Simulation of plankton dynamics and internal nutrient fluxes in a eutrophic shallow lake
The numerical water quality model StoLaM (Stoichiometric Lake Model) is a coupled hydrodynamic-ecological 1-D simulation lake model for the prediction of nutrient cycling, plankton succession and water quality in standing waters.
It integrates the principles of ecological stoichiometry, which means the changing ratios of several nutrients like phosphorus and nitrogen within the foodweb and their impact on the growth rates of plankton organisms with respect to nutrient limitation. On the basis of hydrodynamic processes, the modelling concept takes into consideration the zooplankton and phytoplankton succession, the quantitative cycles of several nutrients (P, N, Si), the exchange between sediment and water, and the role of omnivorous fish in food web interactions.
The basis for model development and validation of simulation results is a three-year survey of the succession of plankton and nutrient dynamics in a eutrophic lake (Strauss 2009).
Fig.1: Schematic diagram of implemented food web interactions in StoLaM including the main nutrient fluxes within the water column.
StoLaM is appropriate for temporary thermally-stratified shallow lakes as well as for deeper lakes. It takes into consideration the sediment-water interactions, more important in shallow lakes due to the low volume-surface ratio, as well as the vertical gradients in shallow and deep water bodies. A one-dimensional vertical hydrodynamic module allows high resolution of the lake internal conditions (e.g. temperature, light, and turbulence) both in time and space using weather data at 10 minutes intervals. This makes possible a realistic simulation of the physical environment required for modelling the nutrient and plankton dynamics in detail.
Fig.2:Measured (upper panel) and simulated (lower panel) temperature profile [Â°C] (left) and oxygen profile [% saturation] for the shallow Lake Alsdorf over a three-year period.
Besides the oxygen concentration and water temperature, several zooplankton and phytoplankton groups are used as state variables in StoLaM. In addition, closed nutrient cycles of phosphorus, nitrogen and silicon within the water column have been mathematically formulated. And because their stoichiometric ratios have a strong influence on the nutrient uptake and regeneration by planktonic organisms, they have been included in the model. The quantity and ratio of dissolved nutrients in the water are affected by external loading and internal processes (e.g. release from the sediment, excretion by benthivorous fish, sedimentation, dinitrogen fixation and denitrification). The uptake rate of dissolved nutrients by phytoplankton is as defined by Monod kinetics, and the phytoplankton growth depends on their internal nutrient contents in accordance with the Droop equation. The growth and nutrient excretion of the zooplankton is determined by their nutrient requirements as well as the stoichiometry of nutrients in the food. The impact of both fish zooplanktivory on the zooplankton as well as the nutrient release from the sediments into the water column by benthivorous fish is described by a special fish module.
As an example, StoLaM was able to simulate the general patterns of the nutrient fluxes and plankton dynamics comparably with the three-year field survey of the shallow Lake Alsdorf. Despite the low water depth, the temperature stratification as well as turbulence-dependent vertical oxygen and nutrient gradients have been precisely predicted by the hydrodynamic module HyLaM.
Fig.3:Simulations and measurements of total phytoplankton biomass (left) and the biomass of the dominant cladoceran species Bosmina longirostris (right) in Lake Alsdorf.
StoLaM has also been tested for lakes with different morphometry and nutrient conditions. For example, two lakes with similar morphometry and water depth (25 m), but different trophic conditions. Due to their different trophic states, the oxygen concentrations in the metalimnion of these two lakes showed contrary pattern: in Lake Perings, a clear metalimnetic oxygen minimum could be found, whereas in Lake Boisdorf the formation of a metalimnetic oxygen maximum has started during summer stratification.
Fig.4:Comparison of computed temperature and oxygen profiles with measured data during a period of summer stratification. Shown are the results for the oligotrophic Lake Boisdorf (left panel) and the eutrophic Lake Perings (right panel).
The water quality model StoLaM can be further used as a prognostic tool for other vertically stratified shallow and deep water bodies. This model can be usefully employed in the process-oriented analysis of lakes as well as in the evaluation of proposed management concepts for standing waters.
Modelling the population dynamics of flagellates with diurnal vertical migration patterns (Strauss 2009):
Due to the high temporal-resolution of the vertical physico-chemical gradient dynamics in StoLaM, the development of bloom-forming summer phytoplankton could be analysed under simulated semi-natural conditions. Simulations of the vertically migrating dinoflagellate Ceratium hirundinella demonstrated that in the nutrient-rich Lake Alsdorf an optimal utilisation of the radiation is of greater importance than the utilisation of additional nutrients from deeper layers of the lake. Nevertheless, diel vertical migration reduces the probability of phosphorus limitation for C. hirundinella even under eutrophic conditions. Field measurements revealed that grazing pressure by the omnivorous rotifer Asplanchna priodonta was the most significant factor affecting the mortality rate for C. hirundinella during mass occurrences. This finding is also quantitatively confirmed by the simulation results (Strauss 2009)
Publications and Presentations:
Strauss T, Gabsi F, Hammers-Wirtz M, Thorbek P, Preuss TG (2017): The power of hybrid modelling: An example from aquatic ecosystems. Ecological Modelling 364:77-88; https://doi.org/10.1016/j.ecolmodel.2017.09.019
Strauss, T. (2009): Dynamische Simulation der Planktonentwicklung und interner Stoffflüsse in einem eutrophen Flachsee. Shaker, Aachen, ISBN 978-3-8322-8501-2, http://dx.doi.org/10.18154/RWTH-CONV-113654
Strauss, T. (2008): Das Gewässergütemodell StoLaM - ein Werkzeug zum Management stehender Gewässer. In: Naturschutz und Freizeitgesellschaft, Bd. 8 (Renaturierung - Programmatik und Effektivitätsmessung), S. 153-171, Academia Verlag, St. Augustin.
Strauss T, Ratte HT (2002): Modelling the vertical variation of temperature and dissolved oxygen in a shallow, eutrophic pond as a tool for analysis of the internal phosphorus fluxes. - Verh. Internat. Verein. Limnol. 28:1508-1511.