vertical migration and mortality of benthos in dredged material—part i: mollusca

Marine Environmental Research 4 (1980-81) 299-319

VERTICAL MIGRATION A N D MORTALITY OF BENTHOS IN DREDGED MATERIAL--PART I: MOLLUSCA

DoN MAURER, RICHARD T. KECK, JEFF C. TINSMAN & WAYNE A. LEATHEM

University of Delaware, College of Marine Studies, Lewes, Delaware 19958, USA

(Received: 25 April, 1980)

ABSTRACT

Benthic invertebrates have man), characteristics which make them prime candidates for burial studies in dredged material. A major concern in dredging and disposal projects is the effect of burial on the survival of benthic invertebrates. The purpose of the research reported in this paper was to determine the ability of estuarine benthos-- in particular three species of molluscs (Mercenaria mercenaria, Nucula proxima and Ilyanassa obsoleta)---to migrate vertically in natural and exotic sediments and to determine the survival of benthos when exposed to particular amounts of simulated dredged material.

Mortalities generally increased with increased sediment depth, with increased burial time and with overlying sediments whose particle size distribution differed from that of the species" native sediment. Temperature affected mortalities and vertical migration. It was concluded that vertical migration is a viable process which can significantly affect rehabilitation of a dredged disposal area. Under certain conditions, vertical migration shouM be considered, together with larval settling and immigration from outside impacted areas, as a mechanism of recruiting a dredge- dump site.

INTRODUCTION

In shallow water aquatic systems, one of the major forces influencing the environment is the dredging industry ( H a n n & Hutton, 1970). Approximately 306 million cubic metres are dredged annually in the United States to maintain navigation channels (Lee, 1976). Construction of power plants, deep water ports and mineral extraction facilities, together with the routine maintenance ofwaterways, is expected to increase the need for dredging (Rounsefell, 1972). Concern over the

299 Marine Environ. Res. 0141-1136/81/0004-0299/$02-50 ~ Applied Science Publishers Ltd, England, 1981 Printed in Great Britain

300 DON MAURER, RICHARD T. KECK, JEFF C. TINSMAN, WAYNE A. LEATHEM

effects of dredged material should not be restricted to shallow water systems because of proposed mineral extraction associated with deep ocean mining (DOMES, 1976). Management agencies have become increasingly concerned about the environmental impact of dredging disposal. As a result, the United States Army Corps of Engineers initiated a comprehensive dredged material research programme to ascertain environmental impact and develop new or improved disposal practices (Boyd et al., 1972).

Since direct burial by dredged material discharged in large quantities within the short interval of a dredging operation is the most obvious impact on benthic organisms (Morton, 1976), the present research was designed to provide experimental evidence concerning the effect of burial on benthic invertebrates. The null hypothesis was that there is no effect of dredged material on vertical migration or mortality of estuarine benthos. Specific objectives of this research included: (1) the determination of the ability of estuarine benthos (selected species of molluscs) to migrate vertically in natural sediments (i.e. similar in particle size distribution to natural habitat type) and exotic material (i.e. a sediment not normally inhabited) and (2) the determination of the survival of benthos when exposed to particular amounts of simulated dredged material.

MATERIALS AND METHODS

Test species Materials and methods are described in greater detail by Maurer et al. (1978). The

selection of test species was based on criteria of wide geographic range, density, frequency of occurrence, collection accessibility, facility of laboratory maintenance and relative mobility.

M. mercenaria is an infaunal, suspension feeding, venerid bivalve that ranges from the Gulf of St Lawrence to the Gulf of Mexico and has also been introduced to the West Coast of the United States and to Great Britain (Abbott, 1974). This species is normally found locally with its siphons in contact with the sediment-water interface. As a result, adults commonly burrow between 5 and 10cm deep with juveniles occurring closer to the surface. This bivalve is considered to be a moderately rapid burrower (43.7 cm/h) by Stanley (1970). In these experiments, M. mercenaria were obtained through laboratory rearing. Their anterior-posterior length was 1.5-2-0cm.

Nucula proxima is an infaunal, deposit feeding, nuculid bivalve that ranges from Nova Scotia to Texas (Abbott, 1974). Because there has been some confusion with a proposed sibling species (Nucula annulata), N. proxima is presently under detailed investigation (S. Howe, personal communication). Pending the results of Howe's study, we chose to follow Abbott's usage of N. proxima. N. proxima is locally

MIGRATION AND MORTALITY OF MOLLUSCA IN DREDGED MATERIAL 301

found 1-2 cm beneath the sediment-water interface. Levinton (1977) considered N. proxima to be an actively moving, near-surface feeder and recorded feeding depths of 7.9 + 3.9 mm in aquaria experiments. N. proxima (0.3-1.3 cm anterior-posterior length) were obtained from grab samples from soft bottom (80-100 ~o silt-clay) subtidal areas in Delaware Bay.

