Defining isoscapes in the Northeast Pacific as an index of ocean productivity

Abstract

AbstractAimWe modelled isoscapes in the Northeast (NE) Pacific using satellite‐based data, with the main objective of testing whether isoscapes defined by a few key parameters can be used as a proxy for secondary productivity.LocationNortheast Pacific; 46–60° N and 125–165° W.Time periodFrom 1998 to 2017 (ongoing).Major taxa studiedZooplankton, with a focus on large herbivores.MethodsApproximately 280 summer zooplankton samples were analysed for carbon (δ13C) and nitrogen (δ15N) stable isotope (SI) ratios. Environmental conditions experienced by zooplankton organisms were extracted from satellite, in situ sensor and model databases. A generalized additive model approach was used to explain the spatial variability of δ13C and δ15N values and to predict isoscapes.ResultsSea surface temperature (SST), sea level anomaly (SLA) and chlorophyll‐a concentration emerged as the significant SI predictors. Modelled isoscapes reproduced patterns observed in δ13C and δ15N value distribution, such as a decrease from the coast to offshore. The contribution of eddies in enhancing local production in the open ocean was also well captured by the models. In the central part of the NE Pacific, higher SI values were correlated with higher large copepod biomass measured by the North Pacific Continuous Plankton Recorder (CPR) survey. However, in the area off the coast of British Columbia, high δ15N variability appeared to be associated with episodic intrusions of coastal waters, demonstrating that caution is needed when interpreting sharp changes in SI ratios.Main conclusionsAlthough the mechanisms driving SI ratio variability are complex, we demonstrate that a few parameters used as a proxy for some of these major mechanisms are able to produce successful isoscape models. This approach was proved useful to provide a qualitative estimate of secondary production, which can be particularly valuable in a region where few data are available.