Oceanographic studies were conducted to describe gross features and variability within and across the SCS and Sulu Sea. Of particular interest are nutrient characteristics and productivity levels in representative shelf, shoal, and oceanic regimes in the basins. These features are important in inferring possible interactions between the shelf reef systems of western Philippines (e.g., Palawan and Sulu Sea) and the SCS. This could have implications on the recruitment of propagules advected from the centrally located shoal reefs in the SCS to the Philippine shelf reef system and the transport of materials (e.g., nutrients) across hydrographic regimes.
 

Samples collected were analyzed for dissolved oxygen, nutrients, and chlorophyll-a. Dissolved oxygen measurements were done on board the ship using the Winkler titration method (Parsons et al., 1984). Samples for nutrient determination were stored frozen in Nalgene HDPE (high density polyethylene) bottles and later analyzed spectrophotometrically (Spectronic Genesys, Milton Roy) following the methods described by Parsons et al. (1984), and using a nutrient autoanalyzer (San Plus System, Skalar). Samples for chlorophyll-a analysis were filtered through Whatman membrane filters (cellulose nitrate, 0.45μm). The filters were then frozen and later analyzed following the method given by Parsons et al. (1984).
　

Phytoplankton production was determined using the 24-h ¹⁴C incubation procedure modified from the JGOFS Protocol (1994). A set of four depths from the surface to below the subsurface chlorophyll maximum was selected at each productivity station. The depths were 5, 30, 70, and 90 m and the productivity stations were Stns. 2, 7, 11, and 14 (Fig. 1). Samples were obtained at dawn using 5-L Niskin samplers. BOD bottles (300 ml) were filled directly from the Niskin samplers and 3 bottles were used for each sample depth. Under low light conditions, 1 ml of 5 μCi Na₂¹⁴CO₃ was added to each bottle. The bottles were then incubated on board the ship using nylon screens to simulate the light levels in the water column where the samples were collected. The light intensities mimicked were 45, 25, 17, and 9% of incident PAR. Incubation was from dawn of the first day to dawn of the following day inside a plastic bin (approx. 1x1x0.5 m³) filled with running seawater. After incubation, the contents of the bottles were passed through GF/C filters and the filters stored inside the freezer until further analysis in the laboratory. The radioactivity of ¹⁴C taken in by the plankton was measured using a liquid scintillation counter (Deckman LS 6500).

Among the Sulu Sea stations, a shallowing of the nutricline by about 10-20 m was observed when moving from the oceanic (Stn. 1) to the shelf stations (Stn. 4) near Palawan (Figs. 3a-3c). This may be due to geostrophic adjustments as water moves to shallower shelf areas. It may also be attributed to the well-documented fortnightly occurrence of large-scale (5-16 km wavelengths) oceanic internal waves generated in the southeastern Sulu Sea by intense tidal flow over the Pearl Bank sill (e.g., Apel et al., 1985; Liu et al., 1985). It has been hypothesized that these internal waves could induce upwelling pulses resulting in higher nutrient concentrations at shallower depths (e.g., Pingree et al., 1981; Pingree et al., 1986; New, 1988). The shallowing of the nutricline translated to a shallowing of the chlorophyll maximum (Fig. 3d).

Vertical profiles of chlorophyll-a showed the presence of a deep chlorophyll maximum (DCM) occurring between 50-80 m in the Sulu Sea and between 40-75 m in the SCS (Fig. 2d). Although the depth of the chlorophyll maximum was similar for both basins, the maximum chlorophyll-a concentration in the Sulu Sea was higher at 1.11 μg× L^-1 than the SCS (0.66 μg× L^-1). In what Herbland and Voituriez (1979) refers to as the “Typical Tropical Structure”, the DCM is found near the nutricline and thermocline, and the peak in the primary production is coincident with or just above the chlorophyll maximum (Cullen, 1982). In this study the DCM was found above the nutricline (Fig. 4). This implies that DCM represents not only a physiological adaptation to lower irradiance but also to greater availability of nutrients (e.g., Banse, 1987; Cullen, 1982). The plankton may be utilizing the nutrients diffusing upward from the deeper parts of the pycnocline and thus trap this flux (e.g., Jamart et al., 1977). Appreciable biomass accumulation will only take place when sustained new production is possible (Harris, 1986). It was also determined that the maximum dissolved oxygen (DO) concentration was found at depths above or coinciding with the DCM in both the Sulu Sea and SCS (Fig. 4). Calculations showed that DO concentrations at these depths were oversaturated by as much as 10%. This value may represent part of the DO released from increased plankton production at the DCM. Similar results were reported by Huang (1992).
　

　
　
 

Fig. 3　 of the nutricline and chlorophyll maxima from oceanic (Stn. 1) to closer to shelf areas (Stn.4) in the Sulu Sea.

　

  Fig. 4 PO₄ and DO profiles vis a vis the deep (subsurface) chlorophyll maxima for the Sulu Sea and South China Sea. 　

production between depths of 30 and 70 m contributed 30-70% of water column production (Table 1). Higher production in the Sulu Sea may be attributed to the physical transport processes (internal wave induced upwelling and turbulent mixing) that provide the euphotic zone of the Sulu Sea with nutrients from the more enriched lower depths.