Estuarine and Coastal Marine Science (1979) 8, 181-193

Seasonal Fluctuations in Coastal Benthic Invertebrate Assemblages

Don Maurer, Wayne Leathem, Peter Khmer and Jeffrey Tinsman College of Marine Studies, University of Delaware, Lewes, Delaware 19958, U.S.A. Received 5January 1977 and in revised form 29 August 1977

Keywords: seasonal; coastal ; benthic; fauna; statistical models ; Delaware coast

Two hundred and eighty-eight quantitative benthic invertebrate samples were collected during four quarterly sampling cruises at two areas off the Delaware coast. Species composition, species dominance (abundance and frequency of occurrence), density, and species evenness were compared. Baaed on cluster analysis three species-site groups emerged: (I) a high energy shoal assemblage characterized by a year-round suite of haustoriids and a few bivalves, (2) a mixed hard bottom-sand assemblage that featured marked seasonal dominance of Myths edulis with a variety of seasonal epifaunal and infaunal species, (3) a medium sand assemblage with a variety of seasonal infaunal species. Similar assemblages have been recognized off New Jersey and New York. Seasonal fluctuations in undisturbed coastal invertebrate assemblages can be so marked that many environmental impact studies are inadequate to assess the effects of human activities.

Introduction

In the last twenty years human activities around coastal and estuarine systems have increased enormously (Cronin, 1967). In many cases these activities have directly or indirectly produced adverse effects on these environments. Legislation at many levels requires environmental impact statements prior to initiating engineering projects (Peterson, 1976). In turn, the requirements of the impact statements have generated environmental studies which have featured, in many cases, benthic surveys. Because the time schedules for manage- ment decisions were commonly rushed, single surveys were conducted. Fortunately, this trend has changed and benthic pollution studies have been designed to follow seasonal and annual patterns (Rosenberg, 197'j).

It is surprising that studies of benthic pollution ecology did not earlier recognize the significance of seasonal fluctuations in their sample designs. Without some knowledge of the degree of seasonal variation in benthic populations, valid estimates of population shifts presumably due to human activities are seriously suspect. In addition to seasonal fluctuations, annual fluctuations and spatial heterogeneity of natural populations must also be studied before definitive conclusions concerning cause and effect in adjacent polluted populations can be accurately drawn. The purpose of this account is to examine the degree of seasonal fluctuation in coastal invertebrates. To a lesser degree spatial heterogeneity is also addressed.

181 o302-3524/79/020181+13 $02.00/o @ 1979 Academic Press Inc. (London) Ltd.

182 D. Maurer, W. Leathem, P. Kitmer &J. Timman

I

38050’ DELAWARE BAY

CAPE HENLOPEN

REHOEOTH BEACH

I

A?-L ANTK OCEAN

/ND/AN R/VCR INLET

!2 !I JO

.Y l 8 .*

6 . .3 .4

0' 2 Sk

.3 3870'

Figure I. Location map of samples. HCS-Hen and Chickens Shoal. SBB-South Bethany Beach.

Methods

Field Twelve stations (3 replicates/station) were established in a grid pattern off Hen and Chickens Shoal (HCS) and South Bethany Beach (SBB), Delaware (Figure I). The area represented at HCS was 6.4 km2 and at SBB it was 12.6 km2. Sampling was conducted seasonally (July and October, 1973 ; January and April, 1974) with a weighted 0.1 m2 Petersen grab yielding 288 samples. Aliquots were taken for sediment analysis (particle size, volatiles). Samples were then washed through a 1.0~mm mesh screen and the residue was preserved in 10%

formalin buffered with hexamethylenamine for laboratory analysis. Samples of surface and bottom water were taken with Nansen bottles equipped with reversing thermometers for salinity and dissolved oxygen determination.

Laboratory In the laboratory benthic organisms were sorted, identified, and counted. Sediment grain size analysis was performed according to procedures outlined by Folk (1968). Dissolved oxygen and salinity samples were analyzed using the Winkler method (azide modification) and induction salinometer.

Coastal benthic invertebrate assemblages 183

The biological index value (BIV) of species was determined according to McCloskey (1970). This index is a measure of species dominance based on number and frequency of occurrence. In addition, the scaled standard deviation (SDN) was computed according to Fager (1972). This value is a direct measure of species evenness. Finally, transformed species counts [In @+I)] were used in numerical classifications including both normal and inverse analyses that used the Canberra metric similarity and group average clustering (ClitIord & Stephenson, 1975). Species which occurred less than 5% were considered as rare species and were not included in cluster analysis.

