Low specific nitrate uptake rate: A common feature of high-nutrient, low-chlorophyll marine ecosystems

Abstract

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 15NO3 uptake, V15NO3, are plotted together with values from coastal upwelling systems, the data diverge into a high V15NO3 track for the coastal systems, and a low V15NO3 track for the three HNLC regions which have V15NO3 values consistent with oligotrophic regions and so are functionally oligotrophic. These values of V15NO3 are too low to allow biomass accumulation and the formation of blooms of diatoms. One possible reason for the lack of high V15NO3 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 V15NO3 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 V15NO3 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 V15NO3 to low values in the Southern Ocean. Inadequate levels of available Fe could be expected to lead to low specific 15NO3 uptake. However, experiments conducted to date in all three HNLC regions fail to show a response ofV15NO3to Fe additions within the first few days of enclosure experiments. Si(OH)4 concentrations are low with respect to half-saturation values, Ks for Si(OH)4) in the eastern equatorial Pacific and in the Southern Ocean where surprisingly high Ks values for Si(OH)4 have been reported.