Oceanographic and Acoustic Survey Focused on Pelagic Habitat and Ecosystem of the Mariana Archipelago

In 2010, scientists in the PIFSC Ecosystem and Oceanography Division conducted a 4-week oceanographic and acoustic survey of oceanic waters in the Mariana Archipelago. The NOAA ship Oscar Elton Sette was used to survey waters of the Commonwealth of the Northern Mariana Islands (CNMI), Guam, and Micronesia. Data were collected to identify physical and biological oceanographic characteristics of the region. The research improves our understanding of the habitat and ecosystem in this economically important area, which supports a variety of pelagic species targeted by fishermen.

To better understand the habitat, a Conductivity-Temperature-Depth (CTD) instrument, oxygen sensor, and fluorometer were deployed from the ocean surface down to 1,000 meters at stations along three meridional transects west ("Western Transect"), south ("Central Transect"), and east ("Eastern Transect") of the island chain (diagram below). As the CTD was deployed, water samples were collected at each full degree along the survey trackline to sample nutrients and chlorophyll-a, and at every 0.5° to sample chloropigments. Discrete chlorophyll-a samples were filtered at sea and chloropigment samples were stored in liquid nitrogen for later laboratory HPLC analysis.

Density of micronektonic organisms in the water column (bioacoustic backscatter measured by sonar) and ocean currents (measured with an acoustic Doppler current profiler) were continuously monitored during the oceanographic survey. Further, zooplankton were sampled every 1° along the three meridional transects using an Isaacs Kidd Mid-water Trawl (IKMT). All samples were collected in the upper 200 m during the night, when zooplankton (and micronekton) density increases as diel migrators move to near-surface waters to feed. To supplement the in situ data, satellite altimetry and ocean color data were acquired and used to learn about eddy structure and productivity levels in the area.

Survey transects for in situ observations of the NOAA Ship Oscar Elton Sette in the Mariana 
            Archipelago, 20 March – 12 April 2010.  Filled squares: CTD with all variables sampled; Empty squares: CTD 
            with only chloropigment sampled; Gray circles: CTD with no sampling. At all sampled CTD stations, acoustic
             transects and IKMT trawls were conducted (once in daytime, twice at night).
Survey transects for in situ observations of the NOAA Ship Oscar Elton Sette in the Mariana Archipelago, 20 March – 12 April 2010. Filled squares: CTD with all variables sampled; Empty squares: CTD with only chloropigment sampled; Gray circles: CTD with no sampling. At all sampled CTD stations, acoustic transects and IKMT trawls were conducted (once in daytime, twice at night).

As shown in the accompanying graph of vertical profiles, data obtained from the CTD casts show a general shallowing of the mixed layer in the southern part of the survey area (left end of profiles in panel a), and a maximum and minimum in salinity at 100-250 m and 300-600 m, respectively (panel b). The salinity minimum shallows and becomes vertically thinner from south to north. The salinity maximum reaches the surface and is wider in its vertical extent to the north. It becomes a subsurface feature to the south, caused by North Equatorial Current (NEC) dynamics. This salinity maximum represents North Pacific Tropical Water that is formed to the north and advected southward along isopycnals and westward by the NEC.

Dissolved oxygen concentrations were relatively high in the upper 100-150 m with a strong gradient from 150-250 m in the south to 400-450 m in the north, deeper along the Eastern Transect than along the Western Transect (panel c). Below these depths, oxygen minimums were at 350-450 and 650 m in the south and north, respectively. Fluorometry results show a very low background concentration of chlorophyll-a (< 0.05 mg m-3), apart from subsurface deep chlorophyll-a maximum situated around 150 m in depth and shoaling to the south (panel d).

Besides evidence from CTD data, the presence of the westward flowing NEC is confirmed by the ADCP records. In the graph of ADCP results, note the variability in direction and small magnitude of currents along the Western Transect in the upper 200 m (panel a). This is the effect of eddy activity in the wake of the island chain. The presence of eddies west of the islands is confirmed by satellite altimetry data. Interestingly, the direction of the current in the upper 200 m reverses along the Western Transect, and along this transect in deeper water (panel b) the current increases in magnitude and flows uniformly to the east.

IKMT samples were dominated by micronektonic organisms (almost half of the samples by weight), followed by gelatinous zooplankton, euphausiids, salps, and shrimps. For the migratory portion of resident micronekton and macrozooplankton, the highest variability in biomass was observed in the latitudinal dimension; this pattern has been widely observed across the equatorial region. Zooplankton diversity and biomass in the trawl tows significantly increased south of 12°N along the Eastern Transect, while ichthyoplankton diversity and abundance were higher along the Western Transect than the Central Transect and Eastern Transect. Ichthyoplankton and micronektonic fish associated with coral reef communities were found west of the island chain, likely carried westward from the islands by the NEC.

Current vectors along the 3 survey transects in the upper 200 m (a) and at 600-800 m depths (b).  From 
            ADCP data.
Current vectors along the 3 survey transects in the upper 200 m (a) and at 600-800 m depths (b). From ADCP data.