biomass range O-25 kg mm *) at Quartermaster Washington, during the period 1 January 1978 to 1 October 1979.
+0.5-m shore levels, whether or not mussels were present [Figure 3(d,g)], and at the One abrupt and dramatic + 1.4-m level in an area with 95O, mussel cover [Figure3(f)]. change in sediment level was observed in December 1978 [an increase of 15 cm in an area with IOO”,, mussel cover at the +0.5-m shore level; [Figure 3(g)], but it was highly localized. The range of fluctuation for each group of gauges over the entire study is plotted against shore level in Figure 4(a). As would be expected from Figure 3, the range of fluctuation was generally less than 4 cm and showed little relationship to shore level (cl> 0.05, Spearman rank correlation; Zar, 1984). Decreases in sediment level were more common than increases over the entire period of observation (Table 3). While there was some indication that increases were more frequent in summer than in other seasons, as would be expected if an annual cycle of sediment movement were occurring (Table 3), this difference was not significant at rr =0.05 (heterogeneity G test; Sokal & Rohlf, 1981). EfJect
of wtussels
Little sediment movement occurred over a period of 18 months high on the shore whether mussels were present or absent [Figure 7(a)]. Lower on the beach, sediment level cycled or fell slightly in two areas without mussels but rose slightly in two areas with mussel cover [Figure 5(b,c]. However, the long-term increase at the lower of these two areas occurred after most of the mussels at that location were swept away by a storm. In the other case, the mussel-covered area was in a swale in the lee of a sand ridge, while the comparable area bare of mussels was about 10 m away on a more exposed sandy slope face.
264
3.
Landahl
TABLE
groups 1979
3. Direction of sediment-level fluctuations by season for sediment-level gauge at Quartermaster Harbor Washington, for the period 1 January 1978 to 1 October
Season Spring Summer Fall Winter Overall
Decreases” 34 19 18 12 83
Increases 18 21 13 8 60
Total
P’
G’
52 40 31 20 143
0.65 0.48 0.58 0.60 0.58
1.17 1.80 0.00 0.03 3.00
“Includes both decreases and observations of no change. ‘ decreases ’ among seasons are not significant at u = 0.05. %oportion of decreases. ‘Value of the G statistic (Sokal & Rohlf, 1983).
Differences
in proportions
of
The range of fluctuation for each group of gauges over the entire study is plotted against mussel density in Figure 4(b). The range of fluctuation showed little relationship to mussel cover (a > 0.05, Spearman rank correlation; Zar, 1984). Lack of ability of even high mussel densities to prevent major sediment movements within the mussel bed is clear from Figure 3(g), which shows sediment movement in an area with 100~~0 mussel cover (estimated biomass 25 kg rnm2) at the +0.5-m level. An abrupt and pronounced increase and a subsequent partial drop in sediment level at this location were recorded in late December 1978, and this increase was accompanied by either the sweeping away or, more probably, the burial of mussels at this spot (on the same date that the increase was noted, tagged mussel clumps in nearby permanent quadrats were found to have been partially or completely buried, resulting in mortality; Landahl, 1985). Sweeping of mussels for distances of up to 6 m by wave action during storms had been observed at a nearby site in November 1977 (Landahl, 1985); burial-induced mortality had also been observed at this time (Landahl, 1985).
Discussion How great is the impact of rare events (e.g. unusual weather conditions, periodic visits by migratory predators) on animal populations? This question is seldom addressed in marine ecological studies (see Sebens & Lewis, 1985), which are often conducted by collecting samples representing static ‘ snapshots ’ of a dynamic system. Rare events are difficult to detect with surveys or periodic sampling, just as they are difficult to capture in photographic snapshots. As discussed previously, one class of rare events-storms-is known to affect benthic invertebrate populations. In my studies I monitored sediment level at permanent gauges to permit the detection of storms, enabling me to assess aspects of their effects on the mussel population at Quartermaster Harbor. To ensure that findings are representative from a temporal standpoint, it is desirable to conduct studies of more than a single year’s duration when investigating marine ecological systems (see Lewis, 1980; Paine & Levin, 1981). For this reason, I recorded sediment level within and outside patches of mussels for portions of two successive years. At Quartermaster Harbor, mussels at low-shore levels were exposed to some risk of burial by storm action [Figure 4(a)]. Burial of tagged mussel clumps also occurred
265
Sediment-levelfluctuatiolr
-10 0
I 3
I 6 Time
from
I 9 I January
I I2 1978
I 15 (months)
I
I8
21
Figure 5. Comparison of sediment-level fluctuation at Quartermaster Harbor, Washington, between 1 January 1978 and 1 October 1979. La) In bare area (slope +24m), and in area with 60”,, mussel cover (slope + 1.9m; estimated biomass 12 kg m “1. (b) In bare area (slope + 1.5 m), and in area with 95”,, mussel cover (swale + 14 m; estimated biomass 23 kg m ’ ). (c) In bare area (slope + 0.6 m), and in area with loo”,, mussel cover (slope +0.5 m; estimated biomass 25 kg mm ‘). Points are relative fluctuations of three-gauge group mean about long-term group mean. The ’ spike ’ in December 1978 at the +0.5 m shore level was accompanied by the complete disappearance of mussels in the vicinities of two gauges -- 0 --, IAocations without mussels; - - n - -, locations with high mussel densities.
