Modelling Phytoplankton Dynamics in Estuaries

In estuaries, which are the transition areas between rivers and coastal waters, the spatial and temporal distribution of phytoplankton cell number density (P) is determined by local specific growth and loss, advection by currents, mixing by turbulence, and sinking if the density of phytoplankton is larger than that of the surrounding water. This thesis discusses the spatiotemporal dynamics of P under different estuarine abiotic conditions. Inspired by field observations, four key issues were addressed, that is, the impact of river discharge, the spatial distribution of suspended particulate material (SPM), seasonal variability of incident light intensity and water temperature, and salinity stratification on P patterns. For this, an idealised model was developed and its output was systematically analysed.
The main features of the observed phytoplankton bloom that locates in the middle reach of the Taw estuary (a vertically well-mixed estuary with a low turbidity level) were captured by the model. The specific spatial pattern of the bloom emerges due to the advection of phytoplankton by large river flow in the narrow upper reach and due to the nutrient-limited phytoplankton growth in the wide lower reach. A sensitivity study to river discharge revealed the presence of three regimes, which are characterized by different ratios of net specific growth rate to flushing rate.
The Ems estuary (between the Netherlands and Germany) was taken as an example for vertically well-mixed estuaries with a pronounced turbidity zone. The model captured the key features of the observed blooms in this estuary, i.e., in the lower reach a spring bloom occurs, which is followed by a secondary bloom in autumn. Analysis of model results showed that the along-estuary distribution of turbidity determines the along-estuary location of blooms and it largely affects the intensity of blooms by controlling the distance between bloom locations and the river head, where nutrients enter the estuary.
In estuaries such as the Columbia River estuary (western US), the degree of vertical salinity stratification and the spatial distribution of phytoplankton cell number density P differ substantially between spring and neap tides. The model captured the main features of the observed P patterns in this estuary, that is, during weak stratification (spring tides) P is almost vertically uniform with values decreasing towards the estuary mouth, whereas during strong stratification (neap tides) high values of P occur in the upper layer and extend seawards. Sensitivity experiments showed that the advection of P by subtidal currents is important in obtaining a smaller along-estuary gradient of P during weak stratification and in obtaining a smaller horizontal gradient and a larger vertical gradient of P during strong stratification. Accounting for stratification controlled vertical distribution of vertical eddy viscosity and eddy diffusivity is necessary for obtaining realistic P patterns if stratification is strong, but not if stratification is weak. A higher loss rate of P due to osmotic stress results in a larger along-estuary gradient of P if stratification is weak and in a larger vertical gradient of P if stratification is strong.