Markensten, H., W. Goedkoop, L. Sonesten, G. Ahlgren, M. Boberg
Abstract:
Plankton net samples and sedimenting matter from mesotrophic Lake Erken (Sweden) were analyzed for carbon (C), nitrogen (N), phosphorus (P), and fatty acid (FA) content (% of dry weight, mg/g DW) to determine what differences and seasonal changes in the quality of food available to pelagic and benthic primary consumers exist. Two central questions were addressed: (i) How do important chemical elements and FA profiles of food particles change during sedimentation, i.e. what are the differences in food-quality regimen for pelagic and benthic primary consumers? (ii) Are some FA in phytoplankton utilized more completely than others?
Plankton net samples (net samples), 25 um mesh, were consistingly higher in C, N, P, and all separate groups of FA than particulate matter collected with sedimention traps (trap samples). The differences between net and trap samples were confirmed by examinations using microscope which showed that net samples consisted mainly of pure phytoplankton and that trap samples were made up of fluffy aggregates of detritus, in which only large species of intact diatoms or diatom fragments could be identified.
The C, N, and P also showed large seasonal variations in the net samples, whereas trap sample contents were surprisingly stable throughout the year. In contrast, FA content variation was more marked in trap samples. The sumFA in net samples ranged from 17-59 mg/g DW and in trap samples from 2-19 mg/g DW. During sedimentation, N decreased more than P, and among the FA, the polyunsaturated FA (PUFA) decreased more than either the saturated FA (SAFA) or the monounsaturated FA (MUFA). In particularly, the w3 PUFA decreased during the summer months to only 1-2% of the net samples, i.e. to only traces approximates 0.01-0.02 mg/g DW. One of the unidentified acids, which appeared immediately before the palmitic acid (16:0) in the chromatograms, showed often higher amounts in the trap samples than in the net samples. In contrast to the identified FA, this acid seemed to be more or less accumulated in the trap samples.
The large seasonal variations in the chemical elements in the net plankton can be largely traced to plankton succession, but temperature and mixing conditions also occasionally influenced. The extremely low chlorophyll concentration in July (clear-water phase) indicates high grazing pressure. The summer months were apparently the most critical for the detrivores when food density and quality reached their lowest levels.
Significant statistical relationships were obtained between PUFA and P content and w3 FA and P content in the net plankton (r^2 = 0.74 and 0.67, respectively), but not in the corresponding data from the trap samples (r^2 = 0.23 and 0.18, respectively). EPA alone did not give a good relationship (r^2 = 0.30). Thus, P seems to be a good predictor of food quality in living phytoplankton but not in dead matter, i.e. detritus.
(Uppsala University, Institute of Limnology, Sweden)
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