Performance of sinking and nonsinking phytoplankton taxa in a gradient of mixing depths

Abstract

According to a recent dynamical model, the depth of a well-mixed water column should have contrasting effects on the abundances of sinking and nonsinking phytoplankton taxa. Because of increasing light limitation, nonsinking taxa should decline monotonically with increasing mixing depth, and because of sinking loss limitation at low mixing depths, sinking taxa should peak at intermediate mixing depths. Along a gradient of mixing depths, the position of this maximum should increase with increasing taxon-specific sinking velocity and decrease with increasing background turbidity. In two field-enclosure experiments, we investigated the effects of mixing depth and background turbidity on a variety of sinking and nonsinking phytoplankton taxa. We exposed the natural, 100-mscreened phytoplankton community of a clear, unproductive, but silica-rich lake to a gradient of mixing depths (1.5-15 m) during 4-6 weeks. To mimic two different background turbidities, the transparent enclosure walls were surrounded by either white or black foliage. Although diatoms suffered from high sedimentation losses at low mixing depths, they dominated biomass at all mixing depths throughout both experiments. Results were largely in accordance with model predictions. Specific gross growth rates of most common taxa were negatively related to mixing depth. In both experiments, the abundances of most sinking taxa showed a unimodal pattern along the mixing depth gradient, while two of three motile taxa declined monotonically with mixing depth. The depths where these taxa reached their maximal abundances were positively related to taxon-specific sinking velocity and negatively related to background turbidity.

Artificial mixing to control cyanobacterial blooms: a review

“…Sedimentation is a significant loss process for diatoms and non-motile green algae (Sommer 1988;Ptacnik et al 2003). Both the depth of the mixed layer and the intrinsic settling velocity of the algae determine the extent of sedimentation losses of the negatively buoyant species in a lake (Visser et al 1996a;Jäger et al 2008).…”

Turbulence increases the average settling velocity of phytoplankton cells

“…Turbulent modifications of C B are negligible for the vast majority of phytoplankton species and turbulence conditions of upper mixed layers (18). Lake experiments where plastic bags are used to generate a gradient of mixing depths clearly reveal a decrease of phytoplankton sedimentation with increasing H (19). At the ocean surface, high turbulence usually results in deep (hundreds of meters) mixed layers with respect to calm conditions (tens of meters), therefore modifying H by a factor of Ϸ10.…”

“…The inner spinning system was a PVC cylinder (4-cm diameter). Outer cylinders of different diameters (19,30, and 114 cm) were implemented and held static in all experiments, except for a case where the 19-cm outer cylinder was spinning in the opposite direction to the inner one. (c) Turbulence was generated by means of a vertically oscillating grid in a cylindrical tank 12.9 cm in diameter and 17.2 cm in height.…”

Temporal changes of phytoplankton community at different depths of a shallow hypertrophic reservoir in relation to environmental variables

“…Variation in phytoplankton communities also occurs with depth in response to environmental conditions (Huovinen et al, 1999;Ptacnik et al, 2003;Reynolds, 2006). The vertical distribution of phytoplankton in lakes appears to be affected by factors that include light, temperature, nutrients, predation, and mixing patterns within the water column, and, thus, their composition and biomass varies with depth (Huisman et al, 1999;Gervais et al, 2003;Ptacnik et al, 2003;Pinilla, 2006).…”

“…The vertical distribution of phytoplankton in lakes appears to be affected by factors that include light, temperature, nutrients, predation, and mixing patterns within the water column, and, thus, their composition and biomass varies with depth (Huisman et al, 1999;Gervais et al, 2003;Ptacnik et al, 2003;Pinilla, 2006). In particular, the influences of the gradient of incident light and mixing patterns in the water column have been studied as niches for different groups of species related to their motility, buoyancy, and size (Huisman et al, 1999;Ptacnik et al, 2003). The combination of nutrient availability and temperature is also a key factor in the spatial and temporal dynamics of phytoplankton, and affects their productivity and growth period (Reynolds, 1984(Reynolds, , 1988Wetzel, 2001).…”