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Lights, camera and acoustics: Assessing macrobenthic communities at a dredged material disposal site off the North East coast of the UK
Journal of Marine Systems 62 (2006) 204 – 216 www.elsevier.com/locate/jmarsys
Lights, camera and acoustics: Assessing macrobenthic communities at a dredged material disposal site off the North East coast of the UK Silvana N.R. Birchenough a,⁎, Siân E. Boyd a , Roger A. Coggan a , David S. Limpenny a , William J. Meadows b , Hubert L. Rees a a
b
Centre for Environment, Fisheries and Aquaculture Science, Burnham Laboratory, Remembrance Avenue, Burnham-on-Crouch, Essex, CM0 8HA, UK Centre for Environment, Fisheries and Aquaculture Science, Lowestoft Laboratory, Pakefield Road, Lowestoft, Suffolk, NR 33 0HT, UK Received 7 July 2004; received in revised form 24 March 2006; accepted 27 March 2006 Available online 21 July 2006
Abstract This study presents the results of a trial assessment based on a combination of sampling techniques at a dredged material disposal site located off the North East coast of the UK, over 2001 to 2004. The site was surveyed with a high-resolution sidescan sonar system producing a mosaic with 100% coverage of the survey area. Benthic communities and sediments were ground-truthed using a Hamon grab with a video camera. Additionally, the area was also sampled in 2003 with a Sediment Profile Imaging (SPI) camera, which complemented other techniques by providing in situ information on sediment quality, and biogenic activities. An assessment is made of the benefits of combining the results from conventional methods, principally using grab samples, with those from acoustic techniques and optical imaging devices to determine seafloor and macrobenthic conditions. This information has the potential to contribute to the enhancement of routine monitoring programmes within UK waters. Crown Copyright © 2006 Published by Elsevier B.V. All rights reserved. Keywords: Sidescan sonar; SPI; Macrobenthos; Dredged material disposal site
1. Introduction Historically, large amounts of industrial wastes (especially colliery waste and fly ash) were disposed of from ships at licenced sites off the northeast coast of England (e.g. Eagle et al., 1979; Bamber, 1984; Khan and Garwood, 1995; Frid et al., 1996; Herrando-Perez and Frid, 1998, 2001; Bustos-Baez, 2003). In most cases disposal of these materials started well before statutory
⁎ Corresponding author. Tel.: +44 1621787211; fax: +44 1621784989. E-mail address: [email protected] (S.N.R. Birchenough).
controls for the protection of the marine environment were enforced (Eagle et al., 1979). The disposal of industrial wastes and sewage sludge was phased out in 1998. Presently, sea disposal from ships is largely confined to dredged material, but the environmental consequence of this activity has been the focus of increasing attention from government, the general public and industry (for review see Bolam and Rees, 2003). In UK waters, the disposal of dredged material is licensed under the Food and Environment Protection Act (Great Britain-Parliament, 1985a) following guidelines laid down by International Convention (Great BritainParliament, 1972a,b, 1985a). This act controls deposits in the sea below the mean high water springs in order to
0924-7963/$ - see front matter. Crown Copyright © 2006 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jmarsys.2006.03.011
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protect human health, the marine environment and legitimate uses of the sea. The licencing authority for England and Wales is the Department for Environment, Food and Rural Affairs (Defra) (previously the Ministry of Agriculture Fisheries and Food (MAFF)). In many cases, openwater disposal to designated sites is the best practicable environmental option and also the only economically realistic one. Criteria which must be satisfied before a licence is granted to dredge and dispose of material at sea include the chemical quality of the material, the quantity to be disposed of, its nature and origin and its predicted impacts at the area of disposal (Rees et al., 2000). Worldwide, dredged material disposal at sea is an activity which is increasing as a consequence of economic development in coastal areas. There have been a large number of associated monitoring studies at disposal sites but, by their nature, reports of such activities do not lend themselves to publication in the peer-reviewed literature. Recent studies include those by Smith and Rule (2001), Zimmerman et al. (2003), Cruz-Motta and Collins (2004), Fredette and French (2004) and Valente (2004). Accessing such information can present a challenge when seeking to draw from previous ‘best practice’ in order to modify or enhance sampling practices elsewhere (Valente, 2004). The Tyne dredged material disposal site (DMDS), also called TY070, is located 5.7 km off the Northumberland coast; this site receives maintenance-dredged material from the Tyne Estuary, that, sometimes exceeds 150,000 t/ year (Fig. 1). Since the commencement of disposal operations, the Ministry of Agriculture Fisheries and Food (MAFF) (Eagle et al., 1979) and subsequently, the Centre for Environment, Fisheries and Aquaculture Science (Cefas) have intermittently surveyed environmental conditions at this site (e.g. MAFF, 1994; CEFAS, 2003). In the past, scientists have relied on traditional sampling tools (e.g. grabs, corers and dredges) to quantify
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sediments and their associated benthic fauna (macrofauna and meiofauna) (e.g. Boyd, 2002; Eleftheriou and McIntyre, 2005). This information only relates to a specific point on the seabed from which the sample was collected and, therefore, inferences regarding the wider distribution of substrata and the associated fauna may be unrealistic, especially in patchy environments (Brown et al., 2002). Whilst there are sound reasons for continuing to apply such approaches (e.g. Rees et al., 1990), with improvements in technology there is increasing interest in using acoustic and optical techniques for assessing anthropogenic effects, particularly in areas of known habitat heterogeneity. A number of acoustic techniques (especially sidescan sonar and more recently multibeam bathymetry) have been widely employed to monitor the physical disturbance of the seabed resulting from aggregate extraction activities (Boyd, 2002; Limpenny et al., 2002; Boyd et al., 2003, 2004, 2005) and to map the distribution of gravel habitats (e.g. Brown et al., 2001, 2002, 2004) in UK waters. However, an ongoing challenge is to establish the extent to which such techniques, used in conjunction with appropriate ground-truthing methodology including optical methods, can be used to improve assessment of the status of benthic communities in relation to natural and manmade influences. Currently, there are a number of groundtruth techniques (i.e. grabs, dredges, and cores), and optical methods include video and stills photography, as well as Sediment Profile Imaging (SPI) (Rhoads and Germano, 1986, 1990; Solan et al., 2003). A limited number of studies further serve to illustrate the combined use of acoustic, optical and traditional methods for seafloor mapping and impact assessment (e.g., Zajac et al., 2000, 2003; Kostylev et al., 2001, 2005; Cutter et al., 2003; Disposal Area Monitoring System (DAMOS), 2006).
Fig. 1. Wet weight in tonnes of maintenance-dredged material licenced for offshore disposal at TY070 DMDS (in Defra unpublished data).
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Table 1 Summary of survey techniques applied at TY070 DMDS Year
SSS
2001 2002 2003 2004
7 9 11 11
Multibeam
1
GRAB
CAMERA
SPI
6 21 21 21
3 5 5
15 (triplicate)
vices to determine seafloor conditions resulting from the disposal of dredged material at a licenced site off NE England. This information has the potential to contribute to the development of improved monitoring approaches within UK waters. 2. Methods
SSS = sidescan sonar survey contains the number of swaths used to cover the area. Camera = video stills used for this work. Grab = number of grabs collected during each survey.
2.1. Study site and data collection
The aims of the present study were to assess the advantages of combining conventional grab sampling methods, acoustic techniques, and optical imaging de-
The disposal site TY070 consists of a segment of a circle of approximately 2.73 km2. This site and its surrounding area were surveyed annually over the period
Fig. 2. A) Map showing the location of the licenced dredged material disposal site (TY070) and grab-sampling stations from the surveys carried out in 2001–2003. B). Map showing the SPI sampling survey in 2003.
