The influence of trophic complexity on preferential uptake of dissolved inorganic and organic nitrogen: a laboratory microcosm experiment

N is a critical element for all living things, including the flora and fauna in aquatic ecosystems. However, N exists in many forms, and some organisms might preferentially use one compound over another. We used ¹⁵N tracers in laboratory microcosm experiments to measure preferential uptake of 4 different N compounds (NH₄-N, NO₃-N, glycine, methionine) by filamentous green algae (Cladophora glomerata; autotrophs), microbes in stream sediment (heterotrophs), and freshwater snails (Physa acuta; consumers) in response to varying trophic complexity. Seven trophic complexity treatments were applied: 3 types of single-organism treatments (filamentous algae, sediment microbes, or snails), three types of 2-organism treatments (filamentous algae + sediment microbes, sediment microbes + snails, filamentous algae + snails), and 1 treatment consisting of all organisms (filamentous algae + sediment microbes + snails). We combined trophic complexity treatments (n = 7) with 1 of 5 ¹⁵N tracers: control (no ¹⁵N), ¹⁵NH₄, ¹⁵NO₃, ¹⁵N-glycine, and ¹⁵N-methionine (enriched to ~3000‰). Each trophic complexity treatment was replicated 4 times for each ¹⁵N tracer treatment to yield a total of 140 microcosms. We incubated microcosms for 5 d after which organisms were rinsed, dried, ground, and analyzed for ¹⁵N content. Overall, uptake of ¹⁵NH₄ was greater than uptake of any other N form for all organisms. However, trophic complexity changed the relative uptake rates of different N forms. Cladophora preferentially used ¹⁵NH₄ when alone, but when in the presence of heterotrophic competitors or consumers, Cladophora ¹⁵NO₃ uptake doubled. Cladophora uptake of ¹⁵N-glycine and ¹⁵N-methionine decreased by ½ in the presence of other organisms. In contrast, when sediment microbes were exposed to Cladophora or Physa, ¹⁵NH₄ and ¹⁵N-methionine uptake rates increased. The N form incorporated by Physa was dependent on the food source present (Cladophora vs sediment microbes). The highest N uptake rates for all N forms were associated with the presence of Cladophora. These data indicate that N uptake is a plastic trait, and organisms can alter use of N forms in response to the presence of competitors or consumers.