AN INCUBATION PROCEDURE FOR ESTIMATING CARBON-TO-CHLOROPHYLL RATIOS AND GROWTH-IRRADIANCE RELATIONSHIPS OF ESTUARINE PHYTOPLANKTON

The carbon-to-chlorophyll ratio of phytoplankton, theta, is difficult to determine by direct chemical measurement because natural waters als o contain particulate carbon due to heterotrophic organisms and detrit us that cannot be separated from the phytoplankton. When growth is bal anced phytoplankton produce new C and chlorophyll in proportion to the ta, but growth will be unbalanced in the short-term when there is accu mulation of C that has not had time to be proportionately allocated to chlorophyll, or when the phytoplankton are adjusting theta to a new l ight regime (i.e. photoadaptation). We conducted incubations in Manuka u Harbour, New Zealand, to estimate theta from increments in C-14 and chlorophyll using highly diluted water (fraction of unfiltered seawate r = 0.05 to 0.1) to greatly reduce grazing by microzooplankton. Estima ted theta ranged from 21.5 to 46.6 mg C (mg chi a)(-1), typical of hea lthy, nutrient-sufficient diatoms. Maximal growth rates varied from ab out 1 to 2 d(-1), and C- and chlorophyll-based growth rates agreed wel l with one another. Growth rates predicted from separate, short-term m easurements of photosynthesis-irradiance (P-I) curves agreed well with light-saturated rates measured in 24 h incubations, but were generall y higher than the 24 h measurements at lower irradiances, possibly due to greater effect of respiration in the longer incubations. Dilution had contrasting effects on chlorophyll and C-14 increments because gra zed chlorophyll was degraded, but grazed C appeared to be conserved in the particulate matter. Failure to use diluted water for the incubati ons would have resulted in large overestimates in theta. We constructe d a model of C-14 tracer flux and chlorophyll production to explore th e consequences of unbalanced growth, e.g. photoadaptation, on estimate s of theta determined using incubations substantially free of grazing. Simulations indicated that accurate estimates of theta can be obtaine d by commencing 24 h incubations prior to sunrise before new C accumul ates, and by avoiding major shifts in the range of light intensities t o which the phytoplankton are adapted. The procedure should be applica ble in other environments provided precautions about sunrise start and avoidance of light shifts and photoinhibiting irradiances are observe d.