Abstract
Abstract. Using in situ data of upper ocean vertical mixing along a transect in the North Atlantic and a one-dimensional phytoplankton growth model, we study the sensitivity of the surface phytoplankton concentration to vertical mixing distributions. The study is divided into two parts. In the first part, the model is calibrated to the observations. The optical model parameters are determined from measurements of the light attenuation. The biological parameters are calibrated to three different reference stations with observed vertical profiles of the chlorophyll a (Chl a) concentration and the nutrient concentration. In the second part, the sensitivity of the three model calibrations to the vertical mixing is studied. Therefore measured vertical mixing profiles are applied to the model. These mixing profiles are based on the measurements along the transect and are treated as a set of possible mixing situations of the North Atlantic. Results show that shifts in vertical mixing are able to induce a transition from an upper chlorophyll maximum to a deep one and vice versa. Furthermore, a clear correlation between the surface phytoplankton concentration and the mixing induced nutrient flux is found for nutrient-limited cases. This may open up the possibility to extract characteristics of vertical mixing from satellite ocean colour data using data-assimilation methods.
Highlights
Thanks to long-term in situ and satellite observations of ocean surface chlorophyll a (Chl a) concentrations, the plankton variability can be studied on timescales longer than seasonal
As comparing the results of highly idealised models, such as the one used here, to in situ measurements and using them for the calibration of model parameters may raise concerns, we provide a rather extensive discussion of the model calibration results
The relatively high standard deviation at some stations can be partially explained by the varying fraction of light which is reflected at the surface due to the zenith angle
Summary
Thanks to long-term in situ and satellite observations of ocean surface chlorophyll a (Chl a) concentrations, the plankton variability can be studied on timescales longer than seasonal. The shallowing of the mixed layer (ML) exposes the phytoplankton to more PAR This enhances growth at the surface and leads to an upper chlorophyll maximum (UCM) given that enough nutrients are available (Behrenfeld, 2010; Lozier et al, 2011). In areas where phytoplankton is nutrient limited, e.g. in the midlatitude Atlantic, an increase in SST will inhibit vertical mixing and lead to stratification. In areas where the phytoplankton is light limited, such as in the northern North Atlantic, a SST increase will reduce the depth of the ML (MLD) and one would expect an increase in phytoplankton The fact that this trend is not observed in high-latitude regions according to data in Boyce et al (2010) indicates that vertical mixing processes are not solely controlled by stratification. Additional information on the data can be obtained from http://oceancolor.gsfc.nasa.gov and http://projects.nioz.nl/stratiphyt, respectively
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