Deep-Sea Research II 53 (2006) 2808–2832 Influence of model geometrical fitting and turbulence parameterization on phytoplankton simulation in the Gulf of Maine Rucheng Tian à , Changsheng Chen Department of Fisheries Oceanography, School for Marine Science and Technology, University of Massachusetts Dartmouth, 838 South Rodney French Blvd., New Bedford, MA 02744, USA Accepted 31 August 2006 Available online 13 November 2006 Abstract Numerical experiments were made to examine the influences of model geometrical fitting and turbulence parameterization on the temporal and spatial distributions of simulated phytoplankton in the Gulf of Maine. The assessment of the role of geometrical fitting was made by running a state-of-the-art Nutrient-Phytoplankton-Zooplankton (NPZ) model with physical fields provided from unstructured-grid, finite-volume coastal ocean model (FVCOM) and structured-grid, finite-difference coastal ocean model (ECOM-si), respectively. The impact of turbulence parameterization was studied by running a coupled NPZ–FVCOM system with various vertical turbulence modules implemented in the General Ocean Turbulence Model (GOTM). Comparisons were focused on three large tidal dissipation regions: Georges Bank (characterized by strong tidal rectification over steep bottom topography and tidal mixing fronts), Bay of Fundy (featuring large semidiurnal tidal oscillations due to the gulf-scale resonance), and Nantucket Shoals (a tidal energy flux convergence zone). For the same given tidal forcing and initial physical and biological conditions, the ability of a model to accommodate the irregular coastal geometry and steep bottom topography is critical in determining the robustness of the simulated spatial and temporal structure of N and P. For the same given external forcing in FVCOM, turbulence parameterizations have less impact on N and P in mixed regions than in stratified regions. In mixed regions, both q–e and q–ql models reproduced the observed vertical mixing intensity. Since biological variables remained vertically mixed in these regions, their structures were little affected by turbulence closure schemes. In stratified regions, q–e models predicted stronger mixing than q–ql models, which produced more nutrient fluxes over the slope and thus influenced the growth and distribution of P around the tidal mixing front. A direct comparison between observed and model-predicted turbulence dissipation rates suggests that q–e models with a mixing cutoff at Richardson number of 1.0 predict more realistic mixing intensity than q–ql models in stratified regions on Georges Bank. r 2006 Elsevier Ltd. All rights reserved. Keywords: Phytoplankton dynamics; Tidal mixing front; Geometrical fitting; Physical–biological modeling; Gulf of Maine 1. Introduction Due to near-resonance geometry and complex topography, the Gulf of Maine (GoM) is ARTICLE IN PRESS www.elsevier.com/locate/dsr2 0967-0645/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2006.08.006 à Corresponding author. Tel.: +1 508 910 6383; fax: +1 508 910 6342. E-mail addresses: rtian@umassd.edu (R. Tian), c1chen@umassd.edu (C. Chen).