Marine £nrironmental Research 6 (1982) 49-68

VERTICAL MIGRATION A N D MORTALITY OF BENTHOS IN DREDGED MATERIAL: PART III--POLYCHAETA*

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

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

(Received: 18 November, 1980)

ABSTRACT

The survival of benthic mvertebrates J?om dredging and disposal activities is a major environmental concern in such projects. The purpose of the research described in this paper was to determine the ability of benthos (polychaetes, Scoloplos fragilis and Nereis succinea) 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, exotic sediment and burial time. Temperature also aJJected vertical migration and mortalities. These experiments, together with other experiments conducted by us and other workers, indicate that polychaetes in particular, and benthos in general, can survive dredging and disposal projects. Under certain conditions several major taxa (polychaetes, molluscs, crustaceans) can be expected to successfully recolonise disposal sites by vertical migration.

INTRODUCTION

Shipping of raw materials and finished products via coastal and inland waterways will continue to be an important activity in aquatic systems within the United States. To successfully pursue and promote this activity, it will be necessary to maintain these waterways with a regular and fairly intensive dredging and spoil disposal programme. Concern over the environmental effects of dredging and spoil disposal is reflected in the high priority ranking that this activity recently received in a national (N OAA) planning document for Ocean Pollution Research, Development

* Part I of this paper appeared in Vol. 4, No. 4, 1981, pp. 299-319 and Part lI in Vol. 5, No. 4, 1981, pp. 301-17. t Present address: Southern California Ocean Studies Consortium, California State University, Long Beach, California 90840, USA.

49 Marine Era'iron. Res. 0141 - 1136/82/0006-0ff49/$02.75 ~t ~ Applied Science Publishers Ltd, England, 1982 Printed in Great Britain

50 DON MAURER e l al.

and Monitoring (ICOPRDM/FCCSET, 1979). Direct burial is one of the most important impacts of dredge spoils on benthos (Morton, 1976). This impact is not necessarily restricted to shallow water systems as deep ocean mining is imminent. An analogy was made between the impact on benthic organisms from severe natural events (Ryan & Heezen, 1976) and the impact from the disturbances caused by deep ocean mining (DOMES, 1976). Consequently, the disposition of buried organisms is an important consideration in predicting survivorship and recolonisation of a dredge and spoil site.

In Parts I and II of this paper (Maurer et al., 198 la, b) we reported the results of a series of laboratory burial experiments undertaken on molluscs (Mercenaria mercenaria, Nucula proxima and llyanassa obsoleta) and crustaceans (Neopanope sayi and Parahaustorius Iongimerus) to determine (i) their ability to migrate vertically in natural and exotic sediments and (ii) their survival when exposed to particular amounts of simulated dredged material. In general, mortalities for molluscs and crustaceans increased with increased amounts of sediment, increased amounts of exotic sediment and increased time. Moreover, there was evidence of synergistic effects on burrowing activity and mortality throughout these experiments. It was concluded that, under certain conditions, vertical migration can be a viable mechanism for recolonising a dredge-dump site. The purpose of this paper is to expand the data base by examining another important component of soft bottom faunas--the polychaetes.

MATERIALS AND METHODS

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

1981a, b). 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.

Scoloplosfragilis is a burrowing polychaete of the family Orbiniidae which ranges from the Gulf of St. Lawrence, Newfoundland, to the Gulf of Mexico and Florida (Pettibone, 1963). S. fragilis occurs intertidally and subtidally throughout the Delaware Bay area in waters of 10"2-33%o and 5.1-11.0 mg/litre of dissolved oxygen. This species locally occurs in clean sand where it reaches its largest size and in organic mud where it is usually smaller. It is found in the upper 15 cm of sediment when sampled at low tide (Brown, 1979). Scoloplosfragilis was collected from the Cape Henlopen, Delaware, sand fiat. This polychaete ranged in size from 2-5 to 5.0 cm (anterior-posterior length).

Nereis succinea is an active polychaete of the family Nereidae which ranges from the Gulf of St. Lawrence to Central America (Pettibone, 1963). This species ranges widely throughout the Delaware Bay region and occurs in a broad range of salinity and sediment types (Maurer & Watling, 1973). N. succinea burrows into sands,

MIGRATION AND MORTALITY OF POLYCHAETA IN DREDGED MATERIAL 51

muds (80-90 cm), clay and peat in the local tidal marshes. N. succinea was collected by hand from muddy intertidal river banks of the Broadkill River, adjacent to the laboratory. This species ranged in size from 5.0 to 12.0crn (anterior-posterior length).

