Transitions between Auhcoseira and Anabaena dominance in a turbid river weir pool

Abstract

The transitions between the diatoms Aulacoseira spp. (Melosira) and the cyanobacteria Anabaena spp. as dominant phytoplankton species in a turbid-river weir pool are shown to depend directly on the establishment or destruction of persistent thermal stratification. A transition from high to low flow through the pool resulted in the establishment of persistent thermal stratification, causing Aulacoseira to sink out of the euphotic zone at a speed of 0.95 m d-l. Concurrently, the slightly buoyant Anabaena grew within the euphotic zone with a specific growth rate of 0.37 d-l, climaxing after approximately 14 d at a population of 20,000-30,000 cells ml I, at which point its biomass may have been limited by the availability of phosphorus. The stratification thus caused the phytoplankton population to separate into two distinct layers, with Anabaena occupying the illuminated surface layer and Aulacoseiru found only in the lower layer below the euphotic depth. Under stratified conditions, the ratio of the surface layer depth to euphotic depth, z,, : z,,, was approximately 1, whereas for a mixed water column that ratio was >3. Access to light appeared to be the main factor determining the dominant phytoplankton species. of the weir. Thanks also to Wendy Minato and Rhonda Smith, who uncomplainingly counted the algal samples, and to Mark Fink for the chemical analyses.

The Influence of a Eutrophic Lake to the River Downstream: Spatiotemporal Algal Composition Changes and the Driving Factors

“…Many existing cases have proven that intensified turbulence of water may lead to algae species shift from buoyant Cyanobacteria to sinking Bacillariophyta [14][15][16][17]. However, these experiments were conducted in the same aquatic systems (either in lakes or in rivers) rather than in the lake-to-river coupled systems.…”

Low-Reynolds-number swimming at pycnoclines

“…This is partly due to the notion that most organisms are too small to be affected by stratification, because the water density varies on a length scale, L ρ ¼ ρ 0 ∕γ ∼ OðkmÞ, much larger than the size of the organism, where ρ 0 is a reference density (e.g., 1;000 kg m −3 ) and γ is the vertical gradient in water density [typical values of γ range from Oð0.01Þ kg m −4 at ocean thermocline (8) to Oð1Þ kg m −4 in fjords and lakes (2,3)]. …”

“…Pycnoclines can trigger a wide range of environmental and oceanographic processes. In oceans and lakes, intense biological activity and accumulation of organisms and particles are associated with pycnoclines (2,3). For example, formation of phytoplankton blooms is often correlated with stratification (3), and these blooms can enhance CO 2 sequestration (4) or disrupt water supply systems (5).…”

Dissolved oxygen response to wind‐inflow interactions in a stratified reservoir

“…With the reduced inflow inertia, diatoms become progressively less adapted to the environment and sink to the sediment (Friedl and Wü est 2002). Significant sinking rates of riverborne diatoms have been observed in many other reservoirs (see, e.g., Shermann et al 1998;Garnier et al 2000;Tsujimura and Okubo 2003). Although the relative contribution of algae/bacteria competition for nutrients and diatom sinking rates still need to be quantified, both mechanisms may explain the reduction of Chl a (and associated DO decline) as the Snake River inflow approached Brownlee Reservoir.…”