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

Abstract

Listen

Translate article

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 Slotsso, 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 Mosso, 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 Mosso, 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 Slotsso and 51% in Lake Mosso. Extracellular organic carbon (EOC) released from the phytoplankton contributed DFAA to the water. In Lake Mosso, 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 Slotsso and Lake Mosso, respectively. The bacterial productivity suggested that DFAA were essential substrates to the bacteria, especially in Frederiksborg Slotsso. The zooplankton biomass in Frederiksborg Slotsso was six times larger than that in Lake Mosso, but cladocerans were dominant in both lakes. The zooplankton grazing probably was an important regulatory factor for the bacterial productivity.