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2001 to 2004 using a variety of techniques, including sidescan sonar, multibeam bathymetry, grab sampling, imaging of the sediment surface using towed video, and SPI (Table 1). A pilot survey was conducted during 2001 to assess the nature of the substrate in the area, and to test additional ground-truth techniques. Grab samples (0.1 m2 Hamon grab) were collected to ground-truth the acoustic surveys through the provision of information on sediment particle size distributions and macrofaunal communities. The grab survey involved the collection of two replicates at each of five stations; i) two located inside the disposal site (codes used: gtdispn and Gtdispn; samples were collected for sediments and fauna), ii) two northeast of the disposal site (codes used: gtsemot and GTNEpat; samples were collected for fauna only and for sediment and fauna respectively) and iii) one west of the disposal site (code used: GTWHOM; samples were collected for fauna only). A sidescan sonar survey was conducted once each year 2002–2004 to provide a 100% mosaic of the site (for methodology of the acoustic survey, see Boyd et al., 2006). A visual analysis of the sidescan sonar mosaic was conducted. The delineation of acoustically distinct areas was aided by the use of still video photographs of sediments collected by grabs, video stills and sediment profile imagery. In 2002–2004 a video camera attached to the 0.1 m2 Hamon grab was used to ground-truth the sediment patterns (Fig. 2-A). A stratified random sampling design was employed for the grab survey, with stations randomly
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positioned within each of the acoustic regions based on the outputs from the sidescan sonar survey. The number of stations within each region was allocated in proportion to the size of the area. A sub-sample was taken from each of the Hamon grab samples for sediment particle size analysis (PSA) following the methodology given in Boyd (2002). The remaining contents of each grab were processed for benthic macrofauna. Sieving, sorting, preservation and laboratory processing of samples were done according to the methodology described in Boyd (2002). In addition to the images collected by the video camera attached to the Hamon grab sampler, a camera sledge was towed along the seafloor to collect video footage at selected locations. This continuous footage was subsequently divided into a series of still images to identify seafloor features of interest at specific locations. The SPI camera survey was conducted in 2003 at a total of fifteen stations, located inside and in the immediate vicinity of the disposal site (Fig. 2-B). The camera used was a Nikon F-801s and it was deployed in the vicinity the stations used for the grab sampling. The SPI camera provided undisturbed, in situ surface and profile images of the upper ∼ 20 cm of the sediment column. 2.2. Data analysis Univariate analysis based on total number of individuals (excluding colonial species) and the total
Fig. 3. Temporal change (2001–2004) of the sidescan sonar mosaic at TY070 DMDS. The segment of a circle delineates the disposal site (centre). Letters on the 2003 mosaic are used to label acoustically distinct areas: A = homogenous sand, B = heterogeneous sediments, C = centre of the DMDS, D = hummocky area and E = sandy muds.
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number of species were calculated from the Hamon grab samples to provide a quantitative assessment of benthic assemblages within each acoustic region over time. Additionally, multivariate analyses of data were conducted with the software PRIMER (Plymouth Routines in Multivariate Ecological Research), version 6 for Windows. Following square root transformation of the abundance data (excluding colonial taxa), non-metric multidimensional scaling ordination (MDS) of Bray–Curtis similarity measures was used to assess changes in species composition (Clarke and Warwick, 2001). Pairwise ANOSIM (Analysis of Similarities), (Clarke, 1993) was also conducted to test for significant differences in macrobenthic assemblage composition over time and among acoustically distinct areas. The SIMPER routine (similarities percentages) within PRIMER was used to determine the contribution of individual species towards the dissimilarity between years and stations, while the BIO–ENV (Biota-Environment) analysis was used to find the best match
between the multivariate sample patterns of an assemblage and from environmental variables associated with those samples (Clarke and Gorley, 2006) over time at study sites. The environmental variables tested were % gravel, % sand and % silt/clay and annual quantities of licenced dredged material disposed into the area. Particle size distributions were analysed using Principal Component Analysis (PCA) to assess spatial and temporal changes at the stations. The variables used were the sorting coefficient, % gravel, % sand and % silt/clay of individual sediment samples. The software to process the SPI images was Image Analyst for Macintosh version 9.0.3. Three replicate images were taken at each sampling station, and the median values of derived measures were calculated. From each image, the sediment type, sediment boundary roughness (SBR), apparent redox potential discontinuity (aRPD) depth and the Benthic Habitat Quality index (BHQ), (Nilsson and Rosenberg, 1997) were calculated.
Fig. 4. Close-up sidescan sonar records for locations A–D in Fig. 4, along with video images showing seabed types: A) homogeneous sandy mud in the offshore part of the survey area (area shown is approx 200 m × 400 m); B) heterogeneous sediments composed of shelly muddy sand, sandy mud, coal and clay (area shown is approx 800 m × 1600 m); C) shelly muddy sand with colliery waste, present inside the disposal site. Also evident are linear trawling marks and possibly anchor drag scars (area shown is approx 300 m × 600 m) and D) discrete patches of soft material (arrowed) to the west of the disposal site (area shown is approx 600 m × 1200 m).
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3. Results 3.1. Acoustic survey interpretation The sidescan sonar mosaic from each of the acoustic surveys is shown in Fig. 3. Five acoustically distinct areas were delineated over the entire study site (2003 image in Fig. 3). The bathymetry in the area gently sloped from around 30 m in the west to 50 m in the east (bathymetric survey reported in Boyd et al., 2006). The seafloor to the west of the disposal site had a hummocky topography (Fig. 3, area D), whereas the seafloor topography to the east was much smoother (Fig. 3, area A). Temporal comparison of the sidescan sonar records shows that there was little change in the broad distribution of sediments and features within the survey area between 2001 and 2004. The surficial sediments in the offshore area east of the disposal site were soft in nature and dominated by relatively featureless, homogeneous sandy muds with small amounts of shell material at the surface (Fig. 3 area A and Fig. 4-A). The video stills showed that burrowing and tube-forming fauna (Turritella, sp. and Antalis sp.) inhabited these sediments. Occasionally, harder linear features were also observed over this substratum. These linear features might be attributable to other impacts (e.g. demersal fishing activity). At the inshore area (Fig. 3 area B), the sediments were observed to be coarser and more heterogeneous with a band of shellier muddy sands and sandy muds, intermixed with coal and clay, dominating the central part of the survey area (Fig. 4-B). In the western part a mixture (Fig. 3, area C) of sand, mud, coal and other rocks are frequently encountered and can be linked with coal measures known to be present in this area. At the disposal site itself, sediments were very mixed. The centre was characterised by a lower acoustic backscatter and sediments mainly consisted of sandy muds and muddy sands. The southern and central part of the site was generally muddy in nature, with some coarse sand present. The sidescan sonar imagery portrays this as a patch of predominantly finer sediment with areas of coarser, mixed material at the periphery, which appears to represent the acoustic footprint of the deposited sediments. The remainder of the site was mainly shelly, muddy sand with varying quantities of colliery waste and the presence of Alcyonium digitatum, the brittlestars Amphiura sp., and Ophiura sp., hydroids and bryozoans (Fig. 4-C). Underwater photography showed an area of mixed sediment extending away from the southwest boundary of the licenced area with the presence of A. digitatum (Fig. 4-D). It was apparent
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that the coarser component of this substratum consisted of gravel and cobble-sized coal particles within a muddy sand matrix. Patches of finer sediment (muddy sand/sandy mud) also appear within this area. From the visual assessment provided by the video and SPI this appeared to be a combination of both disposal operations and natural coal deposits which were exposed at the seabed.
