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

AUGUST 2005 FINAL REPORT
The islands of Tinian and Rota lack complex lagoon systems. The island of Tinian is surrounded by reefs,
but lacks a true lagoon complex. The lagoons of Tinian, save two (off of the Leprosarium at the
southwestern edge of the leaseback area (LBA; see Figures 1-1 and 2-8a), and the northern region of
the Tinian Harbor area), are all adjacent to military leases (USGS 1980; NCCOS/NOAA 2005).
Saipan has five small lagoons located on the western side of the island and two lagoons along the
eastern coastline (USGS 1980; PBEC 1985; NCCOS/NOAA 2005). On the island of Rota, a small “semilagoon”
is located along the entire western coast, and the only true lagoon on Rota can be found at the
extreme southern tip of the island (PBEC 1985; NCCOS/NOAA 2005).
2.7.6 Seagrass Beds
Seagrasses are vascular (flowering) plants adapted to living in a saline environment and grow completely
submerged (Phillips and Menez 1988). Seagrasses are unique as they are land plants that spend their
entire life cycle underwater. Seagrasses grow in muddy or sandy substrates and can develop into
extensive undersea meadows (Phillips and Menez 1988). Seagrass beds are among the most highly
productive ecosystems in the world and are an important ecosystem of shallow-water tropical regions
(Nybakken 1997). Beds are often used as protective habitats or nursery grounds for many organisms that
live in/on sandy or muddy bottoms, in the surrounding waters, or on the plants themselves (Phillips and
Menez 1988; Daniel and Minton 2004). While seagrasses are consumed by only a few species (including
dugongs, sea turtles, mollusks, and some urchins), many organisms feed on the epiphytic algae growing
on the plant structure (Nybakken 1997).
Seagrass beds are widely distributed within the study area. Both Guam and Saipan have extensive
seagrass meadows surrounding the coastlines (NCCOS/NOAA 2005; Figure 2-10), including extensive
beds in Agat Bay (including the Agat Unit of the War in the Pacific National Historical Park; Daniel and
Minton 2004), south of Apra Harbor, and Cocos Lagoon on Guam (Eldredge et al. 1977; Daniel and
Minton 2004). Rota is known to posses a small seagrass bed off its southern shore (Abraham et al.
2004). Tinian possesses seagrass beds along the northwestern, the northeastern, the southwestern and
the eastern coastlines (DoN 2003a). Seagrasses are more scattered on the island of Saipan, with
seagrass beds reported along Tanapag Beach (along the northwest coast) and in the Puerto Rico
Mudflats (northwest shoreline, south of Tanapag Beach) (Tsuda et al. 1977; Scott 1993). Seagrasses
have vanished off the southern coast of Saipan (Abraham et al. 2004). There is no record of seagrass
beds occurring on the islands north of Saipan (Tsuda 2003).
Currently, three species of seagrasses (Enhalus acoroides, Halodule uninervis, and Halophila minor) are
known to occur in the Mariana Islands (Tsuda et al. 1977). Enhalus acoroides, also referred to as tape
grass, possesses long leaves (30 to 150 cm long and 1 to 2 cm wide), white flowers, forms clumps, and
grows best on sheltered coastlines in sandy or muddy substrate in a range from the mean low water to 4
m deep (Phillips and Menez 1988; Daniel and Minton 2004). Halodule uninervis possesses leaves
ranging from 6 to 15 cm long (0.25 to 3.5 mm wide), grows from the intertidal zone to 30 m deep on firm
sand and soft mud, and can survive in a range of environments including highly sheltered bays and along
coral reefs. Halophila minor has small wide leaf blades (0.7 to 1.4 cm long, 3 to 5 mm wide) and is found
in sheltered areas on muddy or sandy substrate in the upper subtidal zone (Phillips and Menez 1988).
2.7.7 Mangroves
Mangroves are a type of wetland that borders estuaries or shores protected from the open ocean (Scott
1993). They are composed of salt-tolerant trees and other plant species and they provide critical habitat
for both marine and terrestrial life. Species diversity is usually high in mangroves, and like seagrasses,
can act as a filter to remove sediments before they can be transported onto an adjacent coral reef (Scott
1993; Nybakken 1997; Thurman 1997).
Mangrove forests are native to the Marianas study area, however, are only present on the islands of
Guam and Saipan (Figure 2-10), with the mangroves of Guam being the most extensive and diverse,
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