The gastropod, llyanassa obsoleta, is an obligate omnivore (Curtis & Hurd, 1979) which occurs on soft bottoms (sand fiats, mudfiats and salt marshes) and ranges from the Gulf of St Lawrence to northeast Florida (Abbott, 1974). It has been introduced to the West Coast of the United States and Canada. This species commonly feeds on edaphic diatoms on the surface of sediment or on decaying fish or crabs, particularly in tide pools. It can migrate vertically into the sediment, depending on the state of the tide in relation to hot or cold weather. In the summer, I. obsoleta can be found locally 1-2 cm deep, if it is not on the surface feeding or schooling with other individuals. I. obsoleta (2.5-3.5 cm, length from nuclear whorls to tip of canal) were collected by hand from the Cape Henlopen sand fiat.

After collection, N. proxima and M. mercenaria were placed in shallow glass pans of their native sediment and I. obsoleta were maintained in a large fibreglass tank. These molluscs were held for 7-14 days to acclimatise them to laboratory conditions. In aquaria burial experiments, mean temperature for the summer ranged from 17.4 to 21-4 °C and for the winter, temperatures ranged from 5 to 10 °C. The salinity range through the experiments was 20-260/00 .

Test sediments Several sediment types were collected for particular purposes in the burial

experiments and thus were treated in several ways. Coarse sand (0.58 mm median sediment size, 0.90 sorting, 0 ~ silt-clay) from the Cape Henlopen sand fiat was used extensively as substratum zero upon which test organisms were placed. This sand served as a marker bed to compare with other sediments used as dredge material. A fine (0.21mm median sediment size), clean sand (0~o silt-clay) was collected subtidally to serve as simulated sand-dredged material. This sand type occurs along coastal Delaware and is widely distributed throughout portions of lower Delaware Bay. As such, it would be frequently encountered in dredging local sand bottoms. Both the coarse and fine sand were sieved through a 1-0 mm mesh screen to remove macroscopic benthic invertebrates and were also air-dried to eliminate eggs, larvae and juveniles which might have passed through the screen.

Fine sediment containing 90-100 ~ silt-clay and 1-4~ carbon was collected subtidally from the University's harbour to serve as simulated mud-dredged material. This sediment is removed from the harbour, as periodic dredging is necessary for navigational maintenance. As such, this sediment type is characteristic of the soft-bottom habitats (creek bottoms, marina canals and semi-enclosed lagoons) which are commonly dredged throughout the area. The fine sediment was also sieved through the 1.0 mm mesh screen into a large concrete holding tank. Since

302 DON MAURER, RICHARD T. KECK, JEFF C. TINSMAN. WAYNE A. LEATHEM

air drying adversely affected the texture of this sediment, it was stored wet. This sediment was inundated periodically with salt water to retain its texture and periodically drained to preclude colonisation of benthic organisms. In addition to the physical characteristics of the simulated dredged materials, sediment types encompassed the range of sediment encountered by the test species.

Aquaria burial experiments involved two general sediment designs. One design, termed experiments (A), used several different depths of either sand or silt-clay. The second design (B) used four sediment types: 100 To sand, 20 ~ silt-clay/80 ~/o sand, 40 ~o silt-clay/60 ~o sand and 100 ~o silt-clay. Since there was considerable difficulty in blending the specified ratio of silt-clay and sand, the measured ratios were cited with the experiments. Nevertheless, the range between the specified and the measured ratios was extremely small. Because of logistical problems in obtaining significant volumes of particular sediment at a given time, experiments were not always symmetrical in testing the same type and volume of sediment.

Experimental protocol Pilot studies: Pilot studies were initially run in 5-cm diameter plexiglass tubes

established within 55-gallon aquaria (Maurer et al. 1978). Test animals were placed on top of 16 cm of substratum zero (native sediment) and allowed to acclimatise for 24 h prior to the addition ofvarious amounts of simulated dredged material added in a slurry form.

A second type of core experiment was performed to determine the terminal depth (up to 85 cm) for species which had shown the ability to exhume themselves from 32 cm of dredged material. Because the cores channelled the animals' movements to the vertical axis and the edge effect of the cores was deemed considerable, the core experiments were discontinued and yielded to a different approach. The core studies are described very briefly in the 'Results' section below.

Aquaria studies: Experiments with aquaria were conducted based on pilot studies with plastic cores. Faunal assemblages were allowed to establish themselves in 16 cm of substratum in the bottom of each of four 55-gallon aquaria. Faunal assemblages included species of molluscs, polychaetes and crustaceans, but we only report on the molluscs in this paper. After 24h, 16 or 32cm of sand or silt-clay or some combination of dredged material was slurried rapidly onto the substratum (within one hour). In experiments (A), measurements and samples were taken after one and eight days; in experiments (B), with M. mercenaria only, data were recorded after one, eight and fifteen days. Control experiments that contained test organisms and substratum zero and no simulated dredged material were performed concurrently with the test experiments. The aquaria studies form the bulk of this paper.

When sampling, dredged material was removed in 4-cm increments. After sieving, the numbers of living and dead animals in each layer were recorded. Animals recovered alive were placed in fresh native sediment and observed for 24 h to assess whether there were any further mortality or inability to assume a natural position in


the sediment. Sand contaminated by mud was discarded and uncontaminated sediments were redried and recycled.

