Phosphorus utilization in nutrient enrichment experiments: A case for diatoms depleting the surface ocean of phosphorus in the oligotrophic North Pacific Subtropical Gyre

K. M. Björkman and D. M. Karl

Department of Oceanography, University of Hawaii, 1000 Pope Road, Honolulu, HI 96822

Abstract

A progressive decline in the inventories of both inorganic and particulate phosphorus (P) in the upper 100 m of the water column has been observed at the Hawaii Ocean Time-series (HOT) site Station ALOHA in the North Pacific Subtropical Gyre (NPSG). Over the past 15 years the average rate of P loss has been 0.34 mmol m^-2 y^-1 and 0.07 mmol m^-2 y^-1 for the inorganic and particulate pools respectively. This appears not be the result of decreased mixing depth, that would limit influx of new nutrients into the upper ocean, but may be caused by changes in the phytoplankton community structure. Nitrogen (N) fixing organisms have been shown to be important contributors to new N within the euphotic zone, and this reduced N stress could select for eukaryotic phytoplankton. This would lead to increased P demand. Large diatoms forming summertime blooms, temporarily displacing the Prochlorococcus type community typically found in the NPSG, have been observed at Station ALOHA. These diatom events appear to lead to a net removal of P from the euphotic zone, as well as driving the inorganic P inventory lower than when diatoms are not present. The Prochlorococcus community that typically dominates primary production has little or no net impact on the standing stocks of P. In nutrient enrichment experiments, where nutrient replete deep seawater was amended to nutrient depleted surface waters, rapid growth of eukaryotic plankton was observed and the nutrients drawn down within a few days. At the end of the experiments the P concentrations in the enriched incubations were below those of the controls, which remained close to or at their original levels. This indicates the capacity for diatom based eukaryotic assemblages to quickly draw down P to concentrations not approached by the Prochlorococcus community. The potential for diatoms to rapidly fall out of the euphotic zone as the bloom declines, increases the likelihood that remineralization of P will occur deeper than for particles settling slower. This diatom P loss combined with a prolonged lack of sufficiently deep mixing, could explain the progressive loss of P from the upper water column of the NPSG.