The seafloor is, arguably, one of the most extensive habitats on the planet and it is significantly understudied. It is home to a variety of benthic organisms that spend much, if not all, of their time on the bottom sliding along or ploughing through sediment. Some organisms are deposit-feeders that ingest sediments, absorb their organic content, and excrete faecal strings or pellets; other organisms are burrowers that actively mix sediments vertically. This process by which organisms mix up sediment, is known as bioturbation, and is ecologically important because it influences nutrient recycling and other biogeochemical processes on the seafloor.
Bioturbation has traditionally been studied using time-lapse imagery or vertical tracers (natural or artificial objects on the seafloor which become buried in the sediment through bioturbation). University of Victoria Biology MSc student Katleen Robert decided to try a new approach, using video camera recordings from an Ocean Networks Canada camera at the Pod 2 site near theUpper Slope junction box, to determine which animals were mixing surface sediments and how fast they are doing it. Working with her supervisor Kim Juniper, Katleen developed a brand new methodology for this process, using imagery from a video camera deployed at 396 m depth in Barkley Canyon. In addition to the video imagery, they made use of:
1. scans from the rotary sonar installed at this location to monitor decimetre-scale sediment structures
2. acoustic Doppler current profiler (ADCP) data, to characterize bottom water currents
3. two push-core samples in order to assess sediment grain size
An instrument platform in Barkley Canyon with an attached ADCP used to monitor bottom water currents.
There are lights connected to the cameras, however, so Katleen had to be cautious about how much she used them as some species, such as black cod (sablefish) are attracted to the lights and may congregate in such high numbers they obscure the benthic animals, or possibly alter the natural behaviours of the organisms they were trying to observe. To avoid any negative effects from light pollution, the team limited light to 1 hour per day. Within this limit, Katleen tested three observation regimes:
1. 1 continuous hour of observation (4 January - 25 February 2010)
2. Two 30 minute periods (24 March - 9 April 2010)
3. 5 minutes every second hour (15 August - 23 October 2010)
The continuous one-hour observations in January and February were used to establish which organisms were present. Camera-mounted lasers, spaced 10 cm apart, and a scaling ruler on the seafloor within the field of view were used to build a perspective grid which could be overlain on the video frames to measure organism size and rates of locomotion.
An example of a perspective grid overlaid on a video frame used in this study (from Robert & Juniper, 2012).
The most commonly observed animals were rockfish, flatfishes (Dover sole and Pacific halibut), skates, fragile pink sea urchins, and an orange anemone. Of these organisms, only the flatfishes and urchins were commonly observed disturbing the sediments. By monitoring their movements, Katleen determined that sea urchins were able to completely overturn the sediments in the field of view, an area of 8.8 m2, in 153 to 213 days while the flatfish took slightly longer, overturning the same area in 227 to 294 days. These numbers taken together represent sediment surface overturning completely at rates of 26.0 to 35.1 m2/year. These numbers are consistent with other observations at deeper locations in the northeast Pacific and northeast Atlantic Oceans.
Fragile pink urchins aggregate in a trawl scar.
Although Katleen determined that individual fragile pink urchins cause little sediment mixing, these animals are known to occasionally aggregate in high numbers, in which case they could have more intensive but localized bioturbating effects. Flatfish overturn much more sediment individually than urchins. Halibut and sole tend to burrow into the surface of the sediment and leave small oval pits about 1-2 cm deep. It is thought that they may dig these pits for protection from predators or to help them capture prey.
The two most common flatfish observed by Robert & Juniper, Pacific halibut (left) and Dover sole (right, inset)
Researchers at Dalhousie University have discovered similar pits in high-resolution sonar images from this same site (Pod 2, near Upper Slope). These pits remained, and were not filled in, for approximately 6 months. Current metre measurements suggest that these pits were not formed by bottom currents which were rarely strong enough to re-suspend sediments and fill in the burrows. Instead, the researchers are now exploring the idea that these pits are made by flatfish that revisit them often and maintain the same pits so they do not become in-filled.
Robert & Juniper’s study illustrates the importance of animals in the deep sea are for mixing sediment, which can happen at surprisingly rapid rates. They suggest that long-term monitoring of bioturbation will provide important insights into the response of seafloor ecosystems to changes in primary productivity and related supplies of organic matter supply to the deep sea. This eventually will help scientists evaluate the impacts of global climate change and human disturbances on deep-sea ecosystems.