occasionally at mid-shore levels in other studies (Landahl, 1985). However, Figure 3(a) and (e‘) suggest that burial was unlikely to occur at the upper limit of distribution of Af~rilus sdzdis at this site, ifmeteorological conditions during this study are typical of those over the long term. Thus mussels at mid- and low-shore levels were at the mercy of large sediment fluctuations C:> 3 cm I when they occurred. Small fluctuations (on the order of 1-2 cm) were probably not of major importance for mussels; as mentioned previously, Kuenen i 1942) rqorted that isolated M. E&II’S are able to survive up to 3 cm of sand burial b!; working their way to the surface. Figure 3(g) shows that even when dramatic changes in sediment level occurred, they ~~xrld be highly localized. When sediment at two gauges at the $ 0.5-m shore level rose
266
J. Landahl
Plate 3. Area ofbeach near the study locality on 13 February 1978, about two weeks after partial resculpturing by a storm. Sand ridge to right has moved shoreward, almost completely burying marker stakes with 10 to 12 cm of sediment.
approximately 15 cm within a two-week period (probably in one day during a storm; see Landahl, 1985), that at the third gauge, only 1 m away, rose only 2 cm. Sediment level in other areas of the beach changed less than 1 cm at this time, decreasing rather than increasing in some cases, and remaining unchanged at the +0.6-m level. These results suggest that storm effects were sometimes extremely important at this site during the period of observation, but were highly variable on a small spatial scale even at a single shore level. My observations indicate that the study site was indeed protected from most storms due to local geography, but occasionally experienced relatively heavy wave action during winter. This resulted in occasional major sediment movements which were unpredictable both on a temporal and on a spatial basis. This storm wave action created rolling topography and occasionally reshaped this topography in small areas (e.g. Plate 3). The most important implication of these results is that occasional storms can have catastrophic effects on marine invertebrate populations even in apparently protected intertidal marine habitats. This conclusion is in agreement with recent studies pertaining to more exposed habitats, including those of Yeo and Risk (1979), Thistle (1981), and McCall (1978). Acknowledgements L. Johnson, V. Gallucci,
E. Dupras, and A. Kohn kindly assisted with fieldwork during this study. P. Jumars, A. Kohn, and R. Paine provided helpful discussion. V. Gallucci,
Sediment-levelfluctuation
267
A. Kohn, L. Johnson, R. Paine, and two anonymous reviewers suggested a number of useful revisions to earlier drafts of this manuscript. B. Kuhne assisted with preparation of the figures. This work was supported in part by an NSF dissertation research grant. References Anderson, F. E., Black, L., Watling, L. E., Mook, W. & Mayer, L. M. 1981 A temporal and spatial study of mudflat erosion and deposition. 3ournal of Sedimentary Petrology 51,729-736. Bailey-Brock, J. H. 1979 Sediment trapping by chaetopterid polychaetes on a Hawaiian fringing reef. 3ournaf of Marine
Research
31,643-656.
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Fager, Field,
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to of
39,361-374.
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Series
6,22>228.
Turnbeaugh, L. R. 1975 Marine Hydraulics. Report No. 6. Vol. II. Quartermaster Harbor marine park study. College of Forest Resources, University of Washington, Seattle. Verwey, J. 1952 On the ecology of distribution of cockle and mussel in the Dutch Waddensea, their role in sedimentation, and the source of their food supply. Archives Neerlandaises de Zoologie 10, 172-239. Yeo R. K. &Risk, M. J. 1979 Intertidal catastrophes: effects of storms and hurricanes on intertidal benthos of the Minas Basin, Bay of Fundy. Journal of the Fisheries Research Board of Canada 36,667-669. Young, D. K. & Rhoads, D. C. 1971 Animal-sediment relations in Cape Cod Bay, Massachusetts. I. A transect study. Marine Biology 11,242-254. Zar, J. H. 1984 Biostatistical Analysis, 2nd edn. Prentice-Hall, Englewood Cliffs, NJ.
