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

AUGUST 2005 FINAL REPORT
rooted vegetation which are alternately inundated and drained by the tides (Day et al. 1989). While salt
marshes can occasionally form in tropical regions along salt flats, they are not known to occur in the study
area (Day et al. 1999). Rather, mangroves, the tropical equivalent of salt marshes, occur within the study
area. Mangroves often line the shores of coastal embayments and the banks of rivers to the upper tidal
limits in tropical environments, especially where the slope is gentle (Myers 1999). Mangroves possess
large roots that spread laterally and consolidate sediments, eventually transforming local mudflats into dry
land (Myers 1999). The extensive root system and nutrient rich waters found in mangroves make them
among the richest of nursery grounds for marine life (Scott 1993; Myers 1999).
On Guam, estuarine habitats occur in areas of tidal intrusion or brackish water, and consist primarily of
mangroves and the lower channels of rivers that are inundated by tides ranging from 75 to 90 cm in
amplitude (Scott 1993). Nine of the Guam’s 46 rivers that empty into the ocean have true estuarine
habitats with elevated salinity levels extending upstream (Scott 1993). While estuarine habitats in the
CNMI are not as widely studied, there are a number of bays and lagoons that probably function as
estuarine habitats. Further discussion of the estuarine environments located within the study area
including sand flats, mud flats, lagoons, and mangroves can be found within this chapter.
2.7.5 Lagoons
A lagoon within the study area can be described as a semi-enclosed bay found between the shoreline
and the landward edge of a fringing reef or barrier reef (NCCOS/NOAA 2005; Paulay personal
communication). By geomorphological definition, true lagoons lie only behind barrier reefs, while moats (a
shallow analogue of lagoons) can lie behind fringing reefs (Paulay personal communication). A lagoon is
formed when a sandbar (or barrier reef) is built up parallel to the coastline and cuts off the inland waters
to the sea, creating a shallow region of water. A lagoon typically contains three distinct zones: freshwater
zone, transitional zone, and saltwater zone (Thurman 1997). Yet, most tropical reef-associated lagoons
are not brackish and lack significant freshwater influence (Paulay personal communication).
The study area contains numerous relatively shallow lagoons (depth ranging from 1 to 15 m) and one
deep lagoon, Apra Harbor (PBEC 1985; Paulay et al. 1997; NCCOS/NOAA 2005; Figures 2-7 through 2-
9). The bottoms of the lagoons are mostly sandy and flat or undulatory. Coral rubble, coral mounds (patch
reefs), seagrass, and algae are found within the lagoons. Coral mounds tend to be more abundant in the
outer lagoons and are widely scattered or absent in the inner lagoons (PBEC 1985; NCCOS/NOAA
2005).
Apra Harbor, the only deep lagoon on Guam and the busiest port in the Mariana Islands, is enclosed by
the Glass Breakwater (Figure 2-7a). The Inner Apra Harbor is a lagoon created by dredging in the 1940s.
Cocos Lagoon, a shallow lagoon (12 m water depth) located on the southern tip of the island is also
encompassed by a series of barrier reefs (Paulay et al. 2002). Sasa Bay, also located on Guam, is a
shallow coastal lagoon populated with patchy corals (Scott 1993). Embayments along the entire western
coastline except for the small regions spanning from Oca Point to Ypao Point and from Orote Point to
Apuntua Point have developed behind fringing reefs and may possess physical characteristics similar to a
lagoon (USGS 1978; Paulay et al. 2002; Figure 2-7a). A similar situation occurs on the eastern coastline
with fringing reefs occurring along the eastern coastline from Fadian Point to Cocos Lagoon (USGS 1978;
Figure 2-7a).
The western coastline of Saipan is lined with sandy beaches protected by a barrier reef which forms
Tanapag and Saipan Lagoons (Scott 1993). Tanapag Lagoon is a typical high-island barrier reef lagoon.
Tanapag Lagoon is located on the northwestern coast of Saipan. Also, on the western coastline of
Saipan, the barrier reefs form two additional lagoons, creating the largest lagoonal system in the Mariana
Islands, Garapan Lagoon and Chalan Kanoa Lagoon (Chandron 1988; Duenas and Associates 1997;
Trianni and Kessler 2002). The maximum width of Saipan Lagoon is 100 m, and the maximum depth is
14 m in the Tanapag Harbor channel, although average depth is only 3 m (PBEC 1985; Trianni and
Kessler 2002).
2-40