Hans J. Hansen

Diel variation in concentration, assimilation and respiration of dissolved free amino acids in relation to planktonic primary and secondary production in two eutrophic lakes

Concentration of dissolved free amino acids (DFAA) and assimilation of the 5 most abundant DFAA (glutamic acid, serine, glycine, alanine and ornithine) were measured at 3-h intervals over 27 h in two Danish, eutrophic lakes. The carbon flux of the amino acid assimilation was compared with the major routes of carbon flux, including primary production, bacterial production and zooplankton grazing. In Frederiksborg Slotssø, the mean DFAA concentration was 275 nM with distinct peaks (up to 783 nM) 3 h after sunrise. Assimilation rates of the 5 amino acids amounted on the average to 2.03 µg Cl−1 h−1, but high values up to 7.41 µg Cl−1 h−1 occurred 3 h after sunrise and at midnight. The mean turnover time of the amino acid pools was 3.2 h. In Lake Mossø, the mean DFAA concentration was 592 nM with peak of 1 161 nM at dusk. The assimilation rate averaged 0.44 µg Cl−1 h−1, and the mean turnover time of the amino acid pools was 39 h. In Lake Mossø, similar turnover times of glutamic acid and serine were determined from the 14C-amino acid tracer technique and Michaelis-Menten uptake kinetics, indicating that the tracer technique gave reliable values of the actual assimilation. The average respiration percentages of the assimilated amino acids were 45% in Frederiksborg Slotssø and 51% in Lake Mossø. Extracellular organic carbon (EOC) released from the phytoplankton contributed DFAA to the water. In Lake Mossø, 81% of the ambient EOC pool was <700 daltons and 9.3% of the EOC was DFAA. This corresponded to about 2.4% of the DFAA pool. Bacterial productivity, determined by means of ‘frequency of dividing cells’ and 35S-SO4 dark uptake techniques gave similar results and constituted 4.5 and 3.7 µg Cl−1 h−1 in Frederiksborg Slotssø and Lake Mossø, respectively. The bacterial productivity suggested that DFAA were essential substrates to the bacteria, especially in Frederiksborg Slotssø. The zooplankton biomass in Frederiksborg Slotssø was six times larger than that in Lake Mossø, but cladocerans were dominant in both lakes. The zooplankton grazing probably was an important regulatory factor for the bacterial productivity.

The Cretaceous‐Tertiary transition in South China

114 m of Uppermost Cretaceous to Lowermost Tertiary continental redbeds and their magneto‐stratigraphy are described. The palaeomagnetic directions suggest that a clockwise rotation of around 50 degrees of the South China block took place later than Lower Paleocene time. The last dinosaur nests occur rather high up in magnetochrone 29R. This suggests that the Nanxiong dinosaur population was present much later than the corresponding population of Aix‐en‐Provence, France. The last nests in France are not present in sediments younger than the upper part of magnetochrone 30N.

Instrumental neutron activation analysis of samples with masses from micrograms to hectograms

The rotating rig at the Danish DR-3 reactor was used for irradiation of samples with masses from a hundred micrograms to several hundred grams. The pneumatic post for uranium delayed neutron counting was used for automatic sequential irradiation of samples subdivided into up to 24 aliquots, each with a volume of 7 cm3. The cumulative samples were measured in polyethylene irradiation containers in a specially designed radial holder.