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

AUGUST 2005 FINAL REPORT
LCPW is also referred to in the literature as Circumpolar Deep Water (CDW) (Pickard and Emery 1982).
Part of the LCPW flows from the South Pacific across the equator, westward around the Marshall Islands
and into the Mariana Trough and Trench (Mantyla and Reid 1978; Kawabe et al. 2003; Siedler at al.
2004). Seafloor ridges prevent the densest water of the LCPW and NPDW from reaching the Mariana
Trench. Otherwise, the water characteristics in the Mariana Trench are identical to abyssal water found
north of the Marshall Islands (2,000 km east). At depths ranging from 5,585 to 10,933 m, the Mariana
Trench seawater temperature ranges from 1.5° to 2.5°C, salinity is approximately 34.7 parts per thousand
(ppt), and dissolved oxygen concentrations are about 4 milliliters per liter (ml/l) (Mantyla and Reid 1978).
2.4.1.3 North Pacific Subtropical Gyre
Approximately half of all primary production is supported by phytoplankton found in the oceans (Falkowski
1994). In the marine environment oceanic provinces, or subtropical gyres, occupy 40% of the earth’s
surface, are located far from land, and account for the majority of primary production (Karl 1999). The
Marianas study area lies within the western region of the NPSG, the most extensive gyre on Earth.
Despite being the largest ecosystem on the planet, the NPSG is remote, poorly sampled, and not well
understood (Karl 1999). The NPSG extends from 15 to 35°N and 135°E to 135°W, and is bounded by the
North Pacific Current to the north, the NEC to the South, the California Current to the East, and the
Kuroshio Current to the west. In total, the NPSG encompasses 2 X 10 7 km 2 , creating the planet’s largest
circulation pattern. Geologically, the NPSG is a very old region in which the present boundaries have
existed since the Pliocene (10 7 years ago) (McGowan and Walker 1985) and is considered a climax
community in which the climate affects the seascape, which in turn controls the community structure and
dynamics (Karl 1999).
The NPSG is comprised of warm (>24°C) surface water containing low nutrient levels, low standing
stocks of living organisms, and a persistent deep-water chlorophyll maximum (Karl 1999). The water
column can be divided vertically into two distinct regions including a light-saturated nutrient-limited layer
at the surface (0 to 70 m) and a light-limited nutrient-rich layer at depth (>70 m). Surface circulation in the
gyre is wind driven, and the overall anti-cyclonic rotation of the NPSG isolates the water within the gyre,
restricting exchange with adjacent current systems (Karl 1999).
Due to the isolated waters within the gyre, the NPSG is thought to be a semi-enclosed, stable, and
relatively homogenous habitat; however, increasing evidence suggests that the NPSG exhibits substantial
physical, chemical and biological variability on a variety of time and space scales (Karl 1999). For
example, regions of the NPSG show extensive mesoscale variability via the formation of discreet eddies,
near-inertial motions, and internal tides (Venrick 1990). In addition, during winter months, tropical
cyclones pass through the NPSG, moving from west to east, deepening the mixed layer and injecting
nutrient rich water into the surface waters fueling ephemeral blooms of phytoplankton (Karl 1999).
2.4.2 Hydrography
Hydrography refers to the scientific study of the measurement and description of the physical features of
bodies of water. The following sections describe in detail the temperature of water at the ocean surface,
the vertical structure of temperature within the water column, and the horizontal and vertical distribution of
the salinity in the study area.
Sea Surface Temperature—The waters of the study area undergo an annual cycle of temperature
change, however this temperature flux is only a few degrees each year, as would be expected from a
tropical climate (Figure 2-5). The temperature throughout the year ranges from about 25° to 31°C with an
annual mean temperature of 27° to 28°C for the years ranging from 1984 to 2003 (NOAA 2004a).
Temperatures increase during the summer and autumn months with peak temperatures occurring in
September/October.
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