Does preferential uptake of dissolved inorganic and organic nitrogen change with trophic complexity?

Nitrogen (N) is a critical element for all living things, including the flora and fauna in aquatic ecosystems. However, N exists in many forms with some organisms preferring one compound over another.  We measured preferential uptake of four different N compounds, ammonium (NH₄), nitrate (NO₃), glycine, and methionine by microbes in stream sediment (heterotrophs), filamentous green algae (Cladophora, autotrophs), and freshwater snails (Physa acuta, consumers) using ¹⁵N tracers in laboratory mesocosm experiments.  Labeled 99% ¹⁵NO₃, ¹⁵NH₄, ¹⁵N-glycine, and ¹⁵N-methionine were added to mesocosms, in conjunction with a control, no isotope, treatment, and allowed to incubate for 5 days after which trophic compartments were rinsed, dried, ground, and analyzed for ¹⁵N content.  Overall, the preferred form of N utilized by organisms depended on the presence of other trophic compartments.  Specifically, Physa utilized NH₄ preferentially when alone, but glycine and methionine N sources were more important when sediment and algae were present.  Similarly, filamentous algae expanded use of organic N when in competition with sediment microbes, as demonstrated by higher ¹⁵N incorporation.  In the absence of competition, filamentous algae preferentially utilized NH₄. Organic N was a more important source of N across compartments in the presence of greater trophic complexity.