Plan-view photos, benthic grabs, and sediment-profile images: Using complementary techniques to assess response to seafloor disturbance

Marine Pollution Bulletin 59 (2009) 26–37

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Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul

Plan-view photos, benthic grabs, and sediment-profile images: Using complementary techniques to assess response to seafloor disturbance Stephanie J.K. Wilson a,*, Thomas J. Fredette b, Joseph D. Germano c, James A. Blake d, Pamela L.A. Neubert d, Drew A. Carey e a

ENSR, 2 Technology Park Drive, Westford, MA 01886, USA US Army Corps of Engineers, New England District, 696 Virginia Road, Concord, MA 01742, USA c Germano and Associates, 12100 SE 46th Place, Bellevue, WA 98006, USA d ENSR, 89 Water Street, Woods Hole, MA 02543, USA e CoastalVision, 215 Eustis Ave, Newport, RI 02840, USA b

a r t i c l e

i n f o

Keywords: Dredged material New England Sediment-profile imaging Plan-view imaging Benthic biology Rhode Island Sound

a b s t r a c t A monitoring survey was conducted in July 2005 at the Rhode Island Sound Disposal Site (RISDS) as part of the Disposal Area Monitoring System (DAMOS) program. The survey included the collection of sediment-profile and plan-view images, and benthic biology grabs. Each of these techniques provides a different, yet complementary perspective on benthic community conditions. These complementary techniques aided in the assessment of the benthic recovery process within RISDS following the placement of dredged material from the Providence River and Harbor Maintenance Dredging Project (PRHMDP). Based on observed patterns of physical, chemical, and biological responses of seafloor environments to dredged material disposal activity it was expected that the benthic community within RISDS would be in an intermediate phase of recolonization (Stage II). Results of the 2005 RISDS survey indicated that in the six months since disposal activities at RISDS had concluded, the biological community at RISDS was recovering relatively rapidly and Stages II and III infauna were present throughout the region. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Open water disposal of dredged material is an environmental concern in New England as well as throughout the world (Fredette and French, 2004; Van Dolah et al., 1984). The effects of dredged material disposal on benthic communities are well documented (Bolam and Rees, 2003). Impacts to the benthie community include direct burial by dredged material (Newell et al., 1998), reduction in community diversity (Jones, 1986; Harvey et al., 1998), and a shift in the dominance patterns within the benthic community (Harvey et al., 1998; Roberts et al., 1998). Biological impacts from dredged material disposal are influenced by a number of factors including the composition of the dredged material, the receiving habitat, and the community composition of the disposal site (Smith and Rule, 2001; Bolam et al., 2006). The timing of disturbance can also influence the identity of colonizers and course of recovery, particularly in New England where there is a strong seasonal influence (Zajac and Whitlatch, 1982; Wilber et al., 2007). In New England, over thirty five years of research indicates that, when carefully managed, ocean disposal of dredged material will have minimal environmental impact (Fredette and French, 2004). * Corresponding author. E-mail address: [email protected] (S.J.K. Wilson). 0025-326X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpolbul.2008.11.019

The US Army Corps of Engineers (USACE) New England District Disposal Area Monitoring System (DAMOS) program is a comprehensive monitoring and management program designed and conducted to address environmental concerns associated with the use of open water disposal sites throughout the New England region. DAMOS monitoring surveys are designed to test hypotheses related to expected physical and ecological response patterns following placement of dredged material on the seafloor at established disposal sites. A monitoring survey was conducted at the Rhode Island Sound Disposal Site (RISDS) in July 2005 as part of the DAMOS program. RISDS, located approximately 16.7 km south of Point Judith, Rhode Island, is an open water disposal site for dredged material from Rhode Island, southeastern Massachusetts, and surrounding harbors (40 CFR Part 228) (Fig. 1). The objective of the 2005 RISDS survey was to assess the benthic conditions within RISDS following placement of approximately 4 million m3 sediment from the Providence River and Harbor Maintenance Dredging Project (PRHMDP) between April 2003 and January 2005. A variety of techniques have been used by the DAMOS program to evaluate benthic community conditions following dredged material disposal. The 2005 monitoring survey included the collection of sediment-profile and plan-view images, and benthic biology grabs. Based on previously observed patterns of physical, chemical,

S.J.K. Wilson et al. / Marine Pollution Bulletin 59 (2009) 26–37

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Fig. 1. RISDS a open water disposal site alternatives in Rhode Island Sound.

and biological responses of seafloor environments to dredged material disposal activity at other sites, our prediction was that the benthic community within RISDS would be in an intermediate phase of recolonization (Stage II) (Rhoads and Germano, 1982, 1986; Rhoads and Boyer, 1982). Specifically, the community was expected to consist of small, tubicolous polychaetes and Ampeliscid amphipods or equivalent fauna. However, depending on the elapsed time between a given cluster of disposal events and the location of monitoring stations, a gradation of recolonization might be expected. The main purpose of this paper is to describe the use of three complementary techniques in assessing the condition of the benthic community at RISDS following dredged material disposal. Although each of these techniques targets different aspects of the benthic ecosystem, the use of all three techniques allows for a comprehensive assessment of the benthic conditions in Rhode Island Sound and provides insight into understanding the recovery

of RISDS following placement of sediment from the PRHMDP. Further, this paper highlights the tiered approach of the DAMOS program and the importance of clearly identifying study objectives and selecting appropriate sampling techniques.

