Benthic Invertebrates of the Nearshore Zone of Eastern Lake Huron, Georgian Bay, and North Channel

J. Great Lakes Res. 10(4):407-416 Internal. Assoc. Great Lakes Res., 1984

BENTHIC INVERTEBRATES OF THE NEARSHORE ZONE OF EASTERN LAKE HURON, GEORGIAN BAY, AND NORTH CHANNEL

David R. Barton

Department of Biology University of Waterloo Waterloo, Ontario N2L 3G1 and Marta Griffiths

Great Lakes Investigations and Surveillance Unit Water Resources Branch Ministry of the Environment 1 St. Clair A venue West Toronto, Ontario M4V 1K6

ABSTRACT. Samples of benthic invertebrates were collected by divers during 1980 from 86 stations in the nearshore zone (depths of 5 to 20 m) of Lake Huron, Georgian Bay, and North Channel. Six general categories of substratum were encountered: rock, gravel, hard clay, sand, silt/sand, and silt. Abundance of invertebrates varied with depth and substratum, ranging from 456 m-2 to 45,701 m-2 • Clay and gravel usually supported the largest populations, rock the smallest. A total of 218 taxa were recognized, including a first record of the naidid Ripistes parasitica in the western hemisphere. With only a few exceptions, individual taxa were distributed throughout the study area. The most abundant groups were Nematoda, Oligochaeta, Mollusca, and Chironomidae. Ephemeroptera and Trichoptera occurred at 83% of 5 and ID-m stations. Nine communities ofinvertebrates were recognized on the basis of ordination analysis. Differences in community composition appeared to reflect degree of exposure to wave action and local geology. Comparison of these results with those of earlier studies illustrated the much greater efficiency achieved through direct sampling by divers. Estimates of invertebrate standing stocks were up to an order of magnitude greater in the present survey, and the variety of organisms was much greater since all types of substrata could be sampled. There were no indications of increased eutrophication or unusual environmental stress within the study area. ADDITIONAL INDEX WORDS: Nematodes, Oligochaetes, mollusks, aquatic insects.

INTRODUCTION

strata, especially gravels and larger stones, which occurs in the nearshore zone (Flannagan 1970). For this reason, virtually all that has been published concerning the benthos of the nearshore zone has been based on samples collected from relatively fine-grained sediments (e.g., Veal and Osmond 1968, Johnson and Brinkhurst 1971, Mozley and Alley 1973, Evans and Stewart 1977, Nalepa and Robertson 1981) despite the abundance of coarser materials at depths less than 20 m (e.g., Rukavina and St. Jacques 1971, Graf 1976).

Despite the development of an extensive literature over the past few decades describing the benthic fauna of the Laurentian Great Lakes (e.g., Cook and Johnson 1974, Loveridge and Cook 1976, Evans and Stewart 1977, Flint and Merckel 1978, Golini 1979, Nalepa and Robertson 1981), the biota of the nearshore zone remains only partially described. None of the wide array of grabs and coring devices which have been used in large lake studies is effective over the entire range of sub-

407

BARTON and GRIFFITHS

408

In the very shallow nearshore, the wave-zone, coarse bed materials harbour a diverse and abundant fauna which consists mainly of immature insects (Krecker and Lancaster 1933, Barton and Hynes 1978). It is not known to what depth this rich insect fauna extends, but there are indications that the distributions of some species are limited by substratum rather than by depth (Selgeby 1974, Kraft and Sabol 1980). This study was undertaken as a step toward filling this major gap in our knowledge of the ecology of the Great Lakes. It consisted of a survey of the benthic fauna of Canadian nearshore areas (depths of 5 to 20 m) of Lake Huron and its largest subdivisions, Georgian Bay and North Channel. The problem of sampling from a wide range of substrata was solved by employing divers using an airlift or hand-driven corers.

