Sediment Profile Imagery - SPI-Scan A New Kind of SPI

SPI-Scan A New Kind of SPI

In order to form and test fundamental community ecology hypotheses or address applications such as impact assessment, conservation, and exploitation of the marine environment, one needs to investigate the complex interactions between sediments, organisms, and water. A host of burgeoning technologies are slowly gaining acceptance to measure and explore this dynamic interface through biological, chemical, and physical approaches. Viollier et al. (2003) and Rhoads et al. (2001) provide overviews of this topic though the technologies involved and the standards used are changing rapidly. Several techniques have allowed benthologists to address ‘big-picture’ questions of geochemical-biological interactions and ecosystem functioning. Betteridge et al. (2003) used acoustic technology to measure sedimentary dynamics in situ at a scale relevant to macrofauna. Their benthic landers recorded water velocities near the seabed while simultaneously quantifying sediment disturbance patterns in high resolution. Benthic chambers have been used to examine the productivity of realistic macrofaunal assemblages under different flow regimes (Biles et al. 2003). Isotopic analysis methods permit food-web and environmental impact investigations (e.g. Rogers 2003; Schleyer et al. 2006) impossible to conduct outside of a laboratory only a few years ago. Short-sequence DNA methods (e.g. Biodiversity Institute of Ontario 2006) are rapidly moving toward automated identification and diversity assessment techniques that hold the promise of revolutionising benthic ecology.
Keegan et al. (2001) described the relationships among workers and authorities evaluating long-established, though often expensive and slow, methodologies with more recent technological developments as sometimes discordant. Gray et al. (1999b) lamented that there is a strong institutional tendency for sediment ecologists to rely on sampling methods developed in the early 1900s! A fine balance needs to be struck. Some degree of paradigm inertia is necessary to maintain intellectual continuity, but it can be taken too far. Physics, as a science, confronted this issue long ago and has widely embraced new technologies after establishing a scientific culture of always linking new techniques to established findings in a period of calibration and evaluation. The pace of this process in biology, as a whole, has quickened over the past few decades and ecology has only recently come to this horizon. This article introduces one such technology, sediment profile imagery (SPI) that is slowly gaining acceptance and currently undergoing its evaluation and calibration period even though it has existed since the 1970s. Like many of the technologies mentioned above, each new capability requires a careful consideration of its appropriateness in any particular application. This is especially true when they cross important, though often subtle, boundaries of data collection limitations. For example, much of our benthic knowledge has been developed from point-sample methods like cores or grabs, whereas continuous data collection, like some video transect analysis methods (e.g. Tkachenko 2005), may require different spatial interpretations that more explicitly integrate patchiness. While remote sampling techniques often improve our point-sampling resolution, benthologists need to consider the real-world heterogeneity at small spatial scales and compare them to the noise inherent to most high-volume data collection methods (e.g. Rabouille et al. 2003 for microelectrode investigations of pore water). New developments in the field of SPI will provide tools for investigating dynamic sediment processes, but also challenge our ability to accurately interpolate point-data collected at spatial densities approaching continuous data sets.
SP imagery as embodied in the commercial REMOTS system (Rhoads et al. 1997) is expensive (>NZ$60,000 at time of writing), requires heavy lifting gear (ca. 66–400 kg with a full complement of weights to effectively penetrate sediments), and is limited to muddy sediments. REMOTS is not well suited to small research programmes, nor operation in shallow water from small vessels, which is, quite possibly, an area where it could be most useful. Studying shallow sub-tidal environments can be a challenging exercise, especially among shifting sands. Macrofaunal sampling usually occurs at the sub-metre scale, whilst the dominant physical factors such as wave exposure and sediment texture can change at a scale of only metres, even though they are often only resolved to a scale of hundreds of metres. In such a dynamic environment, monitoring potentially transient disturbances like a spoil mound requires benthic mapping at fine spatial and temporal scales, an application ideally suited to SPI.