Understanding interactions between nutrient cycles is essential for recognizing and remediating human impacts on water quality, yet multielemental approaches to studying nutrient cycling in streams are currently rare. Here we utilized a relatively new approach (tracer additions for spiraling curve characterization) to examine uptake dynamics for three essential nutrients across a landscape that varied in absolute and relative nutrient availability. We measured nutrient uptake for soluble reactive phosphorous, ammonium-nitrogen, and nitrate-nitrogen in 16 headwater streams in the Catskill Mountains, New York. Across the landscape, ammonium-nitrogen and soluble reactive phosphorus had shorter uptake lengths and higher uptake velocities than nitrate-nitrogen. Ammonium-nitrogen and soluble reactive phosphorus uptake velocities were tightly correlated, and the slope of the relationship did not differ from one, suggesting strong demand for both nutrients despite the high ambient water column dissolved inorganic nitrogen: soluble reactive phosphorus ratios. Ammonium-nitrogen appeared to be the preferred form of nitrogen despite much higher nitrate-nitrogen concentrations. The uptake rate of nitrate-nitrogen was positively correlated with ambient soluble reactive phosphorus concentration and soluble reactive phosphorus areal uptake rate, suggesting that higher soluble reactive phosphorus concentrations alleviate phosphorus limitation and facilitate nitrate-nitrogen uptake. In addition, these streams retained a large proportion of soluble reactive phosphorus, ammonium-nitrogen, and nitrate-nitrogen supplied by the watershed, demonstrating that these streams are important landscape filters for nutrients. Together, these results (1) indicated phosphorus limitation across the landscape but similarly high demand for ammonium-nitrogen and (2) suggested that nitrate-nitrogen uptake was influenced by variability in soluble reactive phosphorus availability and preference for ammonium-nitrogen.

Ecological stoichiometry requires accurate measurement of carbon : nitrogen : phosphorus ratios in organic matter. Currently, carbon : nitrogen : phosphorus ratios are typically determined by analyzing one sample for carbon : nitrogen and another sample for percent phosphorus. Not only does this approach require multiple samples and accurate sample weights, but it also obscures natural variability. We developed a method which allows us to obtain carbon : nitrogen : phosphorus ratios from a single, small sample of organic matter. The analysis consists of a single persulfate digestion in sealed vials that converts the organic carbon (C), nitrogen (N), and phosphorus (P) to carbon dioxide gas (CO 2 ), nitrate ion, and phosphate ion. The combination of 3% persulfate in 0.15 M NaOH provides a digestion that is initially alkaline but becomes acidic after about 30 min. In the final acidic solution, CO 2 is driven into the vial headspace and is measured on a gas chromatograph-mass spectrometer. Nitrate and phosphate are measured on an ion chromatograph and by using standard colorimetric methods, respectively. We assessed the method by evaluating recovery of C, N, and P from chemical standards and by comparing values obtained with this method to those obtained by current, conventional methods. We used cultured algae and pond seston on filters, leaf litter, and zooplankton samples for the method comparisons. The method is precise at low mass; we were able to confidently measure C : N : P ratios on individual Daphnia magna. The method showed high recovery for standards and produced C : N : P ratio values similar to those found by other analytical approaches and to values in the literature.

Aquatic environments vary widely in aspects other than their physicochemical properties that could alter the toxicity of novel contaminants. One factor that could affect chemical toxicity to aquatic consumers is their nutritional environment as it can strongly affect their physiology and life history. Nutrition has the potential to alter an organism's response to the toxin or how the toxin interacts with the consumer through its food. Here we determined how growth and survival responses of Daphnia to an emerging contaminant, silver nanoparticles (AgNPs), are affected by the presence of food and its stoichiometric food quality. We used a series of survival tests, each slightly modified, to determine whether variable toxicity in different nutritional environments resulted from algal sequestration of AgNPs in a nontoxic form or from changes to the nutritional status of the test animals. We found that the presence of algae, of good or poor quality, reduced the toxicity of AgNPs on animal growth and survival. However, the decrease in AgNP toxicity was greater for animals consuming P-rich compared to P-poor food. We found evidence that this effect of food quality was due to greater algal uptake of AgNPs by P-rich than by P-stressed algae. However, we also found animal nutrition, in the absence of algal AgNP binding, could affect toxicity with P-nourished animals surviving slightly better when exposed to AgNPs compared to their P-stressed counterparts. Our results show an important role for algal particles and their P content in determining the toxicity of AgNPs in natural waters primarily due to their binding and uptake abilities and, less so, to their effects on animal nutrition.

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