METHODS The locations of sampling transects are shown in Figure 1. Transects H17, G13, G26, G27, and all North Channel stations were sampled between 24 September and 7 October 1980 by Laidlaw (1980). The remaining transects were sampled by Bioenvironmental Services, Ltd. (1981) between 19 September and 17 October 1980. Stations were selected at depths of 5, 10, and 20 m along each transect except those in shallow or narrow parts of bays (e.g., GI4A, GI4C, GI5A, GI5C, NA, NB). Henceforth, stations will be referred to by transect and depth, e.g., HI-1O indicates lO-m depth on transect HI. Divers collected three samples at each station. On silt or sand, plastic tubes (10-cm inside diameter) were pushed by hand 15 to 20 cm into the bottom, capped,

LAKE HURON-H o

60km

FIG. 1. Location of sampling transects in the nearshore zone of Lake Huron, Georgian Bay, and North Channel. Inset shows transects in Colpoy Bay (G14A-C) and Owen Sound (G15A-C).

409

NEARSHORE BENTHOS OF LAKE HURON

removed from the surrounding sediment, and capped on the lower end before being carried to the surface. Each core of sediment was washed through a sieve (200-",m apertures) and concentrated formalin was added to the residue to make a 10% solution. On rocky substrata samples were collected with an airlift similar to that described by Barton and Hynes (1978) which had been equipped with a 200-",m mesh collecting bag. The area sampled was defined by a section of lO-cm diameter coring tube held firmly against the bottom. These samples were also preserved immediately in 10% formalin. The divers noted the nature of the substratum at each station as proportions of rock, gravel, sand, hardclay, or silt. Invertebrates were sorted from the samples with the aid of a dissecting microscope. Microcrustaceans (Copepoda, Cladocera, Ostracoda) were not removed from the residues. All animals were identified to the lowest practiccal taxonomic level (a complete list is available on request). Oligochaetes were mounted on slides in polyvinylactophenol and allowed to clear before being examined. Identification of molluscs was hampered by extensive decalcification caused by the use of unbuffed preservative. All sample counts were log-transformed before calculating 95% confidence limits (Elliott 1977). Ordinations were performed on geometric means of the three replicates per station using an ordination package (ORDIFLEX) by Gauch (1977). An index of exposure to wave action, E = log (1 + fwd- 2) (Barton and Carter 1982), was calculated for each station. Fetch (f) was estimated as the mean of three measurements of the distance to the nearest land, each 45° apart and centered directly offshore. These measurements were made on Canadian Hydrographic Service charts 2200 and 2201. RESULTS

Six general types of substratum were encountered in the study area: rock (including cobbles, boulders, and exposed bedrock), gravel, hardclay, sand, silt/sand, and silt (Table 1). Hardclay was found only at G5-20, NI8-5, and NI8-1O. Gravel substrata were not found at stations in North Channel. Other substrata were encountered throughout the study area and at all depths. Mean total standing stocks of invertebrates ranged from 456 m-2 at Station N12-20 to 45,701 m-2 at G2-20. With the exception of uniformly low standing stocks at depths of 20 m in North Chan-

TABLE 1. Principal substrata at sampling stations in the nearshore of Lake Huron (H), Georgian Bay (G), and North Channel (N). R = rock, G = gravel, C = clay, S = sand, Si = silt. Depth (m) Station

5

10

20

HI H2 H3 H4 HI7 G2 G4 G5 G6 G7 GI4A

S R R R R R R R G S Si/S S/Si G S/Si S R R R R/Si R R R Si Si S R C

G G R R R G R SilR G S Si/S

S G G R S G S/Si C S/Si Si

GI4C GI5A GI5B GI5C G26 G27 Nll NI2 N13 NI4 NI5 NI6 NI7 NI8 NA NB

G S/Si R SilR R R/Si R R R Si Si Si/S R C R SiiS Si

Si/S R/Si R/Si R S S Si Si Si S Si S

nel, there were no general trends in the distribution of total abundance of invertebrates, either by region or depth (Fig. 2). These seemed to be due to variations in total numbers with type of substratum and depth (Table 2), and the heterogeneous distribution of nearshore substrata (Table 1). Total numbers of invertebrates were extremely variable but, in general, clay and gravel supported the largest standing stocks, and rock the smallest. Abundance was similar at all depths on gravel and decreased with increasing depth on silt. On rock and clay standing stocks were similar at 5 m and 10 m, but lower at 20 m. Significantly more animals were collected at 10 m than 5 m or 20 m on sand. On silt/sand total numbers were significantly lower at 10 m.

BARTON and GRIFFITHS

410

• • •

i

<1 1-5 6-10 11-20 21-40 >40

FIG. 2. Distribution oftotal standing stocks of benthic invertebrates at depths of5, 10, and 20 m in the nearshore of the major subdivisions of Lake Huron. Open circles indicate 20-m stations which were not sampled.