Fig. 5. A) Principal Components Analysis (PCA) of sediment characteristics (sorting, % gravel, % sand and % silt/clay) for ground-truth samples at TY070 over 2001–2004. B) PCA ordination based on acoustically distinct areas delineated at TY070.
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3.2. Sediments In the first year of study (2001) the sediment composition was observed to be an admixture of sand, silt/clay and gravel at all the stations. In the centre of the dredged material disposal site (stations gtdispn and GTdisp), the sediments were mainly a combination of sandy mud and muddy sands, whereas east of the licenced area (station gtnepat), sediments were predominantly composed of muddy sand. The centre of the licenced area was characterised by the presence of sandy muds and muddy sands. East of the disposal site (area E) was characterised by muddy sands. Sediments to the west of the survey area (area B) were, in contrast, predominantly composed of sandy gravel over time. Additionally, area A was observed to be patchy sandy gravels with muddy sands. Principal Components Analysis (PCA) of the sorting coefficient, % gravel, % sand and % silt/clay showed the W–E changes in samples over time (Fig. 5-A). A relatively even spread of points can be seen in the ordination; there is a tendency for clustering (left side of
the ordination) of some of the samples from 2002, 2003 and 2004 (Fig. 5-A). The distribution of the samples along the PC 1 axis accounted for 63% of the variability and could be explained by the inverse variation in % sand. PC2 represented 31% of the variability, which was associated with the % silt/clay content. The spatial representation of acoustically distinct areas can also be seen in Fig. 5-B. There is a tendency for clustering on the left side of the plot of the majority of samples from the acoustic areas E, C and D. 3.3. Biological composition A total of 172 taxa were identified over the 4 years of the study. In 2001 the pilot survey of the area was designed to assess differences between the centre of the licenced dredged material disposal site and adjacent areas. Hamon grab samples were collected to target and ground-truth the acoustically distinct areas and also to provide an indication of the status of macrobenthic assemblages. Fig. 6 shows the values of total abundance of individuals and total number of species for the
Fig. 6. Temporal changes in the macrobenthos collected at the Tyne DMDS over 2001–2004. A) Total abundance of individuals. B) Total number of species.
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Fig. 7. A) Non-metric multidimensional scaling ordination using Bray Curtis similarities measure computed for double square-root transformed species abundances for 2003, B) 2004, C) MDS for all years overlain with dredged material quantities (extracted from Fig. 1) and D) MDS for all years overlain with % gravel.
different acoustic areas sampled over the 4 years of the study. In 2001, samples were only collected from within 4 acoustic areas (i.e. areas B, C, D and E). Over these 5 distinct acoustic areas the values of total abundance of individuals ranged from 45–374/0.1 m2 and the total number of species ranged from 20–50/0.1 m2. Lower values of both univariate metrics were observed at area B (west of the dredged material disposal site) in comparison with areas C, D and E over 2002–2004. For simplicity the results of multidimensional scaling using Bray–Curtis similarity coefficients were separated over years to assess the pattern of variation. There is an E–W orientation in both ordinations for 2003 and 2004 (Fig. 7-A–B). An aggregation of samples from the disposal site and adjacent areas is observed to the right side of the ordination, while other samples from the disposal site were more widely dispersed in both years (Fig. 7A–B). Results showed that community composition was best represented by a combination of deposited material, % gravel, and % silt/clay (BIO–ENVanalysis; r = 0.483). Fig. 7 C–D show the faunal data overlain with these
environmental variables (e.g. dredged material quantities and % gravel). The species distribution surrounding the disposal site appears to be influenced by the disposal operation. This is indicated by the separation of samples located in the disposal site and immediate vicinity (Fig. 7-C). Additionally, elevated % gravel (Fig. 7-D) was noted at areas within the disposal site itself and also at areas A and D. The result of the two-way nested ANOSIM provided evidence of significant differences (r-statistic = 0.26; p < 0.002) over time and over acoustically distinct areas (r=0.31; p<0.001) during the study period. SIMPER results (see Boyd et al., 2006) showed that the highest values for overall dissimilarity were found for the comparison between 2001 and 2002 (72.11%) and 2001 versus 2004 (73.72%). It is clear that the differences observed over time are attributable to the changes in abundance of a number of taxa, resulting from the disposal activities at the site. Examples of species showing appreciable changes include the polychaete Lagis koreni, which had a high average abundance during the first year of the study and lower abundances
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Fig. 8. Sediment profile images from the July 2003 survey at TY070.
subsequently. The dissimilarity between the acoustically distinct areas was also analysed. The highest overall dissimilarity was encountered between areas C and B (73.62%) and E versus B (72.80%). Decreased abundance at area B can be attributed to the type of substratum encountered (i.e. sandy gravel) and the high abundance of the carnivorous polychaete Lumbrineris gracilis. 3.4. Sediment profile imaging SPI images from July 2003 are shown in Fig. 8 for stations located in the centre of TY070 and surrounding areas. Surface layers of sediment were comprised predominantly of fine sand. Some medium sand and fine muddy material was seen throughout most of the stations sampled. A varying shelly fraction was also visible at the sediment surface at many of the stations surveyed. This material was seen as the highly reflective fragments scattered across the sediment/water interface in the surface photographs. There was a certain amount of re-suspension of fine material at many of the stations sampled (an artefact
of SPI technology: R. Valente pers comm.). An obvious example of this was observed in the surface and profile images taken at station 19 (Figs. 8 and 9). Coarser sediments were recorded at stations 22 and 25, while boulder field/bedrock outcropping were also recorded at stations 36, 37 and 39, west of the disposal area. Surface boundary roughness indicates unevenness of the sediment surface as a result of physical activity and bioturbation processes (Rhoads and Germano, 1986). Values ranged from a low of 0.33 (the very flat sediment surface present at station 33 in the disposal area) to a high value of 1.58 (a biogenically-roughened sediment-water interface at station 30, located east of the disposal area). High roughness values were largely attributed to biogenic features. For example, in photographs taken at station 18 (Figs. 8 and 9), surface roughness was due to the activities of decapods whose burrows were evident in several SPI and surface photographs (stations 18, 30, and 32) or to tube formation and bioturbating activity of infaunal animals (e.g., polychaetes and holothurians).
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Fig. 9. Surface images from July 2003 at the Tyne DMDS. B = decapod, C = Alcyonium digitatum, D = coarse substratum and E = Buccinum sp.