After collection, Scoloplos fragilis were replaced in shallow glass pans of their native sediment and Nereis succinea were maintained in a large fibreglass tank. These species were held for 7-14 days to acclimatise them to laboratory conditions. In aquaria burial experiments, the mean temperature for summer ranged from 17.4 to 21.4°C and for winter, temperatures ranged from 5 to 10°C. The salinity range through the experiments was 20-26%°.

Test sediments Coarse sand!(0.58 mm median sediment size, 0.90 sorting, 0 ~ silt-clay) 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 (0.21 mm median sediment size), clean sand (0~o silt-clay), which would be frequently encountered in dredging local sand bottoms, served as simulated sand- dredged material. Both the coarse and the fine sand were seived 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 (90-100~ silt-clay and 1-4~ o carbon), characteristic of soft- bottom habitats (creek bottoms, marina canals and semi-enclosed lagoons) served as simulated mud-dredged material. The fine sediment was also sieved through the 1-0 mm mesh screen into a large concrete holding tank. Since 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. Sediment types used in the experiments also encompassed the range of sediment encountered by test species in the field.

Aquaria burial experiments involved two general sediment designs. One design, termed experiments (A), used different depths of sand or silt-clay. The second design (B) used four sediment types; 100~o sand, 20~o silt-clay/80~ sand, 4 0 ~ silt- clay/60 ~o sand and 100 ~ silt-clay. Since there was difficulty in blending the specified ratio of silt-clay and sand, the measured ratios were cited with the experiments. Nevertheless, the range between the proposed ratios 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 per time.

Experimental protocol Pilot studies." Pilot studies were initially run in 5 and 10cm diameter plexiglass

tubes established within 55-gal aquaria (Maurer et al., 1978). Test animals were placed on top of substratum zero (native sediment) to determine the terminal depth

52 DON MAURER et al.

(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.

Aquaria studies: Based on pilot studies, experiments with aquaria were conducted. Faunal assemblages were allowed to establish themselves in 16cm of substratum in the bottom of each of four 55-gal aquaria. Faunal assemblages included species of molluscs, polychaetes and crustaceans, but we are only reporting on polychaetes in this paper. After 24 hours' acclimatisation in the experimental aquaria, some combination of dredged material was slurried rapidly (within 1 h) onto the substratum. In the (A) experiments, measurements and samples were taken after 1 and 8 days; in the (B) experiments, data were recorded after 1,8 and 15 days. Control experiments that contained test organisms and substratum zero and no simulated dredged material were performed concurrently with the test experiments. The bulk of this paper is concerned with the aquaria studies.

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 was 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 at 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 situation was analysed with a X ~ 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. If the total number of animals were too small (i.e. core data), the X 2 test was not used.

Results." core experiments ScoloplosJragilis: In a series cff burial experiments conducted in 5 cm diameter

cores at summer temperatures for 1 week, there was significant percentage migration

MIGRATION AND MORTALITY OF POLYCHAETA IN DREDGED MATERIAL 53

of Scoloplosjragilis in 32 cm of sand. Seventy-one per cent of thirty-one specimens reached the upper two layers by 1 week. Mortality was 22-6 % for the experiment.

In another series of experiments conducted in 10 cm diameter cores at summer temperatures for I week in 30, 50 and 90 cm sand percentage migration of S.fragilis was significant in all depths. Seventy-one per cent, 75 % and 16 ~o of the specimens attained the top layer in 30, 50 and 90 cm of sand, respectively. Mortality was 0 ~o in 30cm of sand, 44.3 % in 50cm and 55.6 in 90cm.

Nereis succinea: In similar experiments with Nereis succinea percentage migration was significant in all depths. Sixty-six per cent, 70 Or/o and 27 ~0 of the specimens attained the top layer in 30, 50 and 90 cm of sand, respectively. Mortality was 25 '~o, 39 ~o and 50 % in the three depths.

Day 1 and Day 8 aquaria burial experiments (A) Scoloplos frafalis--Summer temperatures: For summer temperatures in 20 to

32 cm of sand and 8 to 16 cm of silt-clay (53-56 %), the percentage migration of S. ]ragilis ranged from 84 to 90 % in sand and from 8 to 10 % in silt-clay by Day 1 and from 88 to 95 % in sand and 0 % in silt-clay by Day 8 (Table 1). Percentage migration was significant in all sand experiments but not in silt-clay. Percentage migration was markedly higher in sand than silt-clay even though the highest thickness of silt-clay was 50 ~o less than the highest thickness of sand. In the silt-clay, polychaetes were mainly restricted to substratum zero by Day 1, whilst in sand, although the distribution was skewed to the lower layers, some individuals had migrated to upper layers. By Day 8, this pattern was even more marked; polychaetes were exclusively restricted to substratum zero in silt-clay and were frequently distributed throughout upper layers of sand (Table 1).

