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Publication

Unraveling the coupled large-scale suspended sediment and phytoplankton dynamics in a turbid and tide-dominated estuary

Book - Dissertation

Estuaries often show regions in which suspended particulate matter (SPM) and/or phytoplankton accumulate. Predicting the location of these regions and the corresponding magnitude of the SPM and phytoplankton concentration is of great importance for managing the estuary; it may prevent the system from evolving towards a (hyper-)turbid state, a condition in which phytoplankton growth, which forms the basis of the food chain, is very limited. To predict the location and magnitude corresponding to accumulation of SPM and phytoplankton, we have to understand the interconnection between SPM and phytoplankton concentration. On the one hand, SPM is often a limiting factor for estuarine phytoplankton growth by deteriorating the light climate and thereby limiting photosynthesis. On the other hand, various authors showed that phytoplankton may determine the SPM concentration by, for example, the excretion of sticky substances. These substances may impact flocculation and thereby the settling velocity and dynamics of SPM flocs. Additionally, they may stabilize the bed and thus change the erosion properties, which also impacts the SPM concentration. While most literature focuses on the small-scale impact of biological flocculants, its influence on the SPM profiles on the large spatial- and temporal scale is still largely unknown. In this thesis, we study the interconnection of SPM and phytoplankton on the large temporal- and spatial scale, applied to a turbid, tide-dominated, and nutrient-rich estuary, being the Scheldt estuary. To this end, we combine a model approach and analysis of multi-annual observations covering the entire domain of the Scheldt estuary. First, we extend a hydro-sediment transport model by a flocculation model and showed that flocculation might significantly impact the estuary-scale SPM distribution. Next, we apply the model to show that the impact of biotically-induced flocculation and erosion on the estuary-scale seasonality in the SPM distribution is rather limited. Seasonality in freshwater discharge might explain the observed seasonality in SPM. Next, we construct a conceptual model to show that temporal variability in light climate (cf. SPM) may drastically reduce the time-averaged PP and exponential phytoplankton growth and delay the onset of a spring bloom by ~ weeks. Last, we combine our previous results to show that a multi-annual change in mortality rate, and not in the SPM alone, may explain the observed multi-annual evolution in phytoplankton blooms in spring in the Scheldt estuary. Although we apply our modeling framework to the Scheldt estuary, it may also be applied to other turbid, tide-dominated, and nutrient-rich estuaries.
Number of pages: 202
Publication year:2022
Keywords:Doctoral thesis
Accessibility:Open