Planktothrix in Lake Ammer

The aim of the project is to investigate the role of physical forcing, resource availability, and organismic interactions for the spatial and temporal distribution patterns of plankton in lakes. The research is focused on the distribution patterns of the buoyant cyanobacterium Planktothrix rubescens in Lake Ammer and compares the abundance of P. rubescens with the temporal and spatial variability of abiotic conditions and of phyto- and zooplankton. The main hypotheses are that:

  1. transport by internal wave motions has a substantial influence on the horizontal distribution patterns of P. rubescens and also affects the distribution of other phyto- and zooplankton;
  2. vertical water column motions associated with internal waves cause fluctuations in the vertical layering of P. rubescens and thus alters its competitive abilities;
  3. horizontal differences in habitat conditions, i.e. limited vertical water column depth in bays and resource gradients near river inflows, result in longer-term characteristic horizontal distribution patterns of P. rubescens and other plankton;
  4. layers of toxic P. rubescens may interfere with the vertical migration of zooplankton.  

These research questions will be addressed in extensive field experiments measuring horizontal and vertical distribution patterns of plankton and abiotic conditions at temporal scales ranging from minutes to several weeks. In-situ measuring techniques for plankton and abiotic parameters, providing sufficient temporal and spatial resolution, will be combined with water sample analyses to support them. The distribution of P. rubescens will be measured by using our newly developed in-situ technique that combines information from optic and acoustic instruments. The field experiments will be complemented with 3D and 1D model approaches. The intension of the modeling work is to support the interpretation of the field data by performing numerical experiments that investigate the response of horizontal distribution patterns of P. rubescens to physical forcing, patchy nutrient distribution (e.g. river inflow) or the presence of a shallow bay and by studying the implications of water column depth, internal wave induced fluctuations in light intensity, and grazing for the layering of P. rubescens in a vertical water column.