Control experiments showed no mortality in silt-clay and sand, respectively. Mortalities of S.jragilis ranged from 5.4 to 9.1% in sand and from 42 to 90 % in silt- clay by Day 1 and 20 to 51% in sand and 50 % in silt-clay by Day 8. Mortality was markedly higher in silt-clay than in sand and increased with time in silt-clay. By Day 1, mortalities were exclusively restricted to substratum zero for both sediment types and only one mortality was recorded outside of substratum zero by Day 8.

Scoloplos fragilis-- Winter temperatures: For winter temperatures in 16-36 cm of sand and 32 cm of silt-clay (93-95 %), the percentage migration of S. fragilis ranged from 10 to 51% in sand and was 73 % in silt-clay by Day 1 and from 79 to 86% in sand and was 2 ~ in silt-clay by Day 8 (Table 2). Percentage migration was not significant in 16 cm sand by Day 1 or 32 cm silt-clay by Day 8. Percentage migration increased with time in sand and decreased in silt-clay. However, percentage migration was higher in silt-clay than in sand by Day 1. By Day 1, the occurrence of polychaetes was skewed towards the lower layers in sand, but was normally distributed in silt-clay. By Day 8, the pattern was reversed.

Control experiments showed mortalities of 7.5 % of eighty specimens and 3.3 ~ of one-hundred-and-eighty specimens for silt-clay and sand, respectively. Mortalities

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Percentage migration was generally higher in sand than in silt-clay for both temperatures with the exception of 32cm of silt-clay by Day 1 under winter temperatures. Percentage migration in sand was usually higher in summer, particularly by Day 1, than in winter. In silt-clay, the pattern was reversed with percentage migration higher in winter. In general, relatively few individuals reached the topmost layers in sand or silt-clay for both temperatures except for 16 cm of sand under winter temperatures.

Under both temperature conditions, there was generally an increase in mortality with time, with some exceptions. Mortality was considerably higher in silt-clay under summer temperatures by Day 1 and Day 8. In contrast, mortality was slightly higher in the sand under winter temperatures than summer temperatures for the same periods.

Day 1, Day 8 and Day 15 aquaria burial experiments (B) Scoloplos fra~iis--Summer temperatures. For summer temperatures in four

different sediment types of silt-clay and sand, percentage migration of S. Jragilis ranged from 19 to 29 ~ by Day 1, 13 to 88 ~o by Day 8 and from 4 to 68 ~o by Day 15 (Table 3). Percentage migration was significant in all sediment types by Day 1 but was only significant for all experiments in 100 ~ sand by Day 15. There was a general reduction in percentage migration with time for all sediment types except 100To sand. Percentage migration was generally higher in sand than sediment with silt- clay. Except for sand, the specimens were mainly distributed in the lower layers or substratum zero by Day 15. In sand, the polychaetes had migrated into upper layers by Day 15.

Control experiments showed 10, 13, 13 and 10~/o mortality for 100~ o silt- clay/60 ° o sand, 20 ~ silt-clay/80 °, o sand and 100 ~/o sand, respectively. Mortality for S../ragilis ranged from 20.8 to 51-7 O~. by Day i, 9.8 to 72-1 O//o by Day 8 and from 28.7 to 97.5 °. o (Table 3). There was a general increase in mortality with time except for 100 ° 0 sand. The rate of mortality varied considerably with time in the various sediment types. Mortalities were mainly restricted to substratum zero for all experiments.

Day 1 and Day 8 aquaria burial experiments (A) Nereis succinea--Summer temperatures: For summer temperatures in 24-28 cm

of silt-clay (55-57 ~o), percentage migration of N. succinea ranged from 88 to 98 ~o by Day 1 and from 85 to 88 °, o by Day 8 (Table 4). Percentage migration was significant for all experiments. Percentage migration was generally similar with

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time. Although the occurrence of polychaetes was generally distributed throughout the layers of all experiments, polychaetes attained the topmost layer on Day 1, but not by Day 8.

Control experiments showed no mortality for forty-one specimens. Mortalities ranged from 2.4 to 11.6 ~o by Day 1 and 12.2 to 14.6 o~ by Day 8 (Table 4). Except for a single specimen, mortalities were restricted to substratum zero.

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Control experiments showed no mortality for twenty specimens in sand or clay. Mortality ranged from 0 to 15 ~ by Day 1 and from 15 to 35 ~o by Day 8 (Table 5). There was an increase in mortality with time in sand. Mortalities were restricted to substratum zero in all experiments except for a single specimen in silt-clay by Day 8.

