Circumpolar plant nutrient covariation in the Southern Ocean: patterns and processes

Abstract

Phosphate from the NODC data set is substituted for nitrate in a regression relationship with silicic acid based on the NODC data set to broaden a previous, spatially-limited analysis of nitrogen and silicic acid fluxes in the Southern Ocean (30° S to Antarctica). The log slope of the phosphate-silicic acid relation, an index of plant nutrient utlization rates, provides a complete circumpolar view and supports more detailed speculation on the processes that contribute to the spatial and temporal patterns. A log PO4-Si slope criterion of -1.6 parallels a previous log NO3-Si slope criterion of -0.5 where low log NO3-Si slopes more negative than the criterion were attributed to higher ammonium flux. Three regions are distinguished by these criteria including an area of mixed low and high log slopes south of the Antarctic Divergence, an areaof predominantly low log slopes between the Antarctic Divergence and the Antarctic Polar Front, and an area of predominantly high log slopes north of the Antarctic Polar Front. The middle low log slope region is most restricted in austral winter and spring and most broadly distributed in austral summer and fall. Based on recent information from the Southern Ocean, the processes that are likely contributors to the occurrence of high or low log slopes are extended to include not only ammonium fluxes, but also particulate Si:N molar ratios (and therefore implied Si:N uptake ratios) ranging up to 4:1 and possible irori limitation. The invoked processes apply both to NO3-Si and PO4-Si relationships except that excreted phosphorus atoms are not preferentially selected from the total phosphorus pool as occurs for ammonium from the total nitrogen pool. The continuous presence of low log slopes in various regions of the Southern Ocean throughout the year suggests a strong potential for ammonium preference in response to a dominating ammonium supply or to iron limitation and/or for relatively high Si:N uptake ratios to occur throughout the annual cycle. The nutrient relationships from the Southern Ocean provide a record of the integrated processes that influence net plant nutrient utilization rates over the growth season. However, since a given log slope can represent a variety of process combinations, more information will be required to accurately interpret the exact sequence of processes that contribute to the nutrient signature in the Southern Ocean at a given time and place.