The inventory, seasonality and stoichiometry of the major phosphorus pools in the Sargasso Sea

Abstract

Between 1995 and 1997 samples were gathered on 31 cruises at the Bermuda Atlantic Time-series Site (BATS) for the measurement of soluble reactive phosphorus (SRP), dissolved organic phosphorus (DOP) and particulate organic phosphorus (POP). The optimization of a Technicon Auto-Analyzer II increased the sensitivity of SRP measurements and established a method detection limit (MDL) of 11 nmol kg⁻¹. The mean SRP concentration in the upper 120 m for all cruises was approximately 16 nmol kg⁻¹. No seasonal trend was seen in the SRP pool or the DOP pool. The mean DOP concentration was 79 nmol kg⁻¹ at the surface and declined only slightly to 67 nmol kg⁻¹ at the 300 m depth. The POP pool was the smallest pool (less than 12 nmol kg⁻¹ during the two year period) but it was the only pool to have a seasonal cycle. However, each pool did have an inter-annual difference. When our data were combined with an earlier data set from Ammerman et al. (2001) the fall season had a significantly lower POP concentration (p<0.001) compared to the other seasons. Analysis of elemental ratios revealed the dissolved organic carbon (DOC):DOP, dissolved organic nitrogen (DON):DOP, particulate organic carbon (POC):POP and particulate organic nitrogen (PON):POP ratios were double the Redfield ratio during the two years, and that in the fall these ratios were three to four times the Redfield ratio. The apparent increase in the Redfield ratios was possibly due to loss of phosphorus (P) during the fall.