2. Methods The July 2005 survey at RISDS was performed by a team of investigators from ENSR, CR Environmental, and Germano and Associates. The survey was conducted 30 June-3 July 2005 aboard the F/V Shanna Rose to assess the benthic condition within RISDS following placement of sediment from the PRHMDP. Field activities included the collection of sediment-profile images, plan-view images, and benthic biology grabs. The field team collected SPI and plan-view images at 30 stations within RISDS (Fig. 2) and at 15 reference stations (Fig. 3). Five

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Fig. 2. RISDS with target sediment-profile and plan-view image stations and benthic biology stations indicated.

groups (A, B, C, D, and E) of five stations each were located within the basin area of the site and an additional five stations were spread along the berm formation. The groups of stations located within the basin area were randomly selected within a 150 m radius of identified disposal activity. The five stations located on the berm were spaced approximately equidistant along the feature. As part of the 2005 survey, three reference areas were surveyed, east of the disposal site (REF-E), southwest of the disposal site (REF-SW), and northeast of the disposal site (REF-NE), to provide a basis of comparison between RISDS sediment conditions and the ambient sediment conditions in Rhode Island Sound (Fig. 3). Five reference stations were located randomly within a 300 m radius of the centers of each of the three references areas. At least three replicate SPI images and one plan-view image were collected at each of the 45 stations. Benthic biology grabs were collected at seven randomly selected stations located within RISDS (Fig. 2) and five randomly

selected stations within the three reference areas (Fig. 3). One replicate was collected at each station selected for benthic infaunal community analysis. 2.1. Data collection Positional data, comprised of horizontal positioning (x- and ydimensional data) and time (t-dimensional data), were collected using a TrimbleÒ AG-132 Differential Global Position System (DGPS) unit. This system received and processed satellite and land-based beacon data and provided real-time vessel position. HYPACKÒ hydrographic survey software was used to acquire, integrate, and store all positional data from the DGPS as well as bathymetry and station data. Sediment-profile imaging was used to provide data on the physical characteristics of the seafloor as well as the condition of the benthic infaunal community. The technique involved the deploy-

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REF-NE-02

Site 69a REF-NE

REF-E

REF-NE-04

REF-NE-05

REF-NE-03

REF-NE-01

RISDS REF-SW

0

1

2 Kilometers

0

REF-NE

100

200 Meters

REF-SW-01 REF-SW-02 REF-E-01 REF-E-05

REF-SW-03

REF-E-04 REF-E-02

REF-SW-04 REF-SW-05

REF-E-03

REF-SW

0

100

200 Meters

REF-E

0

100

200 Meters

SPI Reference Stations SPI & Benthic Biology Reference Stations RISDS Reference Area Projection: Transverse Mercator

Coordinate System: RI State Plane (m)

Datum: NAD 83

Fig. 3. Rhode Island Sound reference areas with target sediment-profile and plan-view image stations and benthic biology stations indicated.

ment of an underwater camera system to photograph a cross section of the sediment-water interface. Acquisition of high-resolution SPI images was accomplished using a NikonÒ D100 digital single-lens reflex camera mounted inside an Ocean Imaging Systems Model 3731 pressure housing system. The pressure housing sat atop a wedge-shaped prism with a front faceplate and a back mirror. The mirror was mounted at a 45° angle to reflect the profile of the sediment-water interface to the camera. As the prism penetrated the seafloor, a trigger activated a time-delay circuit that fired an internal strobe to obtain a cross-sectional image of the upper 15–20 cm of the sediment column. Plan-view underwater images were also collected at each station sampled with a second camera mounted to the sediment-profile camera frame. An Ocean Imaging Systems Model DSC6000 plan-view underwater camera (PUC) system was attached to the Model 3731 camera frame and used to collect plan-view photographs of the seafloor surface. The PUC system consisted of a

NikonÒ D-70 camera encased in a titanium housing, a 24 VDC autonomous power pack, a 500 W strobe, and a stainless steel trigger cable. As the camera apparatus was lowered to the seafloor, the weight attached to the trigger cable contacted the seafloor prior to the camera frame hitting the bottom and triggered the camera to obtain an image of the sediment surface. The field of view for the plan-view images ranged from approximately 0.6 m2–3.1 m2, depending on the length of the trigger wire. Sediment grab samples were collected for benthic community analysis and for characterization of sedimentary parameters including, total organic carbon (TOC) and grain size. A 0.04 m2 Ted Young-modified Van Veen grab was used to collect the grab samples. Grain size and total organic carbon (TOC) samples were collected from the grab using a 2.5 cm diameter tube. After the grain size and TOC samples were removed, all sediment remaining in the grab was washed into a clean bucket and sieved through a 0.5 mm mesh screen. The material retained on the sieve was then

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placed in an appropriate sample container and preserved with 10% formalin. After 48 h the benthic samples were transferred out of the formalin, rinsed on a 0.5 mm sieve with freshwater and preserved in an 80% ethanol solution. To facilitate the sorting process, all samples were stained in a solution of Rose Bengal. 2.2. Data analysis Computer-aided analysis of each SPI image was performed to provide measurement of the following set of parameters: sediment type (grain size major mode and range), penetration depth, apparent redox potential discontinuity (RPD) depth, and infaunal successional stage. The sediment grain size major mode and range were estimated visually from the images using a grain size comparator at a similar scale. Results were reported using the phi scale. The presence and thickness of disposed dredged material were also assessed by inspection of the images. The depth to which the camera penetrated into the seafloor was measured to provide an indication of the sediment density or bearing capacity. The penetration depth can range from a minimum of 0 cm (no penetration on hard substrata) to a maximum of 20 cm (full penetration on soft substrata). The mean apparent RPD depth was measured by assessing color and reflectance boundaries within the images. The apparent RPD depth provides a measure of the integrated time history of the balance between near surface oxygen conditions and biological reworking of sediments (Rhoads and Germano, 1982). Sediment particles exposed to oxygenated waters are oxidized and lighten in color to brown or light grey. As the particles are moved downwards by biological activity or buried, they are exposed to reduced oxygen concentrations in sub-surface pore waters and their oxic coating slowly reduces, changing color to dark grey or black. The apparent RPD depth increases when biological activity is high; when it is low or absent, the apparent RPD depth decreases. Infaunal successional stage is a measure of the biological community inhabiting the seafloor. Successional stage was assigned by assessing which types of species or organism-related activities were apparent in the images. Current theory holds that organism-sediment interactions in fine-grained sediments follow a predictable sequence of development in response to organic enrichment (Pearson and Rosenberg, 1978) or disturbance (such as dredged material disposal; Rhoads et al., 1978), and this sequence has been divided into three stages (Rhoads and Germano, 1982, 1986). Stage I assemblages consist of pioneering organisms, such as tube-dwelling polychaetes and oligochaetes, that typically appear shortly after the disturbance. These functional types are usually associated with a shallow redox boundary and shallow bioturbation depths, and are characterized by high rates of recruitment and high ontogenic growth rates. In the absence of further disturbance, infaunal deposit feeders eventually replace these early successional assemblages; the start of this process is designated as Stage II. Typical Stage II species are shallow dwelling bivalves and tubicolous amphipods. Stage III taxa, in turn; represent higher successional stages typically found in low disturbance areas. Many Stage III taxa feed at depth in a head-down orientation. The bioturbation activities of these deposit feeders are responsible for aerating the sediment and causing the redox horizon to be located several centimeters below the sediment-water interface. It is possible for Stage I or Stage II species to be present at the sediment surface while Stage III species are present at depth. In those instances, where two types of assemblages are visible, the successional stage is designated at Stage I on Stage III (I on III) or Stage II on Stage III (II on III). Although the actual species which participate in this successional sequence may vary according to substrate composition, pollutant load, disturbance intensity, as