The apparent redox potential discontinuity (aRPD) depths (the visible line between oxygenated and reduced sediment) recorded from the SPI images are presented in Table 2. The evident aRPD depth was greatly dependant on the presence/absence of bioturbating fauna. Where animals were present (stations 18, 19, 21, 30 and 32), the aRPD layer was relatively deep and uneven, and the re-working of sediment observed in the sediment profile and oxygenated voids were generally present at depth within the sediment. At stations where the excavating, irrigation, burrowing and feeding activities of such animals were absent (stations 28, 29 and 33), a shallower aRPD layer occurred, reflecting the presence of reduced sediment recently placed from a dredged material disposal operation. Station 29 revealed a very limited amount of oxygenated sediment present, with an associated low level of biological activities (Fig. 8). These stations were all located inside the disposal site boundary. Due to the presence of coarse/rocky substrata, it was not possible to gauge the depth of the aRPD layer at stations 36, 37 and 39 (Fig. 8). It is important to highlight the variation in different aRPD depths within the disposal site itself, with both relatively
high and low values recorded. This may be attributable to the degree of historical disposal activity at the site. The stations located outside the disposal site returned consistently high aRPD values (Table 2). The presence of burrows (attributable to the excavating activities of several infaunal species) were clearly seen in many of the profile images (e.g. station 34 with faunal activity at depth); in some cases substantial sections of these burrows were visible. A large decapod (Fig. 9 image 19 B) and a pelican foot shell (Aporrhais pespelecani) were also visible on the sediment profile image for station 26 (Fig. 8). Colonies of the anthozoan A. digitatum (soft coral) were visible in the surface images at station 27, 36 and 37 (Fig. 9). These indicate the presence of suitable substrata for attachment (i.e. gravel, shelly material or coal), although these stations were located south west of the licenced area. In the surface image taken at station 30, surface tracks produced by the whelk Buccinum sp. were evident (Fig. 9 image 30 E). The Benthic Habitat Quality Index was calculated for the stations surveyed following the methodology proposed by Nilsson and Rosenberg (1997) (Table 2). Successional stages varied from stage I environments (e.g.,
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Table 2 Results from the analysis of SPI images, including sediment type, penetration, Sediment Boundary Roughness (SBR), apparent redox discontinuity depth (aRPD), and BHQ Stages (calculated from the Benthic Quality Index) Stations SED. type Penetration (cm) SBR aRPD depth BHQ stage 18 19 21 22 25 26 28 29 30 32 33 34 36 37 39
4–3 phi 3–2 phi 4–3 phi 3–2 phi 4–3 phi 4–3 phi 4–3 phi >4 phi 4–3 phi 4–3 phi >4 phi >4 phi >4 phi >4–3 phi 3–2 phi
14.59 5.36 10.40 5.80 13.43 8.53 5.21 5.55 17.54 15.28 12.43 16.26 – – –
1.35 1.06 0.50 1.00 1.62 1.55 1.02 1.08 1.58 0.96 0.33 1.40 – – –
5.84 5.39 7.08 4.93 10.31 1.56 2.97 1.43 7.38 8.74 2.88 2.68 – – –
II–III II II II–III II–III I I–II I III III I I I–II I–II I–II
stations 26, 29 and 33, located within the disposal site, were characterised by shallow aRPDs and the absence of discernible biogenic features) to Stage III (stations 18, 30 and 32 outside the disposal area) with their characteristically deep aRPDs, well-developed faunal communities and prominent biogenic features such as burrows and feeding mounds. 4. Discussion Marine benthic habitats are vulnerable to the influence of a wide range of anthropogenic activities (e.g. dredged material disposal, aggregate extraction, windfarm developments, oil and gas exploitation and fishing impacts). Recent developments in seabed mapping techniques driven by continuous improvements in acoustic systems offer the potential to radically improve singlepoint sampling approaches to monitor the impacts of such activities. These opportunities provide benthic ecologists with new avenues for studying the structure and dynamics of benthic communities at multiple spatial scales (Zajac et al., 2003). The Tyne dredged material disposal site is used as an exemplar to establish the extent to which such a combination of techniques could be used in future routine environmental monitoring programmes undertaken in UK waters. Acoustic techniques have been routinely employed for many years as a tool in disposal site monitoring but the resolution, affordability and accessibility of the technology has greatly increased in recent years (Kenny et al., 2003; Van Lancker et al., 2003). In this study the utility of
acoustic techniques was demonstrated by: i) the facilitation for rapid coverage of large areas of seabed, ii) facilitating the delineation of sedimentlogically and biologically distinct areas and iii) providing information on the footprint of disposal activity. Examination of the data from grab samples confirmed a distinct area of the seabed within the disposal site, characterised by sediments which were largely muddy in nature, but also included patches of coarse sand. Other substrata present over the survey area also demonstrated agreement between the particle size distribution and SPI images. The particle size distribution data provided the quantitative information to ground-truth the sediments of the area. The use of SPI complemented the PSA information with an in situ representation of sediment structure and quality at each station. On a smaller spatial scale, a video camera attached to the Hamon grab provided instantaneous information on the undisturbed image of the surface sediments, including any associated epifauna. This information clearly only gave a localised and general indication of the footprint of the disposal activity. However, the images at certain stations in the vicinity of the disposal site revealed apparently undisturbed conditions at the sediment surface. Evidence of a legacy of disposal activity was provided by the vertical SPI images, and the combination of the two sources of information was therefore helpful in evaluating overall seabed status. The Hamon grab (covering 0.1 m2) provided pointsample information on fauna and sediment composition. These data allowed a quantitative analysis over the different areas and, to a degree, identified changes occurring within and in the near vicinity of the disposal site between 2002–2004. A decline in number of individuals and species over time was observed for area B (west of the disposal site), when compared to areas C, D and E. Multivariate analysis indicated a certain degree of similarity in terms of faunal distribution over time including some of the stations located at the centre of the disposal site (Fig. 7). However, stations located within the western and southern part of the disposal site were relatively dissimilar; this was also observed in the sidescan images. As expected the BIO–ENV analysis identified an influence on the assemblage associated with the disposal of dredge material and sediment type (e.g. especially variability in % gravel and silt/clay). The disposal site was directly influenced by a combination of coarse and soft sediments. The areas outside and vicinity of the disposal site were mainly composed of soft material and medium sands. SPI has found wide application worldwide (e.g. in the U.S.A., a clear example is the Disposal Area Monitoring System program (DAMOS, 2006), (O'Connor et al.,
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1989; Rumohr, 1995; Keegan et al., 2001; Solan et al., 2003; Fredette and French, 2004; Valente, 2004) but has only been used to a limited extent in the UK. The present pilot study off the Tyne confirmed that SPI can provide valuable information to assist in interpreting impacts occurring both within and in the near vicinity of the disposal site that could not be discerned from the sidescan sonar or grab sample data alone. The attributes of SPI include: i) assisting in mapping thin layers of deposited dredged material which are not clearly detected by acoustic technology, ii) providing complementary information on behaviour and disruption of benthic organisms and sediment quality status and iii) supplying near real-time data return. Therefore, by incorporating the use of SPI in future surveys we can envisage a more informed and efficient monitoring exercise. We conclude that the combination of approaches employed in this study has potential for wider application around the UK coast. With the exception of SPI, the techniques employed in this study have been widely used at other disposal sites continuously around the UK coast, although not always in synchrony. Procedures for integrating these approaches, exemplified by the present study appear, to offer significant benefits. Clearly, the scope for all combination surveys will be limited by local circumstances (e.g. the presence of bedrock provides an obvious constraint on SPI use) and the objectives of the investigation. However, the capability to link point sampling with a wider spatial perspective should, in general, be especially advantageous. In the particular case of the Tyne, the study provided complementary evidence of the localisation of impacts arising from disposal, but also provided additional insights into the heterogeneous character of the environment in the wider region, and the interaction between natural and anthropogenic influences, including those attributable to earlier disposal activities. Acknowledgments This work was funded by the UK Department for the Environment, Food and Rural Affairs (Project code AE1033, BA004, AE0261 and AE1224). Authors would like to thank Joe Costelloe, Stiofan Creaven and Brendan O' Connor (Aqua-fact, Limited Galway) for their support with the SPI interpretation. Thanks are also given to Stacey Faire for constant encouragement during the revision of this manuscript and Mrs Jacqueline Rhodes for assistance in the compilation of references. The manuscript was greatly improved with comments and discussion with Dr Joe Germano, Mr Ray Valente and Dr Ruth Parker.
