Phytoplankton nitrogen-15 content

Labeling of the phytoplankton nitrogen pool in this linear fashion was contrary to the expected result. The solid line in Figure 4 shows the expected asymptotic increase in 15N that was calculated by using the DIN concentrations in the epilimnion, the addition rate of 15NH4, and the assumption of a 1-week nitrogen-turnover time (87) in the phytoplankton. A similar asymptotic increase in 15N content of phytoplankton, with the phytoplankton reaching isotopic equilibrium in 1 week, was measured in an isotope-addition experiment done in limnocorrals in nearby Toolik Lake (Kipphut, G. W. Whalen, S. C. unpublished data). 

Fig. 2. Model output showing the percentage of different food components available to filter feeders with varying water transport (0-7 water column turnovers per day). The nitrogen content of all food available to filter feeders is shown below, and is much higher when the system is "closed" (0 turnovers). Fig. 2a) shows food proportions and quantities under upwelling conditions when all food is derived from macrophytes. Fig. 2b) depicts downwelling conditions when phytoplankton is an additional component (After Wickens and Field, 1985).

Ki0rboe, T. (1989). Phytoplankton growth rate and nitrogen content Implications for feeding and fecundity in a herbivorous copepod. Mar. Ecol. Prog. Ser. 55, 229—234. 

Redfield (1934), who analyzed the major elemental content of many samples of mixed plankton (phytoplankton and zooplankton) caught in nets towed through the surface ocean. They compared the carbon, nitrogen, and phosphorus composition of these collections to concentration profiles of dissolved inorganic carbon (DIC), NOs, and P04 throughout the water column. This pioneering research demonstrated that these three elements are continually redistributed in the ocean by selective removal into plankton cells and their remains (i.e., fecal pellets), which are then efficiently respired as they sink through the marine water column. 

Iron is concentrated most by cyanobacteria followed closely by phytoplankton (Jones et al., 1978) 7S). Copper is concentrated most by phytoplankton and next by cyanobacteria. Primitive photosynthesizers such as the cyanobacteria are especially rich in non-heme iron, which is involved in the reduction of C02, molecular nitrogen and many other substances. It has been speculated that during the evolution of the plant kingdom, the ratio of iron to other polyvalent metals decreased because the latter became more and more involved in metabolism, chiefly in oxidation reactions in the cells (Ochiai, 1983)76). Therefore, cyanobacteria contain much more iron than other plants. It has been also concluded from analyses of individual fossils that the evolution of different algal groupings in the Precambrian was accompanied by a decrease in the iron content and simultaneous enrichment in copper and others (Udel nova et al., 1981)77). Copper has been interpreted to be a marker element of the younger Proterozoic as far as its biological association is concerned. Thus, the two elements iron and copper cover the important period of the Earth s history, between 3.8 — 1.5 and 1.5 — 0.6 Ga resp. (Ochiai, 1983)76>. 

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