Measurements and analysis Sediment samples were collected for determination of grain size (Folk, 1968). The

numbers of live and dead animals at substratum zero and the sediment layer of dredged material were counted. In turn, the percentage of the total organisms and the cumulative percentage at substratum zero and the sediment layer were calculated. The number of animals that migrated from substratum zero, the percentage mortality and the percentage of animals per layer were obtained from these calculations.

A goodness-of-fit for an observed frequency distribution to the expected frequency distribution was conducted on the vertical migration data. The null hypothesis is that there would be no vertical migration from substratum zero with sediment overburden. This appears to be a binomial distribution in which P is the probability of remaining at substratum zero and Q is the probability of vertical migration. This situation was analysed with a 7. 2 test (P < 0.05). In these experiments, data were grouped by numbers of animals per species observed in each layer of dredged material and compared against a hypothetical random distribution. A rule of thumb for the application of the chi-square test is that all hypothetical frequencies must be up to 20 ~o or a little less than five (Li, 1964). As a result, it was necessary to pool observations when the total number of animals observed did not total five for each layer of dredged material. The first and second layers were combined with the third and fourth, and so on, to achieve the necessary grouping. If the total number of animals were too small (i.e. core data), the Z 2 test was not used.

RESULTS

Core burial experiments Mercenaria mercenaria: In a series of fifteen 2-h and 24-h experiments, with water

temperatures of 22 to 25 °C in 1-2 cm, 3-4 cm, 7 cm and 14--16 cm of sand, there was an increase in the mean vertical distance that Mercenaria mercenaria migrated with increasing depth. The vertical migration distance, when covered with 1-2 cm of sand, was 0.8-1.6 cm, 3-4 cm of sand resulted in 1.5-2.0 cm of movement, 7 cm of sand gave 2.9 cm and animals covered with 14-16 cm of sand had a mean vertical migration distance of 3-6-4" 2 cm. The mean distance of migration was highest in the deepest sediment. Some bivalves achieved the surface of the topmost layer. These results indicated that M. mercenaria were able to migrate at least through 16 cm of sand under summer temperatures within a short period of time.


e~

t -

z

< z

Z

k)

0 ~ 0 ~ 0 0 0 ~ 0 0 0 ~ 0 0 0 0 0 ~ 0 ~

. o 9 0 0 0 ~ e.,~ 0 0 0 0 ~ 0 ~ 0

o o o ~ o ~ ~ o o o o o o o o o o . . . . . . . . .

. : . • • ~ . , ~ •

0 .o o

~ . ~ - ' ~ - o ~ .,~--. o. o o o o . . . . . . ~ o . .

9 o 9 o

d

0

N

Q

L.

E

z u


In another series of l-day and 18-day experiments (N = 4) at water temperatures of 19 to 22°C in three depths of sand (30, 50 and 85 cm), there was an increase in per cent mortality with time and depth. There was no mortality by day 1. By days 4, 11 and 18, mortality averaged over all sediment depths was 17, 63 and 78~, respectively, with no mortalities amongst controls. In terms of sediment depth, per cent mortality averaged over all time periods was 31, 40 and 55~o for 30, 50 and 85 cm, respectively. These results indicated that mortalities could be expected after 11 days amongst clams covered with more than 30 cm of sand. The terminal depth for M. mercenariawas at least 85 cm as some were able to migrate vertically through all depths by day 1, with a few bivalves reaching the top in 50 cm by day 4 and day 18. The specimen that reached the surface of 85 cm by day 18 perished shortly after.

Aquaria burial experiments Mercenaria mercenaria. Day 1 to day 8 burial experiments (A)

Winter temperatures: For winter temperatures in 12-32 cm of sand and 32 cm of 93-95 ~o silt-clay, the percentage of M. mercenaria that migrated from substratum zero (per cent migration) ranged from 36.4 to 44.1 ~o (-¢: = 39.5 ~o) by day 1 and from 55 to 82 ~o (.'~ = 72.7 ~) by day 8 (Table 1). Per cent migration was higher by day 8 than day 1, and it was higher by day 8 for 32 cm of sediment load than 16 cm. There was a significant migration from substratum zero for all categories by day 1 and only for sand in 28-32cm by day 8 (Table 2). The uppermost layer from which bivalves were recorded was only the top layer by day 8. The percentage ofclams that attained the top layer was highest in the shallow sand. The occurrence of bivalves was skewed to the bottom layers in sand and silt-clay by day 1. By day 8 there was a bimodal distribution with some clams in both the upper and lower layers (Table 1).

Control experiments showed 10~ mortality in silt-clay and sand for 40 and 40clams, respectively. In test experiments, mortalities ranged from 4 to 15~o (:~ = 9"7~o) by day 1 and from 15 to 45~ (,~ = 25.1) by day 8, Mortalities increased with time in the sand and were highest in 28-32 cm of sand. Mortalities tended to occur in lower sediment layers for all experiments.

