MEPS 242:153-167 (2002)  -  doi:10.3354/meps242153

ABSTRACT: Benthic suspension feeders repackage particulate matter into organic-rich biodeposits (feces and pseudofeces) that are released into near-bottom flows. Where suspension feeders are found in dense assemblages, they physically alter the near-bottom flow field and thus may affect the local pattern of particle deposition. This dispersal pattern will control the spatial flux of biodeposits, and potential effects on sediment properties and surrounding benthic fauna. We quantified the downstream dispersal of biodeposits produced by an array of horse mussels Atrina zelandica fed either cultured phytoplankton (P) or a mixture of phytoplankton and silt (PS), a diet range that mimics the natural variation in near-bed seston quality. Experiments were run in a laboratory flume at a free-stream velocity of 4.5 cm s^-1, a value typical of field conditions. Diet had a significant effect on A. zelandica feeding activity and the dispersal of biodeposits. Compared to the P biodeposits, a greater fraction of the PS biodeposits was ejected well above the horse mussel array into the accelerated overlying flow layer (62% compared with 35%). However, more of the PS biodeposits settled either within the A. zelandica array or in the 120 cm working section downstream of the array (43 vs 17% of P biodeposits). Despite overall similarity in size, PS biodeposits had a settling velocity twice that of the P biodeposits. Of biodeposits settling downstream of the array, more than 70% settled within 20 cm of the trailing edge, regardless of diet. The density of biodeposits decreased exponentially with distance from the array at a rate that was similar for both diets. Monte Carlo simulations with a simple boundary layer model generally reproduced this pattern, but also highlighted the importance of initial height above bottom as well as flow complexities in the downstream wake region. Thus, the quality and quantity of A. zelandica biodeposits reaching the bed depends on the interaction among behavioral responses to available diet, settling velocity and boundary layer flows. Such interactions should have strong but localized effects on the incorporation of pelagic productivity into the sediments and their consequent impacts on benthic community structure.

KEY WORDS: Biodeposits · Bivalve · Benthic-pelagic coupling · Suspension feeding · Atrina zelandica · Diet quality · Boundary layer · Fecal pellets