Evaluation of NEMO–PISCES v2 biogeochemical model with field data from a north-south gradient in the Northeast Atlantic Ocean & Growth of Emiliania huxleyi and Thalassiosira oceanica under 3 irradiance regimes

Abstract

Global ocean biogeochemical models are powerful tools that help to study the carbon cycle and to predict its future climate and biogeochemical changes. Recently, the EC-Earth model was supplemented with the community ocean model NEMO (Nucleus for European Modelling of the Ocean) coupled to PISCES (Pelagic Interactions Scheme for Carbon and Ecosystem Studies). Yet, validation of model outcomes was urgently needed. It was the aim of the present study to validate EC-Earth runs using a dataset on water column physics, chemistry and phytoplankton biomass along a transect in the North Atlantic ocean. To this end the capability of the model to simulate vertical profiles of the aforementioned parameters along this transect was studied. During a first model run, water column characteristics were compared to depth field data for salinity, temperature, density, Chl a and nutrient concentrations. In a second run several NEMO- related physical parameters had been modified and the response of phytoplankton dynamics to the new conditions was investigated. At high latitudes, the first model run did not realistically predict Mixed Layer Depths (MLD). This resulted in a failure to predict the correct timing and magnitude of the phytoplankton spring bloom. In contrast, the ”sensitive” MLD criterion used by the model to work well at low latitudes. The 2nd model run improved both phytoplankton dynamics and nutrients distribution, implying that the adjustments in water column physics had improved model calculations. Finally, a constant overestimation by the model regarding relative diatom abundance in nutrient-rich and coastal regions was observed. The present study contributed to the validation of NEMO-PISCES v2 model in the North Atlantic. It also underlines the need for more validation studies, especially in the crucial high-latitude sections of the North Atlantic, where the quality of model calculations was found to be relatively poor.

Two phytoplankton species, a diatom and a haptophyte, typical for the North Atlantic ocean were acclimated to three different irradiance - temperature regimes, simulating stable (5 μmol m − 2 s − 1 / 17 o C) and dynamic water column conditions (0-20 μmol m − 2 s − 1 / 9 o C and 50-1250 μmol m − 2 s − 1 / 13.5 o C). An effort has been made to simulate as closely as possible the conditions observed during the summer 2009 and spring 2011 STRATIPHYT cruises. Specific growth rates were estimated using microscopy, fluorescence, Chlorophyll a (Chl a) extraction and spectrophotometry. Furthermore, the photoacclimation abilities of the two species were assessed by maximum PSII efficiency (f v /f m ), Chl a content per cell and optical density (OD) per cell. Results showed no distinct differences between the studied species, Thalassiosira oceanica and Emiliania huxleyi. On one hand, based on growth rates and photoacclimation characteristics it is not possible to tell if one species would dominate over the other in any of the three completely different conditions that were studied. On the other hand clear differences in growth and photoacclimation responses were observed between the constant low irradiance and the dynamic low and dynamic high irradiance experiments. These results imply that although both species exhibited high photoacclimation potential under extreme and fluctuating irradiance conditions, this did not translate into significantly different growth rates. We can conclude that light intensity is not sufficient to explain the differences in the occurrence of these species in the field.