Short‐term displacement of Planktothrix rubescens (cyanobacteria) in a pre‐alpine lake observed using an autonomous sampling platform

Abstract

Short‐term changes in temporal and spatial distributions of the toxic cyanobacterium Planktothrix rubescens in Lake Zurich were investigated using high‐resolution data sets acquired with an autonomous surface vessel (ASV). Data profiles were recorded with a multi‐parameter probe while the ASV navigated along 1.5 km toward a waypoint located on the other side of the lake by using a global positioning system. We deployed the ASV seven times on five consecutive days during the stratification period (July 2011) to generate cross‐sectional views of temperature, light, oxygen, and phycoerythrin and chlorophyll fluorescence from surface to 18 m. The data were also used to compute daily photosynthetic rates. Data showed a daily reshaping of the P. rubescens distribution in the metalimnion on both horizontal and vertical axes, from patches to a shore‐to‐shore spreading. A thermocline depression observed after 16 h of sustained winds forced the accumulation of P. rubescens on the downwind shore. The compression of the metalimnion and its downward shift by 6 m within 24 h suggested the modulation of a longitudinal seiche following the wind event. This passive transport of the metalimnetic P. rubescens population resulted in a 90% light reduction, and a decrease of the averaged daily photosynthetic rate from +21 mmol O 2 m −2 d −1 to −10 mmol O 2 m −2 d −1 . Negative photosynthetic rates were computed on 2 d out of 5 d, meaning that the transport of P. rubescens by seiches significantly affected the balance between oxygen production and utilization in Lake Zurich, especially because it is the dominant primary producer.

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