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Coulometric Measurement of Primary Production with Comparison Against Dissolved Oxygen and 14C Methods in Seasonal Study

“Coulometric Measurement of Primary Production with Comparison Against Dissolved Oxygen and 14C Methods in Seasonal Study” Marine Ecology Progress Series 71 (1991): 97-102.

Irwin, Brian

Biological Oceanography Division, Bedford Institue of Oceanography, Dartmouth, Nova Scotia, Canada B2Y 4A2

ABSTRACT: Consumption of carbon dioxide, as measured by direct coulometry in light and dark bottles, was used to estimate the photosynthetic rate of natural assemblages of phytoplankton incubated in situ. A total of 30 experiments were made during a 12 mo period in Bedford Basin, Nova Scotia. For comparison, simultaneous estimates of primary production were made on each occasion using both the light-and-dark-bottle oxygen method and the 14C method. Primary production values fell in the range < 1 to 50 mmol C m-3 d-1. Throughout this range, excellent agreement was found between primary production as estimated by the 14C and gross primary production estimated by change in dissolved carbon dioxide. Release of labeled carbon into solution accounted for 2 to 19% of total primary production as measured by the 14C method, with the lower values occurring in the spring bloom period. When the carbon-based results were compared with the oxygen-based ones, they agreed to within a mean apparent photosynthetic quotient of 1.6. High apparent PQ’s in winter are attributed to an overestimation of oxygen primary production and an underestimation of oxygen 14C primary production. Direct coulometry offers considerable promise as a technique for measuring photosynthesis by phytoplankton.

The methods available for measuring primary production may be separated into 2 broad classes (Platt et al. 1989): in vitro methods, where a small sample is incubated in a closed bottle; and bulk property methods, where production is inferred from changes in the chemical properties of the water column. The first class applies to short time scales and discrete depths, the second to longer time scales and a finite interval of depth (Platt 1984). Among the in vitro methods, the 14C method offers the highest precision, but its accuracy has been called into question (Williams & Jenkinson 1982), the dissolved oxygen method has lacked the precision to make useful measurements at low production rates. Furthermore, uncertainty in the appropriate value of the photosynthetic quotient (PQ) has confounded attempts to verify the oxygen method against 14C (Platt & Harrison 1986, Smith et al, 1986).

The introduction of a coulometric titration technique to measure changes in dissolved carbon dioxide (Johnson et al. 1985, 1987) offers the possibility of an independent check on the validity of the 14C method. Bender et al. (1987) have compared the 14C method, the coulometric method and the dissolved oxygen methods in a series of experiments on samples drawn from laboratory mesocosms. They concluded that the results for gross production, as measured by the dissolved oxygen and coulometric methods, were similar, but that primary production as measured by the 14C method was an underestimate of gross production. Moreover, the artifacts associated with containment in bottles were small regardless of method.

In this paper, the results are presented of a comparison of all 3 in vitro methods of measuring primary production from a series of observations on natural assemblages of coastal phytoplankton covering a complete annual cycle. Excellent agreement was found between the 14C method and the coulometric method, suggesting the latter as a viable alternative to the former, especially in circumstances where it is not possible or convenient to use radioactive isotopes.

Methods. Sampling: Bedford Basin lies at the inner end of a fjord-like inlet. (Halifax Harbour) on the Atlantic coast of Nova Scotia. All samples were collected at a fixed station over the deepest part of the Basin (75 m) at 44° 31.3’N, 63° 38.3’ W between July 1986 and June 1987. Previous studies (Irwin et al. 1983, 1988a, 1988b) have shown this area to have high biomass and production rates especially in spring and summer.