Photosynthetic measurcmcnts in the central North Pacific: The dark loss of carbon in 24-h incubations’

Half-day incubations ( noon-sunset) and 24-h incubations ( noon-noon) with ‘“C-labeled bicarbonate gave similar values for net primary production in the central gyre of the North Pacific (ca. 28”N, 155”W). Photosynthetic rates per light hour were nearly twice as high in 6-h as in 24-h incubations. The discrepancy seems to be due largely to dark respiration in the 24-h incubations, and the data can be used to estimate dark loss of ‘“C. The interpretation of measurements of primary production in natural samples of seawater with the 14C technique (Stecmann Nielsen 1952) is not always straightforward. In the present case uncertainty arose over the observation that half light-day (noon-sunset, ca. 6 h) incubations with 14C often gave results similar to full 24-h incubations. That is, photosynthetic rates per hour of il-lumination were often nearly twice as high for noon-sunset incubations as for 24-h in-cubations. A few full light-day (12-15 h) incubations were also carried out and these gave values similar to the 24-h incubations and much less than twice the 6-h, noon-sunset treatments. Ryther ( 1954, 1956a,b) showed that dark respiration could have a large influence on productivity measurements and on this basis he accounted for discrepancies be-tween measurements with the 14C method and the oxygen light-dark bottle method in tropical oceanic waters. In his laboratory experiments with phytoplankton cultures, 10 to 40 % of the cellular 14C from photo-synthesis was lost in 4-h darkness. The oxy-gen light-dark bottle method showed a dc-Cline with the age of the culture in the ratio of gross photosynthesis to dark respiration, i.e. a variation in the ratio with growth rate