Summer temperatures: For summer temperatures in 20-32 cm of sand and 24 to 32 cm of 51-57 ~o silt-clay, per cent migration ranged from 69 to 83 ~o (-( = 77.3 ~) by day 1 and from 88 to 96 ~o (.~ = 92-8 ~o) by day 8 (Table 3). Per cent migration was higher by day 8 than day 1 for all experiments. There was a significant migration from substratum zero for all experiments except in 20-24 cm of 51-57 ~o silt-clay (Table 2). The uppermost layer from which bivalves were recorded was the top layer for five of eight categories. The percentage of clams that attained the top layer was normally higher in sand than silt-clay. The occurrence of bivalves was distributed relatively evenly throughout layers of sand, but was skewed more towards the bottom layers in silt-clay by day 1. By day 8 the occurrence of clams in

306 DON MAURER, RICHARD T. KECK, JEFF C. T1NSMAN, WAYNE A. LEATHEM

TABLE 2 X1 VALUES RELATED TO VERTICAL MIGRATION OF TEST SPECIES IN BURIAL EXPERIMENTS

Sediment load (cm) Da b' 1 Day 8 Day 15

Mercenaria mercenaria (A) Winter temperatures Sand 12-16 53'9* 1 I. 1 Sand 28-32 272.9* 76.4* 93-95 % Silt-clay 28-32 59.5* 2.9 Mercenaria mercenaria (A) Summer temperatures Sand 16-20 48.2* 301-2* Sand 28-32 48.6* 441-8* 51-57 ~[Silt-clay 20-24 14.9 32.5" 54-55 % Silt-clay 28-32 50.5* 53-8* Mercenaria mercenaria (B) Summer temperatures 92-99 % Silt-clay 32-36 135.3" 37.6" 57-65 % Silt-clay 32-36 108-7" 87.6* 17-21% Silt-clay 32-36 166.4" 67-7* Sand 32-36 61.6* 87-9* Nucula proxima (A) Summer temperatures 53-54 % Silt-clay 4-8 26. I * 184.2* 51-56% Silt-clay 12-16 109.6" 130.5" 51-52 o/Silt-clay 28-32 83. I * 326.2" llyanassa obsoleta (A) Summer temperatures Sand 16-20 43.6* 26.5* Sand 28-32 79.8* 46-8* 48-57 Silt-clay 20-24 10.3" 31.2" 54-55 % Silt-clay 28-32 14.8" 5-6*

40-7* 75"8*

120'1" 76'3*

* Statistical value significant at 95 ~ confidence level.

sand was skewed towards the upper layers with a shift in the same direction for clams in silt-clay.

Control experiments showed no mortality in silt-clay or sand for 45 and 94clams, respectively. Mortalities in test experiments ranged from 3 to 13-3 ~o (.¢ = 7.6 %) by day 1 and from 1.1 to 45.6 % (,? = 26.6 %)by day 8 (Table 3). Mortalities generally increased with time and were normally higher in silt-clay than in sand. Between day 1 and day 8, increases in mortalities were greater in silt-clay than in sand. By day 1, mortalities generally occurred in lower layers; by day 8, mortalities were associated with higher sediment layers.

For both temperatures, per cent migration increased with time (Tables 1 and 3) However, per cent migration was generally lower under winter conditions than summer ones, which was particularly marked by day 1. Moreover, the percentage of clams that reached upper layers was higher in the summer than in the winter. This result was most apparent in the sand experiments.

For both temperatures there was an increase in mortality with time and depth


(Tables I and 3). Mortalities were commonly higher in silt-clay than in sand with the exception of 32cm of sand, day 8, winter temperatures (Table 1).

Day 1 to day 8 to day 15 burial experiments (B) Summer temperatures: For summer temperatures in 32-36cm of four different

sediment types, the per cent migration of M. mercenaria ranged from 55 to 80 (,~ = 69.0~) byday 1, from 71 to 9 4 ~ (,~ = 81.5%) byday 8, and from 68 to 86~o (:~ = 74-8 ~) by day 15 (Table 4). Per cent migration generally increased between day 1 and day 8 and decreased slightly between day 8 and day 15. There was a significant migration from substratum zero for all experiments (Table 2). Bivalves reached the top layer only by day 8 (17-21 ~o silt-clay) and day 15 (sand). The occurrence of clams was generally restricted to the lower half of layers for all sediment types by day 1, but this shifted to the upper halfofthe layers by days 8 and 15. Day 15 in 17-21 ~ silt-clay was an exception to this pattern. By day 15 the number of clams from sand in the uppermost layers was greater than the number from 92-99 ~ silt-clay.

Control experiments showed 4 ~ (N = 90), 3 ~o (N = 65), 3°,0 (N = 65) and 4~o (N = 90) mortality for 92-99~o silt-clay, 35-43~ silt-clay, 17-21 ~o silt-clay and sand, respectively. Mortality of M. mercenaria ranged from 0 to 1.6 ~o (-~ = 0.7 ~) by day 1, from 26.9 to 69.5 ~, (.? = 40.5 ~) by day 8, and from 47.3 to 91.7~ (:~ = 68.7 ~o) by day 15. Mortality increased with time for all sediment types. Increase in mortality was generally greater between day 1 and day 8 than between day 8 and day 15, with one exception. There were no apparent trends of mortality with sediment type. By day 15, the highest mortality occurred in 17-21 ~o silt-clay and the lowest in 92-99 9/0 silt-clay.

