SUMMARY.
1. Growth and reproduction were measured for Daphnia pulex (Leydig) fed Scenedesmus obliquus (Turp.) Kutz. grown with nitrogen‐sufficient and nitrogen‐deficient media. The incorporation of carbon into the body of D. pulex was traced using 14C‐labelled algae and biochemical fractionation.
2. Scenedesmus cultured on N‐defkient media allocated more carbon to lipid and less to protein than when cultured on N‐sufficient media.
3. In a short‐term feeding experiment, Daphnia accumulated more lipid when fed N‐deficient algal cells high in lipid content than when fed algae grown on N‐sufficient media that were lower in lipid.
4. Animals grew faster and produced larger broods when fed algae grown on an N‐sufficient medium over an 8‐day period. Daphnids fed N‐deficient algae had higher quantities of body lipid, suggesting it was not a lack of energy which limited their growth and reproduction.
5. Daphnia fed algae grown on N‐deficient media allocated more lipid to eggs, and their offspring lived longer under starvation conditions.
Phytoplankton in the downlake epilimnion of Normandy Lake, a central Tennessee reservoir, responded to summer N deficiency by increasing relative rates of lipid synthesis from 10–15% up to 20–35% of the total photosynthetic C fixation. Phytoplankton in more N-sufficient areas of the reservoir (downlake in a metalimnetic chlorophyll peak and uplake near the river inflow) maintained lower rates of lipid synthesis, generally [Formula: see text] of the total fixed C, throughout the summer. NH4 enrichment of N-deficient phytoplankton inhibited photosynthesis and significantly depressed the high lipid synthesis rates; however, NH4 enrichment had no effect on the phytosynthesis or lipid synthesis of N-sufficient phytoplankton. Our results document, for the first time, the occurrence of high lipid synthesis rates associated with the N limitation of natural phytoplankton assemblages. This relationship has previously been observed only in laboratory algal culture studies.
Past studies of organic matter processing in stream ecosystems have focused on the fate of allochthonous terrestrial leaf detritus. In streams with a reduced canopy, submerged macrophytes may provide a significant source of organic matter to the microbial community and higher trophic levels. We compared mass loss patterns and microbial dynamics between a submerged macrophyte, Sagittaria platyphylla, and a deciduous leaf, Populus deltoides. Mass loss rates were higher for the submerged macrophyte, though exponential decay values indicated that both are "fast" decomposers. Bacterial abundance was not significantly different between plant types, but bacterial productivity was significantly higher in Sagittaria. Although fungal biomass was higher overall for Populus, it was not significantly different from that of Sagittaria until day 30. Relative to fungi, bacteria made up 4% and 7% of the peak microbial biomass on Populus and Sagittaria, respectively. Aquatic hyphomycete sporulation was detected only on Populus. These results suggest that in systems where submerged macrophytes are abundant, they can provide a carbon source quantitatively comparable to that of riparian leaf detritus, but that qualitative differences in leaf structural composition cause a shift toward a more significant role for bacterial decomposers.
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