Coastal
and Shelf
Sediment-level on a ‘ Protected
Science (1988) 26,255-267
Fluctuation ’ Sand-Gravel
in a Mussel Beach
Bed
John Landahl” Departmerzt
of Zoology,
Received
University
16 Jurze 1986 and in revised
Keywords:
of Washingtorz, form
16 Augztst
sediment movement; mussels; Mytilzts;
Seattle,
WA
98195,
U.S.A.
1987
storms
Studies of a dense population of the blue mussel (Mytilus edulis) between an abrupt lower boundary at + 0.6 m above MLLW, and an indistinct upper limit at + 2.3 m above MLLW on a sand-gravel beach at Quartermaster Harbor, Vashon Island, WA, evaluated the effects of physical and biological factors on patterns of abundance and distribution. Winter storms caused little sediment movement high on the shore either inside or outside patches of mussels, but large fluctuations ( > 3 cm) at mid- and low-shore levels sometimes caused burial and mortality. Even high mussel biomass (25 kgmm2) did not prevent large-scale sediment level change. Introduction The annual cycle of sediment movement on exposed temperate sand beaches, in which the level of the beach face falls in winter and rises in summer, is well known (Bascom, 1980), and its role in the ecology of sand-beach communities has been investigated with respect to the adaptations of shore plants (Markham &Newroth, 197 1). Seasonal changes in sediment elevation on intertidal flats in protected inshore waters have also been documented (e.g. Frostick & McCave, 1979; Pickrill, 1979; Anderson et al., 1981). However, as the annual range of sediment fluctuation reported in these habitats is typically 5 cm or less, it is tempting to assume that sediment movement is not a problem for epifauna, flora, or infauna. It is commonly held that beds of animals or plants exert a stabilizing effect on sediment even on unprotected bottoms subject to strong wave action or water currents by slowing water movement and preventing scouring (e.g. Fager, 1964; Young & Rhoads, 1971; Bailey-Brock, 1979), but this has been shown to depend on the density of the bed (Eckman et al., 1981; Eckman, 1982). Protruding ‘roughness elements ’ such as worm tubes, seagrass blades, or rocks, can either stabilize or destabilize sediment in turbulent flows for reasons discussed in detail by Nowell and Church (1979); Eckman et al. (1981); and Eckman (1982). These investigators concluded that sparse or isolated ‘roughness elements ’ actually increase scouring of sediment in their immediate vicinity, although at high densities (i.e. at least one-twelfth of the bed area occupied by ‘ roughness elements ‘), a ‘ skimming ’ flow which does not erode the bed can indeed form. Eckman et al. (1981 j “Present Service,
address: Environmental Conservation Division, National 2725 Montlake Blvd E., Seattle, WA 98112, U.S.A.
Marine
Fisheries
255 0272-7714,‘88/030255+
13 $03.0010
@ 1988 Academic
Press Limited
256
J. Landahl
and Eckman (1982) argue that natural benthic plant and animal densities are usually too low to produce hydrodynamic sediment stabilization, and suggest that mucus binding of sediment may produce the stabilization of sediment reported by various investigators in association with beds of tubes or seagrasses. Among the benthic invertebrate species forming beds in intertidal and shallow subtidal soft-sediment habitats is Mytilus edulis, the blue, bay, or edible mussel (e.g. Field, 1921; Verwey, 1952). Beginning in 1977, I conducted a series of studies of the ecology of an intertidal mussel bed on a sand-gravel beach at Quartermaster Harbor, Vashon Island, Washington. These studies concerned distribution patterns and interactions of mussels, their associated epifauna, and co-occurring infauna (see Landahl, 1985). Initial observations suggested that the level of the beach face might fluctuate in a seasonal manner similar to that reported for protected intertidal mudflats (e.g. Frostick & McCave, 1979; Anderson et al., 1981). As such seasonal beach-level fluctuations could have had important implications for mussel persistence and distribution, sediment level was monitored within and outside patches of mussels at several shore levels for periods of up to 20 months during 1978 and 1979. Sediment accumulation among mussels due to increased siltation resulting from slowing of water movement could also have had effects on nearby infauna. Mortality of mussels resulting from burial beneath sediment had been observed at this locality in November 1977 (Landahl, 1985). Storms are known to have major impacts on soft-sediment benthic invertebrate assemblages (McCall, 1978; Yeo & Risk, 1979; Thistle, 1981; Orensanz & Gallucci, 1982), are reported to destroy mature mussel beds in Europe (Newell, 1970,1979), and are known to result in passive transport of at least one sand-beach bivalve (Mya arenaria; Matthiessen, 1960). However, few studies have attempted to assess the effects of storms on soft-sediment mussel beds in North America. Monitoring of sediment level provided information on storm impacts on M. edulis as well as on siltation within mussel beds.
