Different forms of carbon, nitrogen, and phosphorus influence ecosystem stoichiometry in a north temperate river across seasons and land uses

Natural and human features on land result in differential loadings of carbon (C), nitrogen (N), and phosphorus (P) to rivers that influence within ecosystem processing. However, little is known about how land use, together with seasonal changes in climate and hydrology, influence the relative proportions of C, N, and P in rivers. To evaluate the spatial and temporal patterns in ecosystem-level C: N: P stoichiometry, we sampled 13 sites once per season for 3 yr along the main stem of a north temperate river with winter ice-cover that flows across a gradient of forested, urban, and agricultural landscapes. We found that C concentrations were rather stable along the continuum, whereas N and P rapidly increased downstream due to urban and agricultural land uses. The flow-weighted C: N: P ecosystem stoichiometry ranged from 2319: 119: 1 in the most upstream site to 368: 60: 1 at the outlet. The dominant form of N generally shifted from dissolved organic nitrogen in upstream forested reaches to nitrate in more impacted, downstream reaches, and winter stoichiometry was enriched in inorganic N and dissolved P forms. Concentrations of all three elements were generally lower in spring during year-high flow due to dilution. The spatial and temporal variation in stoichiometry in this north temperate river covered much of the range previously observed between litter ratios and the Redfield ratio. This suggests that even moderate human impacts can have profound effects on riverine ecosystem stoichiometry, and that these effects are modulated by seasonal trends in temperature and hydrology.

Automated summary

The study found that carbon concentrations were stable along the river continuum, while nitrogen and phosphorus rapidly increased downstream due to urban and agricultural land uses. The dominant form of nitrogen shifted from dissolved organic nitrogen in forested reaches to nitrate in downstream areas. The spatial and temporal variation in stoichiometry of the north temperate river was influenced by human impacts and seasonal trends in temperature and hydrology.