Nucula proxima Day 1 to day 8 burial experiments

S,tmmer temperatures: For summer temperatures in 4 to 32cm of 51-56 ~ silt- clay, the per cent migration of N. proxima ranged from 32 to 95 ,% (:~ = 66.0 ~) by day 1 and from 55 to 100 ~ (.? = 71.0 ~) by day 8 (Table 5). Per cent migration was higher by day 8 than day I except for bivalves in the 16 cm experiment. There was a significant migration from substratum zero for all depths of silt-clay (Table 2). N. proxima reached the top layer in the 8- and 16-cm experiments by day 1 and day 8 and never in the 32-cm experiments. The vertical distribution of bivalves varied with the sediment load. In 8 cm, the distribution was skewed to the top layer; in l 6 cm, it was bimodal and in 32cm, it was skewed to the bottom layers (Table 5).

Control experiments showed no mortality for two hundred individuals. In test experiments, mortalities ranged from 0 to 49-5 ~ (.~ = 18.2 ~) by day 1 and from 40.6 to 80 9/0 (-? = 57.7 ~o) by day 8 (Table 5). Mortality increased with time for all depths of silt-clay. There was a progressive increase in mortality with increased dept h by day 8. By day 8, mortalities were generally distributed throughout layers

308 DON MAURER, RICHARD T. KECK, JEFF C. TINSMAN. WAYNE A. LEATHEM

Z

e~

I -

z

. ~

..~ -.,..

~ ~.~

N

~ , ~ ~ ~ - - ~

.-- @ q~

MIGRATION AND MORTALITY OF MOLLUSCA iN DREDGED MATERIAL 3 0 9

~ . ~ . ~ ~

w

. m t . .

t . -

.=...

e~

- t

E t . .

- .5

E

II

tt~ N

e., ,7 <

._.."

<

{... <

M <

,7

310 DON M A U R E R , R I C H A R D T. K E C K , JEFF C. T I N S M A N , W A Y N E A. L E A T H E M

M I G R A T I O N A N D M O R T A L I T Y O F M O L L U S C A I N D R E D G E D M A T E R I A L 311

oo

z <

a: e~

I.-.

I.-

im Z ta

i;

° O U - i

02

,.Z.

N ..o

E 'r"

8.

Z

II


.< f -

h.

.<

Z

0 0 0 ~ 0 0 0 0 0 0 0 ~

0 ~ 0 0 0 0 0 ~


N

~ ~ ~ ~ , ~

~ ' q II ~ ' q LI

o

E

ea

N

6 E

E

II


of the 8- and 16-era experiments, but were skewed to the lower layers in the 32-cm experiment.

Ilyanassa obsoleta Day 1 to day 8 burial experiments

Summer temperatures: For summer temperatures in 16-32 cm of sand, 20-24 cm of 48-57~ silt-clay and 28-32cm of 54-55~o silt-clay, the per cent migration of I. obsoleta ranged from 79 to 100 ~ (g = 86.3 ~) by day 1 and from 79 to 92 (:~ = 85.0 ~) by day 8 (Table 6). Increases in per cent migration between day I and day 8 were slight with time compared with those recorded for the bivalves. There was a significant migration from substratum zero in all categories, except for 28-32 cm in 54-55 ~o silt-clay (Table 2). More snails were recorded in upper layers in day 1 than in day 8. A higher percentage of snails attained higher layers in 32 cm of silt-clay than in 32 cm of sand.

Control experiments for sand and silt-clay showed no mortality for 48 and 24 specimens, respectively. In test experiments, mortalities ranged from 4.1 to 20-8 (g = 11.3 ~) by day 1 and from 35-7 to 80.9 ~ (g = 54.3 ~) by day8 (Table 6). Mortality increased with time for all experiments. Mortalities were lower in silt-clay than sand, and changes in mortality between day 1 and day 8 were highest in sand. Mortalities were primarily recorded in lower layers for both sediment types by day 1, but were more widely distributed by day 8.

llyanassa obsoleta is a common intertidal gastropod found on fine and silty sand. Ilyanassa spends considerable time on the surface of sediments, but may burrow diurnally and burrows seasonally for longer periods of time with the onset of decreasing water temperatures. For these experiments, survival and vertical migration of I. obsoleta were higher in silt-clay than in sand. This appears to be consistent with its sediment association under natural conditions.

DISCUSSION

The majority of research on the burial of benthic organisms involves bivalve molluscs (Glude, 1954; Pfitzenmeyer & Drobeck, 1967; Dunnington, 1968; Schafer, 1972; Oliver & Slattery, 1973; Saila et al., 1972). The dynamics of the burrowing of bivalves has been studied, particularly by British workers, much of which has been summarised by Trueman (1975). The effects of turbidity and suspended material on bivalves were presented by Stern & Stickle (1978).