Materials
and Methods
Study area Quartermaster Harbor is, as the name suggests, a protected embayment, It is formed by Maury Island and the south end of abutting Vashon Island (latitude 47”23’N, longitude 122’29’W). These islands lie in central Puget Sound, a fjord connected with the Pacific Ocean by the Strait of Juan de Fuca (Figure 1). The study was conducted on an alluvial fan at the mouth of a small creek midway between the mouth and the back of the harbor. This fan was well protected except from south winds. Studies were conducted on a wide beach on the northern side of a sand-gravel point (area approximately 1 km2) which was entirely submerged at high tide. Winds at Seattle-Tacoma International Airport (about 15 km to the north-east of the study area) were from the south and south-west about 35”,, of the time between 1960 and 1969, but reached speeds in excess of 30 kmph (16 kt) from these directions only about 2’,, of the time (Turnbeaugh, 1975). This site is most exposed to a wind from the south, because its longest fetch (10 km) is in that direction (Turnbeaugh, 1975). Turnbeaugh (1975) calculated that a 65 km h-r (35 kt) south wind of >3 h duration would produce waves 1.8-m high with a period of 5 s at this site. However, his study suggests that considerably smaller waves (0.7 m) more frequently occurred during storms in the period 1960-69, based on the wind observations at the airport.
257
Sediment-level fluctuation
122O40'
122'30'
122"20'
122°10'
48"OO'
47030'
47'20'
47"20'
122'40'
122"30'
122"20'
122'10'
Figure 1. Map of Puget Sound, Washington, showing lcjca tion of the study site at Quartermaster Harbor (QH), Vashon Island. Latitudes are N; longin Ides are W
Although apparently well protected on the basisof its geography, the presenceof up to 50”,, gravel in the substrate (Landahl, 1985) suggestedthat the study areawas exposed to more wave and current action than the innermost region of the harbor, where the shores were considerably muddier. The beach surface wasirregular and characterized by rolling topography consisting of ridges and swaleswith a few shallow ponds.
To study possible seasonalbeach-level fluctuations, gaugesconsisting of square cedar stakes 1.9 cm on a side and 0.6-m long to which plastic centimetre scaleshad been glued (Plate 1) were used to monitor sediment level. Gauges of this nature are not ideal in that they constitute isolated, protruding ‘ roughness elements ’ and hence can be expected to increase sediment scour in their immediate vicinity while submerged if current velocity is sufficiently high (Eckman et al., 1981). However, no scour pits were noted at the bases of
the gauges,indicating that current velocities were usually low at the shore levels at which they were placed, or that scour pits were filled in by wave action during the advance and retreat of the tide. Gauges were set in place in groups of three in areasof high mussel density (60-loo”,, mussel cover; estimated ‘field weight ’ biomass 12-25 kg rn-‘, seebelow) at three shore levels ( + 0.5, + 1.4 and + 1.9 m) to permit assessmentof the effect of musselson sediment movement. One group was set in place in December 1977, one in January 1978, and one in July 1978(Table 1). In July and August 1978, four more groups of gaugeswere set in place
258
.7. Landahl
-
--
Plate 1. Sediment-level gauge at Quartermaster encrusted mussels and shell debris.
TABLE
groups groups
Harbor
surrounded
by barnacle-
1. Dates of establishment and numbers of observations for sediment-level gauge at Quartermaster Harbor, Washington; each group consisted of three gauges; all were in slope microhabitats except group 6, which was in a swale
Group
Shore level (m)
Mussel cover (“0)
1 2 3 4 5 6 7
+2,4 +1.5 +0,6 +0.3 + 1.9 + 1.4 f0.5
0 0 0 0 60 95 100
Estimated biomass (km m- ‘) 0 0 0 0 12 23 25
Distance between gauges (ml 10 1 2 10 2 2 1
Starting date 14 Jan. 29 Aug. 29 Aug. 29 Aug. 19 Dec. 15 Jan. 23 Jul.
1978 1978 1978 1978 1977 1978 1978
Ending date 11 Aug. 29 June 29 June 11 Aug. 10May 11 Aug. 11 Aug.