Environmental setting

Hen and Chickens Shoal (HCS) (Kraft, 1971) is an ebb tide shoal at the mouth of Delaware Bay. Irregular, semidiumal tidal currents attain 5.2 km h-1 on the ebb tide at the bay mouth causing massive coastal erosion and sediment transport. The beach slope grades to a shallow, relatively flat, bottom (g m), which then rises on the shoal. The shoal deepens rapidly into a series of deep holes (24 m) and elongate depressions. Sediment size analysis for samples collected July 1973 are presented in Table I. Sediment was generally medium sand (0.2 to 0.6 mm diameter). However, large cobbles and small boulders occurred in holes and depressions or pockets of fine sand contained some silt and clay (Table I).

The situation off South Bethany Beach (SBB) differed from HCS in that the beach slope grades gradually to a shallow bottom (g-12 m) marked with shallow depressions and low

TABLJI I. Depth and sediment measures at Hen and Chickens Shoal (HCS) and South Bethany Beach (SBB)

Median size Depth HCS Stations (mm) % Gravel % Sand % Silt % Clay % Volatiles (m)

ii

9 IO II

SBB S:tions I 2 3 4

i

T

0'210 0.506 0.288

0.598 0.215 0'212

0.615 0.466

0.465 0.276 0.486 0.288

0.524 0.885

0.319 0.622 0.213

0-280 0-290 0.027 0.486

o-375 o-164

0.342

5'3 2'1

23.6

20'1

7'3 16.6 -

19'3 20.9

30'3 21'0

7'7 -

I.3 13.6 -

3'9 -

100'0

74'7 94'7 62.2 98.1 98.0 67.2 80.9

79'5 98.2 78.8

75‘9

66.6

77'1 89.2

95'2 40'5 83.1 98.6

29'7 97'1 97'8 83.6

97.6

- 20'0

3'2 9.8 x-8 2'0

;:4"

3'9 I.8 1.9 3'2

3'1 1'9

;:; 28.0

3'3 I.4

59'2 - 2’2

11.4 2-4

- 9.8 - 3'5

6.8

3'4

- -

- -

- 30.2

- 11'1

-

5’0

0.43 3'74 2.03 I.28

9'2 0'21 0.71 2.05 0.42 o-54 o-18 0.82

0.46 0’52 0.65 0.68

3.63 0'52

0.42 5’52 0.62 0'22

2.5 1'0

6.1

24'4 6.7

18.3 6.7 6.7

24'4 24'4

9.8 9'4 9'1 7'9

8.2 12’5

9.1 13'7

6.4 11.3 10’1 12.8

7'9 14’6 13'4 m-7

Median size = 50th sediment percentile, yO Gravel = yO of ee&nent >2*o mm, o/O Sand - yO of Sediment 0.0625 mm-z.0 mm, o/O Silt = o/O of Sediment 3.9 w-o.0625 mm, % Clay = yO of Sediment <3*9 m.

184 D. Maurer, W. Leathem, P. Kinner &“J. Timman

ridges. The submarine topography here was considerably more homogeneous than at HCS. Sediment at SBB was mainly medium sand with some scattered gravels. Several stations (5, 8, II) in a shallow trough contained appreciable amounts of silt and clay.

The range of mean water temperatures at HCS was 3.7 “C! (January) to 21.4 “C (October) and 4.6 “C to 20.3 “C at SBB during the same sampling period.

Salinity was consistent and showed little fluctuation throughout the tidal cycle, between surface and bottom samples, and between stations. The range of mean salinity at HCS was 27*~%~ (April) and 29.8 (October), and at SBB 285%, (July) and 30.7%~ (November).

Dissolved oxygen varied considerably seasonally, but showed no consistent trends between stations, surface and bottom samples, or throughout the tidal cycles, It was not a limiting factor.

Results

General fauna1 composition

A total of rg7 species was collected. For purposes of brevity, the species list together with the seasonal occurrence of all species was not presented here, but can be found in a 1976 report by the Sussex County, Delaware Engineers. Annelids comprised 37.6% (74) of the species, arthropods 35% (6g), pelecypods 7.6% (IS), gastropods 4.6% (g), ectoprocts 6-1 y0 (12), and other taxa 9.1 o/0 (18). Annelids consisted almost entirely of polychaetes, and arthropods consisted mostly of amphipods and decapods. Other taxa included cnidarians, rhynchocoels, and echinoderms. One hundred and forty-two species and 155 species were recorded at HCS and SBB, respectively. There were no significant seasonal differences in the per cent of species per major taxa within areas or between areas.

A total of 15 2go individuals was recorded from both sites. Pelecypods comprised 45.6% (6969) of the individuals, annelids 27.6% (4221), arthropods 24’5% (3774), gastropods 0~3% (134), and other taxa 1*50/O (222). Counts of other taxa did not include colonial forms (ectoprocts). One species comprised 58% (4045 individuals) of the pelecypods (&?. e&is). There was a decrease in the number of individuals from July to April at both sites. At HCS 4622 individuals were collected in July, 1956 in October, 1560 in January, and gr3 in April. At SBB 1956, 2454, 889, and 878 individuals were taken for the same time periods.

