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

AUGUST 2005 FINAL REPORT
The NEC, which provides the bulk of water passing the Mariana archipelago, is composed primarily of
plankton-poor water. Detailed information on the oceanic zooplankton community in the waters of the
Marianas MRA study area is practically nonexistent (Uchida 1983). Rather, data gathered in waters
surrounding the study area must be explored to gain insight into the zooplankton communities within the
study area. Total zooplankton biomass at the surface examined for the western Pacific and adjacent seas
found that zooplankton biomass was the lowest within the NEC, reaching concentrations of only 1.35
grams (g) wet weight/m 2 (Vinogradov and Parin 1973). Vinogradov and Parin (1973) also surveyed
zooplankton biomass in the tropical Pacific, and at their station nearest the study area (13°31’N,
139°58’E), zooplankton biomass was very low (11.7 mg/m 3 ).
Studies on the neritic plankton have centered around Apra Harbor and Piti Reef on Guam. However, the
majority of studies have been performed in conjunction with more general environmental surveys, and
thus no long-term surveys have been conducted. In general, abundance of zooplankton is highly variable
with respect to location and time (both throughout the day and month to month) (Uchida 1983). In Apra
Harbor, the commercial port contains the highest levels of zooplankton abundance and is dominated by
copepods (Uchida 1983). Other organisms in the harbor include fish larvae, decapod zoeae (freeswimming
larvae), and pteropods (Uchida 1983). In Tanapag Harbor, Saipan, the diurnal zooplankton
community is dominated by copepods and the nocturnal zooplankton community by larval crustaceans
(Uchida 1983).
2.6 BENTHIC HABITATS
Deep sea benthic habitats include seamounts, hydrothermal vents, the abyssal plain, and trenches. The
bottom sediments covering the sea floor in much of the study area are volcanic or marine in nature
(Eldredge 1983). In the Marianas Trench, the seabed is composed mostly of sand and clays (Ogawa et
al. 1997). Sediments found on the narrow shelves along the Marianas archipelago are a combination of
volcanic and calcareous sediments derived from calcareous animal skeletons (Eldredge 1983).
2.6.1 Seamounts
Seamounts are undersea mountains that rise steeply from the ocean floor to an altitude greater than
1,000 m above the ocean basin (Thurman 1997). Generally, seamounts tend to be conical in shape and
volcanic in origin, although some seamounts are formed by tectonic movement and converging plates
(Rogers 1994). The study area contains seamounts of both types. The seamount topography is a striking
difference to the surrounding flat, sediment covered abyssal plain, and the effects seamounts can impart
on local ocean circulation are complex and poorly understood (Rogers 1994). However, around
seamounts increased levels of phytoplankton, primary production, and pelagic and demersal fish (Zaika
and Kovalev 1984; Fedorov and Chistikov 1985; Greze and Kovalev 1985; Parin et al. 1985; Rogers
1994) are correlated with current pattern alterations and Taylor columns (circulation vortices) (Darnitsky
1980; Boehlert and Genin 1987; Rogers 1994).
The large ranges in depth, hard substrate, steep vertical gradients, cryptic topography, variable currents,
clear oceanic waters, and geographic isolation all combine to make seamounts a unique habitat for both
deep-sea and shallow water organisms (Rogers 1994). Thus, seamounts are capable of supporting a
wide range of organisms (Wilson and Kaufman 1987). To date, Richer de Forges et al. (2000) conducted
the most extensive species identification on seamounts. Richer de Forges et al. (2000) found a range of
108 to 516 species of fish and macro-invertebrates from three areas of seamounts in the southwest
Pacific (Tasman Sea, Coral Sea). Approximately one third of species found were new to science and
potentially endemic. The number of species encountered versus the sampling effort showed that more
species are probably present on the seamounts they investigated. Richer de Forges et al. (2000) noted
that there were significant differences in the species composition between groups of seamounts found at
a same latitude and approximately 1,000 km apart. Such differences in seamount communities suggest
that species dispersal is limited to clustered seamounts and that seamount species have localized
distributions (Richer de Forges et al. 2000).
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