Research Technical Report

Potential Impacts of the Monterey Accelerated Research System (MARS) Cable on the Seabed and Benthic Faunal Assemblages 2020

Kuhnz, L.A., K. Buck, C. Lovera, S. Litvin, P.J. Whaling, and J.P. Barry (November 2020)

Monterey Bay Aquarium Research Institute, MBNMS Permit Report. 56 pp.
DOI: 10.13140/RG.2.2.12907.57122

SUMMARY

This report summarizes efforts to assess the condition of the MARS cable and its potential effects on surficial sediments and biological communities, based on an initial biological assessment in 2004, Post-Lay Inspection and Burial Survey (PLIB) in 2007, and comprehensive surveys performed in 2008, 2010, 2015 and 2020. The most recent study was conducted 13 years after the cable was installed. Note that this report supersedes information included in prior impact survey reports (2008, 2010, 2015). The sampling program was designed to:

• Observe the condition of the cable and cable path along the 60 km cable route,
• Assess the potential impacts of the MARS cable on surficial geological conditions and benthic biological assemblages on a local scale (0–50 m from the cable) and at a regional scale (km), based on video transects and sediment samples.

We conclude that the MARS cable has had little detectable impact on seabed geomorphology, sediment qualities, or biological assemblages. Specific conclusions include:

• Over most of its length the cable remains buried, with some evidence of change since installation
  • The cable has remained buried along shallow portion of the cable route.
  • Sediment has filled the cable trench in deeper areas, which is now nearly imperceptible in most locations.
  • Along the rocky section where the cable was not buried, some minor spans in the cable are present due to small-scale bathymetric complexity. No major spans or suspensions were present.
  • Trawling occurred near the MARS node at least three times between 2009–2010, damaging two instruments and an auxiliary cable. We detected that an unburied 1350 m long section of cable has moved 135 m to the SW, likely due to these, or subsequent trawling events.
• Minor differences in mean grain size were detected in relation to the MARS cable.
• The organic carbon content of sediments increased near the MARS cable at two depths, possibly due to natural variation, effects of the cable, or both.
• Local-scale variation in benthic megafaunal communities near (within 50–100 m) the MARS cable as minor or undetectable for the first three comprehensive surveys (2008, 2010, 2015). In 2020, the density of megafauna had increased at two of 10 stations and was significantly greater along the cable route than the undisturbed area just 50 m away. As in earlier surveys the cable was still exposed at both stations, with increasing numbers of attached anemones. The primary faunal change, however, was the large number of sea stars and sea cucumbers observed feeding on dead and dying midwater pyrosomes that had accumulated near the cable at a single station. Pyrosomes had not been observed previously along the cable route, and appeared to accumulate near the cable due to small-scale, near-bottom currents.
• In 2008, prior to energizing the cable, Longnose skates (Beringraja rhina) were significantly more abundant along minor suspensions of the cable (2–10 cm above the bottom, for intermittent short distances (1–3 m) over about 600 total linear m) along the rocky portion of the cable route near 300 m depth. B. rhina may have responded to mild electromagnetic fields generated by components of the cable. In 2010, 2015, and 2020 when the cable was powered, skate densities along the cable did not differ from an area 50 m away.
• The MARS cable has had little to no detectable effect on the regional-scale (i.e., km) distribution and density of macrofaunal and megafaunal assemblages.
• Faunal patterns compared before and after cable installation among three control stations and one cable station within each of three regions (Shelf: < 200 m, Neck: 200–500 m, Slope: > 500 m) indicated a very minor influence of the MARS cable installation on benthic biological patterns.
• Natural spatial and temporal variation in the density and distribution of benthic macrofauna and megafauna appears to be greater than the effects of the MARS cable.