Number of species, density, and SDN

At HCS the number of species ranged from 2 in January to 50 in October (Table 2). The mean number of species per season was essentially the same in July, October, and January, with a marked decline in April. There was a statistically (ANOVA) significant (P<o*og) difference between April and January and October. At SBB the number of species ranged from 6 in July to 35 in October (Table 3). The number of species was statistically the same for all seasons.

At HCS mean density ranged from 40/m2 in April to 6g20/m2 in July. The mean density per season was highest in July (r282/m2) with a marked decline to October (5+4/ms) followed by a gradual decline through January and April (431 and 250/m2). There was a statistically significant difference (ANOVA) between the summer density and the densities of the other seasons. At SBB density ranged from 6/m2 in January to 3750/m2 in October (Table 3). Mean density per season was generally similar in July and October with a modest decline in January and April. There was a statistically significant difference between January and April and July and October.

Cuastal benthic invertebrate assemblages 185

TABLE 2. Number of species, density (N/m*), and scaled standard deviation (SDN) per HCS station (3 replicates/station) by season

Station

Sample I 2 3 4 5 6 7 8 g IO II 12

HCS July I973 No. ofspecies 6 26* 8 IS* 4 IO IS* 26+ 30* 7 8 36* Density 270 6gzo 486 1690 290 280 1620 1960 540 250 93 990 SDN 0.296 0.028 0.058 0.059 0.110 0.217 0.057 0.094 0.468 0.336 0.473 0.612

HCS October 1973 No. of species 3 37* 7 50* 4 5 33+ 31* IO+ 12* g* 14 Density 636 2050 7oo 770 566 340 456 283 66 96 203 363 SDN 0-008 0’209 0.062 0.709 0.087 0.064 0.014 0.571 o-413 o-397 0.434 0.532

HCS January 1974 No. of species 3 47+ 5 * 12 9 38* 27’ IS+ g 7 2

Density 236 1780 123 6;: 3oo 273 940 256 ro3 76 263 I43 SDN 0.000 0.678 0.040 0.533 0.176 0.076 0.667 0.611 0.466 0.599 0.065 owoo

HCS April r 974 No. of species Density 8: 3;:

* 3 9 4 4 * * II 7+ 7 150 ‘I3 333 210 63: 6:: 439 II0 203 40

SDN o-ooo 0.656 0.107 0.497 0.136 0.091 0.566 0.562 0.753 0.617 0.136 0.457

*No. of species includes colonial species which are not included in estimates of density or SDN.

TABLB 3, Number of species, density (N/m*), and scaled standard deviation (SDN) per SBB station (3 replicates/station) by season

Sample I 2 3 4 5 6 7 8 g IO II 12

SBB July I974 No. of species 14 26* 21’ 25+ 6 15 8 21* 17’ 14 15 Density 243 323 340 190 36 6:: 563 3526 196 180 203 3ro SDN 0.575 0.678 0-577 0.748 0.534 0.488 0.527 0.022 0.769 0.568 0.213 0.451

SBB October 1973 No. ofspecies 23* xg 17* 25* IS* 8 * 19’ 16’ 19’ rg Density 333 273 90 270 200 86 93056 37;: 493 320 703 750 SDN o-619 0,354 0.380 0.644 0.531 0.408 o-469 0.181 0.406 o-499 0.404 0.515

SBB January 19% No. ofspecies 6* IS* 6

4::

l 19+ 23 19 13* 41* 21* r4+ Ig Density 66 123 6 100 403 433 106 496 153 123 450 SDN 0.225 o-692 l * 0.388 0.747 0.455 0.531 0.580 0.714 0.574 0.583 0.083

SBB April I974 No. ofspecies 17 16~ 2g* 22’ 14* 21* 15

6: 15 x8* II 20

Density I90 166 376 486 66 240 I33 346 326 213 316 SDN 0.267 0.642 0.636 0.202 0.684 0.459 0.653 0.523 0.567 0.509 0.338 0.533

*No. of species includes colonial species which are not included in estimates of density or SDN. **Was not computed due to inadequate number of individuals.

At HCS the scaled standard deviation (SDN) ranged from o-00 in January and April to 0.753 in April (Table 2). The mean SDN per season was lowest in July and generally similar the other seasons. At SBB mean SDN values ranged from 0’022 to o-769 in July (Table 3). Mean SDN values per season were always higher at SBB than HCS. In general they were similar for all seasons (0.514, 0.450, o-507, 0.501).

