The deep sea is a dark, cold environment that includes a variety of habitats from the mid-water region to the abyss; these are populated by a wide array of animals that are specially adapted to live under the tremendous water pressure and low oxygen level of this harsh environment. In recent years, improved sampling techniques and technologies have shown that the diversity of deep-sea fauna is greater than once thought.

Based on the topography of the seafloor, the deep sea begins at the continental shelf break, at a depth of approximately 200 meters (650 feet). Beyond the shelf break, the continental slope descends through the deep sea to the ocean floor.

Of the total seafloor area in the sanctuary, approximately two-thirds (9,164 square kilometers, or 3,538 square miles) are located in the deep sea. This region also encompasses two distinct pelagic zones: the mesopelagic zone, which starts 200 meters below the surface and extends to about 1,000 meters; and the bathypelagic zone, from 1,000 meters down to the seafloor.

Although the deep sea encompasses 98 percent of all living space on the planet by volume, it is among the least understood ecosystems because of the challenges inherent in accessing it. Marine scientists claim that we know more about space than we do about this remote environment.

To reach the ocean's depths, scientists require a platform from which to deploy sampling or observational gear. Today, in addition to net sampling, manned and unmanned research submersibles are deployed from research ships to collect data and make observations in this remote environment.

The deep sea comes very close to shore in the sanctuary through the heads of several canyons (Monterey Canyon is the largest), making deep water relatively easy to access here.

Conservation and management issues affecting sanctuary resources in the deep sea include seabed disturbance, nonsustainable fishing and carbon dioxide (CO2) sequestration.

• Bottom trawling: This practice is widely believed to have negative impacts on benthic habitats, such as modification of the substrate, disturbance of soft-bottom communities and removal of non-target fish species. The structure of entire seafloor communities is at risk from this fishery practice. Currently, there is an incomplete picture about the extent of these impacts in the sanctuary, but the use of trawl gear is an ongoing source of concern.
• Nonsustainable fishing: In June 2006, the Pacific Fishery Management Council and NOAA Fisheries closed large portions of the continental slope to trawling in order to protect essential fish habitat (EFH) for groundfishes. In addition, closures called Trawl Rockfish Conservation Areas (RCAs; from 183 to 274 meters' depth in the Monterey Bay National Marine Sanctuary) have been established to minimize the bycatch of overfished rockfishes. RCA boundaries, however, may change during the year. Trawling is not allowed in state waters.
• CO2 sequestration: International agencies are investigating deep-sea carbon sequestration as one possible mitigation technique for emissions of CO2, one of the primary greenhouse gases involved in global warming. Scientists at the Monterey Bay Aquarium Research Institute (MBARI) have conducted some of the first experiments aimed at understanding the chemistry and physics of CO2 in the deep sea and the ecological effects of CO2 sequestration. The release of CO2 on the seafloor caused a change in seawater chemistry and high mortality of organisms living in the sediments. Results indicate that large changes in seawater chemistry will cause high mortality rates for nearby deep-sea communities.

Monitoring

Archiving of Mid-Water and Benthic Survey Data at Moss Landing Marine Laboratories
Since the early 1970s, faculty and students in Marine Ecology, Invertebrate Zoology and Ichthyology at Moss Landing Marine Laboratories (MLML) have participated in class cruises to survey the fishes and invertebrates in shallow-benthic (50 and 150 meters), deep-benthic (650 meters) and mid-water (400 and 800 meters) habitats in Monterey Bay. Sampling is conducted using various types of trawling equipment.

Available biological information includes identification of the organisms to the lowest possible taxa, their numbers, and sometimes sizes or size ranges. Survey data are entered into a database, and maps and charts are scanned.

As of March 2005, 158 invertebrates species (and 358 others identified to genus or higher) and 231 fish species (and 55 other fish taxa) have been observed. Definite, predictable changes in fish assemblages were observed with depth. During some warm-water events, such as El NiÒo events, fishes more commonly observed in southern California were collected from sanctuary waters.

Patterns and Dynamics of Benthic Soft-Sediment Faunal Communities
Since 1994, scientists at MBARI have been monitoring megafaunal populations on the deep seafloor (200 to 1,000 meters) at Smooth Ridge in Monterey Bay to determine patterns of abundance and measure changes over time. These data will provide a measure of the interannual variation in community structure in the region.

One interesting finding of this monitoring program is the horizontal patchiness in the abundance of some species. For example, four shelf-dwelling rockfish species are much more abundant at one end of the 200-meter transect than at the other end. In addition, the structure of the seafloor community is variable from year to year, particularly along the shallower (200 meters) transect.

Abyssal Fauna Associated With a Whale Fall in Monterey Canyon
In 2002, MBARI scientists discovered an unusual deep-sea community associated with the remarkably well-preserved carcass of a gray whale, Eschrichtius robustus, at 2,891 meters' depth in the axis of Monterey Canyon.

As a result of the whale fall, an intense burst of approximately 20,000 kilograms (44,092 pounds) of organic material was exported to the typically food-poor seafloor. Particularly noteworthy was the absence of large scavengers, the presence of a number of opportunistic deep-sea species and the abundance of unusual polychaetes - two of which actually digest bone and are new to science.

Since its discovery, the whale fall has been revisited, which has allowed scientists to document faunal community changes in one of the deepest large food falls known to date. MBARI continues to monitor this and other whale fall communities. Whale falls provide opportunities to study localized effects of organic enrichment at depths as well as the unusual animals that rely exclusively on these very transient habitats.

Davidson Seamount Exploration
In 2002 and 2006, a diverse group of scientists explored and began characterizing Davidson Seamount; one of the largest seamounts on the West Coast located beyond the sanctuary boundary. For more information, see the Seamounts Overview.