Percentage migration was lower by Day 1 for winter temperatures than for summer ones (Tables 4 and 5). The number of organisms that achieved the top layer was higher in winter temperatures than in summer temperatures by Day 8. In silt- clay there was a suggestion of an increase in mortality with time under summer temperatures, but not winter temperatures.

Day 1, Day 8 and Day 15 aquaria burial experiments (B) Nereis succinea--Summer temperatures: For summer temperatures in four

different sediment types, percentage migration of N. succinea ranged from 31 to 84~/by Day 1, 64 to 83% by Day 8 and from 45 to 75~o by Day 15 (Table 6t. Percentage migration was significant in all conditions except in 17-21 ~o silt-clay by Day ! and in 100 °J o sand by Day 15. Percentage migration generally decreased with time except for 20 ~o silt-clay/80 °, o sand. The distribution of N. succinea throughout the layers varied considerably with sediment type and time, but this species of polychaete was a very active burrower.

Control experiments showed 22, 2, 2 and 22 ~o mortality for 100 ~o silt-clay, 40 ",, silt-clay/60 % sand, 20 ~o silt-clay/80 % sand and 100 ~o sand, respectively. Mortality of N. succinea ranged from 5.8 to 29-10/~o by Day 1, from 17.4 to 58.4 ~ by Day 8 and from 25.5 to 61.4~o by Day 15 (Table 6). In general, there was an increase in mortality with time. The mortality rate was generally higher between Day I and Day 8 than between Day 8 and Day 15. Mortalities were restricted to substratum zero in 100 ~o silt-clay and sand; however, in addition to high numbers in substratum zero, mortalities occurred scattered throughout upper layers in the other two sediment types.

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DISCUSSION

A review of recent literature indicates that British and European workers have been generally active in research on benthic invertebrate locomotion in general (Trueman, 1975) and polychaetes in particular (Trueman, 1966; Ockelmann & Vahl, 1970; Seymour, 1971; Trevor, 1976, 1977, 1978). These studies show a wide variety of adaptive features among annelids to cope with shifting sediment and that certain families and genera and species within families are more effective burrowers than other annelid taxa. These studies provide some background upon which to compare burrowing performance of polychaetes under certain conditions of burial.

Species level Trevor (1976, 1977) demonstrated that Nephtys cirrosa and Nereis diversicolor

were both effective burrowers. Trevor (1977) noted that the response of Nereis diversicolor to disturbance was usually rapid swimming rather than burrowing activity. This species could not be induced to burrow again immediately after the conclusion of a digging period. After removal from burrows N. diversicolor usually crawled over the substratum for several minutes. This prolonged interval of locomotive activity would make any conclusions regarding fatigue and the associated reduction of the digging cycle frequency as burial progressed difficult (Trevor, 1977).

In a laboratory study of the effect of suspended (bentonite) solids on San Francisco Bay organisms, Peddicord et al., (1975) reported that mortalities of Neanthes succinea ( = Nereis succinea) were significant in the clear water controls and, although higher in the experimental aquaria, bore no relationship to increasing suspended solids concentration. During the course of their experiments, the worms were covered with bentonite in which they failed to establish tubes or burrows. No definite conclusions concerning the tolerance of N. succinea to suspended solids were drawn (Peddicord et al., 1975).

Brown (1979) considered Scoloplosjragilis as an active burrower. She based this conclusion on field and laboratory evidence. In the field, S. Ji'agilis was able to maintain its burrows on shifting sand bars. Core counts indicated that S. ji'agilis occurred at least 15cm deep but generally in the top 5cm. Laboratory studies showed that within 30 min S../ragilis was sampled from 7-3, 8-3, 12-1 and 15. l cm below the surface. Myers (1977) reported on the burrowing activity of another species of Scoloplos--S. rohustus. According to him, S. robustus is active throughout a range of 2-13 cm sediment depth. His account also contains useful burrowing data on a variety of taxa.

Diaz & Boesch (1977) stated that surprisingly little attention has been devoted to the upward migratory abilities of benthic fauna. Among several taxa in a study of the impact of fluid mud dredged material on benthic communities of the James River, Virginia, the largest and deepest burrowing oligochaete, Branchiura sowerbyi, was

MIGRATION AND MORTALITY OF POLYCHAETA IN DREDGED MATERIAL 65

adjudged to be the species best able to cope with the high rates of sedimentation associated with dredged material disposal. The polychaete Nephtys incisa attained the surface in less than 24h from 21 cm of dredged material from Rhode Island South (Saila et al., 1972). Another polychaete, Streblospio benedici, was able to reach the surface through 6 cm of sediment which emphasised important differences in the relative ability of polychaetes to migrate vertically through spoil overburden.

