The relative role of protozoans in the flux of phytoplankton nitrogen through pelagic food webs

Master Thesis


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University of Cape Town

Experiments were carried out during a cruise in the southern Benguela upwelling region in April 1989 to budget the nitrogen flux through the different microplanktonic compartments leading to copepods. Uptake of nitrate and ammonium by three different size classes of phytoplankton (net-, nano-, and picoplankton) was measured using ¹⁵N isotope techniques. Microzooplankton grazing on autotrophic picoplankton and nanoplankton was quantified by predator:prey dilution experiments. Between 7 and 52 copepods in species assemblages representative of the natural communities were incubated in 11 samples of ambient seawater to examine grazing rates on chlorotic and non-chlorotic microplankton. Copepod and microzooplankton excretion rates were also measured using ¹⁵N isotope techniques. Two experiments were performed at night and two during the day. Nitrogen uptake and regeneration studies revealed that phytoplankton of all size classes showed a consistent preference for ammonium, although nitrate was quantitatively more important for netplankton. Microzooplankton excretion fulfilled most of the phytoplankton ammonium demand, while copepod excretion was only detectable at night. Competition between microzooplankton and mesozooplankton for phytoplankton prey was minimal, in that the former appeared to graze mainly < 2 μm phytoplankton. Nevertheless, microzooplankton grazers had a significant impact on phytoplankton standing stocks. Microzooplankton grazing rates represented about 5 % of phytoplankton biomass under diatom bloom conditions and an average of 46% under post-bloom conditions. On the other hand, copepods removed 18% of phytoplankton biomass under bloom conditions and only 1 % under post-bloom conditions. Copepods appeared to demonstrate a preference for protozoan prey over phytoplankton, in that the percentage of carbon ingested as protozoans exceeded the percentage of carbon available as protozoans. Quantitatively, protozoans made up a highly variable component of the copepod diet. For example, at station 12, where the plankton assemblage was dominated by oligotrichous ciliates, 80 % of the ingested nitrogen ration consisted of protozoa. However, the total ingested ration at this station was only 0.6% of that at station 2, with a bloom assemblage, and it is unlikely that such a diet could support a large production potential. On average only 3 % of the nitrogen ingested by protozoa was subsequently transferred to copepods. Microbial pathways thus appear to have a minor role in the· transfer of nitrogen to higher trophic levels, their function being mainly the regeneration of nitrogen for primary producers.

Bibliography: leaves 78-95.