Assessing the value of BGC Argo profiles versus ocean colour observations for biogeochemical model optimization in the Gulf of Mexico

Abstract

Abstract. Biogeochemical ocean models are useful tools subject to uncertainties arising from simplifications, inaccurate parameterization of processes, and poorly known model parameters. Parameter optimization is a standard method for addressing the latter but typically cannot constrain all biogeochemical parameters because of insufficient observations. Here we assess the trade-offs between satellite observations of ocean colour and biogeochemical (BGC) Argo profiles, and the benefits of combining both observation types, for optimizing biogeochemical parameters in a model of the Gulf of Mexico. A suite of optimization experiments is carried out using different combinations of satellite chlorophyll and profile measurements of chlorophyll, phytoplankton biomass, and particulate organic carbon (POC) from autonomous floats. As parameter optimization in 3D models is computationally expensive, we optimize the parameters in a 1D model version, and then perform 3D simulations using these parameters. We show first that the use of parameters optimized in 1D improves the skill of the 3D model. Parameters that are only optimized with respect to surface chlorophyll cannot reproduce subsurface distributions of biological fields. Adding profiles of chlorophyll in the parameter optimization yields significant improvements for surface and subsurface chlorophyll but does not accurately capture subsurface phytoplankton and POC distributions because the parameter for the maximum ratio of chlorophyll to phytoplankton carbon is not well constrained in that case. Using all available observations leads to significant improvements of both observed (chlorophyll, phytoplankton, and POC) and unobserved variables (e.g. primary production). Our results highlight the significant benefits of BGC Argo measurements for biogeochemical parameter optimization and model calibration.

Cetacean‐mediated vertical nitrogen transport in the oceanic realm

“…Additionally, to determine the relationship between species' habitat preferences and the magnitude of nutrient transport among meso-(water column 200-1000 m depths), bathy-(water column deeper than 1000 m depth), and benthopelagic layers, we compared oceanic (seaward of the 1000-m isobath) and continental slope (200-1000 m depth) regions. Finally, we discuss the available surface nitrogen concentrations throughout the oceanic Gulf of Mexico from an existing BGC model (Wang et al 2020).…”

“…The results of the cetacean-mediated transport model (N cet ) were compared to the spatially resolved ammonium model output (converted to mmol N m À3 ) from a coupled physical-BGC model for the Gulf of Mexico Large Marine Ecosystem (N BGC ; Wang et al 2020). Only nitrogen derived from ammonium was considered in this study because it is assimilable by phytoplankton without nitrification.…”

“…Only nitrogen derived from ammonium was considered in this study because it is assimilable by phytoplankton without nitrification. The BGC model ran from 2010 to 2015 and it was calibrated using BGC Argo float data, covering the entire Gulf of Mexico large marine ecosystem using a three-dimensional irregular grid with 36 depth observations at each latitudeÀlongitude (see Wang et al 2020 for detailed information). Daily model output was available but did not contain the contribution of cetaceans to the nitrogen stock.…”

“…Even though 80% of the nutrients coming from the shelf may be denitrified prior to export beyond the 50-m isobath (Xue et al 2013), the amount that is advected may provide more than 90% of the open-ocean nitrogen in the Gulf of Mexico (Kelly et al 2021). However, current biogeochemical (BGC) models in the region do not include the contribution of cetaceans (Shropshire et al 2020;Wang et al 2020), which excrete ammonium (NH 4 + ) that can be readily assimilated, both through excretion and egestion. Nineteen cetacean species occupy the oceanic Gulf of Mexico (Hayes et al 2019), but no attempt to estimate their ecological significance in the transfer of nutrients has been made, particularly for nitrogen.…”