186 D. Maurer, W. Leathem, P. Kinner &J. Tinsman

TABLE 4. Rank based on total number of individuals and the biological index value (BIV) per season of ten top species at HCS

Number of individuals Total Rank according to BIV

July Oct. Jan. April Number Rank July Oct. Jan. April

Mytilus edulis 3463 480 102 3 4048 I 3 9 8 9 Parahaustorius longimerus 384 682 3.52 276 1694 2 2 I I I

Tharyx acutus I 95 258 166 520 3 IO 3 4 Tellina agilis

3 146 35 41 50 272 4 I 4 2

Nereis succinea 4 54 '17 27 202 5 9 5 5 :

Lysianopsis alba 19 18 I43 21 201 6 4 7 Unciola serrata 42 87 44 14 187 Hannothoe extentiata 36 38 4.4 13 131 i 2 Ei

: i 9 7

Sabellank vulgaris IO 53 37 I IO1 9 7 6 7 9 Spio setosa 4 8 0 89 101 9 8 IO IO 4

Dominance

The top ten species were determined for each area. They accounted for 81.3% and 61% by number of individuals at HCS and SBB, respectively. The biological index value (BIV) for each species was determined seasonally to obtain a measure of dominance (Tables 4 and 5). There was considerable difference between the areas as only two species were among the top IO species at both stations (Tellina agilis and Sabellaria vulgaris). Mytilus edulis accounted for 44.6% of the fauna at HCS, but its BIV was no higher than 3 and normally 8 or lower. Parahaustorius longimerus and Tel&a agilis consistentIy received the highest values (generally I or z) at HCS followed by Tharyx acutus, Lysianopsis alba, and Uncibla serrata. The occurrence of Mytilus edulis, T. acutus, N. succinea, and S. setosa was very seasonal (Table 4).

At SBB Tellina agilis received the highest BIV (I) for all seasons (Table 5). Nephtys picta, Spisula solidissima, and Spiophanes bombyx were important codominants with values generally between z and 4. In contrast to HCS, values at SBB showed relatively small variation per species throughout the seasons.

Based on BIV, T. agilis was a dominant species in both areas during most seasons. Its codominants included T. a&us, P. longimerus, L. alba, and U. set-rata at HCS and Spiophanes bombyx, Nephtys picta, and Spisula solidissima at SBB. Two bivalves which occurred in high numbers scored relatively low in dominance values. Mytilus edulis occurred almost

TABLE 5. Rank based on total number of individuals and the biological index value (BIV) per season of ten top species at SBB

Number of individuals Total Rank according to BIV

July Oct. Jan. April Number Rank July Oct. Jan. April

Tellina agilis 246 Nucula proxima 1046 Protohaustorius wigleyi 101

Sptiphanes bambyx 63 Corophium tuberculatum 84 Nephtys picta 2

Spskula sol&&ima 98 SabeEa& vulgar& 1 Polygordius sp. 0

Nemertea sp. A 2

490 249 8 II

164 67 104 20

III 68 6: 23 40

137 16 0 6 0 84

216 0

35 23

6; 24

0

124 16

1201 I I I I I

1065 2 s 7 10 8 367 3 4 5 5 5 210 4 3 4 198

:, 7

i : 8

193 6 it

2 2

1% I54 ii

2

9 4 i i

130 9 IO 9 9 I02 IO 8 9 7 i

Coastal benthic invertebrate assembkpes 187

exclusively at HCS (I individual at SBB) and was a dominant species only in the summer. Nucula proximu occurred sporadically in both areas, but 1031 individuals were collected at one station in the summer at SBB.

Cluster analysis

Normal classijcation

When clustering was performed on the data, three site groups emerged at HCS: near shore, mid-shore, and offshore. Figure z depicts the dendrogram for October at HCS. This pattern was similar for all seasons with a higher number of discrepancies in the spring. However, there were a few stations and/or replicates which were not consistent. In contrast, no site groups emerged at SBB or they were not interpretable in terms of near, mid, and offshore or sediment differences.

t .--.. 1 I I I I 9.3-l

cw7 0.30 042 0.55 0.67 0.80 0.93

X SIMILARITY

Figure 2. Dendrogram from normal cluster (site group) analysis of October HCS samples. Stations 1-12; replicates 1-3.

Inverse classification

When clustering was accomplished, two species groups emerged at HCS. One species group included S. solidissima and haustoriids (Parahaustorius longimerus, P. attenuatus, P. holmesi, Protohaustorius wt&eyi, Acanthohaustorius millsi, A. intermedius). This group persisted through all seasons. The second and larger group included two components; epifaunal species such as M. edulis, N. succinea, U. serrata, Harmothoe extenuata, and Sabellaria vulgaris, and infaunal species such as Tell& agilis, T. acutus, L. alba, and Spio setosa. In contrast to the first species group, members of the second species group were more ephemeral in that they showed marked seasonality in occurrence. Figure 3 depicts the dendrogram for July at HCS.

