Streamwater nutrients stimulate respiration and breakdown of standardized detrital substrates across a landscape gradient: Effects of nitrogen, phosphorus, and carbon quality

Abstract

Elevated streamwater nitrogen (N) and phosphorus (P) concentrations can stimulate microbial activity on detrital C and accelerate its breakdown in stream ecosystems. Our study evaluated whether nutrient–detrital relationships are robust across a moderately altered land-use gradient and can be used to identify functional impairment of stream ecosystems. We tested the relative importance of N vs P as likely drivers of these responses, whether responses differed for labile or recalcitrant standardized substrates, and whether responses were detectable across streams with other stressors, which can potentially mask nutrient effects. Two studies were conducted in 23 sites in southeastern US streams. These streams differed in land use and exhibited low-to-moderate gradients in N and P. In study 1, we used 9 sites to compare the relationships between nutrient (N and P) concentrations and microbial respiration and breakdown of 2 standardized C substrates: recalcitrant oak wood veneer and labile cellulose sponge. Both of these substrates are low in nutrient content but differ structurally. In the best supported models, respiration and breakdown rates were positively related to streamwater P, but not N, after 4 wk of stream incubation. Microbial respiration increased 4.2 and 1.2× and breakdown increased 1.8 and 2.3× on cellulose and wood, respectively, across the P gradient. Temperature (+) and specific conductivity (−) were also in top models for wood respiration. Respiration and breakdown were highly correlated for both substrates, indicating the importance of microbial processing in driving breakdown rates. In study 2, we used 23 sites to test for association between landscape nutrient (N and P) gradients and wood veneer breakdown and whether detrital stoichiometry was a better predictor of breakdown than streamwater nutrient concentrations. Wood breakdown was related to P, but not N, and increased 4.1× in 12 wk across the P gradient. Wood nutrient content (increased %N and %P, reduced C∶N and C∶P) was also related to streamwater P and better predicted breakdown (C∶P r2 = 0.75, C∶N r2 = 0.87) than streamwater nutrient concentrations. Streamwater P concentrations appeared to stimulate breakdown to a degree that indicates impaired stream function. Our study showed that standardized detrital substrates responses to nutrients 1) were greater to streamwater P than N concentration gradients, 2) occurred on both labile and recalcitrant substrates, and 3) were detectable across landscape gradients with other stressors (e.g., temperature, specific conductivity). These responses likely reflect effects of excess nutrients on diverse C resources in these streams. Wood veneers integrated streamwater nutrient effects, were resistant to physical abrasion, and exhibited significant mass loss even when detritivores were excluded, indicating their value in stream functional assessments under a wide range of stream conditions.