TABLE 2. Geometric mean total standing stocks of invertebrates (± 95% confidence limits, individuals m-2) on different substrata.

TABLE 3. Mean number of taxa per sample (± 95% confidence limits) on different substrata.

Depth (m)

Substrata Rock

Gravel Clay Sand Silt/Sand Silt

5 6,068 ± 1,446 15,772 ± 6,920 28,372 ± 10,159 4,918 ± 1,536 8,442 ± 2,152 13,677 ± 3,181

10

Depth (m)

20

7,868 ± 1,838 3,188 ± 910 15,919 ± 4,187 18,781 ± 5,699 21,541 ± 9,636 9,002 ± 4,359 11,018 ± 1,134 4,957 ± 2,075 11 ,964 ± 4,292 4,224 ± 943 4,240 ± 1,424 3,010 ± 1,052

Substrata Rock

Gravel Clay Sand Silt/Sand Silt

5

19.3 ± 2.7 22.2 ± 2.2 39.6 ± 5.4 11.4 ± 1.2 16.6 ± 2.0 24.8 ± 2.8

10

20

20.4 ± 1.8 25.4 ± 2.2 35.6 ± 6.0 13.0 ± 1.2 11.0 ± 1.8 14.6 ± 4.1

12.3 ± 1.4 20.0 ± 2.6 10.0 ± 1.4 10.5 ± 2.1 16.2 ± 3.0 9.8 ± 1.8

NEARSHORE BENTHOS OF LAKE HURON

The mean numbers of taxa per sample showed similar trends with depth on the different substrata (Table 3). The numbers of taxa were similar at all depths on gravel and sand, decreased with increasing depth on silt, were significantly lower at 20 m on rock and clay, and were lower at 10 m than at 5 or 20 m on silt/sand. Hardclay supported more taxa at depths of 5 m and 10m than any other substratum, although it should be emphasized that clay was sampled only at one station at each depth. The greatest variety of animals at 20 m was found on silt/sand and gravel. Qualitatively, the fauna of the nearshore zone included 218 recognized taxa and was fairly homogeneous throughout Lake Huron, Georgian Bay, and North Channel. With only a few exceptions, animals which were found at five or more stations were also found in each region, Le., common species were common throughout the entire study area. Exceptions included Manayunkia speciosa and Crangonyx gracilis which were not found at any Huron station, Gammarus pseudolimnaeus and Asellus intermedius which were not collected in North Channel, and Hexagenia spp. which were found only in North Channel. Most larval Chironomidae were early instars which could not be assigned to species, thus specific distributions within this important group remain in doubt. The species collected during this survey include a number of new records of Oligochaeta for Lake Huron. These include Peloscolex curvisetosus, Tubifex ignotus, Piquetiella michiganensis, Pristina longiseta, and Ripistes parasitica. The specimens of R. parasitica found on transect NA are apparently the first reported from the western hemisphere (J. K. Hiltunen, pers. comm. Great Lakes Fishery Laboratory, 1451 Green Road, Ann Arbor, MI 48105); all of the others have frequently been collected in other Laurentian Great Lakes (e.g., Spencer 1980). The most abundant groups in the nearshore fauna were Nematoda, Oligochaeta, Mollusca, and Chironomidae. Amphipods, Pontoporeia hoyi and Hyalella azteca, were found throughout the study area but were abundant at only a few stations in Lake Huron and North Channel. Crayfish (Decapoda) were rarely collected but were observed at all stations which had rocky substrata. Insects other than chironomids, mainly Ephemeroptera and Trichoptera, were an important component of nearshore communities, occurring at 82% of all stations at depths of 5 m, 83% of those at 10 m, and 32% of 20-m stations. The mean abun-

