Effects of small-scale turbulence on copepods: The case of Oithona davisae

We report the effects of small-scale turbulence on the feeding rates of the marine copepod Oithona davisae. Laboratory experiments were conducted under a range of turbulence dissipation rates between 1024 and 101 cm2 s23. Net enhancements of feeding were observed only at the lowest, whereas negative net effects appeared only at the highest, turbulence intensities. These results contrast with expectations from an encounter-based model for this copepod species that predicted positive feeding enhancements at all turbulence intensities. This disagreement suggests the presence of detrimental effects at moderate and high turbulence intensities, very likely driven by either a lower mechanosensor perception capability or lower capture success. In comparison to other ambush copepods, O. davisae appears much more sensitive to the presence of turbulence, which might be the result of its strict ambush behavior, whereas copepods like Acartia tonsa or Centropages typicus, which can switch into different feeding modes, appear to benefit more from turbulence. The response of O. davisae feeding to turbulence in our experiments agrees with recent field observations on changes in the vertical distribution of Oithona as a function of wind-driven turbulence events. Hence, O. davisae seems to choose those depths where small-scale turbulence favors feeding. Although there was some early evidence suggesting that small-scale turbulence could affect zooplankton behavior and feeding (e.g., Singarajah 1975; Alcaraz et al. 1989), the seminal theoretical work of Rothschild and Osborn (1988) stimulated an intensified effort at quantifying this interaction (e.g., Sundby and Fossum 1990; MacKenzie and Leggett 1991; Saiz et al. 1992). In the last 15 yr, both laboratory experimentation (e.g., Saiz 1994; Landry et al. 1995; Caparroy et al. 1998) and modeling exercises (e.g., Kiørboe and Saiz 1995; Visser and MacKenzie 1998) have demonstrated that the increase in particle contact rates from turbulence affects such processes as aggregate formation and planktonic trophic interactions. Theoretically, the (root mean square) turbulence velocity adds to the particle relative motion, enhancing encounter as an inverse function of the velocity difference between particles (Rothschild and Osborn 1988; Kiørboe and Saiz 1995). In the case of zooplankton, the translation of this enhancement in encounter into higher ingestion rates depends on the feeding behavior of the predator (and very likely on the prey response to turbulence). Suspension-feeding and cruising zooplankton seem to benefit less by turbulence than ambush and pause-and-travel predators (Kiørboe and MacKenzie 1995; Kiørboe and Saiz 1995). Furthermore, turbulence effects depend on the turbulence dissipation rate («). While higher turbulence intensities could shift the Kolmogorov scale down into smaller