No. of obs 1979 1979 1979 1979 1979 1979 1979 Total
26 16 14 15 24 29 19 143
at similar shore levels in areas lacking mussels (Ol’, cover, biomass of necessity 0 kg m-‘) (Table 1). In August 1978 the shore levels of 16 approximately equally-spaced permanent marker stakes along a transect perpendicular to the water line spanning an elevation range from + 0.3 m to + 4.2 m were determined with reference to MLLW using transit, stadia rod, and tape. Elevations of the sediment-level gauges were estimated with reference to
Sediment-levelfluctuation
259
thesemarkers. All groups were on ridge slopes(seeLandahl, 1985) except group 6, which was in a swale. Sediment-level gaugesat a given shore level were spacedequal distancesapart, but this spacing interval was not the sameat all shore levels (Table 1). Readings were recorded at intervals of 10-80 days (usually every month or every fortnight) until August 1979, when several gaugeswere destroyed by drift logs. Prior to this time a number of the wooden stakesdeteriorated or lost their scales,but it wasalways possibleto replace them with new gaugesset to the original depth in the samelocation. From the data on sediment level over time, the range of fluctuation for each group of gaugesover the entire study could be calculated. Mussel density
Aggregation of musselsinto clumps and the presence of individuals as small as a few millimetres in shell length within these clumps made non-destructive censusing of the mussel population impossible. In situations of this nature, Pielou (1974) has suggested measuring a correlate of population size rather than counting individuals. For this study, cover class(defined asthe proportion of the substrate in a quadrat occupied by a speciesof interest) was chosen as the primary index of musselpopulation density. Cover classis a measure commonly employed in plant ecology, but seemsappropriate for the study of populations of sessileorganisms in general. With regard to M. edulis, musselcover can be considered to vary with mussel population size and several other factors, including mussel-layer depth, mean body size, and degree of aggregation. As a first approximation, musselcover can be said to be roughly proportional to population size, but obviously this simplification will be most accurate when mussel-layer depth, meanbody size, and degree of aggregation are all constant. This will be true most often in comparing measurements collected at a single shore level in a single season.In the present study, measurements collected at different shore levels or in different seasonswill sometimesbe compared; in these casesthe approximation may be lessaccurate. To obtain an objective estimate of cover class,this parameter was measuredin 0.25m2 quadrats by placing a rectangular brassframe (dimensions approximately 0.4 m by 0.6 m) divided into 54 squares(‘ subquadrats ‘) each 7 cm on a sideon the substrate. Subquadrats were scored either as unoccupied or as occupied by one or more live mussels. This presence/absencetechnique was an efficient field procedure which overestimated mussel cover at low but non-zero densities, becausea single individual could be scored aspresent in several adjacent subquadrats. However, estimates were much more accurate either at high or at precisely zero musseldensities such asthose chosenin this study. Plate 2 shows an 0.25m2 area of beach estimated ashaving IOO”, musselcover by the presence/absence method. To determine the relationship between mussel-cover measurementsand mussel biomass, 19 quadrats between the + 1.2- and + 1.4-m shore levels were selected. In July 1978, musselcover in each quadrat wasestimated, after which all musselsin each quadrat, together with attached shell fragments and associateddebris, were collected and weighed. For convenience, this estimate of whole wet weight biomasswill be termed ‘ field weight ‘. Linear regression with transformed Y values (seeZar, 1984) was used to fit a curve to the measurements for mussel cover and biomass. As mussel cover is, of necessity, zero when biomassis zero, the regressionline wasforced through the origin (seeZar, 1984). As percentage cover (C) is a measureof area, which is proportional to length squared, while biomass( W) is a measureof volume, which is proportional to length cubed, the model
260
J. Landahl
Plate 2. 0,25-m2 absence method
area of beach estimated as having 100°, mussel using sampling frame divided into 54 subquadrats.
cover
by presence/
where b is the regressioncoefficient and 3/2 (which can alsobe expressedas 1.5) is the ratio between the length exponents 3 (for volume) and 2 (for area), was chosenasappropriate to fit the data. The appropriate Y transformation was therefore w’ = w?“i (2) After fitting to estimate the value of b, the slope of the regressionline, equation (1) was used to convert musselcover measurementsto estimatesof biomass. The Spearman rank correlation coefficient (see Zar, 1984) was used to assessrhe relationships between the range of sediment-level fluctuation and shore level, and between range of fluctuation and musselcover. Results Mussel dekty
The regression model W= bC”5 provided a good fit to the data for mussel cover as a function of biomass(r2 = 0.76; Figure 2). The estimate for b was 0.025 (upper and lower 95”” confidence limits 0.017 and 0.034, respectively). Although mussel cover did not exceed 50°, between the + 1.2- and + 1.4-m shore levels, it sometimesreached lOO”,, at the +0.6-m level, where maximum ‘ field weight ’ musselbiomassreached 40.3 kg m-’ in small areas(about 100 cm2; seeLandahl, 1985), indicating biomassestimates projected using this equation are realistic outside the range of the actual measurements.
