PHYTOPLANKTON DYNAMICS IN OCEANIC WATERS OFF KE-AHOLE POINT, Hawaii
Phytoplankton activity in an oligotrophic environment was studied on six cruises over a 14-month period. Phytoplankton biomass and productivity displayed considerable temporal variability despite the relative constancy of the physical and chemical environment. No evidence of seasonality or diurnal variability in phytoplankton biomass was observed. Annual average (+ s.d.) depth-integrated values (0-260 m) for chlorophyll a, phaeopigment, ATP, and primary productivity were 24.55 + 10.31 mg {center_dot} m{sup -2}, 11.81 + 7.20 mg {center_dot} m{sup -2}, 3.00 + 1.78 mg {center_dot} m{sup -2}, and 8.79 + 7.82 mg C {center_dot} m{sup -2}, h{sup -1}, respectively; over the year these parameters were seen to vary over ranges of 3X, 6X, 10X, and 26X, respectively. The mean depths of the chlorophyll and phaeopigment maxima were 85 + 9 m and 95 + 11 m, respectively; the pheopigment maximum was always located 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 {micro}m fraction. Low ambient nutrient levels were typical at the depth of the chlorophyll maximum, indicating that nutrient assimilation was actively occurring in that layer. Elevated nutrient levels were typical at the deeper phaeopigment maximum layer. The results of sinking rate and size fractionation experiments, together with evidence of physiological viability in this layer suggest that phytoplankton sinking and possibly its association with the nutrient regime influence the accumulation of biomass in this region. Productivity biomass ratios (mg carbon {center_dot} mg chlorophyll a{sup -1} {center_dot} h{sup -1}) were consistently low and indicative of strong nutrient limitation. Variations in phytoplankton biomass did not account (p > 0.10) for the high variability in photosynthetic activity among the six site visits; neither did the slopes or upper depth limits of the nitrate or phosphate gradients (as indicators of the supply rate of new nutrients) show any correlation (p > 0.10) with the observed primary productivity. 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). It is postulated that variations in the supply of regenerated nutrients via grazing (indexed by phaeopigments) were primarily responsible for the observed temporal variability in photosynthesis. Indications of a close coupling between grazing and phytoplankton activity in these waters is supportive evidence for 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 this phytoplankton community since the latter are probably grazed by small filter feeders which produce amorphous, slow-sinking, rather than encapsulated, fast-sinking fecal material.
- Research Organization:
- Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
- Sponsoring Organization:
- Earth Sciences Division
- DOE Contract Number:
- DE-AC02-05CH11231
- OSTI ID:
- 993755
- Report Number(s):
- LBL-11277; TRN: US201023%%585
- Journal Information:
- Deep Sea Research, Journal Name: Deep Sea Research
- Country of Publication:
- United States
- Language:
- English
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