Bacterial production in the western equatorial Pacific: implications of inorganic nutrient effects on dissolved organic carbon accumulation and consumption

Bacterial production and concentrations of dissolved and particulate organic carbon (DOC and POC), in the upper 200 m water column were measured in the western equatorial Pacific Ocean (0° S-8° N, 137°-159° E). To test the effect of inorganic nutrients (NH 4 + and PO 4 -3 ) on the turnover rates of glucose, nutrient enrichment experiments were performed with 0.8 μm filtrates of the surface seawater. Depth profiles ofbacterial biomass, production and turnover rates ranged 1.4-9.4 mgC m -3 , 0.34-4.06 mgC m -3 d -1 and 0.13-0.36 d -1 , respectively. They were comparable to those reported in the central equatorial Pacific. POC varied 4-fold ranging from 13 to 54 mgC m -3 . DOC values at the surface waters and the aphotic zone (> 100 m) were 112-128 and 48-54 μM, respectively. In comparison, the differences of DOC between the upper ( 50 m) parts of the euphotic zone in this area (40-60 μM) were greater than those observed by other researches in the central equatorial Pacific (10-30 μM). The enrichment experiments showed that surface water bacteria could not utilize excess glucose efficiently without inorganic nutrients, indicating that availability of inorganic nutrients might play an important role in regulating accumulation and consumption of the bio-reactive DOC in the water column. We propose that the high DOC accumulated at the surface waters probably could be ascribed to either the very slow consumption of DOC or higher DOC production due to the shortage of (or very low) inorganic nutrients supply or both. The ecological implications of our findings for oceanic DOC distribution and bacterial production may be far reaching, if other so-called labile DOC molecules show similar dependency on inorganic nutrients.