261
Sediment-level$uctuation
A-
30
/
'E 252 2 P 0 3 0 z -c a, z 2-
Y= 0.025X R2=0.76
20-
/
/
‘I2
n=19
15IO5-
0
0
20 Area
40 covered
60 by mussels
60
100
P/.1
Figure 2. Relationship between estimated biomass (Y, ‘ field weight ‘-see text) and mussel cover (X). Line fitted by linear regression through the origin using transformed Y values; dashed lines indicate 95”,, confidence bands for the regression line. Cl, Observed values; 0, levels of mussel density investigated.
TABLE 2. Patterns of change observed Harbor, Washington, for the period Mussels Shore
level
+ 1.9-2.4
+ 14-1.5 +0.5-0.6 f0.3
SD, standard
deviation
absent
Mussels
Pattern
SD”
Constant Long-term decline Annual cycle Long-term increase
0.41 2.10
(in cm) for sediment
Patterns
for sediment-level gauge groups at Quartermaster 1 January 1978 to 1 October 1979
0.95 0.67
level fluctuation
present
Pattern Constant Long-term increase Long-term increase with ‘ spike ’ -
SD” 0.47 0.69 2.36 -
of each group
of sediment movement
Four temporal patterns of sediment movement were evident at individual locations at Quartermaster Harbor (Table 2). For easeof comparison, sediment-level records are presented beginning 1 January 1978. To reveal seasonalvariations, data are presented as the difference between the individual observations and the mean sediment level over the entire period of observation for each group of gauges.The region within 2 cm of the longterm mean for eachgroup of gaugesis indicated. The 2-cm region hasbeen chosenbecause Kuenen (1942) reported that burial by more than this amount of sediment is fatal to M. edulis.
The first pattern was constancy, in which mean sediment level varied little over the period of observation. Constancy was observed only at the highest shore levels, whether or not musselswere present [Figure 3(a,e)]. The second was an annual cycle of small amplitude (about 1 cm); this wasobserved only at the +0.6-m shore level in an area free of mussels[Figure 3(c)]. The third was a long-term decline (decreaseabout 5 cm); this was observed only at the + 1.5-m shore level in an area free of mussels[Figure 3(b)]. The fourth was a long-term increase (change about 3 cm); this was observed at the + 0.3- and
262
.7. Landahl
I5
(e)
(a)
IO -
............+.+.
...................... ...
$----J.’ .............
I
-10
I
I
I
I
15 -
I
I
t..
I
.....
.*.;;i;;;;‘;‘i”i’. .,
*..?..+v
I
..............
l . +, ....... m
I
I
I
(b)
I (f)
0
I 3
0
3
6 Time
9 from
12 I January
1978
15 I6 (months)
I 6
I 9 Ttme from I January
I I I2 I5 1978bnonthsl
I I6
21
Figure 3. Relative fluctuation of readings of three gauges at Quartermaster Harbor, Washington, about their group mean during the period 1 January 1978 to 1 October 1979. (a) Constancy of sediment level at the f2.4m shore level (no mussel cover) (n = 78). (b) Long-term decline of about 5 cm in sediment level at the + 1.5-m shore level (no mussel cover) (n = 48). (c) Annual cycle of sediment level (amplitude about 1 cm) at the +0.6-m shore level (no mussel cover) (n = 42). (d) Long-term increase of about 2 cm in sediment level at the +0.3-m shore level (no mussel cover) (n = 45). (e) Constancy of sediment level at the + 1.9-m shore level (60”,, mussel cover; estimated biomass 12 kg mm ‘; fl= 72). (f) Long-term increase ofabout 3 cm in sediment level at the + 1.4-m shore level (95”. mussel cover; estimated biomass 23 kg mm’; n =87). (g) Long-term increase of about 3 cm in sediment level with ‘ spike ’ in December 1978 at the +0.5-m shore level (lOO”,, mussel cover; estimated biomass 25 kg m-a; n = 57); the ’ spike ’ reflects a sudden increase and subsequent sudden decrease in sediment level at two of the three gauges of the group, and was accompanied by the complete disappearance of mussels in the vicinity of the affected gauges. Symbols show relative fluctuation of readings of three gauges about their group mean over the period of observation, dashed line is relative fluctuation of three-gauge group average about long-term group mean, and dotted lines indicate region within 2 cm of long-term group mean.
