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Environmental Impact of the ATOC/Pioneer Seamount Submarine Cable

Kogan, I., C.K. Paull, L. Kuhnz, E.J. Burton, S. Von Thun, H.G. Greene, and J.P. Barry (November 2003)

Technical Report, 80p.

EXECUTIVE SUMMARY

A submarine cable extends between Pioneer Seamount and the Pillar Point Air Force Station in Half Moon Bay, California (Howe 1996). The cable is known as both the ATOC (Acoustic Thermometry of Ocean Climate) and Pioneer Seamount cable. The cable was used to transmit data from a passive, acoustic hydrophone array on Pioneer Seamount to shore. Approximately two thirds of the cable lies within the Monterey Bay National Marine Sanctuary and is permitted (MBNMS-2001-031) through December 31, 2003. This report documents cable surveys performed by the Monterey Bay Aquarium Research Institute (MBARI) in 2002-2003. Survey objectives were to describe the state of the cable and document its effect on the seafloor and on benthic organisms.

MBARI carried out this study in partnership with NOAA-OAR (National Oceanic and Atmospheric Administration - Oceanic and Atmospheric Research) and NOAA-NOS (National Ocean Service). MBARI researchers were interested in documenting the environmental impacts of submarine cables, a topic of importance to the scientific community for which little data is published and publicly available. NOAA-OAR has responsibility over this particular cable and the requirement to survey it prior to permit expiration. NOAA-NOS interest in this survey was in assessing the environmental impact of submarine cables and general seafloor characterization within the Monterey Bay National Marine Sanctuary.

The Pioneer Seamount cable was installed in October 1995 as part of the Acoustic Thermometry of Ocean Climate (ATOC) project. NOAA-OAR installed a hydrophone array on Pioneer Seamount to passively record sound in the ocean and officially took responsibility for the cable on December 7, 2001. The presence of this cable has resulted in 80 references consisting of presentations, scientific publications, articles for the general public, and student projects. The cable is currently damaged and has not been transmitting data from the seamount since September 25, 2002.

A total of 13 sites along the 95 km cable route were surveyed using MBARI ROVs Ventana and Tiburon equipped with cable-tracking tools during research cruises on February10-14, 2003 and July 28-August 1, 2003. Quantitative comparison between cable and control sites was performed at nine stations. Survey locations were chosen to target representative substrate and habitat types, features of interest, and for logistical reasons. Side scan sonar data collected on October 21-25, 2002 from the R/V Zephyr helped select these sites. A total of 42 hours of video footage and 138 push cores were collected over 15.1 km of seafloor. Approximately 12.1 km of the cable was observed (13% of the cable route).

Video observations indicated the nature of interaction between the cable and seafloor. Most of the cable has become buried with time in sediment substrates on the continental shelf (water depths <120 m) whereas much of the cable remains exposed on the seafloor at deeper depths. Burial depth on the continental shelf ranged from 0 to 27 cm and averaged approximately 10 cm. Burial depth may fluctuate due to shifting substrate and buried cable may become exposed during storms. The cable is exposed in rocky environments of the nearshore region and on all of Pioneer Seamount.

Where the cable was exposed, its condition was assessed. Video images from the rocky nearshore areas, where wave energies are greatest, showed clear evidence that the cable has been damaged. Here, evidence of abrasion included frayed and unraveled portions of the cable's armor. In many places the cable occupied vertical grooves in the rock that were apparently cut by the cable. Incisions ranged from 6.6 cm (diameter of double armored cable) to 45 cm wide. The greatest incision and armor damage occurred on ledges between spans in rocky areas with irregular bathymetry. Snagged kelp was seen intertwined with frayed cable in the near shore areas.

The most notable suspensions were in rocky areas with irregular bathymetry. Such rocky areas occur at both ends of the cable. Suspensions up to 40 m long and greater than 1 m high were seen in the nearshore rocky area and up to 25 m long and 2 m high were seen on Pioneer Seamount. Unlike the nearshore rocky region, neither the rocks nor the cable appeared damaged along outcrops on Pioneer Seamount. Short (~10 cm) suspensions were also common bridging low spots associated with irregular topography in sediment substrate areas. Multiple loops of slack cable, added during a 1997 cable repair operation, were found lying flat on the seafloor at 950 m water depths. Several sharp kinks in the cable were seen at 240 m water depths in an area subjected to intense trawling activity. Cable crossings were seen in 13 m water depth at 37�� 29' 50" N, 122�� 33' 04" W and in 344 m water depth at 37�� 29' 54" N, 122�� 30' 30" W.

The main observed biological differences between cable and control areas were the number of organisms attached or adjacent to the cable. Anemones colonized the cable and were more abundant in cable transects at most soft sediment sites. Where the cable was buried, the presence of linear rows of anemones proved to be a reliable indicator of the cable's position. Coarse extrapolation of transect data suggests over 50,000 anemones may live in the modified habitat created by the cable. Echinoderms and sponges were also seen living on the cable. At three of nine stations, flatfish and rockfish congregated near the cable. The cable has had no apparent effect on infaunal abundance. Other differences between cable and control sites were probably due to patchiness of animals. Considerable care was taken to count megafauna in video transects and macrofauna from the top 5 cm of push cores. Few differences were found between cable and control sites at the 95% confidence level. The cable may also subtly affect local hydrodynamic conditions that concentrate shell hash and drift kelp near the cable.

See 2006 Journal Article for additional information.

URL: http://montereybay.noaa.gov/research/techreports/trkogan2003a.html    Reviewed: March 04, 2014
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