Abstract
Abstract. An established one-dimensional Shelf Sea Physics and Primary Production (S2P3) model 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 conductivity–temperature–depth (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.
Highlights
Shelf seas are ocean regions where water depth is less than a few hundred metres ( ∼ 200 m) and represent only ∼ 10 % by area of the global ocean
Away from advective sources such as the shelf break or plumes from rivers, horizontal processes can be neglected in comparison to vertical processes; S2P3 does not consider advective fluxes, it can make a good representation of the dynamics in the water column for temperate shelf seas (Sharples et al, 2006; Sharples, 2008; Marsh et al, 2015)
Changes in the timing and magnitude of the spring zooplankton bloom coincide with the changes of the timing and magnitude of the phytoplankton blooms (Fig. C7b). These changes in the plankton communities over the year due to different values of θmNax, have an effect on the values of DIN (Fig. C8c, f), with the largest differences shown during springtime at the surface (Fig. C8c). These differences agree with the results found by Ayata et al (2013), where it was demonstrated that taking into account photo-acclimation and variable stoichiometry of phytoplankton growth in marine ecosystem models produces qualitative and quantitative differences in phytoplankton dynamics
Summary
Shelf seas are ocean regions where water depth is less than a few hundred metres ( ∼ 200 m) and represent only ∼ 10 % by area of the global ocean. Steele, 1974; Wroblewski et al, 1988; Anderson, 2005), have shown good agreement with observations in terms of chlorophyll and PP, by simulating the timing and magnitude of the spring phytoplankton bloom in different regions of the ocean Despite their relative simplicity, NPZ models can be a better option to approach an understanding of the physics and biology of an ecosystem, which leads to a further development of the S2P3 v7.0 model where the simplest assumption of a fixed proportion of phytoplankton being grazed and remineralised into the DIN pool (grazing rate) is developed into an NPZ framework (S2P3-NPZ model), using a Holling type 2 or Ivlev grazing functional response of zooplankton grazing on phytoplankton (Franks, 2002), which shows a saturating response to increasing food. Model outputs are compared with observations of phytoplankton responses to physical forcing to illustrate the importance of different processes’ representation
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