188 D. Maurer, W. Leathem, P. Kimzer &J. Tinsman

I

016 0.21 0.26 0.31 0.36 0.41 046

% SIMILARITY

Figure 3. Dendrogram from inverse cluster (species group) analysis of July HCS samples.

In the fall at SBB a species group consisting of haustoriids (A. milk’, P. w@yi, P. attenuatus), S. bombyx, T. agilis, S. solidissima, and Echinarachniusparmu emerged. Exclusive of this species group at one particular season, no other consistent species groups were recognized. Each season showed a different combination of species with many similarities and differences to those at HCS.

The coincidence of occurrence between species groups and site groups was examined. At HCS haustoriids occurred at the near shore and mid-shore sites. Acanthohaustorius millsi and A. intermedius primarily occurred in the near shore site whereas Parahaustmius attenuatus, P. holmesi, and P. longimerus mainly occurred in the mid-shore site. These haustoriid associations were recognized through all seasons. Based on frequency of occur- rence and abundance, P. longimerus was the dominant species.

In the HCS offshore site, M. edulis formed the substratum for a variety of species. Many of these species occurred in the near shore site group. Although colonial species were not included in the cluster analysis, ectoprocts were associated with M. edulis at the offshore site. In addition, the M. edulis species group showed marked seasonality. Mytilus edulis was the single dominant species in summer and progressively declined to less than r/m2 by the next spring. At HCS several species including the dominants were particularly important seasonally, i.e. summer (M. edulis, Polydora spp., Pherusa @jir&), fall and winter (N. succinea, S. vulgaris, S. benedicti, U. serrata) and winter and spring (T. a&us, S. setosa, A. oculata, G. americana, G. capitata). A few species (T. agilis and P. longimerus) occurred at all sites throughout the year. Although Tell&a agilis and P. longimerus were abundant year-round, their peak development was in October and January. In summary, one species- site group was characterized by a year-round suite of haustoriids which occurred mainly at the HCS mid-shore and near shore sites. The second species-site group featured the marked seasonal dominance of M. edulis with a wide variety of epifaunal and infaunal species which varied seasonally at the offshore and near shore stations. The second group contained

Coastal benthic inwertebrate assemblages 189

many species more widely distributed than members of the first group with the exception of P. longimerus.

No definite site groups or species groups were recognized from the cluster analysis at SBB. Haustoriids were rare here compared to HCS except for Protohaustorius euigleyi, which occurred in high numbers. Again, Acanthohaustorius millsi and A. intermedius were also mainly restricted to the near shore site. Other species which occurred more frequently and in greater abundance at SBB than HCS were N. picta, S. bombyx, Polygordius sp., Ampharete acutifrons, S. solidissima, T. agilis, C. tuberculatum, and E. parma.

Average number of species, density, and SDN by site and season

From the cluster analyses the areas were divided into subsites and analysed further. The mid-shore site (Stations I, 3, 5, 6) at HCS contained the lowest mean number of species (38 to 7.3) with no apparent seasonal trend (Table 6). The highest mean number of species was recorded at the HCS offshore site (Stations 2, 4, 7, 8) (22-37.8 species) with peak abundance in October and January. For HCS there was a statistically (ANOVA) significant (P<o.og) difference in the mean number of species between offshore and the other two sites. At SBB the mean number of species at the offshore site (Stations 3, 4, 9, IO) was higher than the near shore (Stations I, 6, 7, 12) and mid-shore sites (Stations 2, 5, 8, II), but there was no seasonal pattern. The near shore site at SBB generally contained a higher mean number of species than the HCS site except during the summer (Table 6). The seasonal peak at SBB was in October.

Density was highest at the HCS offshore site (457 to 3048/m2) and lowest at the HCS near shore site (99-468/m2). F or HCS there was a statistically significant difference in density between the offshore site and the other two sites. Mytilus edulis accounted for the offshore peaks and P. longimerus, U. serrata, and T. agilis for the near shore ones. Mean density at SBB was higher than HCS for the near shore and mid-shore sites (Table 6). These values were statistically significant in July and October. There was a marked decline in density between October and January at the SBB mid-shore site. A mixed group of

TABLE 6. Mean number of species, density, and scaled standard deviation (SDN) (&standard deviation) by site and season

HCS SBB

Nearshore Mid-shore Offshore Nearshore Mid-shore Offshore -

Mean number of species/station July 20'3ztI4'9 7.ok2.6 22.o~t4.6 15.8+ 2.2 23’519’o zr.o* 2.3 October 11’3& 2'2 4.8i1.7 37.8xt8.5 21.3k11.1 17'3Lt2.9 19’3+ 4'0 January 8o1t 4.9 7’3 f4.o 37.3 18.1 16.84 7’4 15.3f2.6 22'3+14'4 April 8*5f 1.9 3.8io.s 23'0&9'4 18.3f 2.8 12.5Lk-3'1 21.0* 6.1

