[Enclosure experiments about the hydrodynamics effects on the plankton].

Area of Science:

• Aquatic Ecology
• Hydrodynamics
• Plankton Biology

Context:

• Investigating the influence of varying surface flow velocities on plankton communities.
• Experiments conducted over a year (August 2007-July 2008) using controlled flow enclosures.
• Flow velocities tested: 0.002, 0.10, 0.15, and 0.30 m/s.

Purpose:

• To elucidate the effects of hydrodynamics on the biomass and distribution of Spirogyra, Chl-a, and zooplankton.
• To understand the mechanisms by which shear stress and turbulence influence plankton reproduction, light exposure, and inter-species interactions.

Summary:

• Spirogyra biomass increased significantly (2.3x to 31.3x) under flow velocities of 0.15 and 0.30 m/s compared to static conditions.
• Chlorophyll-a (Chl-a) concentrations decreased under flow (45-54% of static levels at 0.10-0.15 m/s), while zooplankton biomass was substantially reduced (6-38% of static levels) at higher velocities.
• Proposed mechanisms include shear stress promoting Spirogyra reproduction, altered light availability for phytoplankton, Spirogyra allelopathy, and reduced zooplankton grazing efficiency due to turbulence.

Impact:

• Demonstrates that specific flow conditions can dramatically alter phytoplankton dominance and community structure.
• Highlights the complex, non-linear responses of different plankton groups to hydrodynamic forces.
• Provides insights into factors regulating primary productivity and food web dynamics in flowing waters.