Temporal variability in dissolved phosphorus concentrations in the subtropical North Pacific Ocean

Dissolved phosphorus pool measurements were made on approximately monthly intervals during the period October 1988-November 1994 at an oligotrophic, deep-ocean station near Hawaii (Station ALOHA; 22°45ʹN. 158°W), Data include depth profiles (0-200 m) of soluble reactive phosphorus (SRP), measured by two independent techniques (a high-sensitivity, magnesium-induced coprecipitation (MAGIC) method, and standard autoanalysis procedure), and total dissolved phosphorus (TOP) based on UV photo-oxidation treatment followed by automated SRP analysis. Considerable variability in SRP was observed both with depth and with time over the 6-yr period. Upper water column (0-100 m) SRP concentrations varied in time from > ISO to < 10 n M, and generally conformed to one of two distinct concentration VS, depth patterns: "Type I" profiles were characterized by uniform SRP concentrations with depth (concentration gradients s 0.05 !-Lmol m -4 SRP) and "Type II" profiles were characterized by distinct near-surface (0-30 m) SRP concentration maxima. The Type I profiles were further divided into low (Type I-L) or high (Type I-H) categories based on whether the average SRP concentration was < 60 or > 60 nM, respectively. Throughout the entire 6-yr observation period, the upper water column (0-100 m) inventory of SRP displayed a systematic decrease from - 10 to - 5 mmol m - 2 p. a result that is consistent with the hypothesized role of N2 fixation and P control of new and export production at Station ALOHA. Three independent, but not mutually exclusive, models are presented to explain these time-varying SRP concentrations: (I) convective mixing; (2) atmospheric deposition; and (3) upward P flux. The upward P flux model including both passive (upward particle flux) and active (biological transport) processes, is the most consistent model for the available data set