Constraining the response of phytoplankton to zooplankton grazingand photo-acclimation in a temperate shelf sea with a 1-D model -towards S2P3 v8.0

Abstract. An established 1-dimensional model of Shelf Sea Physics and Primary Production (S2P3) has been developed into three different new models: S2P3-NPZ which includes a Nutrient-Phytoplankton-Zooplankton (NPZ) framework, where the grazing rate is no longer fixed, but instead varies over time depending on different functions chosen to represent the predator- prey relationship between zooplankton and phytoplankton; S2P3-Photoacclim which includes a representation of the process of photo-acclimation and flexible stoichiometry in phytoplankton; and S2P3 v8.0 which combines the NPZ framework and the variable stoichiometry of phytoplankton at the same time. These model formulations are compared to buoy and CTD observations, as well as zooplankton biomass and in situ phytoplankton physiological parameters obtained in the Central Celtic Sea (CCS). Models were calibrated by comparison to observations of the timing and magnitude of the spring phytoplankton bloom, magnitude of the spring zooplankton bloom, and phytoplankton physiological parameters obtained throughout the water column. A sensitivity study was also performed for each model to understand the effects of individual parameters on model dynamics. Results demonstrate that better agreement with biological observations can be obtained through the addition of representations of photo-acclimation, flexible stoichiometry, and grazing provided these can be adequately constrained.