Sediment Profile Imagery - Benthic Disturbance Mapping

Benthic Disturbance Mapping

SPI has been used to model the integrity and performance of capped dredge-spoil sites (NOAA 2003) and containment sites (e.g. Parliamentary-Commissioner 1995; Gowing et al. 1997). Detailed acoustic surveys of spoil disposal sites are inherently limited to a vertical resolution of ca. 10 cm (Ramsay 2005). There is considerable evidence that spoil overburden of less than 10 cm affects macrofaunal species (Chang and Levings 1976; Maurer et al. 1982; Maurer et al. 1986; Chandrasekara and Frid 1998; Schratzberger et al. 2000; Cruz-Motta and Collins 2004). Backscatter and high-frequency side-scan sonar techniques may provide faster characterisation of spoil extent, but only when the spoil’s acoustic reflectivity or topology is sufficiently distinct from native sediments. SPI devices produce imagery of the sediment/water interface with sub-millimetre resolution. SPI therefore offers the capability to examine dredge spoil mound morphology, compaction, winnowing, integration with native sediments, and, potentially, biological activity at a scale relevant to the macrofaunal assemblages under study.
SPI can be applied to other, perhaps more common, benthic disturbance investigations as well. To illustrate, consider a benthic ecological impact study for a hypothetical shellfish mariculture facility. There are an enormous variety of study approaches. Existing information and the available resources inevitably constrain every design. With little information on bottom type, a simple, one-off, spatial impact study like that shown in Figure 5 with eight sites along an isobath, taking three replicate grabs from each, is fairly common and moderately powerful. Prior data gathering including bathymetric, diver, towed-camera, ROV, or side-scan sonar observations would probably alter site placement and greatly enhance overall information and value. Collecting such data over even a small site such as this one requires considerable resources and will probably cause a gap of several days to allow data processing between the first field days and the grab sampling events (It is this delay that precludes, or reduces, the value of studying transient events in hydrodynamically energetic areas). Collecting a large number of point data from an SPI device is easily done where the resulting snapshots of the benthic character are automatically placed on a map of the study area in real time. This approach allows rapid categorisation according to one or more variables of interest. In waters <30 m deep it is not unreasonable to expect to collect the 170 SP images indicated in Figure 6 and produce a rough benthic classification map in a single field day. The categories may be based on sediment texture, overburden, specific detritus, biota, etc. Sampling effort can then be allocated to focus on the variability of communities among the gross habitat differences by using grabs as habitat replicates with varying lag. This type of approach produces a broader understanding of the system and permits more informed decisions by increasing the generality of the grab sample data. The SPI evidence can effectively increase the extent from one dimension to at least two. Correlation between physical and biological data collected from the grabs also allows more data to be extracted from the SP imagery by identifying specific features (infaunal species, tubes, mounds, etc.). Furthermore, a detailed analysis of ARPD depths can then be presented as geochemical environment contours.

Rhoads and Germano (1982) compare SPI techniques with three other studies off the east coast of the USA. Their work put SPI within an accepted ecological framework and subsequently widened its appeal and value as a standard monitoring tool. Solan et al. (2003) review the broader conceptual shift from traditional “kill ‘em and count ‘em” methodologies in benthic studies and show how integrating SPI and other optical and acoustic technologies with traditional sampling has fundamentally added to our understanding of several benthic processes. Although most SPI studies remain in the ‘grey literature’ (Keegan et al. 2001), a growing number and variety of applications is appearing. SPI-produced data were as informative as macrofaunal samples along an organic enrichment gradient in a temperate system (Grizzle and Penniman 1991). Other studies include those by Germano (1992) who investigated dredge-spoil disposal in Auckland’s Hauraki Gulf, and Heip (1992) who summarised the value of SPI alongside meio- and macrofaunal sampling near an ocean drilling platform off the German Bight. Rumohr and Schomann (1992) found that the SP imagery provided important clues and context for interpretation of otherwise enigmatic benthic data. Early work using SPI to identify hydrocarbon contamination (Diaz et al. 1993) was later refined to include more accurate and precise measurements by spectroscopy (Rhoads et al. 1997). Smith et al. (2003) investigated fishing trawl impacts using SPI, whilst Solan and Kennedy (2002) demonstrated the use of time-lapse SPI for quantifying ophiuroid bioturbation. Diaz and Cutter (2001) used the same method for quantifying polychaete bioturbation through transient burrow formation and its relationship with oxygen penetration into sediments. NOAA (2003 and references therein) report the widespread use of SPI for habitat mapping, dredge material cap monitoring, and oxygen stress (Nilsson and Rosenberg 1997) in estuarine, coastal, and deep water environments. Beyond pure research, SPI is a technique well suited to tiered monitoring and compliance. It is now widely accepted as a standard technique (Rhoads et al. 2001). Clearly, the applications of SPI are diverse and scientifically robust when properly applied, but some practical problems limit its wider use. Keegan et al. (2001) summarise that SPI is “...not developed as a replacement for conventional benthic monitoring tools, but as a survey and reconnaissance technique to optimise the efficiency of benthic monitoring programs.” They further state:

“...SPI is only now getting the widespread recognition it deserves. While this has something to do with acknowledged limitations in image interpretation, there remain certain impediments linked to the size and weight of the device, as well as to its restriction to use in muds and muddy sands. The relatively high cost of the most basic SPI assembly is perhaps most telling of all...SPI has tended to be used in activities promoted more by government and the wealthier commercial environmental consultancies than by the more traditional research sector.”

Development of the SPI-Scan system, also known as rSPI (rotational SPI) by Brian Paavo and Benthic Science Limited addresses the problems of mass and expense to enable lake and coastal users to economically deploy SPI systems from small vessels.