Effects of increasing current velocity, turbidity and particle-size selection on the feeding activity and scope for growth of Ruditapes decussatus from Ria Formosa, southern Portugal

This study examines the feeding activity of the infaunal bivalve Ruditapes decussatus L., in relation to increasing current velocities and concentrations of suspended particulate matter (SPM). An annular flume generated free-stream velocities of 0.6, 3, 8, 17, 34 and 36 cm s(-1) (estimated shear stress ranging from 0.001 to 3.8 Pa) and feeding rate was measured in terms of algal cell depletion (clearance rate) in the water column. Maximum clearance rate, ca. 2.5 lh(-1) ind(-1) for standard 0.3 g dry tissue mass individuals, occurred at the lower current velocities, up to 8 cm s(-1) and declined with increasing velocities, specially above 17 cm s(-1) when shear stresses were >0.89 Pa and sediment was eroding and moving. At low current velocities (<8 cm s(-1)) suspension feeding caused a significant depletion in the algal concentration in the water column within 10 cm of the bed. At very low current velocities ( < 1 cm s(-1)) the vertical profile showed that the lowest algal concentrations occurred at a height of 10 cm. Therefore the inhalent and exhalent siphons of the clam draw in and eject water at different heights in the water column, thus minimising the intake/recirculation of algal cell depleted water This strategy allows the maintenance of a higher filtration rate and scope for growth when compared with suspension feeders where inhalent and exhalent water currents are at the same height above the bed. At slow current velocities R. decussatus therefore maintains high feeding rates, in part by regulating the distribution of algal cells in the water column. There was a marked decline in clearance rates of clams with increasing concentration of suspended particulate matter, and this was accompanied by copious mucus production. Small particles (3-7 mu m) were retained at a lower efficiency at lower SPM concentrations (ten compared to 100 mg l(-1)) probably reflecting adaptation to a turbid natural environment containing a mixture of phytoplankton, suspended benthic algal cells and silt. (C) 2000 Elsevier Science B.V. All rights reserved.