Biological and chemical characteristics of high-chlorophyll, low-temperature water observed near the Sulu Archipelago

Abstract

Abstract According to ocean-color remote sensing data, a low phytoplankton biomass exists in the central parts of the Sulu and Celebes seas, whereas higher concentrations of chlorophyll a are observed in the waters near the Sulu Archipelago, which separates these water bodies. These high-chlorophyll a waters have lower temperatures than those of surrounding waters, indicating vertical mixing of nutrient-rich subsurface water that is attributable to the topographical features of the Sulu Ridge. Differences in photosynthetic parameters, primary productivity and the phytoplankton community structure between the high-chlorophyll area near the Sulu Archipelago and the oligotrophic basin area in the Philippine, Sulu and Celebes seas were investigated in November 2002 during the R.V. Hakuho-Maru KH-02-4 cruise. Entrainment of subsurface water and vertical mixing by intense tidal flow from the Sulu Archipelago to the Celebes Sea created a low-temperature, high-nutrient condition at the southeastern edge of the Sulu Ridge. Chlorophyll a concentrations greater than 2 μg l−1 were observed at stations 5–15 km offshore at the edge of the Sulu Ridge. This high-chlorophyll tongue in the Celebes Sea showed high primary productivity: it was 1.6–18 times higher than those measured in oligotrophic central basin regions of the Philippine, Sulu, and Celebes seas. Microscopic observation of these high-chlorophyll waters showed the dominance of chain-forming large centric diatoms. Extension of high-chlorophyll waters was not observed at the Sulu Sea side, where the surface water salinity was lower than in the Celebes Sea and consequently prevented intrusion of the Celebes Sea water into the surface layer. Occasional extensions of productive, chlorophyll-rich surface water from the archipelagic zone to the surrounding basin area seems to be an important factor causing spatiotemporal variability in the phytoplankton biomass in the southern Sulu Sea and the western Celebes Sea.