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Lights, camera and acoustics: Assessing macrobenthic communities at a dredged material disposal site off the North East coast of the UK Silvana N.R. Birchenough a,⁎, Siân E. Boyd a , Roger A. Coggan a , David S. Limpenny a , William J. Meadows b , Hubert L. Rees a a
b
Centre for Environment, Fisheries and Aquaculture Science, Burnham Laboratory, Remembrance Avenue, Burnham-on-Crouch, Essex, CM0 8HA, UK Centre for Environment, Fisheries and Aquaculture Science, Lowestoft Laboratory, Pakefield Road, Lowestoft, Suffolk, NR 33 0HT, UK Received 7 July 2004; received in revised form 24 March 2006; accepted 27 March 2006 Available online 21 July 2006
Abstract This study presents the results of a trial assessment based on a combination of sampling techniques at a dredged material disposal site located off the North East coast of the UK, over 2001 to 2004. The site was surveyed with a high-resolution sidescan sonar system producing a mosaic with 100% coverage of the survey area. Benthic communities and sediments were ground-truthed using a Hamon grab with a video camera. Additionally, the area was also sampled in 2003 with a Sediment Profile Imaging (SPI) camera, which complemented other techniques by providing in situ information on sediment quality, and biogenic activities. An assessment is made of the benefits of combining the results from conventional methods, principally using grab samples, with those from acoustic techniques and optical imaging devices to determine seafloor and macrobenthic conditions. This information has the potential to contribute to the enhancement of routine monitoring programmes within UK waters. Crown Copyright © 2006 Published by Elsevier B.V. All rights reserved. Keywords: Sidescan sonar; SPI; Macrobenthos; Dredged material disposal site
1. Introduction Historically, large amounts of industrial wastes (especially colliery waste and fly ash) were disposed of from ships at licenced sites off the northeast coast of England (e.g. Eagle et al., 1979; Bamber, 1984; Khan and Garwood, 1995; Frid et al., 1996; Herrando-Perez and Frid, 1998, 2001; Bustos-Baez, 2003). In most cases disposal of these materials started well before statutory
⁎ Corresponding author. Tel.: +44 1621787211; fax: +44 1621784989. E-mail address: [email protected] (S.N.R. Birchenough).
controls for the protection of the marine environment were enforced (Eagle et al., 1979). The disposal of industrial wastes and sewage sludge was phased out in 1998. Presently, sea disposal from ships is largely confined to dredged material, but the environmental consequence of this activity has been the focus of increasing attention from government, the general public and industry (for review see Bolam and Rees, 2003). In UK waters, the disposal of dredged material is licensed under the Food and Environment Protection Act (Great Britain-Parliament, 1985a) following guidelines laid down by International Convention (Great BritainParliament, 1972a,b, 1985a). This act controls deposits in the sea below the mean high water springs in order to
0924-7963/$ - see front matter. Crown Copyright © 2006 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jmarsys.2006.03.011
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protect human health, the marine environment and legitimate uses of the sea. The licencing authority for England and Wales is the Department for Environment, Food and Rural Affairs (Defra) (previously the Ministry of Agriculture Fisheries and Food (MAFF)). In many cases, openwater disposal to designated sites is the best practicable environmental option and also the only economically realistic one. Criteria which must be satisfied before a licence is granted to dredge and dispose of material at sea include the chemical quality of the material, the quantity to be disposed of, its nature and origin and its predicted impacts at the area of disposal (Rees et al., 2000). Worldwide, dredged material disposal at sea is an activity which is increasing as a consequence of economic development in coastal areas. There have been a large number of associated monitoring studies at disposal sites but, by their nature, reports of such activities do not lend themselves to publication in the peer-reviewed literature. Recent studies include those by Smith and Rule (2001), Zimmerman et al. (2003), Cruz-Motta and Collins (2004), Fredette and French (2004) and Valente (2004). Accessing such information can present a challenge when seeking to draw from previous ‘best practice’ in order to modify or enhance sampling practices elsewhere (Valente, 2004). The Tyne dredged material disposal site (DMDS), also called TY070, is located 5.7 km off the Northumberland coast; this site receives maintenance-dredged material from the Tyne Estuary, that, sometimes exceeds 150,000 t/ year (Fig. 1). Since the commencement of disposal operations, the Ministry of Agriculture Fisheries and Food (MAFF) (Eagle et al., 1979) and subsequently, the Centre for Environment, Fisheries and Aquaculture Science (Cefas) have intermittently surveyed environmental conditions at this site (e.g. MAFF, 1994; CEFAS, 2003). In the past, scientists have relied on traditional sampling tools (e.g. grabs, corers and dredges) to quantify
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sediments and their associated benthic fauna (macrofauna and meiofauna) (e.g. Boyd, 2002; Eleftheriou and McIntyre, 2005). This information only relates to a specific point on the seabed from which the sample was collected and, therefore, inferences regarding the wider distribution of substrata and the associated fauna may be unrealistic, especially in patchy environments (Brown et al., 2002). Whilst there are sound reasons for continuing to apply such approaches (e.g. Rees et al., 1990), with improvements in technology there is increasing interest in using acoustic and optical techniques for assessing anthropogenic effects, particularly in areas of known habitat heterogeneity. A number of acoustic techniques (especially sidescan sonar and more recently multibeam bathymetry) have been widely employed to monitor the physical disturbance of the seabed resulting from aggregate extraction activities (Boyd, 2002; Limpenny et al., 2002; Boyd et al., 2003, 2004, 2005) and to map the distribution of gravel habitats (e.g. Brown et al., 2001, 2002, 2004) in UK waters. However, an ongoing challenge is to establish the extent to which such techniques, used in conjunction with appropriate ground-truthing methodology including optical methods, can be used to improve assessment of the status of benthic communities in relation to natural and manmade influences. Currently, there are a number of groundtruth techniques (i.e. grabs, dredges, and cores), and optical methods include video and stills photography, as well as Sediment Profile Imaging (SPI) (Rhoads and Germano, 1986, 1990; Solan et al., 2003). A limited number of studies further serve to illustrate the combined use of acoustic, optical and traditional methods for seafloor mapping and impact assessment (e.g., Zajac et al., 2000, 2003; Kostylev et al., 2001, 2005; Cutter et al., 2003; Disposal Area Monitoring System (DAMOS), 2006).
Fig. 1. Wet weight in tonnes of maintenance-dredged material licenced for offshore disposal at TY070 DMDS (in Defra unpublished data).
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Table 1 Summary of survey techniques applied at TY070 DMDS Year
SSS
2001 2002 2003 2004
7 9 11 11
Multibeam
1
GRAB
CAMERA
SPI
6 21 21 21
3 5 5
15 (triplicate)
vices to determine seafloor conditions resulting from the disposal of dredged material at a licenced site off NE England. This information has the potential to contribute to the development of improved monitoring approaches within UK waters. 2. Methods
SSS = sidescan sonar survey contains the number of swaths used to cover the area. Camera = video stills used for this work. Grab = number of grabs collected during each survey.
2.1. Study site and data collection
The aims of the present study were to assess the advantages of combining conventional grab sampling methods, acoustic techniques, and optical imaging de-
The disposal site TY070 consists of a segment of a circle of approximately 2.73 km2. This site and its surrounding area were surveyed annually over the period
Fig. 2. A) Map showing the location of the licenced dredged material disposal site (TY070) and grab-sampling stations from the surveys carried out in 2001–2003. B). Map showing the SPI sampling survey in 2003.
