Seasonal variation in carbon and nutrient mobility in forested, low-order streams of the western Olympic Peninsula

Background/Question/Methods

Situated in the temperate rainforest, the western Olympic Peninsula is an area of dense stream networks, winter-wet, summer-dry precipitation patterns and deep, well-drained soils, high soil carbon content and low decomposition rates. This region boasts some of the highest biomass production rates in North America, and is among the top bioregions in terms of carbon storage. Despite this, previous work on stream carbon and nutrient export is sparse in this region. Our research objective was to characterize seasonal patterns in storage and depletion of C and N export from small (3rd order) streams across the study region. Samples were collected from 10 streams ranging 0.5-7 km2 in area once per season starting in June 2016. Stream gaging stations in the sample reaches provide a high-resolution streamflow dataset used to correlate geochemical variables, including dissolved nitrate (NO₃^-) and phosphate (PO₄^3-) concentrations as well as particulate C and N content. We expect to find high export rates of carbon and nutrients during high streamflow and corresponding litterfall periods with decreasing availability over the water-year to due to depletion of soil reserves and vegetative uptake. Existing datasets of riparian vegetation, channel morphology and were mined to incorporate variation due to differences in basin characteristics.

Results/Conclusions

Dissolved NO₃^- followed the general pattern of mobilization and depletion predicted, with high concentrations and variability (0.2-2.4 mg N/L) during high flow and low concentrations (0.02-0.3 mg N/L) throughout the summer. Winter base flow conditions resulted in higher NO₃^- than summer, but did not reach the concentration levels of peak flows. Observed streamflow showed very strong correlations with NO₃^- concentration within individual sites. PO₄^3- followed the opposite pattern, with lowest concentrations present during highest flows (overall range 0.003-0.04 mg P/L). Particulate C:N and dissolved carbon data is forthcoming. Future work should compare these values to those of additional water years to evaluate the strength of the seasonal pattern over time. In addition, we strongly suggest the addition of high-resolution monitoring over the course of episodic streamflow pulses to determine the rate of flushing at these sites and to further investigate the influence of stream stage on export variation.