Influence of inorganic nitrogen cycling on the δ15N of Lake Ontario biota

The seasonal variability in the delta(15)N of biota at the base of the pelagic food chain in Lake Ontario was studied during the period 1995-1997. Strong seasonal patterns were observed in the delta(15)N of dissolved inorganic nitrogen, different size fractions of particulate organic matter, and zooplankton split into size fractions of 110-210, 210-295, and > 295 mu m. Similar patterns were reflected in the delta(15)N of individual species: Diacyclops thomasi, Bosmina longirostris, Daphnia spp., and calanoid copepods. The seasonal fluctuation of primary producers was a function of the nitrogen enrichment of dissolved inorganic nitrogen during photosynthetic uptake and the delta(15)N of NH4+ and NO3- used by the algae. The large range in delta(15)N of primary producers suggests that recycling of nitrogen is an important determinant of delta(15)N at the base of the food web. The delta(15)N of individual species of zooplankton reflected the known feeding strategies and dietary preferences.