Response of copepods to physical gradients associated with structure in the ocean

We studied the response of the copepods Acartia tonsa and Temora longicornis to spatial gradients of flow velocity and fluid density to determine whether the presence of physical gradients initiated local search for resources or promoted aggregation. Two additional species of copepods, Candacia ethiopica and Labidocera madurae, were also exposed to velocity gradients. A plane jet flume apparatus facilitated the isolation of physical structures that mimic those found in the ocean and permitted high-resolution behavioral observations. All four species significantly increased the proportion of time in the gradient layer region relative to the total time in the observation window (proportional residence time) in response to the velocity-gradient layer. Behavioral changes, such as increased swimming speed and turn frequency, were consistent with area-restricted search behavior. T. longicornis also significantly increased proportional residence time in response to the density-gradient layer, but changes in swimming speed and turn frequency were not significantly different. A. tonsa and T. longicornis appeared to contact the density gradient and swim away or along the boundary. Hence, density gradients may act as a barrier to vertical movement and not as a positive cue for area-restricted search behavior. Velocity and density gradients play important, yet different, roles in defining patterns at fine-to-intermediate scales in zooplankton ecology. Historically, ecological patterns in the plankton have been attributed to large-scale physical forcing; however, recent studies and observations have led to a realization that behavioral interactions of organisms to physical or chemical features at fine-to-intermediate scales may also be important (Franks 1995; Yamazaki et al. 2002; Gallager et al. 2004). Additionally, the ability of copepods to locate patches of high productivity is directly linked to survival (Mullin and Brooks 1976; Daro 1988); hence, behavioral responses of organisms at these scales will influence population dynamics and community structure (Alldredge et al. 2002; Cowles 2004). Ephemeral food patches in the ocean are often associated with steep gradients in physical properties, and behavioral adaptations that limit search to these regions may be advantageous (Poulet and Ouellet 1982; Buskey 1984; Daro 1988; Tiselius 1992; Saiz et al. 1993; Bochdansky and Bollens 2004). Many observations suggest that a large proportion (.75%) of phytoplankton biomass in a water column can be concentrated into one or a few well-defined regions (Holliday et al. 2003). These patches, often termed thin layers, may be 10s of centimeters thick, possibly less (Cowles et al. 1998), and are commonly associated with boundaries sepa1 Corresponding author ([email protected]).