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

AUGUST 2005 FINAL REPORT
The topography of a mud flat is flatter than that of a sand flat, as mudflats require less wave energy to
form (Nybakken 1997). Mud flats are also more stable than their sand counterparts, and are more
conducive to the establishment of permanent infaunal burrowing communities (Nybakken 1997). The
Puerto Rico Mudflats of Saipan (15°13’N, 145°43'E) and mudflats in and around the Apra Harbor
mangrove system are substantial (Scott 1993; Stinson et al. 1997; Paulay personal communication) and
are important feeding grounds for migratory shorebirds (Scott 1993; Figures 2-7 through 2-9).
Softbottom substrates in coastal regions of the study area are not common. This is due to the fact that the
intertidal and subtidal regions are often characterized by limestone pavement interspersed with coral
colonies and submerged boulders (Kolinski et al. 2001). Shorelines are often rocky with interspersed
sand beaches or mud flats (Eldredge 1983; PBEC 1985).
On the island of Guam, the majority of the coastline is comprised of rocky intertidal regions. Interspersed
among this rocky shoreline are 58 beaches composed of calcareous or volcanic sands (Eldredge 1983).
On Rota, the rare beaches are found scattered among limestone patches and are composed of rubble
and sand (Eldredge 1983). The submarine topography surrounding Tinian and Aguijan can be described
as limestone pavement with interspersed coral colonies and submarine boulders (Kolinski et al. 2001).
While the island of Aguijan contains no beaches (Kolinski et al. 2001), the island of Tinian contains 13
beaches (10 located on the west coast and 3 on the east coast). These beaches are not well developed
(except Tinian Harbor on the southwest coast, and Unai Dankulu along the east coast, Figure 2-7c) and
are comprised mainly of medium to course grain calcareous sands, gravel, and coral rubble (“coral-algalmollusk
rubble”) (Eldredge 1983; Kolinski et al. 2001). The west coast of Saipan contains well developed
fine-sand beaches protected by the Saigon and Tanapag Lagoons (Scott 1993). All other beaches of
Saipan consist of coral-algal-mollusk rubble. The coastal area of FDM contains two small intertidal
beaches that are inundated by high tide on the northeastern and western coastlines. Offshore of FDM, at
approximately 20 m, a softbottom, sandy slope extends downward onto the abyssal plain (DoN 2003a).
Most of the other islands in the Marianas also have sandy slopes below the fore reef, typically starting at
30 to 40 m, with some variation (Paulay personal communication).
2.7.4 Estuarine Habitats
Estuaries are bodies of water along coasts and are formed where there is an interaction between
freshwater, saltwater, land, and the atmosphere (Day et al. 1989). Estuaries are among the most
productive natural systems on earth, producing more food per acre than the richest farmland (RAE/ERF
1999). A minority of fish and shellfish species depend on estuaries, although these are often very
abundant and economically important species. Estuaries provide a vital buffer between land and open
water, filtering pollution and protecting surrounding lands from flooding (RAE/ERF 1999). The dominant
feature of the estuarine environment is the fluctuating salinity. By definition, a salinity gradient (from fresh
to saline) exists at some time in an estuary (Nybakken 1997).
There are many types of estuaries in the world. The most common type is the coastal plain estuary which
is formed when rising sea levels invade low-lying coastal river valleys. Examples of coastal plain estuaries
are the Chesapeake Bay and the mouths of the Hudson and Delaware Rivers on the east coast of the
U.S. (Nybakken 1997). Tectonic estuaries are formed when land subsides, allowing water to flood coastal
regions. One example of a tectonic estuary is San Francisco Bay (Nybakken 1997). Fjords, a third type of
estuary, are formed when a valley that has been deepened by a glacier is submerged by oceanic waters.
Fjords are characterized by a shallow sill that restricts water exchange with the ocean and the deeper
waters of the fjord. Finally, the lagoon is an estuarine habitat formed along a coastline behind a sandbar
or reef. Within the study area, estuarine habitats are found in lagoons, embayments, and river mouths.
Steep slopes and complex shorelines of the Mariana Islands (Guam to FDM) form relatively sheltered
coastal bays characterized by silty sediments and turbid waters. Often, these bays are associated with
riverine freshwater discharge (Myers 1999). Bordering estuaries and coastal embayments throughout the
world are unique plant associations. In temperate and subpolar regions, this association is found in the
form of a salt marsh. A salt marsh develops wherever sediment has accumulated to form a transition area
between aquatic and terrestrial ecosystems (Nybakken 1997). They are composed of beds of intertidal
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