Variability of inorganic and organic phosphorus turnover rates in the coastal ocean

Phosphorus is an essential nutrient in pelagic marine ecosystems. Phosphorus cycling in the upper ocean is, however, poorly understood, and few studies have directly investigated the biological utilization of this essential element(1-4). Here, we have determined in situ phosphorus-turnover rates in a coastal marine environment by measuring the activities of two cosmogenic radionuclides (P-32 and P-33, with half lives of 14.3 and 25.3 days, respectively) in dissolved inorganic, dissolved organic and total particulate phosphorus pools over a seasonal cycle. Phosphorus turnover rates within dissolved and particulate pools are rapid and vary over seasonal timescales, suggesting that low phosphorus concentrations can support relatively high primary production. Furthermore, picoplankton, such as bacteria, appear preferentially to utilize certain dissolved organic phosphorus compounds to obtain other associated nutrients, such as carbon and nitrogen. It seems that the significance of the roles of both dissolved inorganic and organic phosphorus in supporting primary production-and, hence, CO2 uptake and particulate organic carbon export-has been hitherto underestimated.