Mean density/station July 468 f394 265 f 173 3048 f2586 43x+182 1022f1673 2264 76 October 182+134 417zt279 889zk 799 518Ik376 1231f1693 293i166 January 146% 83 2331 78 916dc 641 338xkI82 113* 12 285 f 24.5 April wxt 76 194*IoS 457* 270 2x9+ 77 x27* 74 383* 71

Mean SDN/station July 0.472&0.X12 0~170f0~106 0'059fO.027 0~295 ho.233 o.362fo.zg8 0~670~0~102 October 0441 fo.056 0.055 fo*o33 0,525 ko.218 0~503~to~o89 0*368&0.145 0.482j~o.119 January 0*2821to.294 0.073 1to.075 0.622 fo.066 0.324rto.206 0.650=!~0.082 o.q.rgfo.3og April 0~49o-fo~266 0~083~0~058 0.570fo~o65 0.478i-to.161 0.547*0"55 0'47gfo'Ig2

190 D. Maurer, W. Leathem, P. Kinner &f-Y. Tiktnan

amphipods, bivalves, and polychaetes accounted for these distributions. In contrast to HCS, if there was a seasonal peak at SBB, it occurred in the fall.

The lowest SDN values were recorded at the HCS mid-shore site (0~055-0~170) which reflected the dominance of P. longimerus and the highest at the SBB offshore site (o*4rg- 0.670). In general, mean SDN values showed less seasonal fluctuation and were more random in distribution than mean number of species or density. The low SDN summer value at the HCS offshore site was attributable to the dominance of M. edulis. As the number of M. edulis declined, SDN values increased accordingly.

Discussion

When the mean number of species, density, species evenness, and BIV values of HCS and SBB were compared to one another, there were some seasonal differences. July density values at HCS were markedly higher than any other period and site and the mean number of species in April at SBB was significantly higher than at HCS. Mytilus edulis peaked in the summer; U. serrata in the fall; L. alba, N. succinea, and T. acutus in the winter; and S. setosa in the spring at HCS (Table 4), but members of the top ten species at SBB were very consistent in terms of frequency of occurrence and abundance.

However, when HCS and SBB were divided into sites and were viewed in a finer habitat scale, the effects of seasonal and spatial variability were accentuated and several distinct assemblages emerged with marked seasonality as expected in coastal waters of these latitudes (Calder & Brehmer, 1967; Boesch, 1973; Garlo et al., 1975).

Based on depth, bottom topography, sediment type, species composition, and density, HCS mid-shore and offshore sites were distinct from one another and the remaining sites at HCS and SBB (Table I, Figures z and 3). The HCS mid-shore site comprised few species, relatively low densities, and low species evenness, and were seasonally relatively persistent (Table 6). This sand shoal assemblage contained haustoriids with a few burrowing bivalves suited for life in sediment made unstable by powerful currents. The significance of hydro- dynamically active sediments influencing the composition of benthic invertebrates on the mid-Atlantic continental shelf was broadly treated by Pratt (1973). Reduced numbers of species and densities were reported from an estuarine situation where sediment instability was due to the dissipation of wave energy on a shallow bar (Boesch & Rackley, 1974). Haustoriids (Parahaustorius longimerus, A. millsi, Protohaustorius deichmannae) and bivalves ( M. edulis, S. solidissima, T. ugilis) were among the most common taxa from Little Egg Inlet. Except for the mussel these species were very similar to those at the HCS mid-shore site or sand shoal. At Little Egg Inlet, New Jersey, approximately 102 kilometers north of the present study, benthic stations were subject to more surge, greater tidal current velocity, and more shifts in substrate than stations in the ocean (Garlo et al., 1975).

The HCS offshore site had many species, relatively high densities, and high species evenness, and varied considerably seasonally (Table 6). This site contained high numbers of M. edulis in the summer with many epifaunal species associated with the mussels and rocks and many infaunal species associated with bare sand. Off southwestern Long Island, New York, a community dominated by M. edr&.r has been discussed (Steimle & Stone, 1973). They reported that clumps of blue mussels unattached to a substratum commonly contained Harmothoe extenuata, N. succinea, L. squamatus, and N. sayi. The fauna in the sand underlying the mussel clumps consisted of T. agilis, Protohaustorius deichmannae, and E. parma. The clumps of blue mussels and associated species together with members of the sand fauna were very similar to the HCS offshore site. In general the SBB sites

Coastal benthic invertebrate assemblages 191

comprised moderately high numbers of species ranging between those of HCS mid-shore and offshore sites, relatively high densities and high species evenness (Table 6). Various members showed marked seasonality (Polygordius sp., A. acutifrons, S. vulgaris). The species composition of the HCS near shore site was more similar to the SBB sites than the other HCS sites. The medium sand assemblage described off Long Island (Steimle & Stone, 1973) included T. agilis, P. deichmannae, E. parma, u. irrorata, S. solidtisima, A. mills& T. epistomus, and S. bo&yx. Ocean stations off Little Egg Inlet included the above species together with capitellids, A. oculata, N. americana, G. lawrencianus, and S. viridis (Garlo et al., 1975). The latter species has been tentatively reidentified as S. bombyx (Garlo, personal communication). The general composition of species at SBB, off Long Island, and Little Egg Inlet indicated that these assemblages were members of the sand fauna described from the mid-Atlantic continental shelf (Pratt, 1973). This sand fauna was recently studied in deeper water off the Delmarva Peninsula and additional species were identified as characteristic members (Maurer et al., 1976).

