Large-volume (20-liter) bioassays were carried out in order to assess the effects of major nutrients and micro-nutrients on natural phytoplankton populations in water collected from a site close to the mouth of Yaquina Bay, Oregon. Seven long-term experiments were conducted during the years 1975-1976. A stripping technique employing activated carbon was used to reduce the trace metal and dissolved organic load of a portion of the water. Various combinations of major nutrients (nitrate, phosphate, and silicate) and micro-nutrients (trace metals and vitamins) were added to both the carbon-stripped and the non-carbon-stripped water in order to assess the effects of various nutrient substances on the growth rate, biomass yield, and species composition of the phytoplankton populations which developed from small natural inocula. The major nutrients and the micro-nutrients appear to have fundamentally different effects on the species composition and population growth characteristics of the phytoplankton. The micro-nutrients had marked systematic effects on the species composition of the phytoplankton populations which developed in the experimental vessels, while the major nutrients had minimal effects on species composition. The micro-nutrients appeared essential for the achievement of maximal population growth rates and minimal lags, while the major nutrients had no effect on growth rates or lags. Both the major nutrients and micro-nutrients had significant effects on the final biomass yields. There were seasonal variations in the control of final biomass yields by major nutrients and micro-nutrients. / Graduation date: 1977

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. / Graduation date: 1977