Abstract:
A simplified autoradiographic method for estimating species-specific
phytoplankton production rates in mixed natural communities
was evaluated in the laboratory and employed in the field. Laboratory
experiments were designed to test the reliability and variability of
the simplified method. Assays of ¹⁴C uptake by liquid scintillation
spectrometry were used to evaluate assays by autoradiographic silver
grain counts. Linear relationships between the two assays were found
for varying activities within a species. With corrections for
radiation geometry and self-absorption, a linear relationship was
obtained for species from 400 to 50,000 μm³ in cell volume. The
coefficient of variation for microscopic grain counting at densities
from 10 to 70 grains per 100 μm² was 4% of the mean. Counts of
replicate autoradiographs from individual incubations yielded
coefficients of variation of 7 and 14% for grain densities of 11 and
30 per 100 μm², respectively. The frequency distribution of grains
deposited over different cells of a species with varying photosynthetic
activities is indicative of the physiological state of the alga.
Samples from five depths at one station in Yaquina Bay, Oregon
were incubated in situ with ¹⁴C, daily from 2 April to 7 May 1974.
Phytoplankton biomass during the spring bloom reached 7106 μm³ cell
volume l⁻¹ and integral production reached 172 mgC m⁻² hr⁻¹. Total
community carbon uptake increased with biomass during the study but
species-specific uptake decreased as ambient nitrate + nitrite levels
decreased. Species-specific production rates were estimated for 22
consecutive days: 16 April to 7 May 1974. Three taxa, Chaetoceros
debilis Cleve, Thalassiosira decipiens (Grun.) Jorg., and unidentified
flagellates were responsible for over 65% of the community biomass
and over 85% of the primary production after 20 April 1974, Carbon-specific
uptake rates were calculated for 20 species. Maximum carbon-specific
uptake rates (expressed as the percent of the estimated cell
carbon content assimilated per hour) for six abundant diatom species
ranged from 111 to 158%, but average activity over the study period
for 20 species was much lower. Carbon-specific uptake rates (% cell
C hr⁻¹) averaged over all samples for the three dominant taxa were:
Chaetoceros debilis, 22%; Thalassiosira decipiens, 21%; and
flagellates, 54%. Flagellates were always two to three times more
productive than the dominant diatoms on a carbon specific basis.
Photosynthesis:biomass ratios (P/B) of the three dominant taxa
decreased proportionately during the study The percent of total
biomass and percent of total carbon uptake contributed by the three
dominant taxa were proportional over the period studied. Average
daily carbon-specific uptake rates demonstrate that Thalassiosira
decipiens succeeded Chaetoceros debilis due to a change in their
relative production rates.
