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Dissolved reactive and organic phosphorus (DRP and DOP, respectively) in samples from river and coastal areas of Tokyo Bay, Japan, was size fractionated into high-molecular-weight (HMW; <0.1 µm but >10 kDa) and lowmolecular-weight (LMW, <10 kDa) size classes using a stirred-cell ultrafiltration system. The LMW fraction accounted for 54-76% of the bulk DOP. LMW-DOP exhibited conservative behavior during mixing with saline waters, with high concentrations at the river mouth decreasing seaward. HMW-DOP was a rather minor component, accounting for 14-36% of the bulk DOP. Concentrations of HMW-DRP, a trace component of all samples, decreased with increasing salinity. Characterization using two phosphohydrolytic enzymes, alkaline phosphatase and phosphodiesterase, demonstrated the presence of three forms of HMW-DOP: easily hydrolyzable mono-and diesters and unhydrolyzable nonreactive DOP. The nonreactive DOP was a significant fraction (up to 67%) of HMW-DOP. Further size fractionation and characterization revealed the importance of hydrophobic compounds (presumably phospholipids) and phosphate esters as the nonreactive DOP. The esters in the nonreactive fraction are potentially labile but may be protected against decomposition by forming submicron particles and/or macromolecular complexes.

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A persulfate chemical wet oxidation (CWO) method for the determination of particulate phosphorus (PP) was improved and then compared with a conventional high-temperature dry combustion (HTDC) method. In the improved CWO method, the concentration of the digestion reagent, potassium persulfate, was increased from 0.5% to 3% (w/v); the method exhibited high recoveries of P from various test materials under commonly used autoclave conditions (at 120°C for 30 min). The recoveries of P relative to those of the HTDC method were 102 ± 6.7% from standard organic and inorganic P compounds, and 100 ± 3.8% from natural particulate matter analogues including riverine suspended particulate matter (SPM), sediments, plankton, and geochemical reference materials of rock. However, low recoveries of 14% to 69% were observed for reference samples of clay minerals. Comparison using many samples of estuarine and pelagic SPM showed that the CWO method produced PP values consistent with those obtained by the HTDC method, except for some estuarine samples enriched in inorganic matter. The low recoveries were ascribed to the presence of an inorganic P fraction highly recalcitrant to chemical digestion. The procedural blank was lower for the CWO method than for the HTDC method. The filter blanks showed large variation among the filter materials tested; aluminum oxide membrane filters were shown to contain a considerable amount of P that could cause significant contamination in both methods. Analytical precision was equivalent between the two methods. Considering its simplicity and its less time-consuming nature, the CWO method presented here is suitable for PP determination in aquatic environments.

We measured pools of dissolved phosphorus (P), including dissolved inorganic P (DIP), dissolved organic P (DOP) and alkaline phosphatase (AP)-hydrolyzable labile DOP (L-DOP), and kinetic parameters of AP activity (APA) in the euphotic zone in the western North Pacific Ocean. Samples were collected from one coastal station in Sagami Bay, Japan, and three offshore stations between the North Pacific subtropical gyre (NPSG) and the Kuroshio region. Although DIP concentrations in the euphotic zone at all stations were equally low, around the nominal method detection limit of 20 nmol L-1, chlorophyll a (Chl a) concentrations were one order of magnitude greater at the coastal station. DOP was the dominant P pool, comprising 62–92% of total dissolved P at and above the Chl a maximum layer (CML). L-DOP represented 22–39% of the total DOP at the offshore stations, whereas it accounted for a much higher proportion (about 85%) in the coastal surface layers. Significant correlations between maximum potential AP hydrolysis rates and DIP concentrations or bacterial cell abundance in the offshore euphotic zone suggest that major APA in the oligotrophic surface ocean is from bacterial activity and regulated largely by DIP availability. Although the range of maximum potential APA was comparable among the environmental conditions, the in situ hydrolysis rate of L-DOP in the coastal station was 10 times those in the offshore stations. L-DOP turnover time at the CML ranged from 4.5 days at the coastal station to 84.4 days in the NPSG. The ratio of the APA half-saturation constant to the ambient L-DOP concentration decreased markedly from the NPSG to the coastal station. There were substantial differences in the rate and efficiency of DOP remineralization and its contribution as the potential P source between the low-phosphate/high-biomass coastal ecosystem and the low-phosphate/low biomass oligotrophic ocean.