Phytoplankton dynamics in the subtropical Pacific Ocean off Hawaii

Abstract

Phytoplankton activity in an oligotrophic environment was studied over a 14-month period. Phytoplankton biomass and productivity varied considerably in time, despite the relative constancy of the physical and chemical environment. No evidence of seasonality or diurnal variability in phytoplankton biomass was observed. Depth-integrated values (0 to 260 m) for chlorophyll a, phaeopigment, ATP, and primary productivity varied over ranges of 3 x, 6 x, 10 x, and 26 x, respectively, during the period. The mean depths of the chlorophyll and phaeopigment maxima were 85 and 95 m, respectively; the phaeopigment maximum was always at or below that of chlorophyll. Size fractionation studies showed that at this oceanic station about 80% of the phytoplankton biomass occurred in the <5- μm fraction. Low ambient nutrient levels were typical at the depth of the chlorophyll maximum, indicating nutrient assimilation in that layer. Higher nutrient levels were typical at the deeper phaeopigment maximum layer. Sinking rate and size fractionation experiments suggest that phytoplankton sinking and possibly its association with the nutrient regime influence the accumulation of biomass in this region. Productivity-to-biomass ratios were consistently low and indicative of nutrient limitation. The large variation in photosynthetic activity could not be explained by variation in phytoplankton biomass nor by variation in the nutrient gradients. There were significant correlations ( P < 0.01) between depth-integrated phaeopigment stocks and integrated primary production ( r = 0.92) and between integrated phaeopigments and integrated ammonium levels ( r = 0.80). Variations in the supply of regenerated nutrients by grazers (indexed by phaeopigments) may thus account for much of the observed temporal variability in photosynthesis. Indication of a close coupling between grazing and phytoplankton activity supports the commonly held belief that animal excretion products are significant sources of nutrients for phytoplankton in oligotrophic systems. The observed relationship between phaeopigments and primary production may be related in part to the predominance of small cells in the observed phytoplankton community, because the latter are probably eaten by small grazers, which produce amorphous, slow-sinking fecal material.