411

dance of mayflies and caddisflies declined with increasing depth: 698 animals m-2 at 5 m, 329 m-2 at 10 m, and 36 m- 2 at 20 m. The depth and substrate preferences of the most common taxa (those found at 15 or more stations) were examined by comparing frequencies of occurrence at the three depths and mean abundances on the various substrata. Of the forms most frequently collected at 20 m, Stylodrilus herringianus, Vejdovskyella intermedia, Procladius spp., and Monodiamesa spp. were equally abundant on all substrata, P. hoyi was most abundant on gravel, sand, or silt, and Peloscolex jerox was least abundant on gravel. Stylaria lacustris and Heterotrissocladius spp. were found most frequently at 10 m and 20 m, with the former being most abundant on rock and the latter showing no clear substrate preference. Substrate preferences of the taxa most frequently collected at 5 and 10 m were: Cryptochironomus, Cryptotendipes, and Tanytarsusgravel and silty sand; Potamothri vejdovskyi, Ephemera simulans, Microtendipes spp., Polypedilum scalaenum, and Cladotanytarsus spp.gravel; H. azteca and Pseudosmittia group - rock; Piquetiella michiganensis, Ablabesmyia, and Pseudochironomus spp. - rock and gravel. Caenis and Thienemannimyia-group were most common and abundant on rock and gravel at 5 m. Other taxa which were most frequently collected at 5 m either showed no clear substrate preference (e.g., Parakiejjeriella sp.), or tended to be distinctly less abundant on certain substrata (e.g., Dicrotendipes- gravel and sand, Polypedilum simulans-silt, Marstonia decepta-sand). Potamothrix moldaviensis, Slavina appendiculata, Gyraulus parvus, Pisidium spp., and Nanocladius spp. seemed to be indifferent to both depth and substratum. Of these, only Pisidium spp. and Nanocladius spp. occurred at high densities. Since both included several species which could not be separated reliably, broad distributions are not surprising. Ordination was used to group stations with similar benthic communities. Data matrices used in the computations consisted of 10glO transformed counts of the 50 most common taxa (species or genera), larger taxa (e.g., Tubificidae, Gastropoda, Chironomini, etc.) or combinations of larger and individual taxa. Both Principal Components Analysis (PCA) and Reciprocal Averaging (RA) techniques were applied; PCA consistently gave superior results in terms of the amount of

412

BARTON and GRIFFITHS

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12

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1 11

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PC AXIS 1 FIG. 3. Ordination of nearshore stations resulting from Principal Components Analysis of log-transformed station means of major taxa and groups of invertebrates. The five sizes of symbols reflect position of PC Axis 3 in intervals of 20 units. Circles, squares, and triangles indicate depths of 5, 10, and 20 m, respectively. Solid, open, and stippled symbols indicate Huron, Georgian Bay, and North Channel stations, and numbers indicate the sampling transect. Communities detailed in Table 4 are indicated by bold numbers.

variance explained. The strongest ordination obtained, using PCA and log-transformed counts of selected taxa and groups of related organisms, indicated the existence of nine more or less distinctive, groups of stations (Fig. 3). each group had a unique assemblage of invertebrates (Table 4) reflecting the interactions among depth, exposure to wave action, and local geology. Stations in groups 1 and 2 all had boulder or bedrock substrata but those in group 1 were more exposed and supported lower standing stocks of fewer taxa in most of the major groups of invertebrates. Highly exposed stations with more heterogeneous substrata (mixed gravel, cobbles, and

bedrock) (group 3) supported more animals of more kinds. Where bedrock in shallow water was protected from wave-action (group 4), silts covered the rock surface and were inhabited by a rich community dominated by Nematoda, Oligochaeta (Naididae), and Chironomidae (except Orthocladiinae). Invertebrate communities on exposed many substrata (group 5) were less diverse, dominated by Nematoda, Chironomini, and a variety of Oligochaeta. Stations in group 6 were physically similar to those in group 2 but were somewhat less exposed and supported larger total standing stocks of invertebrates despite the lower abundances of Nema-

NEARSHORE BENTHOS OF LAKE HURON

413

TABLE 4. Characteristics of nearshore invertebrate communities ifrom Fig. 3): individuals m-1 (95% confidence limits), depth, substratum (R = bedrock, B = boulders, C = cobbles, G = gravel, Cl = hardclay, S = sand, Si = silt), and Index of Exposure. GROUP 2

3

Nematoda 50±46 Oligochaeta 706 ± 296 Polychaeta 0 Gastropoda 2oo± 140 Bivalvia 13±13 P. hoyi 0 Other Crustacea 403 ± 141 Tanypodinae 842±524 Chironomini 1,885±240 Tanytarsini 454± 161 Orthocladiinae 1,614± 598 Diamesinae 52±46 Other Insecta 254± 128

