MEPS 138:275-291 (1996)  -  doi:10.3354/meps138275

The carbon-to-chlorophyll ratio of phytoplankton, theta, is difficult to determine by direct chemical measurement because natural waters also contain particulate carbon due to heterotrophic organisms and detritus that cannot be separated from the phytoplankton. When growth is balanced phytoplankton produce new C and chlorophyll in proportion to theta, but growth will be unbalanced in the short-term when there is accumulation of C that has not had time to be proportionately allocated to chlorophyll, or when the phytoplankton are adjusting theta to a new light regime (i.e. photoadaptation). We conducted incubations in Manukau Harbour, New Zealand, to estimate theta from increments in ¹⁴C and chlorophyll using highly diluted water (fraction of unfiltered seawater = 0.05 to 0.1) to greatly reduce grazing by microzooplankton. Estimated theta ranged from 21.5 to 46.6 mg C (mg chl a)^-1, typical of healthy, nutrient-sufficient diatoms. Maximal growth rates varied from about 1 to 2 d^-1, and C- and chlorophyll-based growth rates agreed well with one another. Growth rates predicted from separate, short-term measurements of photosynthesis-irradiance (P-I) curves agreed well with light-saturated rates measured in 24 h incubations, but were generally 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 ¹⁴C increments because grazed chlorophyll was degraded, but grazed C appeared to be conserved in the particulate matter. Failure to use diluted water for the incubations would have resulted in large overestimates in theta. We constructed a model of ¹⁴C tracer flux and chlorophyll production to explore the consequences of unbalanced growth, e.g. photoadaptation, on estimates of theta determined using incubations substantially free of grazing. Simulations indicated that accurate estimates of theta can be obtained by commencing 24 h incubations prior to sunrise before new C accumulates, and by avoiding major shifts in the range of light intensities to which the phytoplankton are adapted. The procedure should be applicable in other environments provided precautions about sunrise start and avoidance of light shifts and photoinhibiting irradiances are observed.

Phytoplankton . Carbon-to-chlorophyll ratio . Growth rate . Estuarine