Effect of turbulence modelling, light intensity and tidal flow on phytoplankton in estuaries

Abstract

This study focusses on hydrodynamics and phytoplankton dynamics in estuaries, the transition areas between rivers and seas. Gaining insight into phytoplankton behaviour, in particular the formation of blooms, is important to improve management of estuaries. For this, both complex numerical models as well as idealised models are employed. In this study, an existing idealised numerical model is extended to gain knowledge about the influence of several factors on the phytoplankton blooms in well-mixed (i.e. salinity does not vary in the vertical direction) estuaries. The formulation of turbulent vertical exchange processes, as represented by the vertical eddy viscosity (Aν) and vertical eddy diffusivity (κν) is the first factor investigated. Here, κν and Aν are given a parabolic distribution in the vertical, instead of keeping it constant. The observed differences with the reference experiments are small, about 1 %. The most remarkable difference is that a lower equilibrium phytoplankton concentration is found, which is explained by the development of a lower nutrient concentration in the first weeks of the experiment, which causes less growth in the first weeks. The second factor that was modified is the light intensity: it was given a diurnal variation, with the same mean value as in the original model, instead of keeping it constant in time. This results in a lower phytoplankton concentration, which is explained by the fact that phytoplankton is normally limited by nutrients, but due to the low light intensity at night there is an extra limitation on the growth. By combining these two factors, the effect caused by the diurnal light cycle dominates. No additional effects are observed. The last influence investigated is the role of involving explicit tidal flow in these well-mixed estuaries. A time-dependent velocity profile was used to model the tidal flow. The model did not work with these settings: the tidal flow flushed most of the nutrients to the sea what prevented a phytoplankton bloom from occurring. Analysis demonstrated that the addition of tidal flow requires proper adjustments of the conditions imposed at the seaward boundary.