Differences among the New Jersey, New York, and local areas were as follows. In Little Egg Inlet at the high energy stations M. edulis was the dominant species among species which were very similar to HCS mid-shore stations without M. edulis. Density peaked in the spring (577z/m2) and declined in the summer at the Inlet due to the disappearance of most mussel spat (Garlo et al., 1975). In contrast, M. edulis peaked in the summer off Long Island with a subsequent decline in the fall (Steimle & Stone, 1973). The summer peak may have been because of sampling error (Steimle, personal communication). The HCS site differed from the New Jersey site in that high densities of mussels occurred in the deeper water (25 m) offshore site rather than in the high energy sand shoal. In addition, mussel density was highest in the summer. This summer peak with a decline in the fall was similar to the pattern reported off Long Island.

A fine silty sand assemblage that contained N. proxima, N. incisa, Pherusa afinis, Clymenella torquata, and Leptocheirus pinguis was also described off Long Island (Steimle & Stone, 1973). At depths greater than 9 m in sediment with $5 to 19% silt, capitellid polychaetes were the dominant species (Garlo et al., 1975). Off Little Egg Inlet Tellina agilis was common and N. proxima, Y. limatula, and P. afinis were present in low numbers. In the present study stations 5, 8, and II at the SBB mid-shore site contained 30*2%, I I-IO/~ and 5*oo/o silt-clay respectively (Table I). In addition to other species these stations contained N. proxima, Y. limatulu, and N. in&a. In fact, 1031 individuals of N. proxima were collected at station 8 in July.

These stations formed a shallow trough. The result may be a vertical lens or pocket of silty sand (Z >3% silt-clay) enclosed by medium to coarse sand with less than 1% silt- clay. The microhabitats of silty-sand provide opportunities for colonization by species normally associated with lower energy environments (fine-grained, organic rich bottoms). These silty-sand pockets represent a relatively stable bottom for burrowing and tube building, and an additional source of organic material for invertebrate consumption compared to the properties of medium to coarse sand and pebbles normally produced by relatively high energy. The presence of these silty-sand pockets in an essentially sand bottom environment explains in part the problems experienced in defining benthic communities here. One normally encounters a characteristic suite of sand-dwelling species on the inner continental shelf and silty sand and mud species on the outer shelf (Pratt, 1973). Garlo (personal communication) sampled a trough and found assemblages and their seasonal fluctuations similar from 1972 to 1975. She considered the trough a permanent topographical feature.

192 D. Maurer, W. Leathem, P. Kimer M-7. Tinsman

The pattern of seasonal fluctuation among coastal invertebrates is not always simple and predictable. Patterns vary with life habit, size of the area, hydrography, sediment type, depth, and sampling design. In addition, biotic factors such as predation and spatial competition exert their own effects which influence seasonal patterns. The occurrence of filter-feeding fish such as menhaden or bottom-feeding fish and invertebrates such as flounder and crabs coincident with peaks of meroplankton and setting can greatly alter community structure and seasonal patterns (Levings, 1975). In unpublished laboratory studies we observed extensive predation (25 clams/day) of juvenile hard clams (Mercenuria mercenmia) by the blue crab (Callinectes sapidus) and large numbers of mortalities caused by causal movement of large horseshoe crabs (Limlus polyphemus). Studies on biotic factors influencing the composition of benthic communities on hard and soft bottoms are showing how important these factors are (Dayton, 1975; Hamilton, 1976; Woodin, 1976). The biological significance of the balance between marine invertebrate predators and newly settled larvae has been suggested for some time (Thorson, 1957). These factors together with annual fluctuations (Lie & Evans, 1973) caused by large-scale conditions (climate) produce complex benthic patterns.

Until seasonal, annual, and spatial studies can be conducted, one-time benthic surveys are normally of limited value in providing useful information for management decisions in before and after pollution studies. In view of these natural fluctuations, it would be more prudent to identify major categories of pollution types in representative geographic areas and adequately study these situations temporally and spatially rather than pursue many one-time, small-scale pollution studies for every proposed marina, sewage outfall, or power plant. Finally, because of the natural fluctuations discussed above and the difficulties in assessing them, it is further recommended that experimental approaches (polluted and untreated sediment plots) emphasizing functional aspects of benthic ecology (recruitment, survivorship, and growth) might be a more effective way to pursue pollution studies rather than traditional before and after studies.