well as the existing biological community available for recolonization following disturbance, it is expected they will functionally achieve the same effects over time (Rhoads and Germano, 1982, 1986; Rhoads and Boyer, 1982). Computer-aided analysis of each plan-view image was performed to provide additional information about large-scale sedimentary features, density and patch size of surface fauna, density of infaunal burrows, and occurrences and density of epifaunal foraging patterns on the seafloor of the disposal site and reference areas. The plan-view images provide a much larger field of view than the sediment-profile images and provide valuable information about the landscape ecology and sediment topography in the area. The plan-view images also allow for better interpretation of surface sediment layers/textures or structures detected in the sediment-profile images in light of the larger context of surface seafloor features. Benthic infaunal samples were visually analyzed for penetration depth (10 cm was the maximum and 7 cm was the minimum acceptable penetration depth), depth of the apparent redox potential discontinuity (RPD) layer, sediment color and texture, odor, and observed biota. Samples were sorted using a dissecting microscope to major taxonomic categories, such as polychaetes, arthropods, mollusks, and echinoderms. Following sorting, individual species were identified and enumerated. All specimens were identified to the lowest possible taxonomic category (usually species). Analysis of the final dataset excluded infaunal taxa such as juveniles and indeterminate specimens that could not be identified to the species level, as well as epifauna, shell-borers, and parasites. However, indeterminate organisms of valid benthic infaunal species were included in calculations of total density. The PRIMER statistical package was used to calculate several community diversity indices, including Shannon’s diversity index (H0 ), Pielou’s evenness value (J0 ), and Fisher’s alpha (Clarke and Gorley, 2001). Shannon’s index (H0 ), based on information theory, is the most commonly used diversity index in coastal benthic programs. According to Pielou (1975) and Magurran (1988), Shannon’s index assumes that individuals are randomly sampled from an infinitely large population and that all species are present in the sample. Neither assumption, however, correctly describes the environmental samples collected in most marine benthic programs. Pielou’s evenness index (J0 ) expresses H0 relative to the maximum value that H0 can obtain when all of the species in the sample are equally abundant. Fisher’s alpha model of species abundance (Fisher et al., 1943) has also been widely used and is considered the best index for discriminating community changes among subtly different sites (Taylor, 1978). Fisher’s alpha is a measure of diversity that is independent of sample size. PRIMER was also used to conduct the Bray–Curtis similarity analysis and principal components analysis (PCA). These multivariate analyses were used to identify patterns in the data, such as differences in faunal assemblages within RISDS compared to the reference areas. Data were fourth-root transformed prior to group average sorting with the Bray–Curtis similarity analysis. To facilitate comparison of species composition in the benthic infaunal samples with SPI images and further evaluate the species composition of RISDS relative to the reference areas, all species were assigned to one of six trophic guilds (feeding modes): omnivore/scavenger, sub-surface deposit feeder, interface feeder, suspension feeder, surface deposit feeders, or predator. Assignment of individual species to functional groups was largely based on extrapolation from related species because so little information is available on the biology of coastal benthic invertebrates in the New England region. A list of major species included in each trophic guild is presented in Table 1. Omnivore/scavengers included nephtyid, lumbrinerid, and syllid polychaetes, isopods, and amphipods.

S.J.K. Wilson et al. / Marine Pollution Bulletin 59 (2009) 26–37 Table 1 List of major species in trophic faunal groupings. Trophic group

Taxonomic group

Species

Suspension feeders

Sabellid polychaetes

Chone sp. Euchone incolor Crassicorophium crassicorne Erich thonius fasciatus Dyopedos monacanthus

Amphipods

Anemones Phoronids Bivalves Omnivore/scavengers

Nephtyid polychaetes Lumbrinerid polychaetes Syllid polychaetes Isopods Amphipods

Sub-surface deposit feeders

Bivalves

Capitellid polychaetes Maldanid polychaetes Scalibregmatid polychaetes Surface deposit feeders

Cirratuliform polychaetes

Terebelliform polychaetes Interface feeders

Amphipods

Phoronis architecta Cerastoderma pinnulatum Ensis directus Nephtys sp.