1,479±611 3,966± 1,052 34±50 712 ± 328 178 ± 82 56±52

3,582 ± 852 5,078±3,512 1,520± 1,226 4,139± 1,864 920±392 38±55

4

Total Fauna 2,779±427 Depth (m) 5-10 Substratum R Exposure 0.106

8,220± 1,321 21,874± 3,564 19,468 ± 4,918 9,054 ± 1,647 15,436± 7,572 11,033 ± 3,494 13,102± 3,528 2,898 ± 606 5-10 5-10 10-20 10-20 5-10 5-10 5,20 10 Si/R,G,Cl B,C,G Si/S Si/R S,Si/S R Si/R R 0.077 0.092 0.016 0.089 0.054 0.023 0.031 0.004

5

374±248 716 ± 360 96±87 45±40 1,686 ± 1,020 3,474± 1,284 5OO±214 1,156± 518 5,019± 1,531 7,368 ±2,206 13,335 ± 1,461 4,170± 1,044 2,679± 1,649 11,188±4,201 12,198±4,866 2,172±772 722±320 20± 18 1,208 ±584

6

8,037± 2,773 9,873 ± 2,268 168 ± 150 9,322 ± 4,876 3,458 ± 943 11,524 ± 8,782 0 0 1O± 14 1,248 ± 742 162 ± 98 4,471 ± 2,877 1,187 ± 701 215±113 33 ±61 0 1O± 14 0

14,693 ±2,064 1,428 ± 515 280± 148 350± 108 1,947 ± 907 1,594±912

toda, Bivalvia, and Chironomini. Stations in groups 7 and 8 were mostly located at depths of 10 to 20 m, had similar, moderately low indices of exposure, and had silted substrata. Thus, these could be considered to represent a single group, but they did differ in the abundance of snails, crustaceans, and tanytarsine chironomids. The most sheltered stations (group 9) were located at depths of 10 to 20 m in North Channel, Colpoy Bay, and Owen Sound. These were characterized by few taxa per sample, low total standing stocks, and the dominance of nematodes. P. hoy; and M. speciosa were found at most of these stations. DISCUSSION Previous assessments of the trophic status of Lake Huron, Georgian Bay, and North Channel have been that the former is in the early stages of mesotrophy (Shrivastava 1974), Georgian Bay is oligotrophic (Patalas 1972) and North Channel may be somewhat enriched by organic detritus entering via the St. Marys River (Loveridge and Cook 1976). Gregor and Rast (1982) described the entire nearshore zone covered in the present study as oligotrophic on the basis of Secchi depth and concen-

632 ± 274 123 ±47 158 ± 92

7

8

9

9,219±5,855 18,081±6,713 2,514±992 7,540 ± 2,382 8,300 ± 3,586 838±448 864±824 382 ± 298 334± 176 1,404± 338 432 ± 244 166±82 1,178 ± 928 1,128 ± 522 380± 166 271 ± 172 642 ± 472 238± 139

7,720 ± 7,566 980±534 2,84O± 1,790 376±266 1,810 ± 1,266 1,260±408 4,726 ± 4,394 2,714±1,418

98±80 419±21O 750±386 830 ± 440

33±24 142 ± 62 194 ± 108 230±91

3,320± 1,768 2,416± 1,802 90±242 150±92 1,420 ± 1,330 114±96

541 ±2oo 184 ± 86 38±28

164±88 50±30 94±50

trations of total phosphorous and chlorophyll a. Shrivastava's (1974) conclusions were based on apparent decreases in the abundance of the amphipod, P. hoyi, and increases in the abundance of oligochaetes between the mid-1950s and 1971. As pointed out by Cook and Johnson (1974), these changes may actually have resulted from differences in the season and pattern of sample collection. Comparisons of the results of independent studies of lake benthos are complicated by many factors, including the locations of the sampling stations, the techniques used for collecting and processing the samples, and the season at which samples were collected. Table 5 summarizes the results from all nearshore (0 to 25-m depths) stations included in previous surveys of Lake Huron and the present results from Huron stations with sand substrata. Each of the earlier studies was based on grab samples (sand or silt substrata) and coarse sieves (500 to 600 /Am). With the exception of Shrivastava's (1974) survey, at least part of each study was based on samples collected in August or September. The only group collected in similar numbers in all studies was Mollusca, perhaps because these are relatively large and immobile so are more consistently collected and retained by all

BARTON and GRIFFITHS

414

TABLE 5. Mean standing stocks (and percentage composition) of benthic invertebrates reported from sandy substrata in the nearshore zone of Lake Huron. ND = no data.