Acknowledgements

The authors are grateful to Chris Wethe, Henry Lind, Dick Keck, and Capt. Tom White for their laboratory and field assistance on this project. We would also like to thank Dr Les Watling for his aid in identifying amphipods and his development and implementation of a computer program (cluster analysis) originally obtained from Dr Don Boesch, Virginia Institute of Marine Science. Don Boesch, Robert Diaz, Elizabeth Garlo, and Frank Steimle reviewed earlier drafts and provided valuable, constructive criticism.

Refkrences Boesch, D. F. 1973 Classification and community structure of macrobenthos in the Hampton Roads

A&a, Virginia. Ma&e Biology ~1,226244. Boesch. D. F. & Racklev. D. H. ICU Effects on benthic communities, x-97 m. Retort on Environmental _. --

E&e& of the Se&d Hampi; Roads Bri4gc-Tumd Cowwi;bn to the V&inia Department of Highzaays. Virginia Institute of Marine Science.

Caldex, D. R. & Brehmer, M. L. 1967 Seasonal occurrence of epifauna on test panels in Hampton Roads, Virginia. International Journal of Oceanalqgy and Limtwlogy I, 149-164.

Clifford, H. T. & Stephenson, W. 1975 An Introduction to Numerical Clarti~atkm. New York and London, Academic Press, 229 pp.

Cronin, L. E. ~967 The role of man in estuarine processes. In &tu&es (La&, G. H., ed.). American Association for the Advancement of Science, Publication No. 83, pp. 667-689.

Dayton, P. K. 1975 Experimental studies of algal canopy interactions in a sea otter-dominated kelp community at Amchitka Island, Alaska. Fishery Bulletin 73, 230-237.

Coastal benthic invertebrate assemblages 193

Fager, E. W. 1972 Diversity: a sampling study. America Naturalist 106, 239-310. Folk, R. L. 1968 Petrology of Sedahentary Rocks. Hemphill’s, Austin, Texas, 170. Garlo, E. V., Hondo, J. J. & Miller, G. J. 1975 Benthic invertebrates. In Ecological Studies in the Bays

and other Waterways near Little Egg Inlet and in the Ocean in the Vicinity of the Proposed Site for the Atlantic Generating St&ion, New Jersey. Ichthyological Associates, Inc., Ithaca, New York, pp. 26-36.

Hamilton, P. V. 1976 Predation on Littorina iworata (Mollusca: Gastropoda) by Callinectes sap&s (Crustacea: Portunidae). Bulletin of Marine Science 26,403~409.

Kraft, J. C. 1971 A Guide to the Geology of Delaware’s Coastal Environments. Publication zGLo39, College of Marine Studies, University of Delaware, pp. 1-220.

Levings, C. D. x975 Analyses of temporal variation in the structure of a shallow-water benthic com- munity in Nova Scotia. Internationale Revue der Gesamten Hydrobiologie 60,449-470.

Lie, V. & Evans, R. A. 1973 Long-term variability in the structure of subtidal benthic communities in Puget Sound, Washington, U.S.A. Marine Biology 21, 122-126.

Maurer, D., Kirmer, P., Leathem, W. & Watling, L. 1976 Benthic faunal assemblages off the Delmarva Peninsula. Estuarine and Coastal Marine Science q, 163-177.

McCloakey, L. R. 1970 The dynamics of the community associated with a marine scleractinian coral Internationale Revue der Gesamten Hydrobiologie 55, 13-81.

Peterson, W. R. 1976 The impact statement-Part II. Science r93(424g), 193.

Pratt, S. D. 1973 Benthic fauna. In Coastal and Offshore Environmenta I Inventory Cape Hatteras to Nantucket Shoals. Marine Publication Series No. 2, University of Rhode Island.

Rosenberg, R. 1973 Succession in benthic macrofauna in a Swedish fjord subsequent to the closure of a sulphite pulp mill. Oikos 24, 244-258.

Steimle, F. W. 8%. Stone, R. B. 1973 Abundance and distribution of inshore benthic fauna off south- western Long Island, New York. National Oceanic and Atmosoheric Administration. Technical Report, National Marine Fisheries Service, Special Scientific &epurt Fisheries Series arj, r-50.

Thorson, G. 1957 Bottom communities (sublittoral or shallow shelf), 461-534. In Treatise on Marine Ecology and Palcoecology (Hedgpeth, J. W., ed.). Geological Society of American Memoir 67, I.

Woodin, S. A. 1976 Adult-larval interactions in dense infaunal assemblages: patterns of abundance, &urnal of Marine Research 34, 25-40.