Variation in animal-mediated nutrient cycling across a flood-disturbance gradient in Colorado mountain streams
Animals, such as macroinvertebrates and fish, can remineralize a significant proportion of the available nutrients in certain stream ecosystems. However, the ecological conditions under which animal-mediated nutrient recycling provide a large proportion of whole-stream nutrient recycling are still poorly understood. The purpose of this study was to examine the interaction between hydrologic disturbance caused by a 100-year flood and animal-mediated nutrient recycling in a suite of Colorado Rocky Mountain streams. To evaluate these interactions, a high elevation transect (>3000m) of four stream sites was established. At these sites, excretion rates of ammonium (NH4+) and soluble reactive phosphorus (SRP) were determined for a group of macroinvertebrate focal taxa. Individual animals were incubated for a period of one hour in stream water and samples were analyzed for NH4+ and SRP concentration. Excretion rates were complemented by measuring the demand for nutrients in the system using the TASCC method for quantifying nutrient uptake. Additionally, disturbance indices, abundance and biomass of macroinvertebrates, chlorophyll a concentration, and canopy cover for each site were determined. We expected to find a decrease in abundance and biomass of macroinvertebrates and a lower nutrient demand because of fewer primary producers in sites subjected to higher disturbance intensity.
Analysis of uptake metrics across sites revealed no distinct patterns in relation to disturbance indices. Only one out of the four sites was highly disturbed, and although it displayed low uptake rates and velocities for NH4+ and SRP, as expected, low uptake values were also observed for one of the least disturbed sites. Differences in uptake metrics were positively correlated with chlorophyll a concentrations and negatively correlated with canopy cover in the streams, however single sites often were important in leveraging these results. Significant differences across sites in mass-corrected NH4+ excretion rates were observed for three out of six of the most abundant insect species, yet excretion rates for most taxa varied more with elevation than disturbance. Though precipitation was initially considered to be the driver of stream disturbance and a disturbance gradient was expected to be seen across these sites, our analysis showed relatively rapid recovery in both invertebrate and algal assemblages. The absence of strong differences in nutrient uptake and excretion suggests that streams may also be resilient to catastrophic disturbance with regards to different components influencing nutrient cycling.