Spatio-temporal distribution of suspended particulate matter in the Scheldt estuary (Belgium) and interactions with mesozooplankton

This is a research study on suspended particulate matter (SPM) in estuaries, specifically in the Scheldt estuary in Belgium. The use of image analysis in studying SPM is being explored. The technique as applied by first by Tackx et al.(1995) in quantifying SPM mass and zooplankton body sizes and gut contents was evaluated and improved upon. The conversion of two-dimensional area measurements into biomass (e.g. volumes, weights) was addressed. Furthermore, the technique was used to characterise and differentiate some types of SPM based on their shapes, geometric and fractal dimensions. Semi-automated differentiation between biotic and abiotic particles was demonstrated. Differentiation between detrital particles from different terrestrial plants (e.g. monocotyl vs. dicotyl) was demonstrated to be possible but technical limitations prevent its routine application. Using the image analysis technique plus the classical measurements of dry weight, particulate organic carbon (POC), and chlorophyll a and b concentrations (Chla and Chlb, respectively), an inventarisation of SPM in the Scheldt was done. A longitudinal transect with 16 stations was sampled every month in a period of one year. The transect covers the brackish and freshwater tidal area of the estuary. Results showed a spatio-temporal variation in the SPM in the Scheldt. The temporal variations were due to the seasonal fluctuations in the concentrations of Chla and Chlb. Spatial variations showed a tendency of the brackish stations to be distinct from other stations mainly due to Chla, Chlb and POC concentrations. The image analysis technique was also applied, in parallel with high performance liquid chromatography (HPLC), to quantify zooplankton clearance rates on SPM and phytoplankton, respectively. Based on the gut content approach, both methods were easily comparable and were used to measure clearance rates on phytoplankton (Fph) and SPM (Fspm) by the copepod E. affinis and the cladoceran Daphnia sp. Fph rates were generally higher than Fspm for both taxa, indicating a positive selection for phytoplankton. Finally, the size distribution of SPM in the Scheldt was analysed by the size-spectra approach. Normalised SPM distributions were fitted with linear and polynomial equations, with the latter showing the best fit. Mesozooplankton - prey relationships were tested using the size-based predator - prey relationship model of Sheldon et al. (1977). Problems of fitting the model to biomass data expressed in volumes and in carbon were encountered. However, the model was shown to be applicable to our data, despite the fact that some prerequisites were not met