Nutrient Limitation and the Stoichiometry of Nutrient Uptake in a Tropical Rain Forest Stream

Nutrient limitation assessment is important to understand stream ecosystem functioning. Aquatic primary producers are often limited by nitrogen, phosphorus, or both, as assessed by nutrient diffusing substrata (NDS), a common method for assessing nutrient limitation in streams. But little is known regarding how this method relates to patterns of nutrient uptake at the whole-stream scale. We combined two techniques to examine nutrient limitation in a tropical stream. First, we conducted two NDS experiments using ammonium, nitrate, and phosphate alone and combined, to determine nutrient limitation at substrata scale over several weeks. Second, we conducted whole-stream nutrient addition experiments over the course of a year, using nutrients alone and in combination, to test theoretical predictions about uptake characteristics of limiting and nonlimiting nutrients. NDS results consistently indicated N limitation. Ambient uptake length (S_W-amb) suggested either P limitation (shorter S_W-amb for P than N) or colimitation (similar S_W-amb for both nutrients). The relationship between N uptake and concentration when added alone or with P suggested P limitation, colimitation, or neither, depending on the date. We speculate that the different conclusions arise from differences in the spatial and temporal scale assessed by these techniques and the microbial processes involved, and the potential for physical processes influencing whole-stream uptake estimates. We conclude that nutrient limitation is not as categorical as NDS results often imply, rather habitat, resource, and biotic diversity result in nutrient uptake rates that do not necessarily conform to predictions drawn from fine scale, process-specific bioassays such as chlorophyll-a accrual on NDS.