Species Glude (1954) conducted field burial experiments with another infaunal bivalve,

Mya arenaria. He showed that mortality varied inversely with depth of burial and directly with size. Mortality was highest in silt and lowest in silty sand and lower in winter than in summer. In laboratory experiments, burrowing rates of M. arenaria


were directly affected by temperature (Pfitzenmeyer & Drobeck, 1967). Savage (1976) reported that the optimal burrowing, rate of M. mercenaria (3-1-3.8 cm in shell length) was at a temperature range of 21-31 °C. Burrowing activity reduced gradually below 21 °C and sharply above 31 °C. Savage also showed that there was no seasonal accommodation in burrowing activity with respect to temperature for M. mercenaria, but that median burrowing times of the infaunai bivalve, Spisula solidissima, collected in the autumn were consistently longer than median burrowing times of those collected in the spring at similar temperatures. Reading & McGrorty (1978) reported that the infaunal bivalve, Macoma balthica, migrated within the substratum, apparently in response to day length. This tellinid was nearest to the surface in June and deepest in December.

Experiments cited in Saila et al. (1972) showed that the infaunal bivalve, Arctica islandica, reached the surface of 4-5 cm of dredged material (fine sand) and as much as 14 cm of sand. Clams buried under 8-17 cm of dredged material did not migrate upward, but established air holes to the surface. Oliver & Slattery (1973) observed similar behaviour with another large, infaunal bivalve, Tresus nuttallii, which established air holes through approximately 15 cm of dredged material. Maximum depth of M. balthica was proportional to length of inhalent siphon (Reading & McGrorty, 1978).

N. proxima was reported to have 90 % success in escaping 50cm of its native sediment (Kranz, 1972). In contrast, 40cm of fine sand was totally lethal to N. proxima. Kranz (1972) found that the escape capacity of two relatively small, infaunal bivalves, Yoldia limatula and Donax variabilis, was markedly reduced when buried under 41cm and 21cm of sand, respectively. He noted that the differential effect of the depth of burial was only readily apparent in sand. Kranz speculated that the greater pressures created by the weight of larger amounts of sediment might prevent the bivalves from opening their shells. Saila et al. (1972) stated that the small bivalve, Mulinia lateralis, reached the surface from 21 cm of dredged material, but many individuals required over 24h. Shulenberger (1970) stated that the small bivalve, Gemma gemma, could exhume itself from 23 cm of sand and 5.7 cm of silt. Survival for up to six days prior to exhumation was possible under a variety of conditions. Maurer (1967) found that Transennella tantilla, a small bivalve similar to G. gemma, was able to cope with highly turbid conditions and small amounts (2-3 cm) of clay overburden. In contrast, Gallucci & Kamaratani (1975) reported high mortalities when juveniles of T. tantilla were buried in the field. They projected that similar mortalities would be observed for clams such as Mercenaria mercenaria. Under 10cm of sand, less than 50% of the cockle, Clinocardium nuttallii, reached the surface in 24 h, while none did so under 20 cm of sand (Chang & Levings, 1978).

General Impacts of burial are the most obvious potential effects of dredged material


disposal on benthic organisms (Diaz & Boesch, 1977). There has been considerable concern expressed over the survivorship of benthic organisms subjected to dredging and burial in bays (May, 1973), sand and gravel extraction in coastal waters of the inner continental shelf(Padan, 1977) and mining in the open ocean (DOMES, 1976; Ryan & Heezen, 1976). However, there is a difference of opinion on what the major processes of recolonisation are. Oliver & Slattery (1973) and Oliver et al. (1977), in a series of laboratory and field experiments, stated that they believed that recolonisation is primarily caused by larval settling with little or no vertical migration. On the other hand, Richardson et al. (1977), in a comprehensive field study, stated that recoionisation of benthos into their study area was probably accomplished by organisms burrowing up through the dredged material, by migration into the area, and, to a lesser extent, by reproduction and recruitment from other areas.

In a dredging project conducted at the mouth of Delaware Bay, approximately 191,150m 3 of sand and mud was hydraulically removed from a ferry terminal channel and harbour (Maurer et al., 1974). When disposal began, dredge material was transported in the direction of a trough situated on the west end of the disposal site. Although there was some evidence that fine spoil material was transported to the sea or up the Bay, depending on the tide, and to the adjacent Cape Henlopen flat beaches (Klemas et al., 1974), the bulk of the dredge material was transported towards the trough. The thickest deposits reported in the trough ranged from 4-6 to 5.5m. The 0-9m contour of thickness of spoil occurred about 180m from the deepest overburden. Based on our work (core burial experiments), there is reason to believe that some M. mercenaria would be capable of vertically migrating through that amount of overburden, but not the intervening thicknesses. Moreover, there are no data to support the assumption that N. prox ima or 1. obsoleta could successfully migrate through the 0.9m thickness. In contrast, the adjacent Cape Henlopen beaches contained scattered patches of dredge spoil ( l -2cm thick) upon which large populations of I. obsoleta apparently remained unaffected. Finally, there was evidence of recruitment of benthic invertebrates at the disposal site several months after the cessation of dumping (Maurer et al., 1974). The senior author has had the opportunity to examine the disposal site for several years and has found that predisposal densities and biomass have been attained.

