Quantifying the role of microphytobenthos in temperate intertidal ecosystems using optical remote sensing

In this thesis, we aimed to study the extent to which MPB 1) can be used as indicator for ecosystem functioning and 2) can structure higher trophic levels. A generic method was developed to retrieve MPB primary production rates from multispectral imagery (Landsat 8). Our MPB primary production model based on MPB biomass and mud content derived from satellite remote sensing, ambient temperature measurements and a tide model can be used as a tool to evaluate the effects of, e.g., a decrease in emersion duration and tidal flat area on MPB production. A natural stable isotope analysis showed that MPB forms a major component in the diet of macrofauna and that the relative contribution of MPB in their diet may vary among tidal systems. This highlights the pressing need to preserve these intertidal flats, as decreasing availability of microphytobenthos may have cascading effects up the estuarine food web. We showed that grazing pressure of macrofauna on microphytobenthos in summer/autumn is in the same order of magnitude as microphytobenthos production in early spring. A defaunation experiment showed that removal of macrofauna did not result in higher MPB standing stocks, indicating that grazing by macrofauna does not lower MPB biomass under the studied environmental conditions. This supports the use of remotely sensed information to quantify spatial variation in MPB food availability for higher trophic levels. UAV (drone) images of our experimental defaunation and control plots showed that on the micro scale (centimetre to meter scale), macrofauna altered the spatial heterogeneity of MPB in the top layer of the sediment. The presence of macrofauna decreased the ‘patch size’ of MPB. This indicates that macrofauna may not only increase the erodibility of the sediment by bioturbation, but may also prevent the formation of continuous biofilms that increase the stability of the sediment. Furthermore, a semi-variogram analysis on multispectral satellite imagery (Sentinel-2 MSI) revealed that the location of patches (on the meso scale) varied from winter to summer and that the degree of patchiness increased throughout the year. This information can be accounted for in spatially explicit food web models or sediment transport modelling