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The Burgundy‐blood phenomenon: a model of buoyancy change explains autumnal waterblooms by Planktothrix rubescens in Lake Zürich
Author(s) -
Walsby Anthony E.,
Schanz Ferdinand,
Schmid Martin
Publication year - 2006
Publication title -
new phytologist
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 3.742
H-Index - 244
eISSN - 1469-8137
pISSN - 0028-646X
DOI - 10.1111/j.1469-8137.2005.01567.x
Subject(s) - buoyancy , epilimnion , neutral buoyancy , atmospheric sciences , insolation , geology , atmosphere (unit) , ecology , hypolimnion , meteorology , biology , physics , climatology , thermodynamics , eutrophication , nutrient
Summary• Buoyancy changes of the cyanobacterium Planktothrix rubescens – the Burgundy‐blood alga – were modelled from its buoyancy response to light and irradiance changes in Lake Zürich during autumnal mixing. • The daily insolation received by filaments at fixed depths and circulating to different depths was calculated from the measured light attenuation and surface irradiance. The active mixing depth , z a5 , was determined from the vertical turbulent diffusion coefficient, K z , calculated from the wind speed, heat flux and temperature gradients. The fixed depth resulting in filament buoyancy, z n , decreased from 13 to 2 m between August and December 1998; the critical depth for buoyancy , z q , to which filaments must be circulated to become buoyant, decreased from >60 m in the summer to <10 m in winter. • When z a5 first exceeded z n , in September, P. rubescens was mixed into the epilimnion. In October, z q > z a5 : circulating filaments would have lost buoyancy in the high insolation. Often in November and December, after deeper mixing and lower insolation, z a5 > z q : filaments would have become buoyant but would have floated to the lake surface (the Burgundy‐blood phenomenon) only under subsequent calm conditions, when K z was low. • The model explains the Burgundy‐blood phenomenon in deeper lakes; waterblooms near shallow leeward shores arise from populations floating up in deeper regions of the lake.