Based on our laboratory experiments, there is evidence to support the conclusion of Richardson et al. (1977) that vertical migration of benthos through dredged material can, under certain conditions, be a viable process affecting the rehabilitation of a dredged disposal area. However, there are limitations to invoking this mechanism of recolonisation and in our field studies (Maurer et al., 1974), we saw this as a limited possibility and tend to invoke reproduction and recruitment as the principal mechanism for the recolonisation of molluscs, particularly in the area of deepest sediment accumulation.


The primary measures assessed here were distance of vertical migration, the number of migrating animals and survival of benthic organisms when covered with native and exotic simulated dredged material. There is evidence to support the hypothesis that burial by dredged material strongly affects vertical migration and increases the mortality of estuarine molluscs. However, the effects on migration are not always complete and unidirectional, and the effects on mortality are not always total, as the degree of adversity varies with conditions. Assuming that laboratory experimental conditions are more restrictive and deleterious than field conditions, it is surprising and encouraging to see how hardy the test organisms were in a so-called worst case system. These results will hopefully provide data useful to management agencies in planning to minimise possible harmful effects of open water dredged material disposal. If the dominant benthos are known for an area, then data obtained from this research might provide guidelines on the season and the amount and type of material disposed to obtain an estimate of the percentage of dominant benthic organisms which might survive burial. These results would only be a part of the data base in the decision-making process, but they would provide some rational basis for estimating survivorship under various disposal conditions.

CONCLUSIONS

(1) Mortalities of selected molluscs increased with increased amounts of exotic sediment, depth and time.

(2) Mortalities were generally greater in summer temperatures than in winter temperatures.

(3) There was evidence of synergistic effects on burrowing activity and mortality. Differences in burrowing activity and mortality of selected molluscs were recorded with coincidental changes in time, sediment burden, sediment type and temperature.

ACKNOWLEDGEMENTS

This research was supported by the US Army Engineer Waterways Experiment Station (WES) (Contract No. DACW39-74-R-0019). We would like to thank Dr R. Engler, Mr R. Bingham, Ms S. Palmer, Dr R. Plumb and Dr R. Peddicord, all of WES, who monitored the research. Dr Peddicord was extremely co-operative in helping us to prepare the final version of our report. Ms M. Huntzinger and Mr C. Wethe performed the sediment analyses, and the latter helped with the statistical analyses.


REFERENCES

ABBOT'r, R. T. (1974). American seashells. The marine Mollusca of the Atlantic and Pacific Coasts of North America (2nd edn), Van Nostrand Reinhold Co., NY, 663 pp.

BOYD, M. G,, SAUCIER, R. T., KEELEY, J. W., MONTGOMERY, R. L., BROWS, R. D., MATHtS, D. B. & GraCE. C. J. (1972). Disposal of dredge spoil--Problem identification and assessment and research program development. US Army Engineer Waterways Experiment Station, Tech. Rept. H-72-8, 1-121.

CHANG. B. D. & LEVII~GS, C. D. (1978). Effects of burial on the heart cockle Clinocardium nuttallii (Conrad) and the Dungeness crab Cancer magister Dana. Est. Coast. Mar. Sci.. 7, 409-12.

CURTIS, L. A. & HURD, L', E. (1979). On the broad nutritional requirements of the mud snail, ilyanassa (Nassarius) obsoleta (Say) and its polytrophic role in the food web. J. exp. mar. biol. Ecol., 41, 289-97.

DIAZ, R. J. & BorscH, D. F. (1977). Impact of fluid mud dredged material on benthic communities of the tidal James River, Virginia. US Army Engineer Waterways Experiment Station, Tech. Rept. D-77- 45. 1-38.

DOMES (1976). Progress Report. Deep Ocean Mining Environmental Study--Phase 1. National Oceanic and Atmospheric Administration Technical Memorandum ERL MESA- 15.1-178. Marine Ecosystems Analysis Program Office, Boulder, Colorado.

DUNNINGTO~, E. A. (1968). Survival time of oysters after burial at various temperatures. Proc. Nat. Shell. Assoc., 58, 101-3.

FOLK, R. L. (1968). Petrology of sedimentary rocks. Hemphill's, Austin, Texas, 1-170. GALLUCCl. V. F. & KAMARATANI, R. K. (1975). Mortality of Transennella tantilla due to burial. J. Fish

Res. Bd Can., 32, 1637-40. GLUDE. J. B. (1954). Survival of soft-shell clams, Mya arenaria, buried at various depths. Bull. Dept. Sea

& Shore Fish. Res. Bull., 22, 1~. HANN, R. W. & HUTTON, W. S. (1970). Organic sludges in the Houston ship channel: Their source.

nature, effect and removal. Paper presented to World Dredging Conference, Singapore. KLE~,S, V., MAURER, D., LEATHEM, W., KINNER, P. & TREASURE, W. (1974). Dye and drogue studies of

spoil disposal and oil dispersion, J. War. Poll. Cont. Fed., 46(8), 2026-34. KRANZ, P. M. (1972). The anastrophic burial of bivalves and its paleoecological significance. Ph.D.

