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

AUGUST 2005 FINAL REPORT
A fringing reef borders the Unai Dangkolo white carbonate beach (NCCOS/NOAA 2005). Macroalgae (10
to 50% cover) populate the reef flat while the fringing reef is composed of coralline algae. Corals (10 to
50% cover) are a main constituent of the fore reef and insular shelf (NCCOS/NOAA 2005). Surveys
conducted in 1994, however, report that the inner reef flat supports an extensive (50 to 70% coral cover)
and diverse reef community (25 coral species) (Marine Research Consultants 1999). On the reef front,
there is a spur-and-groove system down to a depth of 10 m, seaward of which the benthos is comprised
of carbonate pavement. Both the spur-and-groove system and the fore reef pavement are densely
populated by corals (36 species of corals). The passage of a typhoon in December 1997 severely altered
the reef flat coral community diversity and cover. Coral cover on the reef flat was reduced from an original
50 to 70% cover to 2% cover. No branching corals remained on the reef flat following the typhoon (Marine
Research Consultants 1999). The recent benthic habitat mapping of the CNMI by NCCOS/NOAA (2005)
reflects the change in reef flat composition. Since NCCOS/NOAA (2005) show relatively abundant coral
cover on the reef front, the fore reef has possibly retained some of its pre-December 1997 characteristics.
The impacts of corallivorous predators on corals have most likely altered the coral composition and cover
on the fore reef (Quinn and Kojis 2003).
2.7.2.4 Coral Communities and Reefs of Farallon de Medinilla
In contrast with the other southern Mariana Islands, the study area at FDM does not include fringing or
fore reefs (Figures 2-9a, 2-9b, and 2-9c). Rather, it has a relatively wide insular shelf (400 to 1,800 m
wide) that supports limited coral cover along all sides except the western side of the island
(NCCOS/NOAA 2005; Smith personal communication). In 2004, 81 species of corals were observed on
reefs at FDM (DoN 2005). Overall, the northwestern nearshore area (eroded submerged cliff face and
reef terrace) of the island supports the highest diversity of marine invertebrates and fishes on FDM (DoN
2005). Most of the coastline of FDM is bordered by steep karstic cliffs which for the most part extend 6 to
9 m below the waterline (DoN 2005). Cliffs on the western shoreline extend more than 20 m below the
waterline. There are numerous underwater caves along the FDM shoreline. Boulders dislodged from the
cliffs border the base of the cliffs. Seaward of the cliff face is a reef terrace that is 30 to 50 m wide and 10
to 25 m deep beyond which is a sandy slope zone. On parts of the western side of the island, a vertical
wall undercut by caves and ledges delimits the seaward edge of the reef terrace and intersects with the
sandy slope habitat. At the southern end of the island, a 2 m deep “finger reef” extends 200 m southward.
The edges of the finger reef are vertical walls that drop down to a 30 m depth. The reef terrace consists of
a spur-and-groove system on the eastern (windward) side of the island where the island forms an isthmus
separating the lower narrow third of the island from the wider upper two-thirds of the island (DoN 2005).
Near the cliff edge on the reef terrace of the eastern side of FDM, there is less than 5% coral cover (DoN
2005). Further offshore, there is 10 to 20% coral cover composed of encrusting Porites and head coral
forming Pocillopora. Coral cover on the boulders is 25 to 30% and comprised of Pocillopora, Porites,
Montipora, and Millepora. Coral cover on the ridges of the spur-and-groove system off the island isthmus
on the windward side ranges from 15 to 25% and is composed of Porites and Pocillopora. There are large
aggregations of the long-spined urchin Echinotrix diadema (hundreds to thousands of individuals) seen
both on the eastern and western sides of the island, and high coral cover is found on boulders along the
reef terrace on the leeward side of the island (50 to 70%, mostly Pocillopora coral heads). Most of the
branching colonies of Pocillopora sp. on the leeward side have broken branches (DoN 2005).
Since 1971, FDM has been a target site for live-fire military exercises (ship-to-shore gunfire, aerial
gunnery and bombing) (DoN 2005). The majority of the ordnance found underwater at FDM during reef
assessments conducted since 1999 occur at the northwestern end of the island (DoN 2005).
2.7.3 Softbottom Habitats
Softbottom habitats are those habitats in which the benthos is covered with a layer of fine sediment
(Nybakken 1997). Commonly identified habitats are beaches, sand flats, and mudflats (Figures 2-7
through 2-9). Sand flats differ from sand beaches in that beaches are intertidal pile-ups along coasts,
while sandflats can be found anywhere away from the coasts. Softbottom habitats can occur on a sloped
seafloor and not only on a flat, horizontal surface (Paulay personal communication).
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