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

AUGUST 2005 FINAL REPORT
SST along the reef flats near the shoreline have been reported to average 2°C higher than those reported
in nearshore waters and may reach temperatures as high as 34°C during periods of extensive low tide
(Eldredge 1983).
Increases in SST caused by El Niño events can influence the distribution pattern of fishes (Lehodey et al.
1997). Further, prolonged high SST will cause the bleaching of corals, coral mortality and induce the
outbreak of coral diseases within the study area (Harvell et al. 1999; Paulay and Benayahu 1999;
Richmond et al. 2002).
Thermocline—The water column in the Marianas MRA study area contains a well-mixed surface layer
ranging from 90 to 125 m. Immediately below the mixed layer is a rapid decline in temperature to the cold
deeper waters. Unlike more temperate climates, the thermocline in the study area is relatively stable,
rarely turning over and mixing the more nutrient waters of the deeper ocean in to the surface layer.
Salinity—The study area lies in a region near the equator of low surface salinity bound to the north and
south by regions of higher salinity (Pickard and Emery 1982). Surface salinity is lower towards the
southern end of the Mariana archipelago and increases towards the north. At a depth of 100 to 200 m,
there is a spike in salinity that corresponds with the input of high saline tropical waters (Eldredge 1983).
Below this region, the salinity drops to a minimum (approximately 34.5 ppt) and corresponds to the influx
of North Pacific Intermediate Water (NPIW). NPIW is formed as cold, fresh, dense water sinks below the
more saline water in the north subarctic Pacific Ocean and can be recognized by its overall lower salinity
and location within the water column (500 to 700 m depth) (Eldredge 1983).
2.5 BIOLOGICAL OCEANOGRAPHY
The physical environment of an area can directly affect the distribution of marine life found within. In this
section, the major groups of organisms found in the Marianas MRA study area are discussed with
particular reference to their geographical distribution and any physical mechanisms that may affect their
distribution. The organisms that comprise the base of the food web and those to which all other oceanic
organisms depend, the plankton, will be specifically discussed here while discussions of the larger
species found in the study area may be found in subsequent chapters.
2.5.1 Primary Production
Primary production is a rate at which the biomass of organisms changes and is defined as the amount of
carbon fixed by organisms in a fixed volume of water through the synthesis of organic matter using
energy derived from solar radiation or chemical reactions (Thurman 1997). The major process through
which primary production occurs is photosynthesis. The intensity and quality of light, the availability of
nutrients, and seawater temperature all influence primary productivity as generated through
photosynthesis (Valiela 1995). Chemosynthesis will also be mentioned in this section since it is another
form of primary production occurring at hydrothermal vent communities along ocean spreading centers in
the study area.
2.5.1.1 Photosynthesis
Photosynthesis is a chemical reaction that converts solar energy from the sun into chemical energy
stored within organic molecules by combining water, carbon dioxide, and light energy to form sugar and
oxygen. In the oceanic system, the majority of photosynthesis is carried out by phytoplankton utilizing a
suite of light harvesting compounds to convert solar energy into chemical energy, the most common
being chl a (Thurman 1997). Rates of photosynthetic production can vary from between less than 0.1
milligram (mg) of carbon (C) per square meter (m 2 ) per day (d) in low primary productivity (oligotrophic)
regions, such as the western equatorial Pacific, to more than 10 mgC/m 2 /d in highly productive areas
(Thurman 1997).
The western Pacific, including the study area, can be considered an oligotrophic region. The water
column surrounding the study area is composed of nutrient depleted surface area overlying a deeper
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