Ptilanthura tenuis Pleurogonium inerme Lysianassidae Oedicerotidae Nucula annulata Nucula delphinodonta Yoldia sapotilla Mediomastus ambiseta Clymenella torquata Euclymene collaris Scalibrema inflatum Aphelochaeta marioni Tharyx acutus Pherusa affinis Terebellides atlantis Ampelisca agassizi Leptocheirus pinguis Unciola irrorata

Spionid polychaetes Predators

Nemerteans Polychaetes

Snails Starfish

Harmothoe extenuata Phyllodoce maculata Pholoe minuta Fargoa bartschi Henricia sanguinolenta

Sub-surface deposit feeders included the burrowing protobranch bivalves, as well as head-down (conveyor-belt) species (capitellid and maldanid polychaetes) and reverse conveyor-belt species (scalibregmatid polychaetes). It should be noted that although capitellid polychaetes are head-down deposit feeders, they are small opportunists, typically found as initial colonizers in disturbed environments, and therefore are classified as Stage I organisms and not Stage III organisms (a category reserved for deep-burrowing, void excavating head-down deposit feeders). The category ‘‘interface feeding” was proposed by Dauer et al. (1981) for organisms such as spionid polychaetes that can collect particles both from the sediment surface and the water column. Often classified as suspension feeders (Dauer et al., 2002), ampeliscids have also been considered to be surface deposit feeders (Mistri et al., 2001). For this study, the ampeliscid amphipods were considered to be interface feeders. Suspension feeders included anemones, sabellid polychaetes, phoronids, and a variety of bivalves and amphipods. Surface deposit feeders included some cirratulid polychaetes, terebelliform polychaetes, and flabelligerid polychaetes. Predators included the nemerteans, some polychaetes, snails, and starfish. 3. Results Results of previous investigations of the ambient sediments in and around Rhode Island Sound, performed as part of the site des-

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ignation investigations, showed the seafloor to be primarily silty sand with patches of gravel (Battelle, 2002, 2003; USEPA, 2004). The grain size major mode at all three reference areas surveyed in 2005 was similar to that found in previous investigations; very fine to fine sands with varying degrees of silt. The sediments with the highest silt component were located in the REF-E area, those with the lowest silt and highest sand fraction were located in the REF-NE area, and those from the REF-SW area were intermediate in silt content (Fig. 4). Some stations in the REF-NE area also had small rocks or shell hash at the sediment surface. The presence of very fine to fine sand with varying degrees of silt, as detected in the sediment-profile images (Table 2), was confirmed in the corresponding plan-view images from the reference areas (Table 3). Examination of the benthic samples also confirmed the presence of silty fine to medium sand at the five reference stations sampled (Table 4). Since the beginning of disposal in April 2003, approximately 4 million m3 of dredged material was placed at RISDS with no interruption in disposal activity. The sediment at the majority of stations sampled within RISDS consisted of recently-deposited dredged material. The layer of dredged material was thicker than the camera prism penetration depth at all stations sampled within the disposal site. The dredged material was composed primarily of fine-grained muds with high water content and low-bearing strength. Three stations had a sediment grain size major mode that was noticeably different than that found at the other stations surveyed (Table 2). Stations A-01 and E-04 had thin surface layers of very fine sand over the mud and Station BE-03 was covered with a layer of small rocks and cobble on the sediment surface. Even though the deposited dredged material was reduced (black in color below the surface oxidized layer), there was no evidence of low dissolved oxygen in the overlying water or sub-surface methane generation at any of the locations sampled. Individual station mean apparent RPD values ranged from 0.8 to 4.6 cm, with deeper values measured at the reference area stations. The averaged mean apparent RPD values at the disposal site and reference area were 1.5 cm and 3.4 cm, respectively (Table 2). The mean apparent RPD value at 80% of the stations within RISDS was between 1 and 2 cm, typical for a recently active disposal site. Mean apparent RPD depths were slightly greater at the reference area and consistent with values measured outside the disposal site in past surveys (Battelle, 2003). With the exception of some of the stations from the REF-NE area and Station BE-03, where compact sands or larger cobble/shell hash prevented adequate prism penetration for infaunal successional stage determination, at least one replicate image at all the stations sampled showed evidence of Stage III taxa (Table 2). In addition to the presence of large sub-surface burrows and feeding voids (Fig. 5), there were also dense assemblages of tubicolous surface fauna and podocerid amphipods (Fig. 6) at both the disposal site and reference area stations. Although the density of burrows from large, infaunal deposit feeders (Stage III) was lower at the RISDS stations, compared to the reference area stations (Fig. 5), the presence of structures consistent with Stage III infauna (burrows, feeding voids) detected in the sediment-profile images was confirmed in the corresponding plan-view images from the disposal site stations (Fig. 6). In addition, while the occasional individual crab and shrimp were seen on the sediment surface at a handful of locations, the foraging tracks of these epifaunal predators were quite common on the sediment surface of all the stations at RISDS and in the reference areas (Fig. 6). Animals and tubes were observed in all grab samples collected at the disposal site and reference area stations. Overall, the reference area stations were populated by a species-rich, dense benthic infauna with high species diversity typical of shallow water habitats in New England. For the five reference area stations combined,

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Fig. 4. SPI images from three reference areas illustrate the sediment grain size major mode in each area as well as the varying percentage of silt. Arranged from left to right in order of highest to lowest silt content: Station REF-E-01 (left), Station REF-SW-03 (middle), and Station REF-NE-03 (right).

