Measuring and interpreting relationships between nutrient supply, demand, and limitation

Stream nutrient uptake and limitation are interconnected by relationships between nutrient supply and demand. We used multiple approaches, including estimates of nutrient supply, measures of stream metabolism derived from dissolved O-2 curves, and nutrient-enrichment experiments, as complementary measures of nutrient supply, demand, and limitation in New Hope Creek, a 3r d -order stream in the Duke Forest of North Carolina. Over the course of 1 y of sampling, NO3- N supply relative to demand (S:D) was large during winter (S:D = 133.4), spring (S:D = 62.0), and summer (S:D = 108.32). Potential N demand, estimated based on ecosystem metabolic rates, was measurable during winter, spring, and summer months, but we measured no N uptake in response to N addition from nutrient-enrichment experiments. In contrast, during autumn, declines in stream NO3- N concentrations to annual minima (autumn concentration range 0.009-0.034 mg/L NO3- N), and low S:D (1.84) induced transient N limitation, and NO3- N enrichment stimulated increased uptake. We demonstrate that nutrient-enrichment experiments fail to detect uptake when nutrient concentrations are nonlimiting even when ambient biological uptake remains an important sink for the nutrient of interest. In addition, we present a conceptual model of 'supply-demand space' to aid interpretation of nutrient-enrichment data. Last, we suggest that simultaneous measures of nutrient supply, demand, uptake, and limitation can help define supply-demand relationships and provide a step toward a more robust understanding of biogeochemical cycling in streams.