COHEN, J.H.*; SINKHORN, E.R.; FORWARD, JR, R.B.; Duke University Marine Laboratory; Duke University Marine Laboratory; Duke University Marine Laboratory: The Role of Light in Diel Vertical Migration: field measurements of copepod migration behavior and the biologically relevant light environment

Behavioral responses of zooplankton to light are regarded as the primary proximate factor regulating diel vertical migration (DVM). The pontellid copepod Calanopia americana is a coastal species that undergoes nocturnal DVM; it resides in deep water or on the benthos during the day as a refuge from visual predators, rising in the water column at night to feed at the surface. Previous laboratory studies have shown that multiple light-related cues (i.e., exogenous light stimuli and endogenous rhythms) could underlie DVM in this species, providing some redundancy in the control of this predator-avoidance behavior. In the present study, DVM behavior of C. americana was measured near the mouth of a temperate estuary, while spectral and quantal measurements of the environmental light field were made simultaneously. The light data were used to calculate the light available for C. americana photobehavior by normalizing the quantal flux at each wavelength based on the spectral sensitivity of this copepod. Two 3 day experiments were conducted 7 days apart, allowing for the affect of tides to be evaluated. A significant diel pattern was found for both experiments, with copepods present in the water column at night, and absent during the day. Tides did not influence this behavior. Copepods initiated migration at sunset when the relative rate of irradiance decrease was greatest. Copepod density decreased around midnight, increased in the early morning, then decreased to daytime levels at sunrise when the relative rate of irradiance increase was greatest. These data agree with laboratory studies in suggesting that both exogenous light stimuli and endogenous rhythms cue DVM in C. americana.