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2001 to 2004 using a variety of techniques, including sidescan sonar, multibeam bathymetry, grab sampling, imaging of the sediment surface using towed video, and SPI (Table 1). A pilot survey was conducted during 2001 to assess the nature of the substrate in the area, and to test additional ground-truth techniques. Grab samples (0.1 m2 Hamon grab) were collected to ground-truth the acoustic surveys through the provision of information on sediment particle size distributions and macrofaunal communities. The grab survey involved the collection of two replicates at each of five stations; i) two located inside the disposal site (codes used: gtdispn and Gtdispn; samples were collected for sediments and fauna), ii) two northeast of the disposal site (codes used: gtsemot and GTNEpat; samples were collected for fauna only and for sediment and fauna respectively) and iii) one west of the disposal site (code used: GTWHOM; samples were collected for fauna only). A sidescan sonar survey was conducted once each year 2002–2004 to provide a 100% mosaic of the site (for methodology of the acoustic survey, see Boyd et al., 2006). A visual analysis of the sidescan sonar mosaic was conducted. The delineation of acoustically distinct areas was aided by the use of still video photographs of sediments collected by grabs, video stills and sediment profile imagery. In 2002–2004 a video camera attached to the 0.1 m2 Hamon grab was used to ground-truth the sediment patterns (Fig. 2-A). A stratified random sampling design was employed for the grab survey, with stations randomly
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positioned within each of the acoustic regions based on the outputs from the sidescan sonar survey. The number of stations within each region was allocated in proportion to the size of the area. A sub-sample was taken from each of the Hamon grab samples for sediment particle size analysis (PSA) following the methodology given in Boyd (2002). The remaining contents of each grab were processed for benthic macrofauna. Sieving, sorting, preservation and laboratory processing of samples were done according to the methodology described in Boyd (2002). In addition to the images collected by the video camera attached to the Hamon grab sampler, a camera sledge was towed along the seafloor to collect video footage at selected locations. This continuous footage was subsequently divided into a series of still images to identify seafloor features of interest at specific locations. The SPI camera survey was conducted in 2003 at a total of fifteen stations, located inside and in the immediate vicinity of the disposal site (Fig. 2-B). The camera used was a Nikon F-801s and it was deployed in the vicinity the stations used for the grab sampling. The SPI camera provided undisturbed, in situ surface and profile images of the upper ∼ 20 cm of the sediment column. 2.2. Data analysis Univariate analysis based on total number of individuals (excluding colonial species) and the total
Fig. 3. Temporal change (2001–2004) of the sidescan sonar mosaic at TY070 DMDS. The segment of a circle delineates the disposal site (centre). Letters on the 2003 mosaic are used to label acoustically distinct areas: A = homogenous sand, B = heterogeneous sediments, C = centre of the DMDS, D = hummocky area and E = sandy muds.
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number of species were calculated from the Hamon grab samples to provide a quantitative assessment of benthic assemblages within each acoustic region over time. Additionally, multivariate analyses of data were conducted with the software PRIMER (Plymouth Routines in Multivariate Ecological Research), version 6 for Windows. Following square root transformation of the abundance data (excluding colonial taxa), non-metric multidimensional scaling ordination (MDS) of Bray–Curtis similarity measures was used to assess changes in species composition (Clarke and Warwick, 2001). Pairwise ANOSIM (Analysis of Similarities), (Clarke, 1993) was also conducted to test for significant differences in macrobenthic assemblage composition over time and among acoustically distinct areas. The SIMPER routine (similarities percentages) within PRIMER was used to determine the contribution of individual species towards the dissimilarity between years and stations, while the BIO–ENV (Biota-Environment) analysis was used to find the best match
between the multivariate sample patterns of an assemblage and from environmental variables associated with those samples (Clarke and Gorley, 2006) over time at study sites. The environmental variables tested were % gravel, % sand and % silt/clay and annual quantities of licenced dredged material disposed into the area. Particle size distributions were analysed using Principal Component Analysis (PCA) to assess spatial and temporal changes at the stations. The variables used were the sorting coefficient, % gravel, % sand and % silt/clay of individual sediment samples. The software to process the SPI images was Image Analyst for Macintosh version 9.0.3. Three replicate images were taken at each sampling station, and the median values of derived measures were calculated. From each image, the sediment type, sediment boundary roughness (SBR), apparent redox potential discontinuity (aRPD) depth and the Benthic Habitat Quality index (BHQ), (Nilsson and Rosenberg, 1997) were calculated.
Fig. 4. Close-up sidescan sonar records for locations A–D in Fig. 4, along with video images showing seabed types: A) homogeneous sandy mud in the offshore part of the survey area (area shown is approx 200 m × 400 m); B) heterogeneous sediments composed of shelly muddy sand, sandy mud, coal and clay (area shown is approx 800 m × 1600 m); C) shelly muddy sand with colliery waste, present inside the disposal site. Also evident are linear trawling marks and possibly anchor drag scars (area shown is approx 300 m × 600 m) and D) discrete patches of soft material (arrowed) to the west of the disposal site (area shown is approx 600 m × 1200 m).
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3. Results 3.1. Acoustic survey interpretation The sidescan sonar mosaic from each of the acoustic surveys is shown in Fig. 3. Five acoustically distinct areas were delineated over the entire study site (2003 image in Fig. 3). The bathymetry in the area gently sloped from around 30 m in the west to 50 m in the east (bathymetric survey reported in Boyd et al., 2006). The seafloor to the west of the disposal site had a hummocky topography (Fig. 3, area D), whereas the seafloor topography to the east was much smoother (Fig. 3, area A). Temporal comparison of the sidescan sonar records shows that there was little change in the broad distribution of sediments and features within the survey area between 2001 and 2004. The surficial sediments in the offshore area east of the disposal site were soft in nature and dominated by relatively featureless, homogeneous sandy muds with small amounts of shell material at the surface (Fig. 3 area A and Fig. 4-A). The video stills showed that burrowing and tube-forming fauna (Turritella, sp. and Antalis sp.) inhabited these sediments. Occasionally, harder linear features were also observed over this substratum. These linear features might be attributable to other impacts (e.g. demersal fishing activity). At the inshore area (Fig. 3 area B), the sediments were observed to be coarser and more heterogeneous with a band of shellier muddy sands and sandy muds, intermixed with coal and clay, dominating the central part of the survey area (Fig. 4-B). In the western part a mixture (Fig. 3, area C) of sand, mud, coal and other rocks are frequently encountered and can be linked with coal measures known to be present in this area. At the disposal site itself, sediments were very mixed. The centre was characterised by a lower acoustic backscatter and sediments mainly consisted of sandy muds and muddy sands. The southern and central part of the site was generally muddy in nature, with some coarse sand present. The sidescan sonar imagery portrays this as a patch of predominantly finer sediment with areas of coarser, mixed material at the periphery, which appears to represent the acoustic footprint of the deposited sediments. The remainder of the site was mainly shelly, muddy sand with varying quantities of colliery waste and the presence of Alcyonium digitatum, the brittlestars Amphiura sp., and Ophiura sp., hydroids and bryozoans (Fig. 4-C). Underwater photography showed an area of mixed sediment extending away from the southwest boundary of the licenced area with the presence of A. digitatum (Fig. 4-D). It was apparent
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that the coarser component of this substratum consisted of gravel and cobble-sized coal particles within a muddy sand matrix. Patches of finer sediment (muddy sand/sandy mud) also appear within this area. From the visual assessment provided by the video and SPI this appeared to be a combination of both disposal operations and natural coal deposits which were exposed at the seabed.