Peddicord et al. (1975) stated that few observations on the response of soft-bodied organisms to either natural or experimental burial were available. They cited studies on the onuphid polychaetes Diopatra cuprea and Nothria elegans--both of which are capable of burrowing upward through an accumulation of 30 cm of sediment-- and the terebellid polychaete, Pista pacifica, which is capable of extending its tube up through at least 25 cm of sediment.

General Several mechanisms of recolonising dredge and disposal sites were cited in

Maurer et al. (1981b). These mechanisms include (1) migration of adults from undisturbed areas, (2) reproduction and larval recruitment from undisturbed areas, (3) residual populations in patches of undredged material from undisturbed areas and (4) vertical migration through disposal material.

In some field and laboratory studies on dredging and disposal activities in Monterey Bay, California, Oliver et al. (1977) reported that most of the infauna was destroyed at the centre of the disturbance. The area was recolonised rapidly by mobile swimming crustaceans and, to a much lesser degree, by a few polychaetes that may also be relatively mobile. The latter include Dispio uncinata and Nephtys caliJorniensis. However, according to Oliver et al., recolonisation was primarily by migration from undisturbed areas or larval recruitment by mobile crustaceans and by opportunistic polychaetes (Armandia brevis, Prionospio pygmaea and P. cirrifera). The later phase was characterised by a gradual increase in the predisturbance fauna of less mobile crustaceans and less opportunistic polychaetes ( Magelona sacculata, Amaeana occidentalis, Nothria elegans). Oliver et al. did not consider animal migration through introduced sediment as a likely means of recolonisation.

In contrast to the situation in Monterey Bay, Richardson et al. (1977) recognised the importance of vertical migration as the principal means of recolonisation at a disposal site near the mouth of the Columbia River, Oregon. They stated that species not affected by dredged material disposal were shelled gastropods and molluscs, cumacea and non tube-dwelling polychaetes. The three species of polychaetes ( Magelona sacculata, Haploscoloplos elongatus and Thalenessa spinosa) unaffected by dredged material disposal were non tube-dwelling, active species capable of considerable migration over the sediment surface and rapid burrowing through the sediment.

In a local field study on dredge and spoil disposal at the mouth of the Delaware

66 DON MAURER el al.

Bay, there was some suggestion of recruitment three months after dredging (Maurer et al., 1974). The 0.9 m contour thickness of spoil occurred about 180 m from the thickest overburden. Based on our work (core burial experiments) there is evidence to state that some S. fragilis and N. succinea would be capable of vertically migrating through that amount of overburden assuming it was sand--at least for S. fragilis. The natural occurrence of N. succinea in mud and clay bottoms (80-90 cm) suggests that it would be able to handle 0.9 m of overburden. In the aquaria burial experiments both species were generally active. Percentage migration of S. fragilis was generally higher in sand than in silt-clay. S.fragilis was much more vulnerable to silt-clay than N. succinea in terms of mortality. Percentage migration of both species was higher in summer temperatures than winter ones.

When this paper is combined with earlier descriptions of the burrowing behaviour of molluscs and crustaceans (Maurer et al., 1980a, b) there is solid evidence to show that some degree of upward mobilisation of benthos should be expected under certain conditions. It is necessary to expand the scope of these studies (l) by including other major taxa, (2) by increasing the number of genera and species within the major taxa (molluscs, crustaceans, polychaetes) to date, (3) by combining species to examine the effects of interactions on burrowing behaviour and (4) by incorporating pore water chemistry studies with burial studies. This type of coverage would place our ability to predict mechanical and chemical effects of dredge disposal on macrobenthos on a very solid base.

CONCLUSIONS

(1) Mortalities of S. j?agilis increased considerably with high percentages of silt-clay.

(2) Mortality of N. succinea and S.fragilis increased with increased time except for the latter in 100~o sand.

(3) Mortality of S.fragilis in silt-clay was higher under summer temperatures than winter ones.

(4) Migration of S.ffagilis in sand was higher under summer temperatures than winter ones.

(5) Mortality of N. succmea in silt-clay was similar under winter and summer temperatures.

(6) Migration of N. succinea in silt-clay was higher under summer temperatures than winter ones.

ACKNOWLEDGEMENTS

This research was supported by the US Army Engineer Waterways Experiment Station (NES) (Contract No. DACW 39-74-R-0019). We would like to thank Dr R.

MIGRATION AND MORTALITY OF POLYCHAETA IN DREDGED MATERIAL 67

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.

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