TABLE 6. Mean total standing stocks (and percentage composition) of benthic invertebrates reported from sand and silt at depths of 10 to 21 m in Georgian Bay (GB) North Channel (NC). Source

Source Teter (1960) Depth (m)

5-25

0-20

Nematoda

4 (0.6) 41 (6.6)

39 (3.1) 819 (64.4) 20 (1.6) 89 (7.0) 50 (3.9) 230 (18.1) 14 ND 1,273

Oligochaeta Gastropoda

ND Sphaeriidae 65 (10.4) Amphipoda 334 (53.4) Chironomidae 100 (16.0) Other insects ND (1.1) 625 Total

Loveridge & Cook 1976

Schytema and Powers Shrivastava This study (1966) (1974) 9-25

5-20

ND 270 (38.5) 90 (12.8) 200 (28.6) 87 (12.4) 55 (7.8)

8,246 (54.4) 3,847 (25.4) 56 (0.4) 240 (1.6) 933 (6.2) 1,556 (10.3) 99

(0.6) 700

This Study

GB

NC

GB

NC

Nematoda

1,489 (50.1)

1,483 (29.0)

6,370 (53.9)

1,159 (31.0)

Oligochaeta

811 (27.3)

1,033 (20.2)

1,814 (15.1)

1,296 (34.7)

Sphaeriidae

59 (2.0)

895 (17.5)

212 (1.8)

186 (5.0)

P. hoy;

39 (1.3)

997 (19.5)

0 (0)

300 (8.0)

Chironomidae

253 (8.5)

363 (7.1)

2,313 (19.2)

623 (16.7)

Other insects

15 (0.5)

26 (0.5)

42 (0.3)

170 (4.5)

2,972

5,113

12,014

3,784

Total

15,148

of the sampling techniques used. The different proportions of other groups (especially nematodes) and the larger standing stocks seem to reflect the more efficient techniques used in the present study. Loveridge and Cook (1976) used tOO-pm sieves to concentrate their Ponar samples from Georgian Bay and North Channel. The large proportions of nematodes in their samples suggests that the loss of small organisms during the sieving process was greatly reduced. Comparison of the results from similar stations in their study and ours (Table 6) suggests that the Ponar may be more efficient at collecting Pontoporeia and Sphaeriidae while underestimating the abundance of chironomids and other insects. Since the station locations were not identical in the two studies, these differences may not be significant. The four-fold larger estimates of total standing stock in Georgian Bay from the present study are in line with the comparisons for Lake Huron. The situation in North Channel is clearly different: the mean estimates of total abundance of invertebrates were similar in both studies, as were the ranges (246 to 18,976 m-2 , Loveridge and Cook; 456 to 15,365 m-2, this study). The reasons for this remain a mystery. In 1978, Barton and Carter (1979) sampled the

benthic fauna of Owen Sound near Transects 15AC. They found a zone of heavily enriched sediments with very large standing stocks, consisting mainly of Tubificidae, in Owen Sound south of Transect GI5A. This zone of enrichment does not appear to have spread further into the bay. In fact, none of the stations sampled during 1980 showed evidence of significant organic enrichment or environmental degradation. Pollution tolerant organisms never dominated the fauna in any part of the study area. Evans and Stewart (1977) felt that frequency of disturbance of the sediments by waves had a major impact on the abundance of benthic animals at depths of 6 and 9 m in Lake Michigan. Barton and Carter (1982) recently demonstrated that exposure to wave action is one of the most important environmental factors influencing the composition of epilithic invertebrate communities in eastern Georgian Bay. Both of these studies were based on sampling from relatively confined areas within short periods of time. Barton and Carter (1982) further found that there were substantial seasonal changes in exposure at any given site, and that these changes were reflected by the epilithic invertebrate community. The present survey took about 1 month to complete and the wind records used in calculating exposure can only be considered an

