The flow of Pacific water to the North Atlantic exerts a globally significant control on nutrient balances between the two ocean basins and strongly influences biological productivity in the northwest Atlantic. Nutrient ratios of nitrate (NO 3 - ) versus phosphate (PO 4 3 - ) have previously been used to complement salinity characteristics in tracing the distribution of Pacific water in the North Atlantic. We expand on this premise and demonstrate that the fraction of Pacific water as determined by NO 3 -  : PO 4 3 - ratios can be quantitatively predicted from the isotopic composition of sub-euphotic nitrate in the northwest Atlantic. Our linear model thus provides a critically important framework for interpreting δ 15 N signatures incorporated into both modern marine biomass and organic material in historical and paleoceanographic archives along the northwest Atlantic margin.

Local and regional modelling of NH 3 surface exchange is required to quantify nitrogen deposition to, and emissions from, the biosphere. However, measurements and model parameterisations for many remote ecosystems – such as tropical rainforest – remain sparse. Using 1 month of hourly measurements of NH 3 fluxes and meteorological parameters over a remote Amazon rainforest site (Amazon Tall Tower Observatory, ATTO), six model parameterisations based on a bidirectional, single-layer canopy compensation point resistance model were developed to simulate observations of NH 3 surface exchange. Canopy resistance was linked to either relative humidity at the canopy level (RH z 0 ′ ), vapour pressure deficit, or a parameter value based on leaf wetness measurements. The ratio of apoplastic NH 4 + to H + concentration, Γ s , during this campaign was inferred to be 38.5  ±  15.8. The parameterisation that reproduced the observed net exchange of NH 3 most accurately was the model that used a cuticular resistance ( R w ) parameterisation based on leaf wetness measurements and a value of Γ s =50 (Pearson correlation r =0.71 ). Conversely, the model that performed the worst at replicating measured NH 3 fluxes used an R w value modelled using RH z 0 ′ and the inferred value of Γ s =38.5 ( r =0.45 ). The results indicate that a single-layer canopy compensation point model is appropriate for simulating NH 3 fluxes from tropical rainforest during the Amazonian dry season and confirmed that a direct measurement of (a non-binary) leaf wetness parameter improves the ability to estimate R w . Current inferential methods for determining Γ s were noted as having difficulties in the humid conditions present at a rainforest site.