Influence of turbulence on settlement of bivalve larvae
Iris E. Hendriks, Luca A. van Duren, Peter M.J. Herman
Introduction
Bivalves in estuaries have a pelagic larval stage. At the end of this stage, larvae need to settle on a suitable substrate in order to begin the adult phase of their life. Larvae are small, about 250 µm, and are weak swimmers in comparison to the tidal currents in the estuary. Their swimming speeds range in mm s-1 while tidal currents range from cm s-1 to up to a meter per second. This discrepancy indicates a strong influence of the hydrodynamic environment on settlement success of bivalve larvae.
Methodological problem
Flume tanks are excellent devices to investigate the effect of boundary layer flow on larval settlement. Flumes however have artificially lowered levels of turbulence and this causes a bias between the ratio of sinking and turbulent motion of small particles in the flume.
Conclusion
• Even with a grid turbulence levels in field conditions are higher than in the flume tank • Increased turbulence levels cause erosion of the viscous sublayer • Settlement of larvae is influenced by turbulence at low flow velocities • Larval settlement and larval mimic settlement differed, indicating active behaviour in the settlement phase
Objectives
• Compare turbulence levels in flume and in field conditions • Investigate the effect of turbulence level on settlement of bivalve larvae
Methods
Turbulence levels
• Field measurements with an Acoustic Doppler Velocimeter (ADV) of ambient turbulence levels in two systems. A wind driven system in Knebel Vig (Denmark) A tidally driven system in the Oosterschelde (the Netherlands). • Flume measurements of ‘normal’ turbulence levels in the flume at NIOO-CEME (fig.3) and increased turbulence levels with a grid (fig.2) throughout the water column.
Results
Turbulence levels
a) 5 cm s-1 b) 5 cm s-1
c) 15 cm s-1
d) 15 cm s-1
Figure 1: Larvae and larval mimics
Figure 2: Turbulence grid in the flume channel
Figure 3: Turning vanes in the bends and pvc pipes at the entrance of the working section act as collimators.
Figure 4. - ‘normal’ flume conditions, -flume conditions with a grid 3 m in front of the test section, -field conditions in Knebel Vig, -field conditions in the Oosterschelde. a) Turbulence Intensity (TI) at low flow conditions (5 cm s1 ) b) Reynolds (Re) stress at low flow velocity c) TI at 15 cm s-1 d) Re Stress at 15 cm s-1
Settlement of larvae at different turbulence levels
Mimics
Settlement of larvae at different turbulence levels
• ‘normal’ and increased turbulence levels in the flume • 2 free stream velocities, 5 and 15 cm s-1 • 500.000 passive larval mimics (polystyrene spheres) per treatment • Planktonic M. balthica larvae (Westerschelde) (fig.1) • Sampling design: 10 cores (0.003 m-2) random in test section
a)
Larvae
b)
Figure 5. Settlement at different turbulence levels in the flume, light colours indicate grid presence, dark ‘normal’ conditions. a) Larval mimics b) planktonic larvae M. balthica
Table 1. Péclet number for field and flume conditions at similar velocities, 4-6 cm s-1
z (mm) Flume, normal Flume, with grid Oosterschelde Knebel Vig 40 50 40 50 40 45 Pe 5.6 1.7 4.1 1.6 0.9 0.7
Vertical transport
Settlement and resuspension processes are determined by two types of motion: + Advective motion such as: sinking, swimming and transport due to shear in a velocity gradient + Random motion caused by turbulent diffusion Peclét number Pe =UL = sinking Kz turbulent mixing
U* = 8.26 mm s-1 Zo = 0.12 mm U* = 5.29 mm s-1 Zo = 0.0009 mm
Larvae differ from polystyrene spheres in swimming and attachment capabilities.
Figure 6 : Introducing turbulence throughout the water column has an effect on estimates of shear velocity (U*), and an even greater effect on the roughness length (Z0). This is probably due to the erosion of the viscous sublayer by turbulence.
Royal Netherlands Academy of Arts and Sciences
Netherlands Institute of Ecology, Centre for Estuarine and Marine Ecology (NIOO-CEME) Korringaweg 7, P.O. Box 140, 4400 AC YERSEKE, The Netherlands TEL. +31 113 577491 - FAX: +31 113 573616 - E-mail: i.hendriks@nioo.knaw.nl - URL: http://www.nioo.knaw.nl