NEARSHORE BENTHOS OF LAKE HURON approximation of the actual conditions throughout the study area. Despite these limitations, the community analysis of our results further illustrates the importance of exposure, and the interaction between exposure and local geology. Other factors which complicated the definition of distinctive communities in the study area were the very great diversity (i.e., species richness) of the benthic fauna, the general lack of dominant species, and the great number of species exhibiting broad tolerances for depth and substrate. The sample size used in this survey (three replicates of 78.5 cm2 each) may have been too small to estimate accurately differences in the abundance of subdominant species which have more restricted substrate preferences. Choice of an appropriate sample size is always a problem in designing benthic surveys (Elliott 1977, Green 1979); the final decision is usually a compromise between statistical adequacy and the resources available for processing the samples. The sample size used in the present survey appeared to be adequate to estimate the standing stock of the dominant species on most substrate, but more effort appears to be needed to assess less abundant species on heterogeneous substrata such as gravel and rock. This problem is by no means unique to this study; the variances of species counts at individual stations were similar to those reported from other benthic surveys (Downing 1979). The most appropriate number and size of samples from various substrata in the Great Lakes will probably have to be determined empirically. Overall, the results of this survey indicate that the nearshore zone of Lake Huron, Georgian Bay, and North Channel is a complex environment which supports a very diverse benthic fauna. The depth zone of 5 to 20 m is the area of transition from the shallow wave-zone characterized by a basically rheophilic community (Barton and Hynes 1978) to the deep, offshore zone dominated by P. hoyi, oligochaetes, and sphaeriids (Teter 1960, Loveridge and Cook 1976). This is not a sharp break; the depth ranges of various characteristic animals, such as Pseudochironomus spp., Pseudosmittia sp. and P. hoyi, seem to vary with local conditions. The broad substrate tolerances observed for most taxa would be highly adaptive in such a dynamic environment. Finally, special mention should be made of the occurrence of Repistes parasitica on Transect A in North Channel. This species was included in a recent key to the Naididae of North America by

415

Hiltunen and Klemm (1980) since it was suspected to be present, but could not be confirmed (J. K. Hiltunen, Great Lakes Fishery Laboratory, 1451 Green Road, Ann Arbor, MI 48105, pers. comm.). It is widely distributed in Europe and the USSR where it typically occurs in association with rooted vegetation (Popchenko 1980, Juget 1980). The specimens from North Channel were found at a depth of 10 m on a bottom of rock with pockets of sand and gravel up to 10 cm thick. No macrophytic vegetation was apparent. A total of 13 specimens was found which gives an estimated abundance of 552 m-2 • We have also seen this species in samples from similar habitat in Thunder Bay, Lake Superior. Both sites are close to major ports, which suggests that R. parasitica may have reached the Great Lakes via ships engaged in international trade. REFERENCES Barton, D. R., and Carter, J. C. H. 1979. Benthic macroinvertebrates from southern Georgian Bay. Waterloo Res. Ins1., Report No. 0ISU.KF 714-80045.

and . 1982. Shallow-water epilithic invertebrate communities of eastern Georgian Bay, Ontario, in relation to exposure to wave action. Can. J. Zool. 60:984-993. Bio-Environmental Services, Ltd. 1981. Lake Huron and Georgian Bay nearshore sediment and benthic survey-1980. ms. report to Water Resources Branch, Ministry of the Environment, Toronto, Ontario. Cook, D. G., and Johnson, M. G. 1974. Benthic macroinvertebrates of the S1. Lawrence Great Lakes. J. Fish. Res. Board Can. 31:763-782. Downing, J. A. 1979. Aggregation, transformation and the design of benthos sampling programs. J. Fish. Res. Board Can. 36:1454-1463. Elliott, J. M. 1977. Some methods for the statistical analysis of samples of benthic invertebrates. Freshwater BioI. Assn., Sci. PubI. 25. Evans, M. S., and Stewart, J. A. 1977. Epibenthic and benthic microcrustaceans (copepods, cladocerans, ostracods) from a nearshore area in southeastern Lake Michigan. Limnol. Oceanogr. 22: 1059-1066. Flannagan, J. F. 1970. Efficiencies of various grabs and corers in sampling freshwater benthos. J. Fish. Res. Board Can. 27:1691-1700. Flint, R. W., and Merckel, C. N. 1978. Distribution of benthic macroinvertebrate communities in Lake Erie's Eastern Basin. Verh. Internat. Verein. Limnol. 20:240-251.

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