Dissertation, University of Chicago, 1-177. LEE, G. F. (1976). Dredged material research problems and progress. Environ. Sci. & Tech., 10, 334-8. LEVINXOS, J. S. (1977). Ecology of shallow water deposit communities Quisett Harbor. Massachusetts.

In: Ecology of marine benthos, (Coull, B. C. (Ed.)), The Belle W. Baruch Library in Marine Science. No. 6, Univ. South Carolina Press, Columbia, SC, 191-227.

LI, J. C, R. (1964). Statistical inference 1. Edwards Brothers, Inc,, Ann Arbor, Michigan. 658 pp, MAUREP,, D. (1967). Burial experiments on marine pelecypods from Tomales Bay, California. Veliger, 9,

376-81. MAUleR, D., BIGGS, R., LEATHEM, W., KINNER, P., TREASU~, W., OTLEY, M., WATLING, L. & KLEMAS,

V. (1974). Effect of spoil disposal on benthic communities near the mouth of Delaware Bay. Delaware Sea Grant Publication 4-74, 231 pp.

MAURER, D. L., KECK, R. T,, TINSMAN, J. C., LEATHEM, W. A., WETHE, C. A., HUNTZINGER, M., LORD, C. & CHURCH, T. M. (1978). Vertical migration of benthos in simulated dredged material overburdens. Vol. I: Marine benthos. US Army Engineer Waterways Experiment Station, Tech. Rept. D-78- 35,1-97.

M AY, E. B. (1973). Environmental effects of hydraulic dredging in estuaries. Alabama Mar. Res. Bull., 9, 1-85.

MORTON, J. W. (1976). Ecological impacts of dredging and dredge spoil disposal: A literature review. Master's Thesis, Cornell University, Ithaca, New York, 1-112.

OLIVER, J. S. & SLATTERY, P. N. (1973). Dredging, dredge spoil disposal and benthic invertebrates in Monterey Bay. Mar. Lab. Rept., Moss Landing, California, 1-130.

OLIVER, J. S., SLAI-rERY, P. N., HULBERG, L. W. & NYBAKKEN, J. W. (1977). Patterns of succession in benthic in faunal communities following dredging and dredged material disposal in Monterey Bay. U S Army Waterways Experiment Station, Tech. Rept. D-77-27, 1-186.

PAD^N, J. W. (Ed.) (1977). Net*' England offshore mining environmental study (Project NO MES). NOAA Special Rept., i - I 39.

PFITZENMEYER, H. T. & DROBECK, K. G. (1967). Some factors influencing reburrowing activity of soft- shell clam, Mya arenaria. Ches. Sci., g, 193-99.


READING, C. J. & MCGRORT¥, S. (1978). Seasonal variations in the burying depth of Macoma bahhica (L.) and its accessibility to wading birds. Est. Coast. Mar. Sci., 6, 135-44.

RICHARDSOn, M. D., C^REV, A. G. & COLGATE, W. A. (1977). Aquatic disposalfield inrestigations Columbia River disposal site, Oregon. US Army Waterways Experiment Station. Tech. Rept. D-77- 30, 1-411.

ROUnSE~ELL, G. A. (1972). Ecological effects of offshore construction. J. Mar. Sci., 2, 1-208. RVAN, W. B. F. & HEEZEN. B. C. (1976). Smothering of deep-sea benthic communities from natural

disasters. Technical Report from Lamont-Doherty Geological Observatory, Palisades. New York to the National Oceanic and Atmospheric Administration, Marine Ecosystems Analysis Program Office, Seattle, Washington, 1-132.

SAILA, S. B., PRATT, S. D. & POLGAR, T. T. (1972). Dredge spoil disposal in Rhode Island Sound. Mar. Tech. Rept. No. 2, Univ. of Rhode Island, 1-48.

SAVAGE, N. B. (1976). Burrowing activity in Mercenaria mercenaria (L.) and Spisula solidissima (Dillwyn) as a function of temperature and dissolved oxygen. Mar. Behar. Physiol., 3, 221-34.

SCHAFER, W. (1972). Ecology and paleoecology of marine enrironment. (Craig, G. Y. (Ed.)). Chicago University Press, 1-568.

S,ULENBERGER, E. (1970). Responses of Gemma gemma to a catastrophic burial. The Veliger, 13, 163-70. STANLEY, S. M. (1970). Relation of shell form to life habits in the bivalvia (Mollusca). Geol. Soc. Amer,

Mere., 125, 1-296. SVERN, E. M. & SI"ICKLE, W. B. (1978). Effects of turbidity and suspended materialin aquatic enrironments

--,4 literature review. US Army Engineer Waterways Experiment Station, Tech. Rept. D-78-21, 1-117.

TRUEMAN, E. R. (1975). The locomotion of soft-bodied animals. In ,4 series of student texts in contemporaryl biology (Barrington, E. J. W. & Willis, A. J. (Eds)), American Elsevier Publishing Company, Inc., New York, pp. 1-200.

vertical migration and mortality of benthos in dredged material—part i: mollusca

Documents