LOW SPECIFIC NITRATE UPTAKE RATE - A COMMON FEATURE OF HIGH-NUTRIENT, LOW-CHLOROPHYLL MARINE ECOSYSTEMS

We have searched for common features of three high-nutrient, low-chlorophyll (HNLC) regions of interest-the Southern Ocean, the eastern equatorial Pacific, and Station P in the northeast Pacific. In each of these areas, the rates of specific NO3 uptake, whether normalized to particulate organic nitrogen (PON) or chlorophyll, are low compared to coastal upwelling systems with comparable nutrient concentrations. When maximum values of NO3 concentration and maximum values of PON-specific (NO3)-N-15 uptake, (VNO3)-N-15, are plotted together with values from coastal upwelling systems, the data diverge into a high (VNO3)-N-15 track for the coastal systems, and a low (VNO3)-N-15 track for the three HNLC regions which have (VNO3)-N-15 values consistent with oligotrophic regions and so are functionally oligotrophic. These values of (VNO3)-N-15 are too low to allow biomass accumulation and the formation of blooms of diatoms. One possible reason for the lack of high (VNO3)-N-15 values in the HNLC regions is that seeding of the large, fast-growing, fast-sinking diatoms is inadequate and primarily due to the lack of a bottom or other recirculation system to assure a supply of these diatoms to the surface regions. Grazing control limits biomass development and may function to hold (VNO3)-N-15 to low values resulting in conditions certain to appear as HNLC. Comparison with model results suggests that deep mixed layers in the Southern Ocean and at Station P may limit (VNO3)-N-15 and that in much of the eastern equatorial Pacific NO3 concentrations are too low for VNO3 to develop to coastal upwelling values. Strong dependence on temperature of N and C uptake in psychrophilic Antarctic phytoplankton and sea-ice algae may be a dominant factor in constraining (VNO3)-N-15 to low values in the Southern Ocean. Inadequate levels of available Fe could be expected to lead to low specific (NO3)-N-15 uptake. However, experiments conducted to date in all three HNLC regions fail to show a response of (VNO3)-N-15 to Fe additions within the first few days of enclosure experiments. Si(OH)4 concentrations are low with respect to half-saturation values, K(s) for Si(OH)4) in the eastern equatorial Pacific and in the Southern Ocean where surprisingly high K(s) values for Si(OH)4 have been reported.