Nutrient limitation occurs when production is limited by the availability of one or more nutrients. In streams, algal growth is often limited by the availability of nitrogen (N), phosphorus (P), or both nutrients (NP). Previous studies have determined that in-stream nutrient concentrations and light levels can affect the degree of algal nutrient limitation. However, less is known about other factors like temperature and current velocity, which clearly influence algal growth but may also interact with nutrient processes. We investigated eight mountain streams along an elevation gradient in Colorado, hypothesizing that higher temperatures and slower current velocities would lead to increased nutrient limitation. To test these predictions, we deployed nutrient diffusing substrate experiments with N, P, and NP treatments during summer 2015, summer 2016, and fall 2016. We continuously logged temperature and obtained point measurements of other environmental covariates. After 2-3 weeks, we measured chlorophyll a (algal biomass) that accumulated on the substrates. We used ANOVA models to determine nutrient limitation for each experiment, and mixed models to determine the overall influence of time point, treatment, and environmental covariates on nutrient effect sizes.
Results/Conclusions
In over half of the experiments, streams were nutrient-limited. NP co-limitation was most common, with primary N-limitation and secondary P-limitation. Across all experiments, results differed by treatment (p < 0.001) and sampling time (p < 0.001). N effect size was negatively affected by in-stream nitrate levels. P effect size was positively affected by discharge, and negatively affected by pH and riparian canopy cover. NP effect size was negatively affected by in-stream nitrate levels, and positively affected by discharge and temperature. We rejected our hypothesis about current velocity and partially rejected our hypothesis about temperature. However, these results are still informative by showing that in addition to algal resources (nutrients, light), other factors can interact with nutrient limitation. Specifically, these experiments emphasize the potentially-important roles of discharge, temperature, and pH. Considering these factors may lead to an increased understanding of nutrient limitation, which is critical for maintaining healthy aquatic ecosystems and preventing high levels of algal biomass accumulation.