Inexpensive spot sampling provides unexpectedly effective indicators of watershed nitrogen status

Abstract

Stream water quality data are essential for understanding watershed processes and managing water pollution, but the effort and expense of stream monitoring limit how many watersheds can be studied. For 59 small watersheds in the Chesapeake Bay drainage, we compared water quality measurements from inexpensive spot sampling to data from costly automated monitoring that used 1–3 yr of continuous flow measurement and weekly, temporally composited water sampling. Mean nitrogen (N) levels ranged from 0.01 to 16 mg N/L among streams. There were important temporal variations in N concentrations at each site, but the differences among sites were much greater. Spot samples were very effective at accurately and precisely placing average stream N levels within the N gradient among streams draining N-enriched watersheds. Among watersheds, nitrate (NO3) and total N concentrations from spot samples were very strongly correlated with means from weekly composite sampling (R2 > 97%). We confirmed this result for independent data for 85 larger watersheds in the Chesapeake Bay Non-tidal Network. NO3 concentration from a single March spot sample was highly correlated (R2 > 92%) with flow-weighted average total N concentration synthesized from five years of monitoring. Spot sampling effectively quantifies average N status across N-enriched watersheds because most N moves as dissolved NO3 in subsurface flow and that flux is much less variable than the episodic surface transport of particulate materials. For questions answered by quantifying average N levels, spot sampling can assess more watersheds at much lower cost than automated sampling, so it should be more widely used to support cost-effective N research and management. For materials that are mainly bound to particulates, such as phosphorus, spot sampling is much less effective.