Fig. 5. A) Principal Components Analysis (PCA) of sediment characteristics (sorting, % gravel, % sand and % silt/clay) for ground-truth samples at TY070 over 2001–2004. B) PCA ordination based on acoustically distinct areas delineated at TY070.
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3.2. Sediments In the first year of study (2001) the sediment composition was observed to be an admixture of sand, silt/clay and gravel at all the stations. In the centre of the dredged material disposal site (stations gtdispn and GTdisp), the sediments were mainly a combination of sandy mud and muddy sands, whereas east of the licenced area (station gtnepat), sediments were predominantly composed of muddy sand. The centre of the licenced area was characterised by the presence of sandy muds and muddy sands. East of the disposal site (area E) was characterised by muddy sands. Sediments to the west of the survey area (area B) were, in contrast, predominantly composed of sandy gravel over time. Additionally, area A was observed to be patchy sandy gravels with muddy sands. Principal Components Analysis (PCA) of the sorting coefficient, % gravel, % sand and % silt/clay showed the W–E changes in samples over time (Fig. 5-A). A relatively even spread of points can be seen in the ordination; there is a tendency for clustering (left side of
the ordination) of some of the samples from 2002, 2003 and 2004 (Fig. 5-A). The distribution of the samples along the PC 1 axis accounted for 63% of the variability and could be explained by the inverse variation in % sand. PC2 represented 31% of the variability, which was associated with the % silt/clay content. The spatial representation of acoustically distinct areas can also be seen in Fig. 5-B. There is a tendency for clustering on the left side of the plot of the majority of samples from the acoustic areas E, C and D. 3.3. Biological composition A total of 172 taxa were identified over the 4 years of the study. In 2001 the pilot survey of the area was designed to assess differences between the centre of the licenced dredged material disposal site and adjacent areas. Hamon grab samples were collected to target and ground-truth the acoustically distinct areas and also to provide an indication of the status of macrobenthic assemblages. Fig. 6 shows the values of total abundance of individuals and total number of species for the
Fig. 6. Temporal changes in the macrobenthos collected at the Tyne DMDS over 2001–2004. A) Total abundance of individuals. B) Total number of species.
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Fig. 7. A) Non-metric multidimensional scaling ordination using Bray Curtis similarities measure computed for double square-root transformed species abundances for 2003, B) 2004, C) MDS for all years overlain with dredged material quantities (extracted from Fig. 1) and D) MDS for all years overlain with % gravel.
different acoustic areas sampled over the 4 years of the study. In 2001, samples were only collected from within 4 acoustic areas (i.e. areas B, C, D and E). Over these 5 distinct acoustic areas the values of total abundance of individuals ranged from 45–374/0.1 m2 and the total number of species ranged from 20–50/0.1 m2. Lower values of both univariate metrics were observed at area B (west of the dredged material disposal site) in comparison with areas C, D and E over 2002–2004. For simplicity the results of multidimensional scaling using Bray–Curtis similarity coefficients were separated over years to assess the pattern of variation. There is an E–W orientation in both ordinations for 2003 and 2004 (Fig. 7-A–B). An aggregation of samples from the disposal site and adjacent areas is observed to the right side of the ordination, while other samples from the disposal site were more widely dispersed in both years (Fig. 7A–B). Results showed that community composition was best represented by a combination of deposited material, % gravel, and % silt/clay (BIO–ENVanalysis; r = 0.483). Fig. 7 C–D show the faunal data overlain with these
environmental variables (e.g. dredged material quantities and % gravel). The species distribution surrounding the disposal site appears to be influenced by the disposal operation. This is indicated by the separation of samples located in the disposal site and immediate vicinity (Fig. 7-C). Additionally, elevated % gravel (Fig. 7-D) was noted at areas within the disposal site itself and also at areas A and D. The result of the two-way nested ANOSIM provided evidence of significant differences (r-statistic = 0.26; p < 0.002) over time and over acoustically distinct areas (r=0.31; p<0.001) during the study period. SIMPER results (see Boyd et al., 2006) showed that the highest values for overall dissimilarity were found for the comparison between 2001 and 2002 (72.11%) and 2001 versus 2004 (73.72%). It is clear that the differences observed over time are attributable to the changes in abundance of a number of taxa, resulting from the disposal activities at the site. Examples of species showing appreciable changes include the polychaete Lagis koreni, which had a high average abundance during the first year of the study and lower abundances
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Fig. 8. Sediment profile images from the July 2003 survey at TY070.
subsequently. The dissimilarity between the acoustically distinct areas was also analysed. The highest overall dissimilarity was encountered between areas C and B (73.62%) and E versus B (72.80%). Decreased abundance at area B can be attributed to the type of substratum encountered (i.e. sandy gravel) and the high abundance of the carnivorous polychaete Lumbrineris gracilis. 3.4. Sediment profile imaging SPI images from July 2003 are shown in Fig. 8 for stations located in the centre of TY070 and surrounding areas. Surface layers of sediment were comprised predominantly of fine sand. Some medium sand and fine muddy material was seen throughout most of the stations sampled. A varying shelly fraction was also visible at the sediment surface at many of the stations surveyed. This material was seen as the highly reflective fragments scattered across the sediment/water interface in the surface photographs. There was a certain amount of re-suspension of fine material at many of the stations sampled (an artefact
of SPI technology: R. Valente pers comm.). An obvious example of this was observed in the surface and profile images taken at station 19 (Figs. 8 and 9). Coarser sediments were recorded at stations 22 and 25, while boulder field/bedrock outcropping were also recorded at stations 36, 37 and 39, west of the disposal area. Surface boundary roughness indicates unevenness of the sediment surface as a result of physical activity and bioturbation processes (Rhoads and Germano, 1986). Values ranged from a low of 0.33 (the very flat sediment surface present at station 33 in the disposal area) to a high value of 1.58 (a biogenically-roughened sediment-water interface at station 30, located east of the disposal area). High roughness values were largely attributed to biogenic features. For example, in photographs taken at station 18 (Figs. 8 and 9), surface roughness was due to the activities of decapods whose burrows were evident in several SPI and surface photographs (stations 18, 30, and 32) or to tube formation and bioturbating activity of infaunal animals (e.g., polychaetes and holothurians).
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Fig. 9. Surface images from July 2003 at the Tyne DMDS. B = decapod, C = Alcyonium digitatum, D = coarse substratum and E = Buccinum sp.