a total of 11,747 individuals were collected representing 119 species. The number of species per sample ranged from 57 (REF-NE03) to 68 (REF-E-01) while the number of individuals per sample ranged from 1,494 (REF-NE-05) to 2,986 (REF-E-01). Species diversity indices were all high at the reference area stations: H0 (3.06– 4.53), J0 (0.50–0.75), and Fisher’s alpha (10.43–14.40) (Table 4). Examination of the benthic samples collected at RISDS indicated that the infauna was represented by fewer species, fewer individuals, and lower species diversity, compared to the reference area stations (Table 4). A total of 1710 individuals were collected from the seven RISDS stations, representing 60 species. The number of species per sample ranged from 17 (C-02) to 32 (D-03) while the number of individuals per sample ranged from 85 (C-02) to 410 (D-05). Species diversity indices were more variable and generally lower at RISDS than at the reference sites: H0 (1.78–4.12), J0 (0.44–0.85), and Fisher’s alpha (4.13–15.53) (Table 4). The one exception was Pielou’s J0 , a measure of evenness or equitability in samples, where the average among samples was actually higher at RISDS locations than at the reference locations (J0 0.65 vs. 0.60). Results from the Bray–Curtis similarity analysis and principal components analysis showed that the seven disposal site stations form a very tight cluster (Fig. 7), separate from the reference area stations. Additionally, Station REF-E-01 stands out from among all other stations sampled, likely due to the high silt component found at this station. The dominant species at the reference area stations was the protobranch bivalve Nucula annulata, followed by the amphipods, Crassicorophium crassicorne, Erichthonius fasciatus, Ampelisca agassizi, Unciola irrorata, and Leptocheirus pinguis, and sabellid polychaetes. The dominant species at the disposal site stations was the syllid polychaete, Euchone incolor, followed by the protobranch bivalve, Nucula annulata, the nephtyid polychaete, Nephtys incisa, and the phoronid, Phoronis architecta. Suspension feeders, including anemones, sabellid polychaetes, phoronids, podocerid amphipods, and several species of bivalves, were the dominant organisms collected at five of the seven disposal site stations and four of the five reference stations (Fig. 8). Animals belonging to the omnivore/scavenger guild (nephtyid, dorvilleid, and lumbrinerid polychaetes and several species of amphipods) were the second most abundant group collected at the disposal site, followed by sub-surface deposit feeders. Interface feeders were the second most abundant group collected at the ref-

erence area. Head-down (conveyor-belt) species, including the capitellid, Mediomastus ambiseta, and the maldanids, Clymenella torquata and Euclymene collaris were present, but in low densities, at all but one of the reference stations, accounting for less than or equal to 1% of the collected fauna. There were no deep-burrowing deposit feeders present in the sediment grab samples collected at the disposal site, i.e. no maldanid polychaetes and only one individual of the capitellid polychaete, M. ambiseta. Overall, benthic conditions across RISDS indicated that the biological community was recovering relatively rapidly and the initial predictions of the benthic community being in at least a Stage II recolonization phase were not only met but exceeded. The SPI and plan-view images showed evidence of Stage III infauna at both the reference and the disposal site, although, as anticipated, their densities were much lower at the disposal site. The presence of dense populations of filter feeding invertebrates in the grab samples collected at RISDS suggested that Stage II organisms dominated the surface sediments. At the reference stations sampled in Rhode Island Sound, an even greater diversity of filter feeding organisms were present, including dense populations of amphipods, bivalves, and polychaetes. There was no evidence of headdown deposit feeding Stage III organisms in the benthic grab samples collected at the disposal site; however, larger surface deposit feeding polychaetes were present. 4. Discussion The objective of the July 2005 survey at RISDS was to assess the benthic recolonization status within RISDS following placement of approximately 4 million m3 of sediment from the PRHMDP. This objective was accomplished by collection and analysis of sediment-profile and plan-view images as well as sediment grabs. SPI data provided in situ information about processes such as bioturbation, trophic stratification, and biogenic irrigation. The addition of plan-view imagery provided information about largescale sedimentary features, density and patch size of surface fauna, density of burrows, and occurrences and density of epifaunal foraging patterns on the seafloor. Additionally, the collection of benthic infaunal grab samples provided further insight into the benthic community structure and species composition, including species abundance and diversity.

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S.J.K. Wilson et al. / Marine Pollution Bulletin 59 (2009) 26–37 Table 2 Summary of SPI results for RISDS survey, July 2005. Station

Grain size major mode (phi)

Mean prism penetration depth (cm)

Mean RPD depth (cm)

Successional stages present (no. of replicates)

REF-E-01 REF-E-02 REF-E-03 REF-E-04 REF-E-05 REF-NE-01 REF-NE-02 REF-NE-03 REF-NE-04 REF-NE-05 REF-SW-01 REF-SW-02 REF-SW-03 REF-SW-04 REF-SW-05 A-01 A-02 A-03 A-04 A-05 B-01 B-02 B-03 B-04 B-05 BE-01 BE-02 BE-03 BE-04 BE-05 C-01 C-02 C-03 C-04 C-05 D-01 D-02 D-03 D-04 D-05 E-01 E-02 E-03 E-04 E-05

4–3 4–3 4–3/>4 4–3 4–3/>4 3–2 4–3 3–2 3–2 3–2 4–3 4–3 3–2 4–3 4–3 4–3/>4 >4 >4 >4 >4 >4 >4 >4 >4 >4 >4 >4 Cobble >4 >4 >4 >4 >4 >4 >4 >4 >4 >4 >4 >4 >4 >4 >4 4–3/>4 >4

11.2 9.6 11.9 9.1 12.1 4.9 4.0 2.6 6.4 5.1 6.7 9.5 8.4 9.7 6.9 8.7 12.5 12.4 8.1 17.2 12 13.6 14.3 16.8 17.3 9.8 14 2.2 12.1 15.3 13.6 15 10.1 13.1 16.3 12.9 14.8 13.2 12.1 11.2 16.1 14.4 12.7 13.0 14.6

2.7 3.6 3.3 3.8 2.1 3.0 2.8 2.6 3.9 3.0 2.7 4.2 4.2 4.6 3.7 1.2 1.2 1.5 1.4 2.0 1.6 1.0 1.1 2.7 1.4 0.8 1.6 1.3 1.3 1.4 1.3 1.7 1.3 1.3 1.3 1.0 1.5 2.2 1.9 1.5 1.8 1.1 1.9 2.0 1.3

