Marine Resources Assessment for the Marianas Operating ... - SPREP

AUGUST 2005 FINAL REPORT
2.7 COASTAL HABITATS
Coastal habitats of the study area encompass part of the subneritic zone, which extends from the
shoreline at high tide to the edge of the insular shelf (200 m isobath) (Kennett 1982; Thurman 1997). The
following discussion of shoreline habitats will focus on the intertidal zone (region of shoreline covered by
water between the high and low tidal extremes), coral communities and reefs, softbottom habitats (sand
beaches, mudflats, and sand flats), lagoons (semi-enclosed bays found around the islands), seagrass
beds, mangroves, and artificial reefs. Since the tidal range in the study area is less than 1 m (Paulay
2003), the shoreline intertidal zone is very narrow around the Mariana Islands.
Biodiversity is high throughout the subneritic zone due to the high variability existing within the habitat
(Thurman 1997). Organisms residing on or in the benthos (epifauna and infauna, respectively) can be
greatly affected by sedimentation, sediment resuspension, vertical mixing, regeneration (recycling of
nutrients), and light penetration (turbidity) (Valiela 1995).
2.7.1 Intertidal Zone
Within the intertidal zone, the shoreline can be divided into three subzones: the high-tide zone, the midtide
zone, and the low-tide zone. In the high-tide zone, benthic organisms are covered by water only
during the highest high tides. Organisms in this zone spend the majority of the day exposed to the
atmosphere. In the mid-tide zone, benthic organisms spend approximately half of the time submerged.
Organisms residing in this zone are exposed during periods of low tides, but are covered with water
during all high tides. Organisms in the low-tide zone are submerged most of the time but may be exposed
to the air during the lowest of low tides.
The islands within study area are volcanic in nature and thus the overall geology reflects this origin
(Eldredge 1983). The intertidal regions along the majority of the coastlines of islands in the study area are
rocky in nature (Rock 1999). Coastlines within the study area are generally lined with rocky intertidal
areas, steep cliffs and headlands, and the occasional sandy beach or mudflat (Eldredge 1983). The water
erosion of rocky coastlines in the study area has produced wave-cut cliffs (produced by undercutting and
mass wasting), and sea-level benches (volcanic and limestone and wave-cut notches at the base of the
cliffs (Eldredge 1979, 1983). Large blocks and boulders often buttress the foot of these steep cliffs in the
Marianas. Wave-cut terraces also occur seaward of the cliffs (Eldredge 1983; Myers 1999).
2.7.2 Coral Communities and Reefs
Islands within the study area (Guam to FDM) support reefs (biogenic or hermatypic coral reefs) as do
islands north of FDM (Anatahan, Sarigan, Guguan, Alamagan, Maug, and Farrallon de Pajaros)
(Birkeland et al. 1981; Eldredge 1983; Randall et al. 1984; Randall 1985; Randall and Siegrist 1988;
Birkeland 1997; Green 1997; Paulay et al. 1997, 2001; Houk 2001; Paulay 2003; Starmer 2005). Reefs
are also found on offshore banks including Tatsumi Reef located 2 km southeast of Tinian, Arakane Bank
located 325 km west-northwest of Saipan, Pathfinder Bank located 275 km west of Anahatan, and Supply
Reef located 18.5 km northwest of Maug Island (Starmer 2005). The degree of reef development
depends on a number of environmental controls including the age of the islands, volcanic activity, the
availability of favorable substrates and habitats, weathering caused by groundwater discharge,
sedimentation and runoff accentuated by the overgrazing of feral animals, and varying levels of exposure
to wave action, trade winds, and storms (Eldredge 1983; Randall 1985, 1995; Randall et al. 1984; Paulay
2003; Starmer 2005). The southern islands (Guam to FDM) are inactive volcanic islands that have
subsided and are covered by massive limestone deposits dating back more than 40 million years
(Birkeland 1997; Randall 2003). The substrate of the younger islands to the north of FDM dates back to
1.3 million years and is not characterized by substantial limestone deposits (Randall 1995, 2003). In the
southern islands, faulting and erosion caused by groundwater discharge have produced large, oblique,
and shallow areas (lagoon, bays) favorable to extensive reef development. This contrasts with the vertical
profile of the uplifted younger islands, where less favorable and fewer macrohabitats are available for reef
development (Randall 1995; Birkeland 1997; Paulay 2003).
2-19