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2. SANCTUARY RESOURCES
The Channel Islands National
Marine Sanctuary is located at the transition between two distinct
biogeographic coastal provinces: the Oregonian and the Californian
provinces (Briggs, 1974). South of Point Conception the cold temperate
waters of the California current flowing from the north meet the
warm temperate waters of the California countercurrent and seasonally
with the Davidson Current, both flowing from the south. The current
patterns affect the distribution of marine fauna. Species representative
of both cold-temperate and warm-temperate regions thrive in the
transition zone, and overlap in their distributions at some locations
within the sanctuary. The environmental conditions and resources
contributing to the ecological significance of the Channel Islands
Marine Sanctuary are described further below.
(a) Environmental Conditions
The sanctuary belongs to a region
referred to by geologists as the Southern California Continental
Borderland. A glance at bathymetric and geological maps confirms
that the Borderland is a complex of basins, canyons, ridges, and
shelves skirting high-cliffed islands. The area is also characterized
by active seismic faults, shallow oil and gas reservoirs, and natural
oil and gas seeps. These are most common in the northern Santa Barbara
Channel and are periodic or unconfirmed elsewhere (California State
Water Resources Control Board, 1979a, 1979b, 1981a; BLM, 1981c).
The submarine geology of the
sanctuary consists of undifferentiated sedimentary rock and quaternary
volcanic rock overlayed with unconsolidated sediments including
gravel, sand, shell rubble, silt, and clay. The rocky shelves around
San Miguel and Santa Rosa Islands are among the widest in California
and are evidence of intense oceanographic dynamics that have precluded
the accumulation of fine sediments (Welday and Williams, 1975).
The abrupt depth change from these shallow shelves to deep trenches
combined with the gradation from deepwater silt and clay deposits
to shallow sand and rock account for the wide variety of intertidal
and subtidal habitats in the sanctuary.
Oceanographic conditions are
another influence on the distribution and diversity of sanctuary
habitats and resources. When the cold California Current reaches
Point Conception, the direction of flow carries it away from the
shoreline, inducing a large gyre effect or eddy in the southern
California bight. The return flow of this gyre, the southern California
countercurrent, moves waters from southeast to northwest, through
the Channel Islands toward the mainland. The resulting gyres and
eddies affect the distribution of marine fauna leading to the presence
of both cold and warm temperate species. Surface and bottom currents
also circulate nutrients and pollutants in the sanctuary. For example,
currents and winds are known to affect turbidity around the islands,
with turbidity generally higher on the north or windward side and
to the east off Anacapa and Santa Cruz Islands (California State
Water Resources Control Board, 1979a).
An upwelling period occurs in
the southern California bight from February or March through August,
during which nutrient-rich deep waters are drawn to the surface
by offshore winds. During the spring, high nutrient levels combined
with increasing day length and light intensity foster exceptionally
high phytoplankton and algal productivity. This increase in the
food supply supports even greater numbers of fish, shellfish, and
other marine life in the sanctuary.
Other aspects of water quality
in the sanctuary have been documented only to a limited extent (NPS,
1982b; California State Water Resources Control Board, 1979a, 1979b,
1981a, 1981b; BLM, 1981c). Owing to the absence of municipal or
industrial point sources, and relative isolation from mainland sources,
water quality around the islands is generally good. However, chronic
oil and tar pollution from non-point sources outside the sanctuary
is reported as an important concern particularly along the north
shore of the islands (California State Water Resources Control Board,
1979a, 1979b). Data collected for the California Mussel Watch Program
at stations along the mainland and at the offshore islands indicate
significant accumulations of heavy metals at Anacapa, Santa Barbara,
San Miguel Island Areas of Special Biological significance (California
State Mussel Watch, 1979 in BLM, 1981c). It is not currently known
if these levels of pollutants are accumulating at higher trophic
levels and whether they are affecting individuals or populations.
Weather conditions in the area
tend to be localized and seasonally variable, but typical of a coastal
mediterranean climate. Over the course of an average year, January
to March are the months receiving the most rain. Associated with
the frequent weather fronts are strong winds, poor water visibility,
and four to six foot (one to two meter) swells. Winter water temperatures
around the Channel Islands do not exceed 55É F to 60É F (13É C to
15É C). Coastal fog and low clouds prevail during April and May
and the sea surface grows favorable and predictable weather conditions
for diving and boating are from June to October when weather fronts
are infrequent and seas are generally calm. Summer fogs are also
common. Average water visibilities around the islands are reported
between 30 to 40 feet (9 to 12 meters) and water temperatures range
from 65É F to 70É F (18É C to 21É C). Stormy and windy conditions
tend to prevail again from October to December (Fagan and Pomeroy,
1980).
This yearly weather pattern
is considered predictable and is largely responsible for the May
to October peak recreational use season. Conditions do vary among
parts of the sanctuary. San Miguel Island, for example, is completely
surrounded by the cold water mass of the California current and
is exposed to the open seas of the Pacific. Nearshore waters are
usually colder, more turbid, and subject to heavy surge. Weather
conditions are reported to change quickly even during the summer,
thus presenting constraints for on-site operations. The Santa Barbara
Island nearshore is also exposed to strong westerly winds, but waters
are more characteristic of the clear and warm conditions of the
California countercurrent. Similar warm and clear conditions prevail
in the vicinity of Anacapa Island with nearshore waters being more
sheltered by the islands to the west. Conditions around Santa Rosa
and Santa Cruz Islands resemble those described for San Miguel Island
with the waters around Santa Cruz being somewhat more protected
and warmer than those around Santa Rosa Island. The general weather
conditions are an important consideration for on-site operations
such as surveillance, search and rescue, and oil spill containment
in the sanctuary. Differences in the sanctuary suggest distinct
safety and Ñcomfortæ zones which can affect the planning of visitor
use and interpretation.
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