I on III (3) I on III (3) I on III (3) I on III (3) I on III (3) I on III (2), IND (1) I on III (3) I on III (1), IND (2) I on III (2), IND (1) I on III (2), IND (1) I on III (3) I on III (3) I on III (3) I on III (3) I on III (3) I (1), Stages I–II (1), I on III (1) I on III (2), III (1) I on III (2), IND (1) I on III (2), IND (1) I on III (3) I–II (1), I on III (2) I(1), I on III (2) I on III (3) I–II (1), I on III (2) I–II (1), I on III (2) I on III (3) II (1), I on III (1), III (3) IND (3) II–III (1), I on III (2) I on III (3) II–III (2), I on III (1) I on III (3) I on III (3) II–III (1), I on III (1), III (1) II (1), II–III (1), III (1) I–II (1), II (1), II–III (1) I on III (3) I on III (3) II (1), I on III (2) I on III (3) I on III (3) I (1), I on III (2) I on III (3) I on III (3) I–II (1), I on III (1), IND

IND, Intermediate.

The results of the July 2005 survey indicated that the biological community at RISDS was recovering relatively rapidly; and Stages II and III infauna were present throughout the region, meeting, if not exceeding, initial predictions. The SPI and plan-view images showed evidence of Stage III infauna present at both the reference and the disposal sites, although as expected their densities were much lower at the disposal site. This was apparent not only from the lower overall RPD values at the disposal site (indicating lower overall bioturbation activity due to lower densities of burrowing and conveyor-belt species), but also the lower densities of burrow openings observed on the sediment surface of the plan-view images from the disposal site (average of 12 per station) versus those from the references areas (more than 100 per station). Despite the deposition of dredged material in layers thicker than the camera prism, larger deposit feeding infauna were able to become established over most of the site. The collection of benthic grab samples aided in the interpretation of biological and recolonization processes at RISDS. The dominant organisms present in the RISDS grab samples were filter feeding sabellid polychaetes, amphipods, phoronids, bivalve molluscs and burrowing, omnivorous nephtyid polychaetes. The presence of dense populations of filter feeding invertebrates in the

RISDS grab samples suggested that Stage II organisms dominated the surface sediments. At the reference stations sampled in Rhode Island Sound, an even greater diversity of filter feeding organisms was present in the grab samples, including dense populations of amphipods, bivalves, and polychaetes. Head-down deposit feeding polychaetes, typical Stage III organisms, were not present in the RISDS grab samples; however, they were a few Stage III organisms identified in the grab samples collected at the reference area stations. Instead, larger surface deposit feeding terebelliform polychaetes were present in the grab samples collected at the disposal site. The presence of dense populations of filter feeding invertebrates at RISDS suggests that Stage II organisms, rather than Stage I organisms, characterized the surface sediments at the recently active RISDS. Therefore, the question arises as to whether these filter feeding organisms were the primary colonizers of RISDS or represented a transitional stage following an earlier colonization by typical Stage I assemblages, such as spionid and capitellid polychaetes. There is good reason to believe that filter feeders may be the primary colonizers at open-ocean disposal sites such as RISDS because the reference areas were also composed primarily of Stage II organisms. At the reference stations sampled in Rhode Island

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Table 3 Summary of plan-view image results for RISDS survey, July 2005. Station

Area (m2)

Sediment type

Infauna

Burrows

Tubes

Tracks

Epifauna

Mudclasts

Ref E-01 Ref E-02 Ref E-03 Ref E-04 Ref E-05 Ref NE-01 Ref NE-02 Ref NE-03 Ref NE-04 Ref NE-05 Ref SW-01 Ref SW-02 Ref SW-03 Ref SW-04 Ref SW-05 A-01 A-02 A-03 A-04 A-05 B-01 B-02 B-03 B-04 B-05 BE-01 BE-02 BE-03 BE-04 BE-05 C-01 C-02 C-03 C-04 C-05 D-01 D-02 D-03 D-04 D-05 E-01 E-02 E-03 E-04 E-05

0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 3.1 3.1 3.1 3.1 3.1 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 1.0 0.9 0.6 0.8 1.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 1.2 0.9 0.9 0.8 0.9

Silty sand Silty sand Silty sand Silty sand Silty sand Silty sand Silty sand Silty sand Silty sand, some gravel Silty sand Silty sand Silty sand Silty sand Silty sand Silty sand Sandy silt Sandy silt Sandy silt Sandy silt Silty sand Sandy silt Silt Silt Silt Silt Silt Silt Gravel Clay and silt Silt Silt Silt Sandy silt Silt Silt Silt Silt Sandy silt Sandy silt Sandy silt Silt Silt Silt Silt Silt

Cerianthids Ind Cerianthid, bivalves Bivalves Bivalves Bivalve Bivalves Bivalve Bivalves Bivalves Bivalves Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes No Yes Ind Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes

>100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 <100 <100 >100 >100 <100 >100 >100 <100 <100 <100 <100 >100 <100 >100 Ind >100 >100 <100 >100 >100 <100 >100 >100 >100 >100 <100 <100 >100 <100 >100

>100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 <100 <100 >100 <100 <100 >100 <100 >100 >100 <100 <100 <100 <100 <100 Ind <100 >100 <100 >100 >100 >100 >100 >100 >100 <100 <100 <100 >100 >100 >100

Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes No Yes Ind Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes

No No Yes Shrimp Yes Starfish Starfish Starfish shrimp, Skate, starfish, crab Starfish, snail Starfish, shrimp Starfish, fish Starfish, shrimp Starfish Starfish, flatfish, crab No No No 1Crab No Dead crab, hermit crab Hermit crab No No No Crab No No No 1 Crab No No No No No No No 1Crab No No Yes Crab and fish No No No

