Historical Proceedings
23
new radiocarbon method. A large series of primary production measurements
done in tropical waters aboard the R/V Galathea had shown that the oxygen
bottle method exaggerates primary production in tropical water by up to 2
orders of value (Steemann Nielsen 1952; 1958; Steemann Nielsen and Jensen
1957); correspondingly exaggerated, by about order of value, appeared to be
also the global evaluations done by Riley after estimations using the oxygen
method. The adepts of this methodology pointed out that the 14C-method
might underestimate primary production. This appeared to be the case - but
on a much lesser scale than the recorded 50-ISO-times difference to tropical
waters between old data by Riley (Oz-method) and Steemann Nielsen C 4 Cdata). From modern experience we can now evaluate the underestimation of
real gross primary production in tropical waters by modifying the 14C-method
used in the 1950s by about 2-2.5 times, but not 10-100 times. The causes of
such a gross overestimation of primary production at that time with the oxygen
method are now also quite obvious. In fact, in tropical waters the phytoplankton density is extremely low in the upper water layer, where Riley measured primary production. Basically the productive popUlation lives at the
upper boundary of the thermocline at 60-80m depth. Near the surface, its wet
biomass is some 5 to 20 mg m- 3 , being significantly less than the biomass of bacterioplankton. In the same way, the primary production in the surface layer of
the tropical ocean is as a rule two to five times less than the production and
respiration of bacteria (Sorokin 1971). Riley (1994) and Ryther (1954), measuring primary production with the oxygen dark and light bottle method,
whose sensitivity was too low to detect the real photosynthetic oxygen production in surface tropical waters, kept bottles in the sea for up to 100h. We
know now that in the tropics after 12-14h of exposure in the bottled sample
the microplankton changes drastically: rapid growth of large bacteria starts,
while the phytoplankton depletes. The difference in oxygen contents in dark
and light bottles in experiments done at that time was created largely by the
difference in their microbial respiration rate: in the light bottles, where the
growth of bacteria was virtually inhibited by the strong solar radiation, it was
less than in the dark ones (Sorokin 1971;Azam and Holm Hansen 1973; Bailey
et al. 1983).
The new 14C-method of primary production measurements very soon
almost completely replaced the oxygen dark-light bottle method, not only in
areas of oligotrophic oceanic waters, where its use was the only way to obtain
realistic data, but also in meso trophic and eutrophic seas and in freshwaters.
The sensitivity of the radiocarbon method was 3- orders of value more
than that of the oxygen method: some 0.02-0.05)lg C 1-1 day-l as opposed to
20-50)lg CI- 1 day-l for the oxygen method. At that time, it was also understood
that primary production should be estimated as an integral value in the whole
water column of the euphotic zone under 1 m 2 of the basin's area. It thus
became necessary to measure photosynthesis rates in deeper, less illuminated
water layers, where the values could be quite low and thus inaccessible for the
oxygen bottle method.
23
new radiocarbon method. A large series of primary production measurements
done in tropical waters aboard the R/V Galathea had shown that the oxygen
bottle method exaggerates primary production in tropical water by up to 2
orders of value (Steemann Nielsen 1952; 1958; Steemann Nielsen and Jensen
1957); correspondingly exaggerated, by about order of value, appeared to be
also the global evaluations done by Riley after estimations using the oxygen
method. The adepts of this methodology pointed out that the 14C-method
might underestimate primary production. This appeared to be the case - but
on a much lesser scale than the recorded 50-ISO-times difference to tropical
waters between old data by Riley (Oz-method) and Steemann Nielsen C 4 Cdata). From modern experience we can now evaluate the underestimation of
real gross primary production in tropical waters by modifying the 14C-method
used in the 1950s by about 2-2.5 times, but not 10-100 times. The causes of
such a gross overestimation of primary production at that time with the oxygen
method are now also quite obvious. In fact, in tropical waters the phytoplankton density is extremely low in the upper water layer, where Riley measured primary production. Basically the productive popUlation lives at the
upper boundary of the thermocline at 60-80m depth. Near the surface, its wet
biomass is some 5 to 20 mg m- 3 , being significantly less than the biomass of bacterioplankton. In the same way, the primary production in the surface layer of
the tropical ocean is as a rule two to five times less than the production and
respiration of bacteria (Sorokin 1971). Riley (1994) and Ryther (1954), measuring primary production with the oxygen dark and light bottle method,
whose sensitivity was too low to detect the real photosynthetic oxygen production in surface tropical waters, kept bottles in the sea for up to 100h. We
know now that in the tropics after 12-14h of exposure in the bottled sample
the microplankton changes drastically: rapid growth of large bacteria starts,
while the phytoplankton depletes. The difference in oxygen contents in dark
and light bottles in experiments done at that time was created largely by the
difference in their microbial respiration rate: in the light bottles, where the
growth of bacteria was virtually inhibited by the strong solar radiation, it was
less than in the dark ones (Sorokin 1971;Azam and Holm Hansen 1973; Bailey
et al. 1983).
The new 14C-method of primary production measurements very soon
almost completely replaced the oxygen dark-light bottle method, not only in
areas of oligotrophic oceanic waters, where its use was the only way to obtain
realistic data, but also in meso trophic and eutrophic seas and in freshwaters.
The sensitivity of the radiocarbon method was 3- orders of value more
than that of the oxygen method: some 0.02-0.05)lg C 1-1 day-l as opposed to
20-50)lg CI- 1 day-l for the oxygen method. At that time, it was also understood
that primary production should be estimated as an integral value in the whole
water column of the euphotic zone under 1 m 2 of the basin's area. It thus
became necessary to measure photosynthesis rates in deeper, less illuminated
water layers, where the values could be quite low and thus inaccessible for the
oxygen bottle method.