ALBERT

Description: This study elucidates the interspecific interactions between competing unicellular predators in an intraguild predation system. The organisms studied were two microzooplankton (MZP) predators competing for the phototrophic dinoflagellate prey Scrippsiella trochoidea. Since the smaller dinoflagellate predator Gyrodinium dominans was also potential prey for the larger predator, the tintinnid ciliate Favella ehrenbergii, the experimental system included the probability of intraguild predation (IGP). The development of the three species was studied in set-ups containing either one of the two predators or both together with their prey. The IG predator F. ehrenbergii grew at a mean rate of 0.77 d-1 independent of the presence of the IG prey G. dominans. High grazing of the IG predator on the smaller IG prey was detected in treatments containing only the two predators. However, when all three species were present, the IG prey displayed significantly higher growth rates (0.42 d-1) compared to treatments containing only the IG prey as predator (0.32 d-1). The results of further experiments allowed the exclusion of mechanical or chemical signals induced by the IG predator being responsible for the observed increase in growth rate of IG prey. Live observations revealed that the IG predator rejected a significant proportion of its S. trochoidea catch after initial uptake. This behavior led to an immobilization of around 26% of the caught cells. We tested if this prey immobilization by the IG predator facilitated prey uptake by the IG prey and thus could be potentially responsible for the higher growth rates of the IG prey. Indeed, the smaller predator selected positively for immobilized prey and reacted with higher grazing and growth rates. Consequently, the IG prey benefitted from this commensalism between IG predator and IG prey and the strength of this pattern predominated IGP in our model system. As both predators co-occur in the same environment their feeding relationship could increase exploitation efficiency of common mobile prey items. Furthermore, such commensalism potentially opens a loophole for a stable coexistence of MZP predators despite their competition.

Description: The Laptev and Eastern Siberian shelves are the world’s broadest shallow shelf systems. Large Siberian rivers and coastal erosion of up to meters per summer deliver large volumes of terrestrial matter into the Arctic shelf seas. In this chapter we investigate the applicability of Ocean Colour Remote Sensing during the ice-free summer season in the Siberian Laptev Sea region. We show that the early summer river peak discharge may be traced using remote sensing in years characterized by early sea-ice retreat. In the summer time after the peak discharge, the spreading of the main Lena River plume east and north-east of the Lena River Delta into the shelf system becomes hardly traceable using optical remote sensing methods. Measurements of suspended particulate matter (SPM) and coloured dissolved organic matter (cDOM) are of the same magnitude in the coastal waters of Buor Khaya Bay as in the Lena River. Match-up analyses of in situ chlorophyll-a (Chl-a) show that standard Medium Resolution Imaging Spectrometer (MERIS) and Moderate Resolution Imaging Spectroradiometer (MODIS) satellite-derived Chl-a is not a valid remote sensing product for the coastal waters and the inner shelf region of the Laptev Sea. All MERIS and MODIS-derived Chl-a products are overestimated by at least a factor of ten, probably due to absorption by the extraordinarily high amount of non-algal particles and cDOM in these coastal and inner-shelf waters. Instead, Ocean Colour remote sensing provides information on wide-spread resuspension over shallows and lateral advection visible in satellite-derived turbidity. Satellite Sea Surface Temperature (SST) data clearly show hydrodynamics and delineate the outflow of the Lena River for hundreds of kilometres out into the shelf seas.