No No No No No No No No No No No No No No No No No No Yes No Yes Yes No No No No No No Yes Ind No No No No No No No Yes No Yes Yes Yes No No No

Table 4 Summary of benthic biology community parameters for reference and RISDS stations, July 2005. Total organic carbon (%)

Number of species

Number of individuals (per 0.04 m2)

Shannon’s H0

Pielou’s J0

Fisher’s alpha

Reference stations REF-E-01 3.92 REF-NE-03 1.80 REF-NE-05 1.82 REF-SW-01 2.93 REF-SW-02 3.29 Average 2.75 Minimum 1.80 Maximum 3.92

0.70 0.34 0.23 0.50 0.32 0.42 0.23 0.70

68 57 67 59 58 62 57 68

2986 1942 1494 2628 2697 2349 1494 2986

3.06 3.50 4.53 3.39 3.26 3.55 3.06 4.53

0.50 0.60 0.75 0.58 0.56 0.60 0.50 0.75

12.39 11.01 14.40 10.71 10.43 11.79 10.43 14.40

RISDS stations B-01 B-04 C-02 C-03 D-03 D-05 E-05 Average Minimum Maximum

1.8 2.3 2.4 2.8 2.3 1.8 1.6 2.1 1.6 2.8

24 28 17 29 32 26 24 26 17 32

237 278 249 85 410 240 211 244 85 410

3.03 3.23 1.78 4.12 3.06 3.16 2.92 3.04 1.78 4.12

0.66 0.67 0.44 0.85 0.61 0.67 0.64 0.65 0.44 0.85

6.67 7.77 4.13 15.53 8.12 7.41 6.97 8.09 4.13 15.53

Sample

a

Grain size (phi)a

5.14 5.43 5.58 5.59 4.71 5.34 5.67 5.35 4.71 5.67

Mean phi value for total sample.

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Fig. 5. SPI images from Station REF-SW-04 (left) and B-03 (right) showing larger sub-surface burrows and denser assemblages of tubicolous amphipods at sediment-water interface of reference area stations compared to RISDS stations.

Similarity

20 40 60 80 100

Ref Ref Ref Ref Ref C-02 E-01 SW-01 SW-02 NE-03 NE-05

E-05

C-03

B-01

B-04

D-03

D-05

Station 10

REF-E-01

8 6 4 D-03 B-04 D-05 B-01 C-02 E-05 2 C-03

PC2 0 -2 -4 -6 -8 -10 -5

REF-SW-01 REF-SW-02 REF-NE-03 REF-NE-05

0

5

10

15

20

PC1 Fig. 7. Similarity (top) and principal components analysis (bottom) of infaunal data from the reference and RISDS stations.

Fig. 6. Plan-view images from Station BE-02 (top) and REF-E-01 (bottom) showing the surface appearance of burrow openings and epifaunal foraging tracks. The extended tentacles of burrowing Cerianthid anemones can be seen on the sediment surface in the bottom image.

Sound, a great diversity of filter feeding organisms were present including dense populations of amphipods, bivalves, and polychaetes. Further, Stage I organisms such as Capitella capitata, Mediomastus ambiseta and spionids, Polydora cornuta and Streblospio

benedicti, often considered as primary colonizers on disposal mounds, are more typical of waters having lower salinities and only rarely occur in open-ocean sites. For example, both of these species are commonly found in upper harbor locations in Boston Harbor, but are rare in outer harbor locations and entirely absent in adjacent Massachusetts Bay (Maciolek et al., 2005a,b). These results suggest that initial colonization of RISDS following dredged material disposal was by small filter feeding organisms that in turn provided a food source and favorable habitat conditions for other invertebrates. Assuming there is good tidal flow, it is reasonable to postulate that small sabellid polychaetes and other

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S.J.K. Wilson et al. / Marine Pollution Bulletin 59 (2009) 26–37

100%

Percent of Individuals

80%

60%

40%

20%

0% B-01

B-04

C-02

C-03

D-03

D-05

E-05

Station Omnivores/scavengers Suspension feeders

REF E- REF NE- REF NE- REF SW- REF SW01 03 05 01 02

Subsurface deposit feeders Surface deposit feeders

Interface feeders Predators

Fig. 8. Percentage of individuals belonging to each trophic guild at RISDS and reference area stations.

invertebrates present in nearby ambient sediments were carried to the disposal site as juveniles and adults in the water column. Dauer et al. (1982) and others have demonstrated that a wide variety of benthic invertebrates are capable of leaving the sediments and moving substantial distances in the water column. After arriving at a new site, individual sabellids worms reproduce continuously by brooding and then releasing clutches of directly developing larvae. In this manner, small sabellids are capable of rapidly establishing large local populations (Rouse and Pleijel, 2001). Migration of other invertebrates from the adjacent ambient sediments via adult movements in the water column or by settlement of planktonic larvae will continue to expand the species richness and diversity at the disposal site. In general, 35–40 percent of the regional metapool of species found at the reference stations also occurred at the disposal site six months after the disposal event. These results indicate that infaunal succession was well advanced at RISDS at the time of sampling. For this study the collection of benthic infaunal samples in addition to sediment-profile and plan-view images adds considerable insight to understanding the recovery of RISDS. By defining the local regional species metapool present at the reference stations, the ambient local fauna can be interpreted both in terms of traditional benthic ecology and the infaunal successional paradigm. The recovery of the disposal mounds can then be understood by comparing the infaunal structure of the disposal site to that of the reference area. The classification of infauna into trophic groups or guilds is another technique that assists in the comparison of traditional infaunal results and profile images. There have been many studies comparing SPI to grab results with varying levels of concordance (Rhoads and Germano, 1982; Nilsson and Rosenberg, 1997; Nilsson and Rosenberg, 2000). In fact, Rhoads and Germano (1982) point out that sediment-profile imaging was never intended to replace grab sampling for a detailed characterization of benthic community structure but that a certain percentage of SPI images should be ground-truthed with traditional benthic infaunal sampling. Although the sediment-profile and plan-view imagery results showed evidence of Stage III organisms being present at RISDS, while benthic community analyses indicated that Stage II organisms dominated the surface sediments at RISDS, we believe that these results are complementary and provide a more complete assessment of site conditions.

