Abstract
Consequences of internal wave motion for phytoplankton and in particular for the distribution and production of the harmful and buoyant cyanobacterium Planktothrix rubescens were investigated based on data from two field campaigns conducted in Lake Ammer during summer 2009 and 2011. In both years, P. rubescens dominated the phytoplankton community and formed a deep chlorophyll maximum (DCM) in the metalimnion. Internal wave motions caused vertical displacement of P. rubescens of up to 6 m and 10 m, respectively. Vertical displacements of isotherms and of iso-concentration lines of P. rubescens from the same depth range coincided, suggesting that P. rubescens did not or could not regulate its buoyancy to prevent wave-induced vertical displacements. Diatoms dominated the phytoplankton community in the epilimnion and were vertically separated from P. rubescens. The thickness of the diatom layer, but not the diatom concentrations within the layer, changed in phase with the changes in the thickness of the epilimnion caused by internal wave motions. Seiche induced vertical displacements of P. rubescens caused fluctuations in the light intensity available at the depth of the P. rubescens layer. The interplay between seiche induced vertical displacements of the P. rubescens layer and the daily cycle of incident light lead to differences in the daily mean available light intensity between lake ends by up to a factor of ∼3. As a consequence, the daily mean specific oxygen production rate of P. rubescens differed by up to a factor of ∼7 between lake ends. The horizontal differences in the specific oxygen production rate of P. rubescens were persistent over several days suggesting that the associated production of P. rubescens biomass may lead to phytoplankton patchiness. The effect of internal seiches on the spatial heterogeneity and the persistence of horizontal differences in production, however, depend on the timing and the synchronization between internal wave motion and the daily course of incident light intensity. Vertical displacements caused by internal waves could be distinguished from other factors influencing the distribution of P. rubescens (e.g. active buoyancy control, production, vertical mixing) by a temperature-based data transformation. This technique may be of general use for separating wave-induced transport from other processes (e.g. sedimentation, vertical mixing) that affect the distributions of dissolved substances and suspended particles.
Highlights
Lakes are dynamic systems and organisms living within lakes are influenced by physical, chemical and biological factors which vary at different temporal and spatial scales
They found that vertical displacements of P. rubescens over a time period of three days altered available light sufficiently to cause a significant impact on production of P. rubescens [13]
This study investigates the impact of internal wave motions on the vertical distribution of phytoplankton in Lake Ammer with particular focus on the consequences of internal waves on the vertical distribution and production of P. rubescens
Summary
Lakes are dynamic systems and organisms living within lakes are influenced by physical, chemical and biological factors which vary at different temporal and spatial scales. Internal wave motions influence the distribution of species of different trophic levels and sizes by causing passive transport or by inducing an active response of the organisms. As internal waves change the vertical position of organisms and the amount of available light, Cuypers et al [6] suggested, that they can influence the growth rate of Planktothrix rubescens (former Oscillatoria rubescens). Garneau et al [13] presented field data showing the effect of short term displacements on the growth rate of P. rubescens They found that vertical displacements of P. rubescens over a time period of three days altered available light sufficiently to cause a significant impact on production of P. rubescens [13]
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