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Sancutary Program Accomplishments Open Ocean & Deep Water Systems The Physical Environment Endangered & Threatened Species
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Interpretation of this seafloor imagery in terms of seafloor geology is now complete and comprises a layer in our GIS (geographic information system) for our Sanctuary project. The resulting maps (e.g., Figure 1) show that rock outcrops occur predominantly on the inner and outer shelves, whereas the mid-shelf is covered with recent sediment. Coastal areas that are uplifting, such as the coast around Santa Cruz, have rock outcrops at the seafloor that have been eroded by wave action in the surf zone. In contrast, coastal areas that are stable or subsiding, such as the central Monterey Bay around Moss Landing, have only recent sands and other sediment at the seafloor. Around the Monterey Peninsula the granites that constitute the structural backbone of the Peninsula crop out on the seafloor. Sedimentary rocks with distinctive acoustic signatures can be recognized and equated to the common rock formations around the Bay, principally the Monterey, the Santa Cruz Mudstone, and the Purisima Formations, from youngest to oldest respectively. The Purisima Formation is the most common rock outcrop on the outer shelf and it forms ledges and hardgrounds for many benthic species that require a hard substrate. Outcrops on the outer shelf are most commonly the result of relatively more resistant cuestas of dipping rock strata that were probably eroded down to their present relief at times of lowered sea level, the last of which occurred prior to 15,000 years ago. The outcrops of the Purisima formation on the outer shelf south of Año Nuevo are populated with vast fields of Brachiopods (mollusk-like animals), based on video transects, and these presumably provide a major food source for fish and mammals of the area. In addition to rock outcrops, coarse sand bodies are prominently outlined by the acoustic imagery and occur mostly in the 20- to 30-m depth range around the Bay. These sand bodies are in the form of shallow, 1-m deep troughs floored by coarse sands molded into one-meter length waves. Such sand troughs are most common in the offshore former Fort Ord area, and the deepest one occurs offshore of Seaside at a depth of 56 m. The sand waves are successfully modeled as being produced by the oscillation of winter-storm-wave driven bottom water during times of the largest waves over the shelf. The seafloor imagery and interpreted geology will be published along with other studies of the seafloor and water-column in a special volume of the journal Marine Geology in summer or fall 1999. Stephen L. Eittreim
The USGS has deployed a moored current and temperature array on a line offshore of Davenport in water depths of 32 m, 70 m, and 120 m. Measurements span the last two years and include near-bottom currents and sediment data. During the first year, an additional mooring was deployed at a depth of about 60 m off Santa Cruz. Across the Davenport shelf, poleward currents dominate the mid-depth and near-bottom flows, even during spring/summer when surface winds and surface currents are predominantly equatorward. Results also suggest that suspended sediment transport on the shelf is mainly storm-driven and that Monterey Bay is the origin of the suspended sediments. In addition to the riverine sources entering Monterey Bay (north of the Monterey Canyon), coastal cliff erosion is likely to play a role. These results, based on data from 1997, should be considered in the context of the El Niño conditions during that period. Observed temperatures show that the entire shelf down to a depth of 100 m was unusually warm, with temperatures of greater than 15° C at depths of 60 m in November and December 1997. During this time, unusually strong poleward currents throughout the water column of up to 40 cm per second were observed, particularly at mid-water depths. The currents shifted abruptly equatorward in early 1998 at the outer-shelf mooring. Near the end of Feb-ruary 1998, temperatures at depth on the shelf and sea level off Davenport dropped over a relatively short period of several days, during a time of both strong equatorward winds and currents at mid-water depths. The USGS efforts are summarized online at: http://walrus.wr.usgs.gov/docs/projects/mbms/. A cooperative program between MBARI and NPS has been underway during 1997 and 1998 to monitor this critical El Niño period and the return to "normal" conditions. Fourteen hydrographic cruises crossing the Sanctuary along CalCOFI (California Cooperative Oceanic Fisheries Investigations) line 67 - a survey line that intersects the coast at Monterey Bay - have been carried out between January 1997 and November 1998 with five additional sections planned over the following year. A strong El Niño signal was clearly present in the subsurface temperatures from mid- to late 1997. Physical oceanographic conditions at the thermocline depths offshore returned to near normal by the time of the 1998 spring upwelling season. Jeffrey D. Paduan |
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