Some of the advantages and limitations of each technique are discussed below. Grab samples provide demographic data about benthic community structure including a species list, abundance counts, species density, and derived metrics such as diversity. With SPI imagery, only limited identification of actual species inhabiting the site is possible. Instead, observers rely on presence of tubes and burrows, sub-surface voids and trails, surface biota, and the occasional readily identifiable organism to establish the successional stage classification. Because benthic assemblages differ widely from one area to another, the organisms actually comprising the infaunal successional stages (I, II, and III) applied in SPI image analysis are not consistent. SPI observers usually assume that small tubes and burrows at or near the surface represent spionid and capitellid polychaetes, typically thought to be initial colonizers in disturbed sediments. These would be categorized as Stage I in the infaunal successional paradigm (Rhoads and Germano, 1986). As we see at RISDS, spionids and capitellids were either rare or non-existent in the infaunal samples. Instead, the dominant organisms were small filter feeding sabellid polychaetes, amphipods, phoronids, bivalve molluscs and burrowing, omnivorous nephtyid polychaetes, typical Stage II organisms. A few larger, deep-burrowing organisms were present, including surface-deposit feeding terebelliform polychaetes, however head-down deposit feeding maldanid polychaetes, typical Stage III organisms that produce sub-surface voids, were absent at RISDS. Sediment-profile image data, on the other hand, provides in situ information about processes such as bioturbation, trophic stratification, biogenic irrigation, and interspecies competition as manifested in small-scale spatial relationships. SPI technology provides for the preservation of animal-sediment relationships, allowing investigators to deduce dynamics from the animal-sediment structure (Rhoads and Germano, 1982). Although the analysis is typically limited to epifaunal or shallow infaunal organisms which are easily observed at the sediment surface (Stages I and II species), the presence of infaunal deposit feeding species (Stage III) can be inferred from their effect on sediment structures (i.e. sub-surface feeding voids) (Rhoads and Germano, 1982). Additional advantages of SPI data include increased horizontal and vertical characterization of fauna distribution. In this study only one grab was taken at each station to develop a list of representative species. The relatively rare occurrence of large burrow openings in the plan-view images demonstrates the unlikelihood

S.J.K. Wilson et al. / Marine Pollution Bulletin 59 (2009) 26–37

of capturing one of these representative Stage III taxa in a 0.04 m2 grab (approximately 1/23 the surface area of the plan-view image). Differences in characterizing the vertical distribution of fauna within the sediment column are also likely to exist because the collection of benthic grab samples is limited to the top 10 cm of the sediment column, whereas the sediment-profile image allows for evaluation of the top 15–20 cm of the sediment column, depending upon penetration. For example the cerianthid anemones seen in plan-view images, create large burrows visible in SPI images, but generally escape below the depth of the grab before a sample can be collected. The results of the 2005 survey suggest that the collection of sediment-profile and plan-view images and benthic infaunal samples allowed for a comprehensive assessment of the benthic conditions in Rhode Island Sound and provided insight into understanding the recovery of RISDS following placement of sediment from the PRHMDP. The conclusion was that the biological community at RISDS was recovering relatively rapidly and Stages II and III infauna were present throughout the region. In conclusion, because sediment imaging and sediment grab sampling techniques target different areas of the benthic ecosystem and sometimes produce different types of results, this study highlights the importance of clearly identifying the objectives of the study and selecting the appropriate sampling techniques. The tiered approach of the DAMOS program allows managers to address program objectives by providing complementary views of the benthic ecosystem through careful hypothesis development and sample plan development. Acknowledgements We would like to acknowledge the US Army Corps of Engineers, New England District, under Contract No. W912WJ-07-D-0002, to ENSR for supporting this project. We would also like to thank Ananda Ranasinghe and Dr. Angel Borja for organizing this special opportunity through the Estuarine Research Federation Conference. In addition, we would like to thank Isabelle Williams and Stacy Doner for their efforts in sorting and identifying the benthic infauna and the field crews and marine technicians from ENSR, CR Environmental, and Germano and Associates for their assistance with the sample collection. References Battelle, 2002. Fall 2001 REMOTSÒ Characterization report, Rhode Island Region, Long-term dredged material disposal site evaluation project, Contract No. DACW33-01-D-0004, Delivery Order No. 02. US Army Corps of Engineers, New England District, Concord, MA, 37 pp. Battelle, 2003. Sediment profile imaging of area E and area W, July 2003, Rhode Island Region, long-term dredged material disposal site evaluation project, Contract No. DACW33-01-D-0004, Delivery Order No. 02. US Army Corps of Engineers, New England District, Concord, MA, 21 pp. Bolam, S.G., Rees, H.L., Somerfield, P., Smith, R., Clarke, K.R., Warwick, R.M., Atkins, M., Garnacho, E., 2006. Ecological consequences of dredged material disposal in the marine environment: a holistic assessment of activities around the England and Wales coastline. Marine Pollution Bulletin 52, 415–426. Bolam, S.G., Rees, H.L., 2003. Minimizing impacts of maintenance dredged material disposal in the coastal environment: a habitat approach. Environmental Management 32, 171–188. Clarke, K.R., Gorley, R.N., 2001. PRIMER v.5: User manual/tutorial. Plymouth Marine Laboratory, Plymouth, United Kingdom, pp. 91. Dauer, D.M., Maybury, C.A., Ewing, R.M., 1981. Feeding behavior and general ecology of several polychaetes from the Chesapeake Bay. Journal of Experimental Marine Biology and Ecology 54, 21–38.

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