TEMPORAL VARIABILITY IN CHLOROPHYLL-A AND PHEOPIGMENT CONCENTRATIONS DURING INCUBATIONS IN THE ABSENCE OF GRAZERS

Changes in chlorophyll a (chl a) and phaeopigment concentrations durin g 24 h incubations in water prefiltered through 2.0 mum Nuclepore filt ers were determined on a weekly basis over a period of 13 mo using wat er from Kaneohe Bay, a subtropical inlet in the Hawaiian Islands, USA. In bottles illuminated at a constant irradiance of 4.0 E m-2 h-1, bot h chl a and phaeopigment concentrations were consistently lower than i nitial values at the end of the incubations. Chl a concentrations decl ined at a lower rate in dark bottles than in light bottles. There was no evidence of a change in phaeopigment concentrations in dark bottles . There was no temporal pattern in the exponential decay rates of phae opigments in light bottles over the course of the 13 mo study, the med ian value being 0.016 m2 E-1. There was, however, evidence of a nonran dom temporal pattern in the chl a decay constants. Winter values were about twice as large as summer values, a result presumably reflecting changes in the physiology and/or species composition of the phytoplank ton community. In about 30% of the incubations phaeopigment concentrat ions were higher than initial values at intermediate time points, in s ome cases by as much as a factor of 2 to 3 during the first 4 to 8 h o f the incubations. These results are believed to have been caused by s tress associated with the effort to remove grazers by filtration. Incl usion of nanoplankton (2 to 10 mum) in the incubation bottles consiste ntly resulted in a higher concentration of phaeopigments in the picopl ankton fraction after 24 h than was the case in control bottles contai ning only picoplankton. In this study as in other work, prescreening t hrough 10 mum filters appeared to be insufficient to eliminate grazing artifacts from phaeopigment photodegradation experiments. Dilution ra ther than filtration may be a more practical way to account for grazin g effects in such studies.