The apparent redox potential discontinuity (aRPD) depths (the visible line between oxygenated and reduced sediment) recorded from the SPI images are presented in Table 2. The evident aRPD depth was greatly dependant on the presence/absence of bioturbating fauna. Where animals were present (stations 18, 19, 21, 30 and 32), the aRPD layer was relatively deep and uneven, and the re-working of sediment observed in the sediment profile and oxygenated voids were generally present at depth within the sediment. At stations where the excavating, irrigation, burrowing and feeding activities of such animals were absent (stations 28, 29 and 33), a shallower aRPD layer occurred, reflecting the presence of reduced sediment recently placed from a dredged material disposal operation. Station 29 revealed a very limited amount of oxygenated sediment present, with an associated low level of biological activities (Fig. 8). These stations were all located inside the disposal site boundary. Due to the presence of coarse/rocky substrata, it was not possible to gauge the depth of the aRPD layer at stations 36, 37 and 39 (Fig. 8). It is important to highlight the variation in different aRPD depths within the disposal site itself, with both relatively
high and low values recorded. This may be attributable to the degree of historical disposal activity at the site. The stations located outside the disposal site returned consistently high aRPD values (Table 2). The presence of burrows (attributable to the excavating activities of several infaunal species) were clearly seen in many of the profile images (e.g. station 34 with faunal activity at depth); in some cases substantial sections of these burrows were visible. A large decapod (Fig. 9 image 19 B) and a pelican foot shell (Aporrhais pespelecani) were also visible on the sediment profile image for station 26 (Fig. 8). Colonies of the anthozoan A. digitatum (soft coral) were visible in the surface images at station 27, 36 and 37 (Fig. 9). These indicate the presence of suitable substrata for attachment (i.e. gravel, shelly material or coal), although these stations were located south west of the licenced area. In the surface image taken at station 30, surface tracks produced by the whelk Buccinum sp. were evident (Fig. 9 image 30 E). The Benthic Habitat Quality Index was calculated for the stations surveyed following the methodology proposed by Nilsson and Rosenberg (1997) (Table 2). Successional stages varied from stage I environments (e.g.,
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Table 2 Results from the analysis of SPI images, including sediment type, penetration, Sediment Boundary Roughness (SBR), apparent redox discontinuity depth (aRPD), and BHQ Stages (calculated from the Benthic Quality Index) Stations SED. type Penetration (cm) SBR aRPD depth BHQ stage 18 19 21 22 25 26 28 29 30 32 33 34 36 37 39
4–3 phi 3–2 phi 4–3 phi 3–2 phi 4–3 phi 4–3 phi 4–3 phi >4 phi 4–3 phi 4–3 phi >4 phi >4 phi >4 phi >4–3 phi 3–2 phi
14.59 5.36 10.40 5.80 13.43 8.53 5.21 5.55 17.54 15.28 12.43 16.26 – – –
1.35 1.06 0.50 1.00 1.62 1.55 1.02 1.08 1.58 0.96 0.33 1.40 – – –
5.84 5.39 7.08 4.93 10.31 1.56 2.97 1.43 7.38 8.74 2.88 2.68 – – –
II–III II II II–III II–III I I–II I III III I I I–II I–II I–II
stations 26, 29 and 33, located within the disposal site, were characterised by shallow aRPDs and the absence of discernible biogenic features) to Stage III (stations 18, 30 and 32 outside the disposal area) with their characteristically deep aRPDs, well-developed faunal communities and prominent biogenic features such as burrows and feeding mounds. 4. Discussion Marine benthic habitats are vulnerable to the influence of a wide range of anthropogenic activities (e.g. dredged material disposal, aggregate extraction, windfarm developments, oil and gas exploitation and fishing impacts). Recent developments in seabed mapping techniques driven by continuous improvements in acoustic systems offer the potential to radically improve singlepoint sampling approaches to monitor the impacts of such activities. These opportunities provide benthic ecologists with new avenues for studying the structure and dynamics of benthic communities at multiple spatial scales (Zajac et al., 2003). The Tyne dredged material disposal site is used as an exemplar to establish the extent to which such a combination of techniques could be used in future routine environmental monitoring programmes undertaken in UK waters. Acoustic techniques have been routinely employed for many years as a tool in disposal site monitoring but the resolution, affordability and accessibility of the technology has greatly increased in recent years (Kenny et al., 2003; Van Lancker et al., 2003). In this study the utility of
acoustic techniques was demonstrated by: i) the facilitation for rapid coverage of large areas of seabed, ii) facilitating the delineation of sedimentlogically and biologically distinct areas and iii) providing information on the footprint of disposal activity. Examination of the data from grab samples confirmed a distinct area of the seabed within the disposal site, characterised by sediments which were largely muddy in nature, but also included patches of coarse sand. Other substrata present over the survey area also demonstrated agreement between the particle size distribution and SPI images. The particle size distribution data provided the quantitative information to ground-truth the sediments of the area. The use of SPI complemented the PSA information with an in situ representation of sediment structure and quality at each station. On a smaller spatial scale, a video camera attached to the Hamon grab provided instantaneous information on the undisturbed image of the surface sediments, including any associated epifauna. This information clearly only gave a localised and general indication of the footprint of the disposal activity. However, the images at certain stations in the vicinity of the disposal site revealed apparently undisturbed conditions at the sediment surface. Evidence of a legacy of disposal activity was provided by the vertical SPI images, and the combination of the two sources of information was therefore helpful in evaluating overall seabed status. The Hamon grab (covering 0.1 m2) provided pointsample information on fauna and sediment composition. These data allowed a quantitative analysis over the different areas and, to a degree, identified changes occurring within and in the near vicinity of the disposal site between 2002–2004. A decline in number of individuals and species over time was observed for area B (west of the disposal site), when compared to areas C, D and E. Multivariate analysis indicated a certain degree of similarity in terms of faunal distribution over time including some of the stations located at the centre of the disposal site (Fig. 7). However, stations located within the western and southern part of the disposal site were relatively dissimilar; this was also observed in the sidescan images. As expected the BIO–ENV analysis identified an influence on the assemblage associated with the disposal of dredge material and sediment type (e.g. especially variability in % gravel and silt/clay). The disposal site was directly influenced by a combination of coarse and soft sediments. The areas outside and vicinity of the disposal site were mainly composed of soft material and medium sands. SPI has found wide application worldwide (e.g. in the U.S.A., a clear example is the Disposal Area Monitoring System program (DAMOS, 2006), (O'Connor et al.,
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1989; Rumohr, 1995; Keegan et al., 2001; Solan et al., 2003; Fredette and French, 2004; Valente, 2004) but has only been used to a limited extent in the UK. The present pilot study off the Tyne confirmed that SPI can provide valuable information to assist in interpreting impacts occurring both within and in the near vicinity of the disposal site that could not be discerned from the sidescan sonar or grab sample data alone. The attributes of SPI include: i) assisting in mapping thin layers of deposited dredged material which are not clearly detected by acoustic technology, ii) providing complementary information on behaviour and disruption of benthic organisms and sediment quality status and iii) supplying near real-time data return. Therefore, by incorporating the use of SPI in future surveys we can envisage a more informed and efficient monitoring exercise. We conclude that the combination of approaches employed in this study has potential for wider application around the UK coast. With the exception of SPI, the techniques employed in this study have been widely used at other disposal sites continuously around the UK coast, although not always in synchrony. Procedures for integrating these approaches, exemplified by the present study appear, to offer significant benefits. Clearly, the scope for all combination surveys will be limited by local circumstances (e.g. the presence of bedrock provides an obvious constraint on SPI use) and the objectives of the investigation. However, the capability to link point sampling with a wider spatial perspective should, in general, be especially advantageous. In the particular case of the Tyne, the study provided complementary evidence of the localisation of impacts arising from disposal, but also provided additional insights into the heterogeneous character of the environment in the wider region, and the interaction between natural and anthropogenic influences, including those attributable to earlier disposal activities. Acknowledgments This work was funded by the UK Department for the Environment, Food and Rural Affairs (Project code AE1033, BA004, AE0261 and AE1224). Authors would like to thank Joe Costelloe, Stiofan Creaven and Brendan O' Connor (Aqua-fact, Limited Galway) for their support with the SPI interpretation. Thanks are also given to Stacey Faire for constant encouragement during the revision of this manuscript and Mrs Jacqueline Rhodes for assistance in the compilation of references. The manuscript was greatly improved with comments and discussion with Dr Joe Germano, Mr Ray Valente and Dr Ruth Parker.
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