Predation risk is believed to shape many aspects of the behavior, morphology and life history of marine organisms. The timing of synchronous larval release, postlarval migrations to adult habitat and diurnal vertical migrations are all considered adaptations to predictable variations in predation risk. However, despite the fact that predation risk is expected to vary predictably over time, this variation remains poorly understood for planktonic animals in the field. In this study, we conducted dock-based predation risk assays using tethered brine shrimp Artemia franciscana. We tested for the combined effects of prey life history stage and diurnal phase by measuring loss of adult and larval tethered individuals. We sampled during day and night, and during large and small amplitude tides on the Pacific coast of Panama. On the Caribbean coast of Panama and the Atlantic coast of Florida (where tidal amplitude is smaller), we sampled during day and night but did not test for an effect of tidal amplitude. Although predation risk differed between sites, the trends were the same at all 3 docks. Predation risk was significantly higher during the day than at night for larvae, whereas adults experienced the opposite trend in risk across the diurnal cycle, although the difference was not significant. Our results demonstrate a temporal gradient in planktonic predation risk across the diurnal cycle that depends on prey life history stage.

Predation is considered an important source of mortality for plankton, but documenting variation in planktonic predation, particularly across interacting environmental cycles, remains logistically difficult, thus our understanding remains limited. To test for the combined effects of prey life history stage, diel or light level phase (including crepuscular periods) and seasonal upwelling on the risk of predation, we deployed tethered adult and larval brine shrimp Artemia franciscana using dock-based plankton tethering units (PTUs). Risk was higher overall during upwelling, but life history stage also interacted with season. There was no seasonal difference in risk for adults. Larvae were at significantly lower risk of predation during non-upwelling than during upwelling. Larvae were also at lower risk during nonupwelling than were adults during either season. During upwelling, there was no significant difference in risk between the two prey categories. With respect to the diel cycle, dusk was safer than daytime. For larvae, the diel pattern in risk remained consistent across seasons while risk for adults at night was slightly lower during upwelling than during nonupwelling. Variation in planktonic predation risk across diel and seasonal cycles differs for different life history stages and thus, generalizations fail to capture the complexity of interactions between factors.

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