263
Sediment-level fluctuation
Shore
level
cm 1
I
Figure mussel Harbor,
4. Range of sediment-level fluctuation as a function (a) of shore level and (b) of cover (range GlOO”,,; estimated biomass range O-25 kg mm *) at Quartermaster Washington, during the period 1 January 1978 to 1 October 1979.
+0.5-m shore levels, whether or not mussels were present [Figure 3(d,g)], and at the One abrupt and dramatic + 1.4-m level in an area with 95O, mussel cover [Figure3(f)]. change in sediment level was observed in December 1978 [an increase of 15 cm in an area with IOO”,, mussel cover at the +0.5-m shore level; [Figure 3(g)], but it was highly localized. The range of fluctuation for each group of gauges over the entire study is plotted against shore level in Figure 4(a). As would be expected from Figure 3, the range of fluctuation was generally less than 4 cm and showed little relationship to shore level (cl> 0.05, Spearman rank correlation; Zar, 1984). Decreases in sediment level were more common than increases over the entire period of observation (Table 3). While there was some indication that increases were more frequent in summer than in other seasons, as would be expected if an annual cycle of sediment movement were occurring (Table 3), this difference was not significant at rr =0.05 (heterogeneity G test; Sokal & Rohlf, 1981). EfJect
of wtussels
Little sediment movement occurred over a period of 18 months high on the shore whether mussels were present or absent [Figure 7(a)]. Lower on the beach, sediment level cycled or fell slightly in two areas without mussels but rose slightly in two areas with mussel cover [Figure 5(b,c]. However, the long-term increase at the lower of these two areas occurred after most of the mussels at that location were swept away by a storm. In the other case, the mussel-covered area was in a swale in the lee of a sand ridge, while the comparable area bare of mussels was about 10 m away on a more exposed sandy slope face.
264
3.
Landahl
TABLE
groups 1979
3. Direction of sediment-level fluctuations by season for sediment-level gauge at Quartermaster Harbor Washington, for the period 1 January 1978 to 1 October
Season Spring Summer Fall Winter Overall
Decreases” 34 19 18 12 83
Increases 18 21 13 8 60
Total
P’
G’
52 40 31 20 143
0.65 0.48 0.58 0.60 0.58
1.17 1.80 0.00 0.03 3.00
“Includes both decreases and observations of no change. ‘ decreases ’ among seasons are not significant at u = 0.05. %oportion of decreases. ‘Value of the G statistic (Sokal & Rohlf, 1983).
Differences
in proportions
of
The range of fluctuation for each group of gauges over the entire study is plotted against mussel density in Figure 4(b). The range of fluctuation showed little relationship to mussel cover (a > 0.05, Spearman rank correlation; Zar, 1984). Lack of ability of even high mussel densities to prevent major sediment movements within the mussel bed is clear from Figure 3(g), which shows sediment movement in an area with 100~~0 mussel cover (estimated biomass 25 kg rnm2) at the +0.5-m level. An abrupt and pronounced increase and a subsequent partial drop in sediment level at this location were recorded in late December 1978, and this increase was accompanied by either the sweeping away or, more probably, the burial of mussels at this spot (on the same date that the increase was noted, tagged mussel clumps in nearby permanent quadrats were found to have been partially or completely buried, resulting in mortality; Landahl, 1985). Sweeping of mussels for distances of up to 6 m by wave action during storms had been observed at a nearby site in November 1977 (Landahl, 1985); burial-induced mortality had also been observed at this time (Landahl, 1985).
