Nitrate and silicic acid in the world ocean: patterns and processes

Two world ocean data sets, GEOSECS and NODC, were analyzed for patterns in the nitrate versus silicic acid regression relation. Significant regional nitrate or silicic acid excesses (i.e. amount of one of these plant nutrients remaining in the water after the other is no longer measurable) tend to occur in some areas of upwelling, shallow sills, deep convection or high rainfall/river outflow. The Southern Ocean is unique in the intensity and complexity of projected (regression intercepts on the nitrate [y] or silicic acid [X] axis) excesses. These global nutrient depletion patterns pertain to regional comparisons of phytoplankton succession and thus to the overall character of plankton community structure. Intercept patterns in different areas are affected by the processes that determine absolute nutrient concentrations at the beginning, and their relative decline during, the growth season. The region south of the southern Polar Front is unlque because winter nutrient concentrations are unusually high around Antarctica due to water mass characteristics and because nutrients generally remain unusually abundant over the growth season partly in response to the cycle of incident solar radiation. The patterns in the regression intercepts in this region largely result from variations in the regression slopes which respond to different weightings of the chemical and biological pathways that determine nitrate and silicic acid concentrations in the upper water column. The nitrate and silicic acid profiles in the Southern Ocean contain information on the temporal average of primary production and of nitrogen recycling up to the tlme of sample collection within the growth season. The interpretation of this information depends on the detail to which the processes that determine the water column concentrations of nitrate and silicic acid are known.