Discussion How great is the impact of rare events (e.g. unusual weather conditions, periodic visits by migratory predators) on animal populations? This question is seldom addressed in marine ecological studies (see Sebens & Lewis, 1985), which are often conducted by collecting samples representing static ‘ snapshots ’ of a dynamic system. Rare events are difficult to detect with surveys or periodic sampling, just as they are difficult to capture in photographic snapshots. As discussed previously, one class of rare events-storms-is known to affect benthic invertebrate populations. In my studies I monitored sediment level at permanent gauges to permit the detection of storms, enabling me to assess aspects of their effects on the mussel population at Quartermaster Harbor. To ensure that findings are representative from a temporal standpoint, it is desirable to conduct studies of more than a single year’s duration when investigating marine ecological systems (see Lewis, 1980; Paine & Levin, 1981). For this reason, I recorded sediment level within and outside patches of mussels for portions of two successive years. At Quartermaster Harbor, mussels at low-shore levels were exposed to some risk of burial by storm action [Figure 4(a)]. Burial of tagged mussel clumps also occurred
265
Sediment-levelfluctuatiolr
-10 0
I 3
I 6 Time
from
I 9 I January
I I2 1978
I 15 (months)
I
I8
21
Figure 5. Comparison of sediment-level fluctuation at Quartermaster Harbor, Washington, between 1 January 1978 and 1 October 1979. La) In bare area (slope +24m), and in area with 60”,, mussel cover (slope + 1.9m; estimated biomass 12 kg m “1. (b) In bare area (slope + 1.5 m), and in area with 95”,, mussel cover (swale + 14 m; estimated biomass 23 kg m ’ ). (c) In bare area (slope + 0.6 m), and in area with loo”,, mussel cover (slope +0.5 m; estimated biomass 25 kg mm ‘). Points are relative fluctuations of three-gauge group mean about long-term group mean. The ’ spike ’ in December 1978 at the +0.5 m shore level was accompanied by the complete disappearance of mussels in the vicinities of two gauges -- 0 --, IAocations without mussels; - - n - -, locations with high mussel densities.
occasionally at mid-shore levels in other studies (Landahl, 1985). However, Figure 3(a) and (e‘) suggest that burial was unlikely to occur at the upper limit of distribution of Af~rilus sdzdis at this site, ifmeteorological conditions during this study are typical of those over the long term. Thus mussels at mid- and low-shore levels were at the mercy of large sediment fluctuations C:> 3 cm I when they occurred. Small fluctuations (on the order of 1-2 cm) were probably not of major importance for mussels; as mentioned previously, Kuenen i 1942) rqorted that isolated M. E&II’S are able to survive up to 3 cm of sand burial b!; working their way to the surface. Figure 3(g) shows that even when dramatic changes in sediment level occurred, they ~~xrld be highly localized. When sediment at two gauges at the $ 0.5-m shore level rose
266
J. Landahl
Plate 3. Area ofbeach near the study locality on 13 February 1978, about two weeks after partial resculpturing by a storm. Sand ridge to right has moved shoreward, almost completely burying marker stakes with 10 to 12 cm of sediment.
approximately 15 cm within a two-week period (probably in one day during a storm; see Landahl, 1985), that at the third gauge, only 1 m away, rose only 2 cm. Sediment level in other areas of the beach changed less than 1 cm at this time, decreasing rather than increasing in some cases, and remaining unchanged at the +0.6-m level. These results suggest that storm effects were sometimes extremely important at this site during the period of observation, but were highly variable on a small spatial scale even at a single shore level. My observations indicate that the study site was indeed protected from most storms due to local geography, but occasionally experienced relatively heavy wave action during winter. This resulted in occasional major sediment movements which were unpredictable both on a temporal and on a spatial basis. This storm wave action created rolling topography and occasionally reshaped this topography in small areas (e.g. Plate 3). The most important implication of these results is that occasional storms can have catastrophic effects on marine invertebrate populations even in apparently protected intertidal marine habitats. This conclusion is in agreement with recent studies pertaining to more exposed habitats, including those of Yeo and Risk (1979), Thistle (1981), and McCall (1978). Acknowledgements L. Johnson, V. Gallucci,
E. Dupras, and A. Kohn kindly assisted with fieldwork during this study. P. Jumars, A. Kohn, and R. Paine provided helpful discussion. V. Gallucci,
Sediment-levelfluctuation
267
A. Kohn, L. Johnson, R. Paine, and two anonymous reviewers suggested a number of useful revisions to earlier drafts of this manuscript. B. Kuhne assisted with preparation of the figures. This work was supported in part by an NSF dissertation research grant. References Anderson, F. E., Black, L., Watling, L. E., Mook, W. & Mayer, L. M. 1981 A temporal and spatial study of mudflat erosion and deposition. 3ournal of Sedimentary Petrology 51,729-736. Bailey-Brock, J. H. 1979 Sediment trapping by chaetopterid polychaetes on a Hawaiian fringing reef. 3ournaf of Marine
Research
31,643-656.
Bascom, W. B. 1980 Waves and Beaches, revised edn. Anchor Books, Garden City. Eckman, J. E. 1982 Hydrodynamic effects exerted by animal tubes and marsh grasses and their importance the ecology of soft-bottom, marine benthos. Dissertation, University of Washington, Seattle. Eckman, J. E., Nowelf, A. R. M. & Jumars, I’. A. 1981 Sediment destabilization by animal tubes. Journal Marine
Fager, Field,
Research
to of
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